HL IB Biology Inheritance PDF

Head to www.savemyexams.com for more awesome resources HL IB Biology Your notes Inheritance Contents Genetic Inheritance & Genetic Crossing Inheritance: Terminology Inheriting Alleles Sex Determinat...

Head to www.savemyexams.com for more awesome resources HL IB Biology Your notes Inheritance Contents Genetic Inheritance & Genetic Crossing Inheritance: Terminology Inheriting Alleles Sex Determination Pedigree Charts Continuous Variation: Skills Dihybrid Crosses & Unlinked Genes (HL) Statistical Analysis of Dihybrid Crosses (HL) Genes & Polypeptides: Skills (HL) Gene Linkage (HL) Identifying Recombinants (HL) Page 1 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Genetic Inheritance & Genetic Crossing Your notes Inheritance: Gametes & Fertilisation Gametes are the sex cells of an organism For example, the sperm and egg (ovum) cells in humans The egg is larger than the sperm as most of its space contains food to nourish a growing embryo The sperm cell contains many mitochondria to release energy for its motion Gametes fuse during fertilization to form a zygote (fertilised egg cell) These sex cells are formed during meiosis and only have one copy of each chromosome and so are haploid cells For humans, that means the sperm and egg cells contain 23 single chromosomes in their nucleus (as opposed to diploid cells which contain 46 chromosomes, or 23 pairs) As there is only one chromosome from each homologous pair there is only one allele of each gene present This allele may be dominant, recessive or co-dominant Sperm cell diagram Egg cell diagram Page 2 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The structure of human gametes - the sperm and egg Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring When a male and female gamete fuse their chromosomes are combined This means the resulting zygote is diploid The zygote contains two chromosomes of each type It will therefore have two alleles of each gene If the two alleles for a particular gene are the same then the genotype is described as homozygous If the two alleles for a particular gene are different then the genotype is described as heterozygous Page 3 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Genetic Crosses in Flowering Plants Gregor Mendel was an Austrian monk Your notes In the mid-19th century, Mendel carried out breeding experiments on large numbers of pea plants whilst looking after the monastery gardens He studied how characteristics were passed on between generations of plants Due to his extensive work on the understanding of inheritance, he is sometimes called the Father of Genetics Mendel carefully transferred pollen from one pea plant to the reproductive parts of another Pollen contains the male gamete and is located on the anther of the flower The female gametes are located in the ovary The plants reproduce sexually and require pollination for fertilisation This technique eliminated any uncertainty from his data since he knew which pollen had fertilised each of the plants He collected the pea seeds from these plants and grew them in favourable conditions to find out their characteristics He also cross-bred offspring peas in order to find out which, if any characteristics would appear in future generations Mendel investigated the height of pea plants, the colours of their flowers and the smoothness of their seed coat Mendel's breeding experiments of pea plants diagram Page 4 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 5 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Mendel's pea plant crosses Mendel's Pea Plant Results Table Page 6 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Mendel found that characteristics were inherited in a predictable pattern All pea plants in the first generation had the same characteristic as one of the parental plants The offspring plants in the second generation had characteristics of both parent plants in a 3:1 ratio Your notes Without knowing it, Mendel had discovered genes, he referred to them as 'units of inheritance' He also discovered that some genes are dominant and some genes are recessive Different forms of the same gene are called alleles A monohybrid trait is one that is controlled by only one gene Generally, we consider that such a gene has two alleles Either: one allele is dominant and the other is recessive Or: the alleles are co-dominant A monohybrid cross starts with pure-breeding parents (homozygous), each displaying a different phenotype This generation is known as the parental generation, denoted as the P generation The purpose of a Punnett grid is to predict the probability of a certain offspring displaying a certain genotype or phenotype In the case where multiple offspring are produced, Punnett grids can predict the numbers of offspring that will display a certain genotype or phenotype after a cross Steps in constructing a Punnett Grid 1. Write down the parental phenotypes and genotypes 2. Write down all the possible gamete genotypes that each parent could produce for sexual reproduction A useful convention is to write the gamete genotypes inside a circle to denote them as gametes (haploid cells) 3. Place each parental genotype against one axis of a Punnett grid (2 x 2 table) 4. In the boxes of the Punnett grid, combine the gametes into the possible genotypes of the offspring This gives the offspring of the F1 generation (1st filial generation) 5. List the phenotype and genotype ratios for the offspring Worked example Sweet peas grow pods that are either green or yellow. The allele for green, G, is dominant to the allele for yellow, g. Construct a Punnett grid to predict the outcome when crossing green and yellow pure- bred plants to show the F1 generation offspring. Using plants from the F1 generation, construct a second Punnett grid to show the outcomes of the F2 generation. Step 1: Write down the parental phenotype and genotypes Green coloured pods Yellow coloured pods GG gg Step 2: Write down all the possible gamete genotypes that each parent could produce Page 7 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Step 3: Place each parental genotype against one axis of a Punnett grid (2 x 2 table) Step 4: Combine the gametes in each box of the Punnett grid Genotypes of the F1 cross between homozygous green (GG) and homozygous yellow (gg) pea plants. All offspring (100%) have the genotype Gg and the phenotype is green. Step 5: Take two heterozygous offspring from the F1 generation and cross them Page 8 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Step 6: Combine the gametes in each box of the Punnett grid Punnett grid showing the results of the F2 generation Phenotype ratio is 3:1 green:yellow, Genotype ratio is 1 GG: 2 Gg: 1 gg Plants can sexually reproduce in different ways: Some plants have the male and female reproductive parts within the same flower Others have male flowers and female flowers on the same plant Others have different male and female plants Plants with male and female reproductive parts on the same plant can be capable of self-pollination and self-fertilisation Farmers and ornamental plant growers can control the way their plants reproduce by artificially pollinating them Page 9 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources If a grower thinks a trait is useful or profitable they may choose to self-pollinate the favoured plants to keep the desirable traits in the next generation Growers can also cross-pollinate by artificial pollination between different plants with favoured Your notes traits, with the goal to create new generation of plants will possess the desirable traits from both parent plants Genetic crosses can be used to predict and plan for these outcomes Page 10 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Inheritance: Terminology Your notes Genotype A gene is a short length of DNA found on a chromosome that codes for a particular characteristic (by coding for the production of a specific protein) Alleles are variations of the same gene As we have two copies of each chromosome, we have two copies of each gene and therefore every individual will have two alleles of each gene One of the alleles is inherited from the mother and the other from the father The alleles may be the same as each other, or they might be different. e.g. an individual has two copies of the gene for eye colour but one allele could code for brown eyes and one allele could code for blue eyes The combination of alleles that an individual organism inherits is its genotype When the two alleles at a locus are the same/identical, an individual is said to have a homozygous genotype When the two alleles at a locus are different the genotype is said to be heterozygous Chromosome diagram Chromosomes showing genes, loci and alleles Page 11 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tip Your notes Make sure to not use the words allele and gene interchangeably. They both have different definitions and need to be used correctly in your exams in order to gain marks. Phenotype The observable characteristics of an organism (seen just by looking - like eye colour, or found – like blood type) is called the phenotype The phenotype of all characteristics is determined by the following factors: The genotype only - the combination of the two alleles for the gene for the characteristic, for example blood group is determined this way The environment only - surroundings such as chemical or radiation exposure, diet or exercise can affect physical characteristics, for example scars and accent are determined this way Interaction between both the genotype and the environment, for example height and skin colour are determined this way Page 12 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Dominant & Recessive Alleles Alleles can be dominant or recessive Your notes A dominant allele only needs to be inherited from one parent in order for the characteristic to be expressed in the phenotype A recessive allele needs to be inherited from both parents in order for the characteristic to be expressed in the phenotype. If there is only one recessive allele, it will remain hidden and the dominant characteristic will show If the two alleles of a gene are the same, we describe the individual as being homozygous (homo = same) An individual could be homozygous dominant (having two copies of the dominant allele), or homozygous recessive (having two copies of the recessive allele) If the two alleles of a gene are different, we describe the individual as being heterozygous (hetero = different) Different forms of allele pairs diagram Alleles are different forms of the same gene. You can only inherit two alleles for each gene, and they can be the same (homozygous) or different (heterozygous). Alleles can be dominant or recessive. Page 13 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Incomplete & Codominance Co-dominant alleles have a combined effect on the phenotype Your notes The alleles are both expressed to an equal extent in the phenotype An example of codominance is in speckled chickens Chickens can have different alleles for gene that determines the colour of their feathers We can denote the gene for colour using the capital letter C The two alleles for this gene are white for white feather colour, and black for black feather colour We denote the two alleles using superscript letters, CW and CB A chicken with the genotype CWCW has white feathers as their phenotype A chicken with the genotype CBCB has black feathers as their phenotype A chicken with the genotype CWCB has a combination of both feather colours, they are called speckled colour chickens Because both alleles are expressed in the phenotype this is called codominance Example of codominance in chickens diagram Diagram showing the phenotypes and genotypes of white, black and speckled chickens, which is an example of codominance Incomplete dominance is similar to codominance because two alleles are expressed together instead of just one dominant allele being expressed However, instead of both alleles being expressed, both alleles are partially expressed leading to a phenotype which is a blend of both phenotypes or an intermediate phenotype between the two An example is incomplete dominance can be found in the four o'clock flower or marvel of Peru (Mirabilis jalapa) Marvel of Peru can have different alleles for gene that determines the colour of their flowers We can denote the gene for colour using the capital letter C The two alleles for this gene are white for white flower colour, and red for red flower colour We denote the two alleles using superscript letters, CW and CR A plant with the genotype CWCW has white flowers as their phenotype A plant with the genotype CRCR has red flowers as their phenotype A plant with the genotype CWCR has a blend of both colours, which is expressed in the phenotype as a pink flower colour Page 14 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Because the flowers are neither white or red, but an intermediate between this two, this is incomplete dominance Your notes Example of incomplete dominance in marvel of Peru diagram Diagram showing the phenotypes and genotypes of white, red and pink flowers of the marvel of Peru, which is an example of incomplete dominance Examiner Tip When referring to different species examples in an exam you can use either the common name or the scientific name to gain marks. For example you could say 'four o'clock flower' or 'marvel of Peru' or 'Mirabilis jalapa' to be awarded the mark. Page 15 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Inheriting Alleles Your notes Phenotypic Plasticity Phenotypic plasticity is the idea that although genotype remains fixed throughout an organism's lifetime, the way that the phenotype is expressed can vary during this time An organism’s internal or external environment can influence gene expression patterns, and therefore phenotype The levels of regulatory proteins or transcription factors can be affected in response to environmental stimuli such as light, and chemicals including drugs and hormones For example, enzymes are activated in response to ultraviolet radiation and increase the expression and production of melanin, leading to skin pigmentation Temperature can also influence gene expression as demonstrated by organisms The Himalayan rabbit (Oryctolagus cuniculus L.) possesses a gene for the development of pigmentation in its fur The gene is inactive above 35°C but active between 15°C and 25°C In the parts of the body that are cooler such as ears, feet and nose the gene becomes active making these areas black Page 16 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Inheriting Recessive Alleles: Phenylketonuria Phenylketonuria (PKU) is an inherited condition caused by a recessive allele on an autosome Your notes It is a condition that can lead to symptoms such as mental disorders and seizures It is caused by a build-up of the amino acid phenylalanine in the body Phenylalanine comes from broken down protein from diet and our cells The enzyme phenylalanine hydroxylase breaks down phenylalanine This enzyme is coded for by the PAH gene PKU is caused by a mutation to the PAH gene that results in a non-functional enzyme so that the phenylalanine does not get broken down In the UK around 1 in 10,000 people are born with PKU In order for a child to have PKU, they must first inherit two recessive alleles from each of their parents Because it is caused by a recessive allele it means that two non-PKU sufferers could have a child with PKU if both parents are heterozygous carriers of the mutated PAH gene An example genetic cross is shown below: A genetic cross between two PKU carrier parents diagram Page 17 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Diagram showing the parental phenotypes, genotypes, gametes and a Punnett square predicting the possible genotypes of their offspring. Both parents are PKU carriers and their offspring have a 25% chance of inheriting the disorder. The genetic cross shown on the Punnett square above demonstrates that the offspring of the PKU carrier parents have a 75% chance of not having PKU and a 25% chance of inheriting 2 PKU alleles and therefore having the condition This pattern of inheritance is the same with any autosomal recessive condition, for example cystic fibrosis Every baby born in the UK and in many other countries around the world are tested for several genetic conditions including PKU The babies have a small prick of blood taken from the sole of their foot a few days after being born in order to be screened for the condition Page 18 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tip Your notes It should be the case that in most exams letters will be chosen for genetic crosses that have very different upper and lowercase appearances. If you are ever asked to use a letter in an exam that has a similar upper and lowercase appearance, such as P and p, make sure to overly exaggerate the difference to ensure there is no ambiguity during marking. Single Nucleotide Polymorphisms & Multiple Alleles Many genes have more than two alleles However, a diploid individual will still only inherit two of the possible alleles Alleles differ from each other by one or only a few bases Even a very small change in base sequence can bring about a large effect in gene function, with a large knock-on effect on the phenotype Even though different alleles of a gene have slightly different base sequences, they still occupy the same locus on the chromosome Since the Human Genome Project, sophisticated techniques can analyse different alleles The exact positions where bases differ between alleles are called SNPs or snips (Single Nucleotide Polymorphisms) An allele can have several SNPs but still only differ by a few bases from its other allele Page 19 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Multiple Alleles: ABO Blood Groups Inheritance of blood group is an example of co-dominance with multiple alleles Your notes This is of critical importance when deciding to give blood transfusions following injury or illness Use of the wrong blood group can cause an immune response that coagulates (solidifies) blood, leading to clots and serious illness/death There are three alleles of the gene controlling a person's blood group instead of the usual two I represents the gene Superscripts A and B represent the codominant alleles, IA for example Lowercase i with no superscript represents the recessive allele IA results in the production of antigen A on the surface of red blood cells IB results in the production of antigen B on the surface of red blood cells i results in no antigens being produced on the surface of red blood cells These three possible alleles can give us the following genotypes and phenotypes Blood Genotype & Phenotype Table We can use genetic diagrams to predict the outcome of crosses that involve the codominant alleles controlling blood groups Worked example Show how a parent with blood group A and a parent with blood group B can produce offspring with blood group O. Punnett square of the inheritance of blood group Page 20 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Punnett square showing the inheritance of blood group with two heterozygous parents, type A and type B Page 21 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Sex Determination Your notes Sex Determination in Humans Sex is determined by an entire chromosome pair (as opposed to most other characteristics that are just determined by one or a number of genes) Females have the sex chromosomes (pair 23 in humans) XX Males have the sex chromosomes (pair 23 in humans) XY Note that the rule XX for females and XY for males applies to mammals, but not to all species All other chromosomes (pairs 1 - 22 in humans) are autosomes and have no influence on determining the sex of offspring Because only a father can pass on a Y chromosome, he is responsible for determining the sex of the child Due to meiosis, half of his sperm cells will carry his X chromosome, half his Y chromosome The chromosome carried by the sperm that fertilises the egg will determine the sex of the child His daughters receive a copy of his X chromosome His sons receive a copy of his Y chromosome Sex determination in humans diagram Page 22 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Sperm cells determine the sex of offspring The inheritance of sex can be shown using a genetic diagram (known as a Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes Page 23 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Sex determination Punnett square Your notes Punnett square showing the inheritance of sex due to the combination of the X and Y chromosomes from each of the gametes Genes carried by X and Y chromosomes The X chromosome is larger than the Y, and has its centromere more central than on the Y chromosome Fewer genes are coded for on the Y chromosome as a result The X carries around 16 × more genes than the Y chromosome Non-sex phenotypic traits, including certain blood clotting factors, are coded for on the X chromosome but not on the Y The Y chromosome carries genes that code for male characteristics One of these genes is the SRY gene which is involved in Development of testes in male embryos Production of testosterone Females don't receive these genes, so instead, ovaries develop and female sex hormones are expressed Page 24 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Sex Linked Disorders: Haemophilia Some genetic diseases in humans are sex-linked Your notes Inheritance of these diseases is different in males and females Sex-linked genes are only present on one sex chromosome and not the other This means the sex of an individual affects what alleles they pass on to their offspring through their gametes If the gene is on the X chromosome, males (XY) will only have one copy of the gene, whereas females (XX) will have two There are three phenotypes for females: normal carrier has the disease, Males have only two phenotypes norma has the disease Haemophilia is a well known sex-linked disease There is a gene found on the X chromosome that codes for a protein called factor VIII. Factor VIII is needed to make blood clot There are two alleles for factor VIII The dominant F allele which codes for normal factor VIII The recessive f allele which results in a lack of factor VIII, meaning a person has haemophilia When a person possesses only the recessive allele f, they don’t produce factor VIII and their blood can't clot normally If males have an abnormal allele, f, they will have the condition as they have only one copy of the gene Females can be heterozygous for the faulty gene and not suffer from the condition but act as a carrier This means that haemophilia is a potentially fatal genetic disease which affects males more than females Examiner Tip The expected notation when writing about sex linked alleles is to use upper case 'X' and 'Y' for the chromosome, next to superscript letters to represent the allele. For example XfXf Homozygous female who has haemophilia or XFXf Heterozygous female who is a carrier XfY Male who has haemophilia Page 25 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes The genetic diagram below shows how two parents with normal factor VIII can have offspring with haemophilia Parental phenotypes: carrier female x normal male Parental genotypes: XFXf XFY Parental gametes: XF or Xf XF or Y Monohybrid Punnett Square with Sex-linkage Table Predicted ratio of phenotypes in offspring 1 female with normal blood clotting : 1 carrier female : 1 male with haemophilia : 1 male with normal blood clotting Predicted ratio of genotypes in offspring: 1 XFXF : 1 XFXf : 1 XFY : 1 XfY Page 26 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tip Your notes Make sure to include all of your working out when constructing genetic diagrams. It is not enough just to complete a Punnett grid, you need to show that you have thought about the possible gametes that can be produced by each parent. Also, remember to state the phenotype as well as the genotype of the offspring that result from the cross. Read the questions carefully when answering sex-linked inheritance questions – is the question asking for a probability for all children or is it asking about a specific sex (males or females). Page 27 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Pedigree Charts Your notes Pedigree Charts Family pedigree diagrams are usually used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family This can be used to work out the probability that someone in the family will inherit the genetic disorder Pedigree chart diagram A family pedigree chart Males are indicated by the square shape and females are represented by circles In this diagram, affected individuals are red and unaffected are blue Shading or cross-hatching may also be used to show affected individuals Horizontal lines between males and females show that they have produced children (which are linked underneath each couple) Roman numerals may be used to indicate generations For each generation the eldest child is on the left and each individual is numbered The family pedigree above shows: Both males and females are affected Every generation has affected individuals Page 28 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The eldest son (in the second generation) is affected That there is one family group that has no affected parents or children The other two families have one affected parent and affected children as well Your notes The study of pedigree charts provides an opportunity to appreciate why marriage between close relatives is prohibited in many countries Reproducing with close relatives increases the chance that both individuals possess harmful recessive alleles that can be passed onto the offspring This causes the offspring to have a much higher chance of inheriting genetic diseases Worked example Worked example: Pedigree charts Below is a pedigree chart which traces the inheritance of albinism across several generations. Albinism affects the production of the pigment melanin leading to lighter hair, skin and eyes. Using the pedigree chart, deduce and explain the following: 1. What type of allele causes albinism? 2. The genotype of individuals named 9 and 7 3. The possible genotypes of 10 and 11 1. Albinism is caused by a recessive allele Explanation: We can tell this from the pedigree chart because expression of the disease skips generation II. Also, person number 9 is an affected individual despite his parents (6 and 7) being unaffected. 6 and 7 must both be carriers of the recessive allele and 9 has inherited one recessive allele from each parent. It is unlikely to be a sex-linked disease as both females and males have the condition 2. The genotype of person 9 must be homozygous recessive (aa) and the genotype of 7 must be heterozygous (Aa) Explanation: 9 is an affected individual with albinism (which is determined by the recessive allele). 7 must be heterozygous in order for him to pass on the recessive allele to person 9 3. The possible genotypes of 10 and 11 are heterozygous (Aa) or homozygous dominant (AA) Page 29 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Explanation: This is because they are unaffected individuals so must possess at least one dominant allele (A), however, it is possible that they each inherited a dominant allele from each parent Your notes Examiner Tip When answering questions about pedigree charts for genetic diseases, it is always useful to remember which phenotype is caused by the recessive allele. You can write these genotypes onto your chart and it will give you a good starting point for working out the possible genotypes of the rest of the individuals in the chart. NOS: Scientists draw general conclusions by inductive reasoning Inductive reasoning is the idea of making generalised conclusions based on specific evidence taken from a small sample For example, we could observe a sample of evidence from a pedigree chart and if that observation deviated from what we would expect we could surmise that the condition could be sex-linked Deductive reasoning is making specific deductions about something unknown based on known evidence For example, if two non-affected parents have a child that is affected by a genetic condition we can deduce that the condition is caused by a recessive allele, and that the parents are both carriers of the allele Page 30 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Continuous Variation: Skills Your notes Continuous Variation Variation can be discrete or continuous Discrete variation is variation that falls into two or more clear-cut categories with no overlap or in- between categories Blood group is an example of discrete variation All human blood is either group O, A, B or AB, each with a Rhesus factor (+ or -) This gives just 8 distinct blood groups: Pie chart showing global blood group distribution Worldwide A, B, O blood group distribution by percentage, 2019 (data varies regionally with ethnicity) Continuous Variation Continuous variation occurs when two or more genes affect the final characteristic For example, height in humans is determined by many genetic factors: Bone length Skeletal muscle structure Ability to absorb food substances effectively Page 31 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Hormone production …As well as environmental factors like diet, exercise, prenatal nutrition, lifestyle etc Most characteristics are determined by more than one gene - a polygenic characteristic Your notes Even grouped data like shoe size appears to be discrete but in fact, peoples' feet vary continuously in size Shoe size is merely a practicality for shoe manufacturers, who cannot make exactly the right-sized shoes for everybody Continuous variation in birth mass results in the population displaying a normal distribution (bell- shaped curve) Environmental factors can also affect birth mass, e.g. mother's diet, presence of a twin, smoking etc Continuous variation occurs when there are quantitative differences in the phenotypes of individuals within a population for particular characteristics Quantitative differences do not fall into discrete categories like in discontinuous variation For example, the mass or height of a human is an example of continuous variation Instead for these features, a range of values exist between two extremes within which the phenotype will fall The lack of categories and the presence of a range of values can be used to identify continuous variation when it is presented in a table or graph Normal distribution curve Page 32 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Graph showing population variation in height: an example of continuous variation with quantitative differences Genetic basis of continuous variation This type of variation is caused by an interaction between genetics and the environment Phenotype = genotype + environment At the genetic level: Different alleles at a single locus have a small effect on the phenotype Different genes can have the same effect on the phenotype and these add together to have an additive effect If a large number of genes have a combined effect on the phenotype they are known as polygenes An example of a continuous polygenic trait is skin colour Skin colour is determined by several genes that cause the production of a protein called melanin The more melanin is produced, the darker the skin pigmentation becomes Skin colour is also influenced by environmental factors such as UV exposure, which can cause the skin colour to become darker Comparison of Continuous and Discontinuous Variation Table Page 33 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 34 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Box & Whisker Plots What are box plots and when should they be used? Your notes Box plots are also known as box-and-whisker diagrams They are used when we are interested in splitting data up into quartiles Using quartiles and drawing a box plot allows us to see what is happening at the low, middle and high points and consider any possible extreme values How to draw a box plot You need to know five values to draw a box plot Lowest data value First quartile Median Third quartile Highest data value Usually on graph paper, box plots are drawn accurately with the five points marked by short vertical lines The middle three values then form a box with the median line inside The median will not necessarily be in the middle of the box The box represents the interquartile range (middle 50% of the data) The lowest data value and highest data value are joined to the box by horizontal lines These are often called whiskers They represent the lowest 25% of the data and the highest 25% of the data You may be given a box plot From which you can read off the five values Calculate other statistics like the range and interquartile range (IQR) Page 35 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes The table gives some information about the heights of 80 girls: Height (cm) Least height 133 Greatest height 170 First quartile 145 Third quartile 157 Median 151 Outlier 122 A box plot of this data is as follows: Box and whisker plot showing height data from a population of girls The width of the box shows the interquartile range and the overall range is shown by the length of the whiskers Outliers Page 36 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Outliers are data points that exist at the extremes above and below the rest of the data In order for a data point to be categorised as an outlier it needs to be more than 1.5 × the interquartile range above the third quartile or below the first quartile Your notes As shown in the worked example above, the outliers are plotted separately to the whiskers of the plot Page 37 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Dihybrid Crosses & Unlinked Genes (HL) Your notes Segregation & Independent Assortment Unlinked genes segregate independently as a result of meiosis Unlinked genes are genes that an organism carries on separate chromosomes Not on homologous copies of the same chromosome An example of a pair of unlinked genes in fruit flies (Drosophila melanogaster) is The gene for curly wings on chromosome 2, and The gene for mahogany eyes on chromosome 3 An example of a pair of unlinked genes in humans is The gene for trypsin (a stomach enzyme) on chromosome 7, and The gene for human growth hormone on chromosome 17 Assortment of chromosomes refers to their alignment in metaphase I of meiosis Each bivalent assorts (aligns) itself independently of all the others Segregation of chromosomes (i.e. how they get separated) is governed by their pattern of assortment Segregation just refers to which pole of the cell the whole chromosomes are pulled to in anaphase I Segregation determines which combinations of alleles end up in which gamete cells by the end of meiosis II We call this Mendel's Law of Independent Assortment which states that alleles of different genes are inherited independently of one another; in other words inheriting a particular allele for one gene doesn't affect the ability to inherit any other allele for another gene By contrast, linked genes (on the same chromosome) tend to be inherited together Linked and unlinked genes diagram Page 38 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The loci of selected genes in the human genome Trypsin and CFTR are linked genes (both on the same chromosome); Human Growth Hormone and trypsin are unlinked genes (both on different chromosomes) Page 39 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Dihybrid Crosses Monohybrid crosses look at how the alleles of one gene transfer across generations Your notes Dihybrid crosses look at how the alleles of two genes transfer across generations i.e. dihybrid crosses can be used to show the inheritance of two completely different characteristics in an individual, for example unlinked genes The genetic diagrams for both types of cross are very similar For dihybrid crosses, there are several more genotypes and phenotypes involved When writing out the different genotypes, write the two alleles for one gene, followed immediately by the two alleles for the other gene Do not mix up the alleles from the different genes For example, if there was a gene with alleles Y and y and another gene with alleles G and g an example genotype for an individual would be YyGg Alleles are usually shown side by side in dihybrid crosses e.g. TtBb Page 40 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes Worked example 1: Dihybrid genetic diagram Horses have a single gene for coat colour that has two alleles: B, a dominant allele produces a black coat b, a recessive allele produces a chestnut coat Horses also have a single gene for eye colour E, a dominant allele produces brown eyes e, a recessive allele produces blue eyes Each of these genes (consisting of a pair of alleles) are inherited independently of one another because the two genes are located on different non-homologous chromosomes Such characteristics are said to be unlinked In this example, a horse that is heterozygous for both genes has been crossed with a horse that is homozygous for one gene and heterozygous for the other Parental phenotypes: black coat, brown eyes x chestnut coat, brown eyes Parental genotypes: BbEe x bbEe Parental gametes: BE or Be or bE or be x bE or be Determining the Alleles Carried by Gametes Based on the Parental Genotypes Using the FOIL (First, Outside, Inside, Last) Method Page 41 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Dihybrid Cross Punnett Square Table Your notes Predicted ratio of phenotypes in offspring 3 black coat, brown eyes : 3 chestnut coat, brown eyes : 1 black coat, blue eyes : 1 chestnut coat, blue eyes Predicted ratio of genotypes in offspring = 3 BbEE : 3 bbEE : 1 Bbee : 1 bbee Page 42 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes Worked example 2: Dihybrid genetic diagram In a separate cross to that shown in Worked Example 1, a horse that is heterozygous for both genes has been crossed with another horse that is heterozygous for both genes. Parental phenotypes: black coat, brown eyes x black coat, brown eyes Parental genotypes: BbEe x BbEe Parental gametes: BE or Be or bE or be x BE or Be or bE or be Dihybrid Cross Punnett Square Table 2 Predicted ratio of phenotypes in offspring 9 black coat, brown eyes : 3 chestnut coat, brown eyes : 3 black coat, blue eyes : 1 chestnut coat, blue eyes Page 43 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes Worked example 3: Dihybrid genetic diagram In the final worked example, a horse that is heterozygous for both genes has been crossed with another horse that is homozygous recessive for both genes. Parental phenotypes: black coat, brown eyes x chestnut coat, blue eyes Parental genotypes: BbEe x bbee Parental gametes: BE or Be or bE or be x be Dihybrid Cross Punnett Square Table 3 Predicted ratio of phenotypes in offspring = 1 black coat, brown eyes : 1 chestnut coat, brown eyes : 1 black coat, blue eyes : 1 chestnut coat, blue eyes Page 44 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tip Your notes For the double-heterozygous cross for unlinked genes above, you're expected to remember the phenotypic ratio 9:3:3:1, and for one heterozygous parent and one homozygous recessive parent, you're expected to remember the phenotypic ratio 1:1:1:1. You won't need to remember the ratio of the genotypes but this can be worked out from a Punnett square like the one above. NOS: 9:3:3:1 and 1:1:1:1 ratios for dihybrid crosses are based on what has been called Mendel's second law Mendel's second law, also known as the law of independent assortment, states that a pair of traits segregates independently of another pair during meiosis when the sperm and egg cells are created during meiosis As the different genes undergo independent assortment, different traits get the same opportunity to be expressed together When Mendel was carrying out his experiments and coming up with his laws it was at a time when DNA and the chemical basis of inheritance was unknown Since then it has been discovered that the law is not universally true, but requires certain conditions in order to occur For example, linked genes on the same chromosome do not follow the same observed ratios of phenotypes than would be expected if Mendel's second law was always correct Make sure that you remember that there are exceptions to all biological “laws” under certain conditions Page 45 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Statistical Analysis of Dihybrid Crosses (HL) Your notes Chi-squared Test & Dihybrid Crosses Use of a chi-squared test on data from dihybrid crosses The difference between expected and observed results in experiments can be statistically significant or insignificant (happened by chance) If the difference between results is statistically significant it can suggest that something else is happening in the experiment that isn’t being accounted for For example, linkage between genes A statistical test called the chi-squared test determines whether there is a significant difference between the observed and expected results in an experiment The chi-squared test is completed when the data is categorical (data that can be grouped) Calculating chi-squared values 1. Obtain the expected and observed results for the experiment 2. Calculate the difference between each set of results 3. Square each difference (as it is irrelevant whether the difference is positive or negative) 4. Divide each squared difference by the expected value and get a sum of these answers to obtain the chi-squared value Analysing chi-squared values To work out what the chi-squared value means, a table that relates chi-squared values to probabilities is used If the chi-squared value represents a larger probability than the critical probability then it can be stated that the differences between the expected and observed results are due to chance If it represents a smaller probability than the critical probability then the differences in results are significant and something else may be causing the differences To determine the critical probability biologists generally use a probability of 0.05 (p = 0.05) Page 46 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources They allow that chance will cause five out of every 100 experiments to be different The number of comparisons made must also be taken into account when determining the critical probability. This is known as the degrees of freedom Your notes Every hypothesis test must begin with a clear null hypothesis (what we believe to already be true) and alternative hypothesis (how we believe the data pattern or probability distribution might have changed) The null hypothesis is denoted H0 and sets out the assumed population parameter given that no change has happened The standard null hypothesis is that there is no significant difference between the expected and observed frequencies, and any difference that does occur is due to chance The alternative hypothesis is denoted H1 and sets out how we think the population parameter could have changed The standard alternative hypothesis is that there is a significant difference between the expected and observed frequencies When a hypothesis test is carried out, the null hypothesis is assumed to be true and this assumption will either be accepted or rejected Page 47 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes An experiment was carried out investigating the inheritance of two genes in rabbits One for coat colour and one for ear length A dihybrid cross revealed the expected ratio of phenotypes to be 9 : 3 : 3 : 1 Several rabbits with the double-heterozygous genotype were bred together and the phenotypes of all the offspring were recorded The ratio of the offspring was not exactly what was predicted, but was reasonably close In order to determine whether this was due to chance or for some other reason, the chi-squared test was used The expected number of each phenotype is the fraction of the total number of rabbits governed by the 9:3:3:1 ratio These are 9/16, 3/16, 3/16 and 1/16 of 160, respectively In order to understand what this chi-squared value of 2.93 says about the data, a table relating chi- squared values to probability is needed Chi-squared critical values table Page 48 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The chi-squared table displays the probabilities that the differences between expected and observed are due to chance The degrees of freedom can be worked out from the results. It is calculated by subtracting one from the number of classes In this example, there are four phenotypes which means four classes, 4 – 1 = 3 This means that the values in the third row are important for comparison For this experiment, there is a critical probability of 0.05 This means that 7.82 is the value used for comparison The chi-squared value from the results (2.93) is much smaller than 7.82 2.93 would be located somewhere to the left-hand side of the table, representing a probability much greater than 0.1 This means that there is no significant difference between the expected and observed results, any differences that do occur are due to chance We accept the null hypothesis Page 49 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes An experiment was carried out on Drosophila genes One gene for body colour B = brown, b = black One gene for wing shape W = straight, w = curved A dihybrid cross revealed the expected ratio of phenotypes to be 9 : 3 : 3 : 1 Several Drosophila with the double-heterozygous genotype were bred together and the phenotypes of all the F2 offspring were recorded The ratio of the offspring was far from what was predicted In order to determine whether this was due to chance or for some other reason, the chi-squared test was used The expected number of each phenotype is the fraction of the total number of flies governed by the 9:3:3:1 ratio These are 9/16, 3/16, 3/16 and 1/16 of 597, respectively In order to understand what this chi-squared value of 235 says about the data, a table relating chi- squared values to probability is needed Chi-squared critical values table Page 50 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The chi-squared table displays the probabilities that the differences between expected and observed are due to chance The degrees of freedom can be worked out from the results. It is calculated by subtracting one from the number of classes In this example, there are also four phenotypes which means four classes, 4 – 1 = 3 This means that the values in the third row are important for comparison For this experiment, there is a critical probability of 0.05 This means that 7.82 is the value used for comparison The chi-squared value from the results (235) is much greater than 7.82 235 would be located somewhere off the far right of the table, representing a very small probability, much less than 0.001 This means that there is a significant difference between the expected and observed results, any differences that do occur are due to a factor other than chance We reject the null hypothesis In this case, that factor is (autosomal) gene linkage Conclusion The alleles for black/brown body colour and straight/curved wings are linked, ie. carried on the same chromosome (autosome) Page 51 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tip Your notes When calculating a chi-squared value it is very helpful to create a table like the ones seen in the worked examples. This will help you with your calculations and make sure you don’t get muddled up! You should also be prepared to suggest reasons why results might be significantly different. For example, there could be linkage between the genes being analysed. NOS: Students should recognise that statistical testing often involves using a sample to represent a population When handling data involving large populations, it is often impossible to collect data on every single member of that population In these scenarios the best option is to take a large sample of the data from a percentage of the population The larger the sample, the higher the chance that the data from the sample is representative of the whole population This is common in ecological sampling, such as when collecting data using a quadrat, or in the context of studying inheritance of within populations In many experiments the sample is the replicated or repeated measurements Page 52 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Genes & Polypeptides: Skills (HL) Your notes Genes & Polypeptides: Skills Use of databases to identify the locus of a human gene and its polypeptide product Following the sequencing of the whole human genome, we now know the exact locus (position) of every gene across the 23 pairs of chromosomes Online databases have been built that are able to locate any known gene or allele Anyone can access these loci One example is the European Molecular Biology Laboratory database (EMBL) Examples of genes that can be located are The CFTR protein, critical to cystic fibrosis, on chromosome 7 HBB, a faulty allele of which is the cause of sickle-cell anaemia, on chromosome 11 If we know the locus of a particular gene, medicine can establish the location of a faulty allele, which is often recessive A faulty allele can be cut out of the chromosome by genetic engineering using recombinant DNA technology Replacing a faulty allele could lead to genetic therapy Location databases of cancer-related genes are often vital information to researchers, doctors and patients involved in cancer genetics Databases can be used to find and compare information about DNA, mRNA and protein sequences diagram Page 53 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The use of databases to compare base sequences (and protein sequences) between species Page 54 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Worked example Your notes Let's take the example of the gene that has a mutated version that causes cystic fibrosis, called CFTR The location of this gene, which can be searched for in the database, is 7q31.2 The 7 represents the fact it is found on chromosome 7 The q represents the fact it is found of the long arm of the chromosome, which is called q The short arm is p 31.2 represents how far away the gene is from the location of the centromere of the chromosome The smaller the number, the closer it is to the centromere We can compare this information to another gene, which encodes a protein that helps with blood clotting, coagulation factor X. If this gene is mutated it can lead to haemophilia The location of the coagulation factor X gene is 13q34 This is found on chromosome 13 A conclusion that can be made about these two genes is that they are found on different chromosomes and so their inheritance is not linked Remember that when genes are located close together they are likely to be inherited together due to autosome linkage For example, a gene located at 3p24.1 is not going to be linked to a gene at 6q13.2 because they are found on different chromosomes However, a gene located at 3p24.1 is likely to be linked to a gene found at 3q27.8, and even more likely to be linked to a gene found at 3p18.9, because they are not only on the same chromosome, but on the same arm of the chromosome and located close together (a similar distance from the centromere) Examiner Tip You may be asked to compare this data in an exam and comment on the likelihood of linkage. Page 55 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Gene Linkage (HL) Your notes Autosomal Gene Linkage Gene loci are said to be linked if they are on the same chromosome Loci (singular: locus) refers to the specific linear positions on the chromosome that genes occupy If genes are on the sex chromosome, they are said to be sex-linked Sex-linked genes have characteristics that generally only affect one gender of a species These genes are usually on the X chromosome because the Y chromosome contains very few genes In humans, colour-blindness and haemophilia are notable examples of genetic conditions that only affect males Linked genes located on the chromosomes 1-22, or any chromosome that is not a sex chromosome (called autosomes) are said to be examples of autosomal linkage The likelihood of genes being inherited together, or the extent to which they are linked, is measured in units called centimorgans, in honour of Thomas Hunt Morgan's work Notation for linked genes A common way of denoting linked alleles is to link them with lines, representing the homologous chromosomes E.g. in a cross between a double heterozygous individual where the dominant alleles G and R are linked and the recessive alleles g and r are linked, the notation would be: Autosomal linkage Dihybrid crosses and their predictions rely on the assumption that the genes being investigated behave independently of one another during meiosis However, not all genes assort independently during meiosis Page 56 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Some genes which are located on the same chromosome display autosomal linkage and stay together in the original parental combination Linkage between genes affects how parental alleles are passed onto offspring through the gametes Your notes Identifying autosomal linkage from phenotypic ratios In the following theoretical example, a dihybrid cross is used to predict the inheritance of two different characteristics in a species of newt The genes are for tail length and scale colour The gene for tail length has two alleles: Dominant allele T produces a normal length tail Recessive allele t produces a shorter length tail The gene for scale colour has two alleles: Dominant allele G produces green scales Recessive allele g produces white scales Without linkage The outcomes for this dihybrid cross if the genes are unlinked are as follows Dihybrid Cross without Linkage Punnett Square Table Page 57 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Predicted ratio of phenotypes in offspring = 1 normal tail, green scales : 1 normal tail, white scales : 1 short tail, green scales : 1 short tail, white scales Your notes Predicted ratio of genotypes in offspring = 1 TtGg : 1 Ttgg : 1 ttGg : 1 ttgg With linkage However, if the same dihybrid cross is carried out but this time the genes are linked, we get a different phenotypic ratio There would be a 1 : 1 phenotypic ratio (1 normal tail, green scales : 1 short tail, white scales) This change in the phenotypic ratio occurs because the genes are located on the same chromosome The unexpected phenotypic ratio, therefore, shows us that the genes are linked The explanation for this new phenotypic ratio is given in the worked example below: Worked example Worked example: Explaining autosomal linkage In reality, the genes for tail length and scale colour in this particular species of newt show autosomal linkage Parental phenotypes: normal tail, green scales x short tail, white scales Parental genotypes: (TG)(tg) (tg)(tg) Parental gametes: (TG) or (tg) (tg) Dihybrid Cross with Linkage Punnett Square Table Page 58 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Predicted ratio of genotypes in offspring = 1 (TG)(tg) : 1 (tg)(tg) Predicted ratio of phenotypes in offspring = Your notes 1 normal tail, green scales : 1 short tail, white scales Examiner Tip When you are working through different genetics questions you may notice that test crosses involving autosomal linkage predict solely parental type offspring (offspring that have the same combination of characteristics as their parents). However in reality recombinant offspring (offspring that have a different combination of characteristics to their parents) are often produced. This is due to the crossing over that occurs during meiosis. The crossing over and exchanging of genetic material breaks the linkage between the genes and recombines the characteristics of the parents. So if a question comes along that asks you why recombinant offspring are present you now know why! Page 59 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Identifying Recombinants (HL) Your notes Identifying Recombinants Genetic diagrams involving autosomal linkage often predict solely parental type offspring (offspring that have the same combination of characteristics as their parents) However in reality recombinant offspring (offspring that have a different combination of characteristics to their parents) are often produced This is due to the crossing over that occurs during meiosis The crossing over and exchanging of genetic material breaks the linkage between the genes and recombines the characteristics of the parents Crossing over of bivalents diagram Page 60 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The process of crossing-over results in recombinant phenotypes that can differ from the parental phenotype. Your notes The frequency of recombinants within a population will nearly always be less than that of non- recombinants Crossing over is random and chiasmata form at different locations with each meiotic division Recombination frequency between two linked genes is greater when genes are further apart on the same chromosome There are more possible locations for a chiasma to form between the genes Identifying recombinants using test crosses Test crosses are often used to determine unknown genotypes Similarly, they can be used to identify recombinant phenotypes in offspring An individual is crossed with a homozygous recessive individual (for both traits) If any of the offspring possess a non-parental phenotype then they are labelled as recombinants These individuals have new allele combinations due to the process of crossing over during meiosis leading to the exchange of genetic material between chromosomes Drawing a Punnett square to show dihybrid inheritance of linked genes A number of sweet pea plants were generated by crossing double-homozygous dominant plants (PL) (PL) with double-homozygous recessive plants (pl)(pl) to produce a 100% heterozygous F1 generation (PL)(pl) as expected Members of this generation then interbreed to produce the F2 generation Alleles: P = purple flowers, dominant to p = red flowers L = long seeds, dominant to l = round seeds Possible Gametes Table Page 61 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes F2 Punnet Square Showing Possible Genotypes According to Mendelian ratios and the Punnett square, the F2 generation should follow the typical 9:3:3:1 ratio Page 62 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources However, in reality, the frequency of recombinant gametes will be much lower than that of parental gametes This affects the resulting offspring phenotypes, with fewer recombinant phenotypes occurring Your notes than expected Expected vs Predicted Phenotypes Table Observations More of the F2 offspring than expected showed the parental phenotypes Fewer plants with recombinant phenotypes were produced than the 9:3:3:1 ratio would suggest The actual ratios found were referred to as 'non-Mendelian' as they didn't follow Mendel’s pattern However, this was not zero; some recombinants were still being produced Possible Theories to Explain These Findings At the time, it was known that many genes were carried on a few chromosomes The idea that certain genes share the same chromosome was being developed by many scientists This suggested that genes could be inherited together, not by the law of independent assortment as put forward by Mendel The idea of linkage of genes was developed to explain the non-Mendelian ratios The frequency of recombinant phenotypes is lower because crossing over is a random process and the chiasmata do not always form in the same place for each meiotic division The frequency of recombinant gametes also depends on the closeness of linkage between the two genes Genes located close together on a chromosome are less likely to be separated by crossing over So recombinants of those two genes will be less frequent Thomas Hunt Morgan later provided proof of linkage to explain non-Mendelian ratios in his experimentation with fruit flies (Drosophila melanogaster) Page 63 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tip Your notes Remember to distinguish between sex linkage and autosomal linkage. The explanation of non- Mendelian ratios falls into the domain of autosomal linkage for IB. Page 64 of 64 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers