Genetics.docx

Genetic Inheritance Gregor Mendel was a 19th century Augustinian monk and teacher who joined a monastery in what is not the Czech Republic. Between 1858 and 1866, Mendel bred peas and brought a fresh approach to the process, elevating it to a scientific footing. This allowed him to deduce principles...

Genetic Inheritance Gregor Mendel was a 19th century Augustinian monk and teacher who joined a monastery in what is not the Czech Republic. Between 1858 and 1866, Mendel bred peas and brought a fresh approach to the process, elevating it to a scientific footing. This allowed him to deduce principles that had eluded others. His work was ahead of its time and not accepted by the scientific community until after his death in 1884. He became known as the father of modern genetics. Statistical analysis of his work in the 20th century suggested that some of his results were too good to be true. However, Mendel knew exactly what was happening in his breeding work, because he picked suitable characteristics to confirm his hypothesis, and designed experiments to demonstrate this. Mendel’s concepts Each characteristic is controlled by a fair of factors known as genes. One pair of genes is inherited from each parent. There are two options to a characteristic, these are called gene alleles. Normally one option is stronger than the other, and the strongest one is said to be dominant, while the weaker one is recessive. - Dominant allele is the allele that is always expressed if it is present - Recessive allele is the allele whose expression is masked by the dominant allele Law of Segregation – Mendel’s First Law This states that each characteristic is governed by a pair of factors (genes). These separate at gamete formation and each gamete only receives one of each pair of factors. - Phenotype is the physical expression of the genotype or genotype plus environment. - Genotype is the genetic make-up of an individual. When carrying out genetic crosses, we use letters to symbolize the genes. The letter used is normally the first letter of the dominant feature. In the case of pea-seed colour, we use Y for yellow and y for green. In the case of pea-seed shape, we use capital R for round seeds and r for wrinkled seeds. If a pea has two identical alleles, e.g., TT or tt, it is said to be homozygous. Homozygous describes the situation when both alleles are identical. If the pea has two different alleles, e.g., Tt, it is said to be heterozygous. Heterozygous describes the situation when both alleles are different. Monohybrid Cross This feature is looking at one feature at a time, e.g., seed colour, height, or sex There are three possible types of cross; - Homozygous dominant crossed with homozygous recessive - Heterozygous crossed with homozygous recessive - Heterozygous crossed with heterozygous Homozygous dominant crossed with homozygous recessive Parental Phenotypes Homozygous X Homozygous dominant recessive Parental Genotypes YY yy Meiosis (Chromosome Separates) Y Y y y Gamete Genotypes y y Y Yy Yy Y Yy Yy All possible random fertilisations using the Punnett Square F1 progeny phenotypes Yy Yy Yy Yy F1 progeny phenotypes yellow yellow yellow yellow Phenotype ratio All Yellow In peas, the allele for yellow seed is dominant to green seed. A pure-breeding pea plant with yellow seeds is crossed with a pea plant that produces green seeds. Procedure - Write down the parents phenotypes - Under this, write their genotypes. (Be careful to use Capital and small letters correctly) - Always write the dominant character first when writing the genotype - Write meiosis, and the alleles are separated, with one going to each gamete - Draw a circle around the gametes - Construct a punnet square and place the gametes as shown above - Put them together, remembering that the capital letter must always go first - Write down each type of progeny genotype - Write down the phenotype of each genotype - Work out the ratio of phenotypes Heterozygous crossed with homozygous recessive Parental Phenotypes Heterozygous X Homozygous recessive Parental Genotypes Rr rr Meiosis (Alleles Separates) R r r r Gamete Genotypes r R R Rr Rr r rr rr All possible random fertilisations using the Punnett Square F1 progeny phenotypes Rr Rr rr rr F1 progeny phenotypes Round Round Wrinkled Wrinkled Phenotype ratio 1 (50% round) : 1 (50%) wrinkled - In peas, the allele for round seed is dominant to wrinkled seed - A heterozygous peas plant with round seeds is crossed with a pea plant that produces wrinkled seeds - The letter R is used because round is dominant to wrinkled - The heterozygous plant is Rr - Wrinkled seed is recessive, so the plant has to be homozygous for this feature Heterozygous crossed with heterozygous Parental Phenotypes Heterozygous X Heterozygous Parental Genotypes Tt Tt Meiosis (Alleles Separates) T t T t Gamete Genotypes T T T TT Tt t Tt tt All possible random fertilisations using the Punnett Square F1 progeny phenotypes TT Tt Tt tt F1 progeny phenotypes Tall Tall Tall Dwarf Phenotype ratio 3 Tall : 1 Dwarf - In peas, the allele for tall stem is dominant to the allele for dwarf stem - A heterozygous tall pea plant is crossed with a heterozygous tall pea plant - Each plant is tall, and tall is dominant, so it has a T - Each is heterozygous, which tells us they have two different alleles, so each must have a t - The genotype of each parent is therefore Tt Filial generations First Parental (P1) Phenotypes Purple Flowers X White Flowers P1 Genotypes PP pp Meiosis P P p p P1 Gamete Genotypes First Filial (F1) Genotypes All Pp F1 Phenotypes All Purple Now the F1 generation are self-fertilised and become the second parental generation (P2) P2 Phenotypes Purple X Purple P2 Genotypes Pp Pp Meiosis P p P p P2 Gamete Genotypes P P P PP Pp p Pp pp All possible random fertilisations using the Punnett Square Second filial F2 Genotypes PP Pp Pp pp F2 phenotypes purple purple purple white F2 Phenotype ratio 3 purple : 1 white The parents involved in a cross are the first parental generation (P1), while the ‘sons’ and ‘daughters’ produced by a cross are called the first filial generation (F1), from the latin filius, and filia, meaning daughter. The crosses above produce the first filial generation. If these offspring are cross-fertilised, they are called the second parental generation (P2), and their offspring are called the second filial generation (F2). This is what Mendel did, and it was this that led him to his deduction about pairs of factors. Sex Determination In most mammals, including humans, XX is female and XY is male. Sex is determined by a monohybrid cross. The cross below shows that there is a 50:50 chance of a baby being a boy or a girl. However, it does not work out exactly like this. Some parents have three boys and no girls, or seven girls and no boys, or any other combination. This shows that genetics is all about chance. Parental Phenotypes Female XX Male XY Parental sex chromosomes XX XY Meiosis (Alleles Separates) X X X Y Gamete sex chromosomes X Y X XX XY X XX XY All possible random fertilisations using the Punnett Square Progeny sex chromosomes XX XY XX XY Progeny phenotypes Female Male Female Male Phenotype ratio 1 (50%) male : 1 (50%) female Incomplete Dominance In cows, a parent with a red coat and a parent with a white coat produce offspring with a mottle colour called roan. Neither allele is fully dominant since both show in the phenotype. This is called co-dominance or incomplete dominance. This is easily recognisable because some of the offspring have a feature which is intermediate between the two parents. Dihybrid Cross A dihybrid cross is the study of crosses involving two characteristics at a time. From studying this type of cross, Mendel devised his second law. Law of Independent Assortment – Mendel’s Second Law Mendel’s second law states that during gamete formation, any member of a pair of factors has an equal chance of entering a gamete with either member of any other pair of factors. Homozygous recessive x heterozygous for both characteristics - In peas, red flower is dominant to white flower. For example, the crossing of peas that are tall, have red flowers, and are heterozygous for both characteristics, with other pea plants, such as those that are short and have white flowers - The heterozygous tall peas with red flowers have the genotype Tt Rr, so the T has an equal chance of combing with R or r to give gametes TR or Tr - The homozygous recessive dwarf white flowered peas tt rr can only produce gametes that are tr - There are parental phenotypes dwarf white and tall red, but also new parental phenotypes: in this case, tall white and dwarf red. Thus, independent assortment has led to variation. Parental 1 (P1) phenotype Dwarf white x Tall Red Parental 1 (P1) genotype tt rr TtRr Meiosis Gamete 1 (P1) genotypes tr TR Tr tR tr TR Tr tR Tr tr TtRr Ttrr ttRr Ttrr First filial generation (F1) Genotypes TtRr Ttrr ttRr ttrr Phenotypes 1 tall red : 1 tall white : 1 dwarf red : 1 dwarf white Phenotype ratio 1 : 1: 1: 1 Dihybrid cross to the F2 Generation First parental phenotypes (P1) Tall Red Dwarf White P1 genotypes TTRR ttrr Meiosis P1 gametes TR TR Cross tr tr First filial generation (F1) All TtRr now F1 becomes parental 2 (P2) Tall red Tall red TtRr TtRr Meiosis TR Tr tR tr TR Tr tR tr TR Tr tR Tr TR TTRR TTRr TtRR TtRr Tr TTRr TTrr TtRr Ttrr tR TtRR TtRr ttRR ttRr tr TtRr Ttrr ttRr ttrr Second filial generation (F2) Genotypes TTRR TTrr ttRR ttrr Phenotypes 9Tall Red 3Tall White 3Dwarf Red 1Dwarf White Phenotype Ratio 9 : 3 : 3 : 1 Above shows a dihybrid cross between a homozygous dominant pea plant and a homozygous recessive pea plant taken to the second filial generation. The ratio of phenotypes of the F2 is 9:3:3:1. That means, 9 are showing dominant features, 3 are showing one dominant feature and one recessive feature, 3 are showing the other dominant and the other recessive feature, 1 is showing both recessive features. Linkage Linked genes are genes that are located on the same chromosome and are therefore inherited together. The characters Mendel examined happened to be on separate chromosomes. That is why he observed independent assortment. If, however, the genes are on the same chromosomes, they will be inherited together. For example, consider the following parental nuclei of a fruit fly. Both father and mother have a pair of chromosomes with alleles for two different genes: Fruit flies can have long wings or vestigial (short) wings and a narrow abdomen or a wide abdomen. Long wings are dominant to vestigial wings and wide abdomens are dominant to narrow abdomens. - If a long-winged, wide-abdomen individual is crossed with a vestigial-winged, narrow-abdomen individual, all the offspring will have long wings and wide abdomens. - If the offspring of this are cross-mated, one may expect all combinations of wing length and abdomen width, but this is not the case. This is because the two genes are linked. Since the two alleles are linked, they behave as one and do not assort independently. The result is that only parental phenotypes are shown in the offspring. - A small proportion of non-parental phenotypes do occur due to a phenomenon known as crossing over. This occurs during the production of gametes by the process of meiosis. This involves the swapping of sections of homologous chromosomes as the chromosomes are pulled apart. **Don’t need to know crossing over in detail** Long Wing Vestigal Wing Long Wing Long Wing Wide Abdomen X Narrow Abdomen Wide Abdomen X Wide Abdomen V V v v V v V v A A a a P2 A a A a V v V v V v A a G2 A a A a F1 individuals crossed with each other F1 V v V v A a A a Long Wing Wide Abdomen V V V V v A A A A a V V v v v a A a a a F2 Genotypes V V V v V v v v A A A a A a a a F2 Phenotypes Long Wing Long Wing Long Wing Vestigial Wing Wide Abdomen Wide Abdomen Wide Abdomen Narrow Abdomen F2 Phenotype Ratio 3 Long Wing Wide Abdomen : 1 Short Wing Narrow Abdomen Sex Linkage Sex-linked refers to genes found on the X or Y chromosome. These genes are called heterosomes because they are different shapes and sizes. The Y chromosome is relatively empty, but it does contain genes to do with sperm production. The other 22 pairs of chromosomes are called autosomes, as each pair is the same size and shape. Sex-linked characteristics are determined by genes that occur on the X chromosome. For example, haemophilia and colour blindness; - Haemophilia: A group of diseases where a person’s blood does not clot properly. Caused by a lack of a particular protein - Colour blindness: An inherited condition where people cannot see the colours red and green properly. - In both conditions, the gene for the normal condition is dominant while the gene for the abnormal condition is recessive Carrier is a female who has an allele for the abnormal condition but does not show it. Phenotypes and Genotypes in Males & Females with regards to Haemophilia Females - XNXN has normal blood clotting - XNXn has the normal blood clotting because the dominant normal allele masks the abnormal allele. This person is a carrier because she carries the condition - XnXn has haemophilia. She has two recessive alleles Males - XNY- has normal blood clotting because even though he has only one allele, it is normal so it is expressed - XnY- has haemophilia because even though he has only one allele, it is abnormal so it is expressed Parents: Female carrier X Male normal XNXn XNY- Gametes: XN Xn XN Y- F1 XNXN XNY- XNXn XnY- Phenotypes Female Male Female Male Normal Normal Carrier Haemophiliac - 25% chance of producing a haemophiliac child - 50% chance of producing a haemophiliac son. - It is the mother that determines if the son is haemophiliac or not, since the father always passes the Y chromosome to his son. Non-nuclear Inheritance - Although the majority of DNA in a cell is found in the chromosomes, there is also DNA found in the nucleus in mitochondria and chloroplasts. This is called Non-nuclear DNA - This DNA is always passed on by the female in the cytoplasm of the ovum (egg). - In animals, the sperm only provides the chromosomal materials. Mitochondrial DNA is used by the sperm to provide energy so it can swim to the egg. Only the nuclear DNA enters the egg - In plants, the pollen grain only contains nuclear DNA. When the male gametes are formed, there is no mitochondrial or chloroplast DNA present Family tree of sex-linked crosses **Important to know all combinations** XNY- A XnXn XnY- B XNXN XnY- XnXN XNY- C XnXN XNY- XNY- XnXN XnXN XnY- XNY- XNXN XnXN Affected Female Carrier Female Normal Female Normal Male Affected Male Xn Abnormal Allele XN Normal Allele - Cross A produces 2 affected males and 2 carrier females - Cross B produces 2 normal males and 2 carrier females - Cross C produces 1 affected male, 1 normal male, 1 normal female, and 1 carrier female Ratios **Important to know all combinations** Monohybrid and Dihybrid crosses Ratio Parental Genotypes All one phenotype Homozygous dominant with homozygous recessive or heterozygous 3:1 (75%:25%) Heterozygous with heterozygous 1:1 (50%:50%) Heterozygous with homozygous recessive 1:2:1 (25%:50%:25%) Heterozygous co-dominant with heterozygous co-dominant 1:1:1:1 (25%:25%:25%;25%) Heterozygous dihybrid with homozygous recessive dihybrid 9:3:3:1 Heterozygous dihybrid with heterozygous dihybrid Sex-linked All offspring normal, all female carriers XNY- with XNXN 50% normal males:50% carrier females XNY- with XNXn 1 affected male, 1 normal male, 1 normal female, 1 carrier female XNY- with XNXn All female carriers and all male offspring are affected XNY- with XnXn All offspring affected XnY- with XnXn Possible gamete genotypes in a dihybrid cross Parental Genotypes TTRR TTRr TTrr TrRR TtRr Ttrr ttRR ttRr Ttrr Gamete Genotypes TR TR, Tr Tr TR, tR TR, Tr, tR, tr Tr, tr tR tR, tr tr Natural Selection and Evolution Variation Variation is the differences between individuals of a species. Causes of Variation - Sexual reproduction: The independent assortment of homologous chromosomes during meiosis ensures genetic variation among gametes - Mutations: This is any sudden change in the amount or structure of DNA Types of Mutations - Chromosome Mutations: A change in chromosome number or alteration of genes within a chromosome. This is potentially more harmful because of the number of genes involved. A change in chromosome number tends to be harmful in animals and humans but beneficial in plants e.g., Down Syndrome. Down Syndrome is a result of having 47 chromosomes (an extra ‘number 21’ in every body cell). This happens because one gamete had an extra copy of this chromosome. During meiosis, homologous chromosomes failed to separate and hence, two of the gametes had no number 21 and two had two copies of the chromosome. - Gene Mutations: A change in bases in the gene. This alters the amino acid sequence of the protein controlled by that gene. The bases may be altered by deletions, insertions, and substitutions. They usually make the gene non-functional or recessive, e.g., Cystic Fibrosis. Cystic Fibrosis is the inability to remove mucus from the lungs. The gene codes for a protein found in cell membranes that controls the flow of chloride ions into and out of the cell. This results in thick mucus that clogs the lungs and stops the correct functioning of the pancreas and liver. Causes of Mutations - Spontaneous mutations: Faulty DNA replication making ‘mistakes’ or, when DNA fails to repair properly - Mutagens: Agents that cause mutations They can speed up the spontaneous rate of mutation e.g., a. Ionising radiation such as X-rays, UV rays, cosmic rays, α/β/gamma rays. These harm DNA indirectly and their effect can accumulate in the body over years. They can also harm gametes. UV alters DNA structure directly and cause mutations in skin cells, e.g., skin cancer b. Chemicals e.g., formaldehyde, tobacco smoke, caffeine and many drugs, preservatives, and pesticides. Many are carcinogenic (cancer-causing) - Some viruses and ageing may cause mutations. Hepatitis B virus can cause liver cancer