AQA GCSE Biology Reproduction PDF

Head to www.savemyexams.com for more awesome resources AQA GCSE Biology Your notes Reproduction Contents Sexual & Asexual Reproduction Meiosis Advantages & Disadvantages of Sexual & Asexual Reproduction D...

Head to www.savemyexams.com for more awesome resources AQA GCSE Biology Your notes Reproduction Contents Sexual & Asexual Reproduction Meiosis Advantages & Disadvantages of Sexual & Asexual Reproduction DNA & the Genome DNA Structure Genetic Inheritance Inherited Disorders Sex Determination Page 1 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Sexual & Asexual Reproduction Your notes Mitosis & Meiosis Mitosis is a type of nuclear division that gives rise to cells that are genetically identical It is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction Meiosis is a type of nuclear division that gives rise to cells that are genetically different It is used to produce gametes (sex cells) Sexual Reproduction 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 The gametes of animals are the sperm cells and egg cells The gametes of flowering plants are the pollen cells and egg cells Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring The formation of gametes involves meiosis Asexual Reproduction Asexual reproduction does not involve sex cells or fertilisation Only one parent is required so there is no fusion of gametes and no mixing of genetic information As a result, the offspring are genetically identical to the parent and to each other (clones) Asexual reproduction is defined as a process resulting in genetically identical offspring from one parent Only mitosis is involved in asexual reproduction Page 2 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Meiosis Your notes Meiosis Cells in reproductive organs divide by meiosis to form gametes (sex cells) The number of chromosomes must be halved when the gametes are formed Otherwise, there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg) This halving occurs during meiosis, and so it is described as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells It starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell After this has happened the cells divide twice so that only one copy of each chromosome passes to each gamete We describe gametes as being haploid – having half the normal number of chromosomes Because of this double division, meiosis produces four haploid cells Page 3 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 4 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The process of cell division by meiosis to produce haploid gamete cells Process Each chromosome is duplicated (makes identical copies of itself), forming X-shaped chromosomes First division: the chromosome pairs line up along the centre of the cell and are then pulled apart so that each new cell only has one copy of each chromosome Second division: the chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart A total of four haploid daughter cells will be produced Importance Produces gametes eg. sperm cells and egg cells in animals, pollen grains and ovum cells in plants Increases genetic variation of offspring Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be different from any others Fertilisation Gametes join at fertilisation to restore the normal number of chromosomes When the male and female gametes fuse, they become a zygote (fertilised egg cell) This contains the full number of chromosomes, half of which came from the male gamete and half from the female gamete The zygote divides by mitosis to form two new cells, which then continue to divide and after a few days form an embryo Cell division continues and eventually many of the new cells produced become specialised (the cells differentiate) to perform particular functions and form all the body tissues of the offspring The process of cells becoming specialised is known as cell differentiation Page 5 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Advantages & Disadvantages of Sexual & Asexual Reproduction Your notes Sexual reproduction Advantages & disadvantages of sexual reproduction table Advantages Disadvantages Increases genetic variation Takes time and energy to find mates The species can adapt to new environments due to variation, Difficult for isolated members of the giving them a survival advantage species to reproduce Disease is less likely to affect the population (due to variation) An additional advantage of sexual reproduction is our ability to use it and control it for our own needs: Natural selection can be speeded up by humans in selective breeding to increase food production We have controlled sexual reproduction in cows and selectively bred them to produce offspring that produce more milk and more meat than they would have under natural conditions Asexual reproduction Advantages & disadvantages of asexual reproduction table Advantages Disadvantages The population can be increased rapidly Limited genetic variation in population - offspring are when conditions are right genetically identical to their parents Can exploit suitable environments quickly The population is vulnerable to changes in conditions and may only be suited for one habitat More time and energy-efficient Disease is more likely to affect the whole population as there is no genetic variation Page 6 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Reproduction is completed much faster than sexual reproduction Your notes As only one parent is needed, asexual reproduction is more time and energy-efficient than sexual reproduction, as asexually reproducing organisms do not need to find a mate Sexual & Asexual Reproduction Some organisms reproduce by both methods depending on the circumstances. For example: Malarial parasites Malaria is caused by parasites that are carried by mosquitoes The parasites are transferred to a human when the mosquito feeds on the human’s blood These malarial parasites reproduce asexually in the human host, but sexually in the mosquito Fungi Many fungi reproduce both asexually and sexually These species of fungi release spores, which develop into new fungi These spores can be produced via asexual or sexual reproduction Spores that are produced via sexual reproduction show variation (they are genetically different from each other) Plants Many plants produce seeds via sexual reproduction but are also able to reproduce asexually They reproduce asexually in different ways: Some plants (e.g. strawberry plants) produce ‘runners’ (stems that grow horizontally away from the parent plant, at the end of which new identical offspring plants form) Some plants (e.g. daffodils) reproduce via bulb division (new bulbs form from the main bulb underground and then grow into new identical offspring plants) Plant runners diagram Page 7 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Some plants grow side shoots called runners that contain tiny plantlets on them. These will grow roots and develop into separate plants Plant bulbs diagram Page 8 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 9 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Some plants develop underground food storage organs that will develop into next years plants – bulbs are an example of this Page 10 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources DNA & the Genome Your notes The Genome The entire set of the genetic material of an organism is known as its genome Biologists now know the entire human genome (they have worked out all the genes that are found in humans) The Structure of DNA The genetic material in the nucleus of a cell is composed of a chemical called DNA DNA, or deoxyribonucleic acid, is the molecule that contains the instructions for growth and development of all organisms DNA is a polymer made up of two strands forming a double helix DNA is contained in structures called chromosomes Chromosomes are located in the nucleus of cells Genes are short lengths of DNA that code for a protein. They are found on chromosomes Genes A gene is a short length of DNA found on a chromosome Each gene codes for a particular sequence of amino acids Page 11 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources These sequences of amino acids form different types of proteins There are many different types of proteins but some example of these could be: Your notes structural proteins such as collagen found in skin cells enzymes hormones Genes control our characteristics as they code for proteins that play important roles in what our cells do The Human Genome Project The Human Genome Project (completed in 2003) was the name of the international, collaborative research effort to determine the DNA sequence of the entire human genome and record every gene in human beings This was a very important breakthrough for several reasons: From a medical perspective, as it has already and will continue to improve our understanding of the genes linked with different types of disease and inherited genetic disorders, as well as the help us in finding treatments The human genome has also made it possible to study human migration patterns from the past, as different populations of humans living in different parts of the world have developed very small differences in their genomes! Page 12 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources DNA Structure Your notes Nucleotides DNA is a polymer (a molecule made from many repeating subunits) These individual subunits of DNA are called nucleotides Each nucleotide consists of a common sugar and phosphate group with one of four different bases attached to the sugar A nucleotide Page 13 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Base Pairing: Basics There are four different nucleotides Your notes These four nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in the base attached There are four different bases: Adenine (A), Cytosine (C), Thymine (T) and Guanine (G) Base Pairing Higher tier only The bases on each strand pair up with each other, holding the two strands of DNA in the double helix The bases always pair up in the same way: Adenine always pairs with Thymine (A-T) Cytosine always pairs with Guanine (C-G) This is known as ‘complementary base pairing’ Page 14 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources DNA base pairs Your notes Examiner Tips and Tricks You do not need to learn the names of the bases, just their letters. Make sure you know which base bonds with which (the complementary base pairs), as this is the most commonly asked question about this topic. Coding for Amino Acids A sequence of three bases is the code for a particular amino acid The order of bases controls the order and different types of amino acids that are joined together These amino acid sequences then form a particular type of protein In this way, it is the order of bases in the DNA which eventually determines which proteins are produced Double Helix The phosphate and sugar section of the nucleotides form the ‘backbone’ of the DNA strand (like the sides of a ladder) and the base pairs of each strand connect to form the rungs of the ladder It is this sequence of bases that holds the code for the formation of proteins Page 15 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The DNA helix is made from two strands of DNA held together by hydrogen bonds Protein Synthesis Higher tier only Proteins are made in the cell cytoplasm on structures called ribosomes Ribosomes use the sequence of bases contained within DNA to make proteins DNA cannot travel out of the nucleus to the ribosomes (it is far too big to pass through a nuclear pore) so the base code of each gene is transcribed onto an RNA molecule called messenger RNA (mRNA) mRNA can move out of the nucleus and attaches to a ribosome (the mRNA acts as a messenger between DNA and the ribosome) The correct sequence of amino acids are then brought to the ribosome and joined together Page 16 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources This amino acid sequence then forms into a protein Your notes Page 17 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 18 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Protein synthesis Examiner Tips and Tricks You do not need to remember the name mRNA. The AQA specification refers to it as a 'template'. Changes to Proteins Higher tier only A change in DNA structure may result in a change in the protein synthesised by a gene If there is a change in the order of the bases in a section of DNA (eg. in a gene), then a different protein may be produced This protein may not function in the same way as the original protein would have (before the change occurred in the DNA) Function of Ribosomes Higher tier only The ribosome ‘reads’ the code on the mRNA in groups of three Each triplet of bases codes for a specific amino acid Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order In this way, the ribosome translates the sequence of bases into a sequence of amino acids that make up a protein Once the amino acid chain has been assembled, it is released from the ribosome so it can fold and form the final structure of the protein Page 19 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The triplet code of DNA (carried by mRNA) is read by the ribosome and amino acids are attached together in a specific sequence to form the protein Protein Structure Higher tier only When the protein chain is complete it folds up to form a unique shape This unique shape enables the proteins to fulfil a specific function. For example, proteins can be: Enzymes – proteins that act as biological catalysts to speed up chemical reactions occurring in the body (eg. maltase is an enzyme that breaks down maltose into glucose) Hormones – proteins that carry messages around the body (eg. testosterone is a hormone that plays an important role in the development of the male reproductive system and development of male secondary sexual characteristics, such as increased muscle mass and growth of body hair) Page 20 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Structural proteins – proteins that provide structure and are physically strong (eg. collagen is a structural protein that strengthens connective tissues such as ligaments and cartilage) Your notes Mutations Higher tier only Mutations are random changes that occur in the sequence of DNA bases in a gene or a chromosome Mutations occur continuously As the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the protein that the gene codes for Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed There are different ways that a mutation in the DNA base sequence can occur: Insertions A new base is randomly inserted into the DNA sequence An insertion mutation changes the amino acid that would have been coded for by the group of three bases in which the mutation occurs Remember – every group of three bases in a DNA sequence codes for an amino acid An insertion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence Page 21 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources An example of an insertion mutation Your notes Deletions A base is randomly deleted from the DNA sequence Like an insertion mutation, a deletion mutation changes the amino acid that would have been coded for by the group of three bases in which the mutation occurs Like an insertion mutation, a deletion mutation also has a knock-on effect by changing the groups of three bases further on in the DNA sequence Substitutions A base in the DNA sequence is randomly swapped for a different base Unlike an insertion or deletion mutation, a substitution mutation will only change the amino acid for the group of three bases in which the mutation occurs; it will not have a knock-on effect An example of a substitution mutation Effects of Mutations Higher tier only Page 22 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed Your notes However, a small number of mutations code for a significantly altered protein with a different shape This may affect the ability of the protein to perform its function. For example: If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site A structural protein (like collagen) may lose its strength if its shape changes Gene Switching Higher tier only Not all parts of DNA code for proteins Some non-coding parts of DNA can switch genes on and off This means they can control whether or not a gene is expressed Variations in these areas of DNA may affect how genes are expressed if a mutation occurs in a section of non-coding DNA that controls gene expression, the expression of these genes may be altered or in some cases, the mutation may cause them not to be expressed at all Page 23 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Genetic Inheritance Your notes Key Terms Table of key terms & definitions for genetic inheritance Page 24 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Monohybrid Inheritance Page 25 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Some characteristics are controlled by a single gene, such as fur colour in mice; and red-green colour blindness in humans Your notes The inheritance of these single genes is called monohybrid inheritance (mono = one) As we have two copies of each chromosome, we have two copies of each gene and therefore two alleles for each gene One of the alleles is inherited from the mother and the other from the father This means that the alleles do not have to ‘say’ the same thing For example, 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 observable characteristics of an organism (seen just by looking – like eye colour; or found – like blood type) is called the phenotype The combination of alleles that control each characteristic is called the genotype Alleles can be dominant or recessive A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype A recessive allele needs to be inherited from both parents in order for the characteristic to show up 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) When completing genetic diagrams, alleles are abbreviated to single letters The dominant allele is given a capital letter and the recessive allele is given the same letter, but lower case Page 26 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Alleles of a gene can carry the same instructions or different instructions. You can only inherit two alleles for each gene, and they can be the same or different Multiple Gene Inheritance Most characteristics are a result of multiple genes interacting, rather than a single gene Characteristics that are controlled by more than one gene are described as being polygenic Polygenic characteristics have phenotypes that can show a wide range of combinations in features The inheritance of these polygenic characteristics is called polygenic inheritance (poly = many/more than one) Polygenic inheritance is difficult to show using genetic diagrams because of the wide range of combinations An example of polygenic inheritance is eye colour – while it is true that brown eyes are dominant to blue eyes, it is not as simple as this as eye colour is controlled by several genes This means that there are several different phenotypes beyond brown and blue; green and hazel being two examples Page 27 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Examiner Tips and Tricks You will NOT be expected to explain the polygenic inheritance of characteristics using a genetic diagram, you just need to be aware that many characteristics are controlled by groups of genes and that this is known as polygenic inheritance. Predicting Inheritance Monohybrid inheritance is the inheritance of characteristics controlled by a single gene This can be determined using a genetic diagram known as a Punnett square A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring From this, the ratio of these combinations can be worked out Remember the dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lower case Example: The height of pea plants is controlled by a single gene that has two alleles: tall and short The tall allele is dominant and is shown as T The small allele is recessive and is shown as t ‘Show the possible allele combinations of the offspring produced when a pure breeding short plant is bred with a pure breeding tall plant’ The term ‘pure breeding’ indicates that the individual is homozygous for that characteristic Page 28 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes A pure-breeding genetic cross in pea plants This shows that all the offspring will be tall ‘Show the possible allele combinations of the offspring produced when two of the offspring from the first cross are bred together’ Page 29 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes A genetic cross diagram (F2 generation) All of the offspring of the first cross have the same genotype, Tt (heterozygous), so the possible combinations of offspring bred from these are: TT (tall), Tt (tall), tt (short) There is more variation in this cross, with a 3:1 ratio of tall : short Page 30 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The F2 generation is produced when the offspring of the F1 generation (pure-breeding parents) are allowed to interbreed ‘Show the results of crossing a heterozygous plant with a short plant’ Your notes The heterozygous plant will be tall with the genotype Tt The short plant is showing the recessive phenotype and so must be homozygous recessive – tt The results of this cross are as follows: A cross between a heterozygous plant with a short plant In this cross, there is a 1:1 ratio of tall to short Page 31 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources How to construct Punnett squares Determine the parental genotypes Your notes Select a letter that has a clearly different lower case, for example, Aa, Bb, Dd Split the alleles for each parent and add them to the Punnett square around the outside Fill in the middle four squares of the Punnett square to work out the possible genetic combinations in the offspring You may be asked to comment on the ratio of different allele combinations in the offspring, calculate percentage chances of offspring showing a specific characteristic or just determine the phenotypes of the offspring Completing a Punnett square allows you to predict the probability of different outcomes from monohybrid crosses Family Trees Family tree 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 Page 32 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources A family tree diagram Males are indicated by the square shape and females are represented by circles Your notes Affected individuals are red and unaffected are blue Horizontal lines between males and females show that they have produced children (which are shown underneath each couple) The family pedigree above shows: Both males and females are affected Every generation has affected individuals 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 Examiner Tips and Tricks You should always write the dominant allele first, followed by the recessive allele.If you are asked to use your own letters to represent the alleles in a Punnett square, try to choose a letter that is obviously different as a capital than the lower case so the examiner is not left in any doubt as to which is dominant and which is recessive.For example, C and c are not very different from each other, whereas A and a are! Predicting Probability Higher tier only A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring From this, the ratio of these combinations can be worked out However, you can also make predictions of the offsprings’ characteristics by calculating the probabilities of the different phenotypes that could occur For example, in the second genetic cross (F2 generation) that was given earlier (see above), two plants with the genotype Tt (heterozygous) were bred together The possible combinations of offspring bred from these two parent plants are: TT (tall), Tt (tall), tt (short The offspring genotypes showed a 3:1 ratio of tall : short Page 33 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Using this ratio, we can calculate the probabilities of the offspring phenotypes The probability of an offspring being tall is 75% Your notes The probability of an offspring being short is 25% Page 34 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Inherited Disorders Your notes Inherited Diseases Some disorders are inherited (passed from parents to offspring) These disorders are caused by the inheritance of certain alleles For example, cystic fibrosis and polydactyly are two genetic disorders that can be inherited: Cystic fibrosis Cystic fibrosis is a genetic disorder of cell membranes It results in the body producing large amounts of thick, sticky mucus in the air passages Over time, this may damage the lungs and stop them from working properly Cystic fibrosis is caused by a recessive allele (f) This means: People who are heterozygous (only carry one copy of the recessive allele) won’t be affected by the disorder but are ‘carriers’ People must be homozygous recessive (carry two copies of the recessive allele) in order to have the disorder If both parents are carriers, the chance of them producing a child with cystic fibrosis is 1 in 4, or 25% If only one of the parents is a carrier (with the other parent being homozygous dominant), there is no chance of producing a child with cystic fibrosis Page 35 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Inheritance of cystic fibrosis if both parents are carriers or if only one parent is a carrier Polydactyly Polydactyly is a genetic disorder that causes someone to be born with extra fingers or toes Polydactyly is caused by a dominant allele (D) This means: Even if only one parent is a carrier, the disorder can be inherited by offspring Page 36 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Inheritance of polydactyly if only one parent is a carrier Impact of Inherited Disease Your notes Embryo screening In vitro fertilisation (IVF) is the process by which embryos are fertilised in a laboratory and then implanted into the mother’s womb A cell can be taken from the embryo before being implanted and its genes can be analysed It is also possible to get DNA from the cell of an embryo that’s already in the womb and analyse its genes in the same way Genetic disorders (eg. cystic fibrosis) can be detected during this analysis This has led to many economic, social and ethical concerns: An IVF embryo (ie. a potential life) might be destroyed if alleles causing a genetic disorder are found in its genes Pregnancy might be prematurely terminated if an embryo already in the womb (also a potential life) is found to have alleles causing a genetic disorder within its genes Arguments for & against embryo screening Page 37 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Gene therapy Gene therapy is the process by which normal alleles are inserted into the chromosomes of an individual who carries defective alleles (eg. those that cause a genetic disorder) It is a developing technology and is not always successful The process raises similar economic, social and ethical concerns to embryo screening: Many people believe that gene alteration is unnatural Many believe it is a good idea as it can help to alleviate suffering in people with genetic disorders Page 38 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Sex Determination Your notes Human Chromosomes Ordinary human body cells contain 23 pairs of chromosomes 22 pairs control characteristics only, but one of the pairs carries the genes that determine sex In females, the sex chromosomes are the same (XX) In males, the sex chromosomes are different (XY) Page 39 of 41 © 2015-2025 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 Page 40 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Determining Sex The inheritance of sex can be shown using a genetic diagram (known as a Punnett square), with the X Your notes and Y chromosomes taking the place of the alleles usually written in the boxes Punnett square showing the inheritance of sex Page 41 of 41 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers