AQA GCSE Biology Combined Science 6.1 Reproduction PDF

Head to www.savemyexams.com for more awesome resources AQA GCSE Biology: Combined Your notes Science 6.1 Reproduction Contents 6.1.1 Sexual & Asexual Reproduction 6.1.2 Meiosis 6.1.4 DNA & the Genome 6.1.6 Genetic...

Head to www.savemyexams.com for more awesome resources AQA GCSE Biology: Combined Your notes Science 6.1 Reproduction Contents 6.1.1 Sexual & Asexual Reproduction 6.1.2 Meiosis 6.1.4 DNA & the Genome 6.1.6 Genetic Inheritance 6.1.7 Inherited Disorders 6.1.8 Sex Determination Page 1 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.1 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 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.2 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 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 4 of 25 © 2015-2024 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 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.4 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 Page 6 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Genes A gene is a short length of DNA found on a chromosome Your notes Each gene codes for a particular sequence of amino acids These sequences of amino acids form different types of proteins There are many different types of proteins but some example of these could be: 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 7 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.6 Genetic Inheritance Your notes Key Terms Table of key terms & definitions for genetic inheritance Page 8 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 9 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Monohybrid Inheritance Some characteristics are controlled by a single gene, such as fur colour in mice; and red-green colour Your notes blindness in humans 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 10 of 25 © 2015-2024 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 Page 11 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Multiple Gene Inheritance Most characteristics are a result of multiple genes interacting, rather than a single gene Your notes 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 Exam Tip 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. Page 12 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Predicting Inheritance Monohybrid inheritance is the inheritance of characteristics controlled by a single gene Your notes 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 13 of 25 © 2015-2024 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 14 of 25 © 2015-2024 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 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’ Page 15 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 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: Your notes A cross between a heterozygous plant with a short plant In this cross, there is a 1:1 ratio of tall to short How to construct Punnett squares Determine the parental genotypes 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 Page 16 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 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 Your notes 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 A family tree diagram Males are indicated by the square shape and females are represented by circles 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 Page 17 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The other two families have one affected parent and affected children as well Your notes Exam Tip 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 Using this ratio, we can calculate the probabilities of the offspring phenotypes The probability of an offspring being tall is 75% The probability of an offspring being short is 25% Page 18 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.7 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 Inheritance of cystic fibrosis if both parents are carriers or if only one parent is a carrier Polydactyly Page 19 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 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: Your notes Even if only one parent is a carrier, the disorder can be inherited by offspring Inheritance of polydactyly if only one parent is a carrier Page 20 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Impact of Inherited Disesase Embryo screening Your notes 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 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) Page 21 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 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 Your notes Many believe it is a good idea as it can help to alleviate suffering in people with genetic disorders Page 22 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 6.1.8 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 23 of 25 © 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 Page 24 of 25 © 2015-2024 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 25 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers