Wellsway school science dept. Curriculum 2011 AQA Additional Science Biology workbook - 1 -

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2 WORKBOOK 7 CELL DIVISION & INHERITANCE Characteristics are passed on from one generation to the next in both plants and animals. Simple genetic diagrams can be used to show this. There are ethical considerations in treating genetic disorders. Candidates should use their skills, knowledge and understanding to: explain why Mendel proposed the idea of separately inherited factors and why the importance of this discovery was not recognised until after his death interpret genetic diagrams, including family trees construct genetic diagrams of monohybrid crosses and predict the outcomes of monohybrid crosses and be able to use the terms homozygous, heterozygous, phenotype and genotype predict and /or explain the outcome of crosses between individuals for each possible combination of dominant and recessive alleles of the same gene make informed judgements about the social and ethical issues concerning the use of stem cells from embryos in medical research and treatments make informed judgements about the economic, social and ethical issues concerning embryo screening. Additional guidance: Candidates should be familiar with the principles Mendel used in investigating monohybrid inheritance in peas. They should understand that Mendel s work preceded the work by other scientists which linked Mendel s inherited factors with chromosomes. Additional guidance: HT only Foundation Tier candidates should be able to interpret genetic diagrams of monohybrid inheritance and sex inheritance but will not be expected to construct genetic diagrams or use the terms homozygous, heterozygous, phenotype or genotype. Subject Content: Cell division a) In body cells the chromosomes are normally found in pairs. Body cells divide by mitosis. b) The chromosomes contain the genetic information. c) When a body cell divides by mitosis: copies of the genetic material are made then the cell divides once to form two genetically identical body cells. d) Mitosis occurs during growth or to produce replacement cells. e) Body cells have two sets of chromosomes; sex cells (gametes) have only one set. f) Cells in reproductive organs testes and ovaries in humans divide to form gametes. g) The type of cell division in which a cell divides to form gametes is called meiosis. h) When a cell divides to form gametes: copies of the genetic information are made then the cell divides twice to form four gametes, each with a single set of chromosomes. i) When gametes join at fertilisation, a single body cell with new pairs of chromosomes is formed. A new individual then develops by this cell repeatedly dividing by mitosis. Additional guidance: Knowledge and understanding of the stages in mitosis and meiosis is not required. Candidates should develop an understanding of the relationship from the molecular level upwards between genes, chromosomes, nuclei and cells and relate these to tissues, organs and systems. For Foundation Tier, knowledge of meiosis is restricted to where the process occurs - 2 -

3 and that gametes are produced by meiosis. Subject content cell differentiation & stem cells j) Most types of animal cells differentiate at an early stage whereas many plant cells retain the ability to differentiate throughout life. In mature animals, cell division is mainly restricted to repair and replacement. k) Cells from human embryos and adult bone marrow, called stem cells, can be made to differentiate into many different types of cells, eg nerve cells. l) Human stem cells have the ability to develop into any kind of human cell. m) Treatment with stem cells may be able to help conditions such as paralysis. n) The cells of the offspring produced by asexual reproduction are produced by mitosis from the parental cells. They contain the same alleles as the parents. Subject Content - Genetic variation a) Sexual reproduction gives rise to variation because, when gametes fuse, one of each pair of alleles comes from each parent. b) In human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex. In females the sex chromosomes are the same (XX); in males the sex chromosomes are different (XY). c) Some characteristics are controlled by a single gene. Each gene may have different forms called alleles. d) An allele that controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele. e) An allele that controls the development of characteristics only if the dominant allele is not present is a recessive allele. f) Chromosomes are made up of large molecules of DNA (deoxyribo nucleic acid) which has a double helix structure. g) A gene is a small section of DNA. h) Each gene codes for a particular combination of amino acids which makes a specific protein. i) Each person (apart from identical twins) has unique DNA. This can be used to identify individuals in a process known as DNA fingerprinting. Additional guidance: Knowledge and understanding of stem cell techniques is not required. Candidates are not expected to know the names of the four bases or how complementary pairs of bases enable DNA replication to take place. HT only - Candidates should understand that genetic diagrams are biological models which can be used to predict the outcomes of crosses. Subject content - Genetic disorders a) Some disorders are inherited. b) Polydactyly having extra fingers or toes is caused by a dominant allele of a gene and can therefore be passed on by only one parent who has the disorder. c) Cystic fibrosis (a disorder of cell membranes) must be inherited from both parents. The parents may be carriers of the disorder without actually having the disorder themselves. It is caused by a recessive allele of a gene and can therefore be passed on by parents, neither of whom has the disorder. d) Embryos can be screened for the alleles that cause these and other genetic disorders. Additional guidance: Knowledge and understanding of genetic fingerprinting techniques is not required. Attention is drawn to the potential sensitivity needed in teaching about inherited disorders. Knowledge and understanding of embryo screening techniques is not required

4 1 Structures in the nucleus. Discuss with your teacher the role of the nucleus and how the nucleus, chromosomes, genes and DNA are all linked. Using labelled diagrams show this relationship in the space below: If you have finished.. Find out how DNA s coded information affects an organism s inherited characteristics: DNA codes for the production of specific PROTEINS which in turn produce all our inherited characteristics

5 DNA 1. (Optional practical.) Using the method provided, prepare some DNA from Kiwi Fruit or Onion. 2. Write down 2 reasons why we might we need to extract DNA from cells. a. To match it with DNA taken from a crime scene b. Paternity / maternity testing to prove that a child is/is not related to its alleged parents. 3. An interesting calculation! Calculate how many times to the moon and back a human s DNA would reach if it was removed from each cell and each strand was laid end to end. Each cell nucleus in a human holds about 2 metres of DNA. A typical adult is comprised of approx. 50 trillion cells (A trillion is ). The distance from the Earth to the moon is kilometres. 50 x 2 trillion m of DNA in all our cells = m of DNA = km of DNA in all our cells / = times!! 2 Genetic Fingerprinting Discuss Genetic fingerprinting with your teacher. The DNA in your cells is as unique as your fingerprints. Unless you have an identical twin your genetic fingerprint is different from anyone else s 1. Complete the following paragraph: DNA can be analysed by genetic FINGERPRINTING. This involves CUTTING the DNA into short PIECES and then separating them into bands. The PATTERN of the bands can then be COMPARED to show SIMILARITIES and differences in the DNA. WORD LIST: - 5 -

6 Cutting compared fingerprinting patterns pieces similarities 2. Look at the genetic fingerprints below. The fingerprints labelled 1-7 are suspects in a murder investigation. The fingerprint in the middle of the picture was DNA found at the scene of the crime. Answer the questions below: a. Who do you think committed the murder? b. Explain your answer clearly: 3. Go to the following website: In the interactive menu option complete the Whodunit? scenario (DNA fingerprint game) The 3 suspects are Jason Chua, Melvin Tan and Danny Lee. a. Write down who was most likely to have committed the crime & explain your answer: Melvin Tan his genetic fingerprint shows he was at the crime scene b. Why can you not be certain that your suspect committed the crime? It only shows he was at the crime scene not that he actually committed the murder. 4. Read P292 Biology For You and write down who you think is the guilty suspect

7 Suspect 2 genetic fingerprint matches that from crime scene. 3 Cell Division Mitosis Watch the video or PowerPoint presentation on Mitosis & answer the questions below: 1. Write down the three reasons why it is so important that cells must divide. a. Growth addition of new cells b. Replacement of dead / damaged cells & repair of tissues c. Asexual reproduction in some animals and many plants 2. How many sets of chromosomes are present in normal body cells? Where does each set originate from? TWO one from each parent s gamete (egg or sperm) 3. How many daughter cells are formed from each cell which divides by mitosis? TWO 4. How does the number of chromosomes and genetic information in each daughter cell compare with that in the parent cell? Same number of chromosomes and genetic information is identical 5. What is the modern term for the production of identical offspring by mitosis? Cloning 6. Draw a diagram showing the production of 2 daughter cells from one parent cell: - 7 -

8 4 Cell Division Meiosis Watch the video or PowerPoint presentation on Meiosis & answer the questions below: 1. What is the purpose of meiosis? To produce HAPLOID GAMETES i.e. sex cells with HALF the number of chromosomes as normal body cells; e.g. egg, sperm, pollen. 2. What is the purpose of gametes (sex cells)? To fuse together at fertilisation. The fertilised egg now has the normal number of chromosomes and can divide and grow into a new individual. 3. Where does meiosis take place? In GONADS special sex glands; e.g. ovaries, testes, anthers. 4. TRUE or FALSE: meiosis takes place in egg and sperm cells. (If the answer is false, write out a correct statement about egg and sperm): FALSE meiosis takes place in GONADS and produces egg and sperm cells. 5. How many sets of chromosomes are present in the cells which produce gametes? TWO they are DIPLOID. 6. How many sets of chromosomes are present in gametes? Why is this important? ONE they are HAPLOID. this is important so that the normal DIPLOID number of chromosomes is restored when the sex cells fuse at fertilisation. 7. How many times does a cell divide during meiosis to produce a gamete? TWO 8. How many daughter cells are formed from each cell which divides by meiosis? FOUR (Usually in human females 3 of these break down to leave one egg, per month). 9. How does the number of chromosomes and genetic information in each daughter cell compare with that in the parent cell? Number of chromosomes is HALVED and the genetic information is shuffled during meiosis so that the daughter cells (the gametes) are always genetically different. 6. Draw a diagram showing the production of 4 daughter cells from one parent cell: - 8 -

9 5 Cell differentiation & stem cells Read your textbook or support material on growth in animals and plants. 1. Describe the main differences between growth in animals and plants in terms of: a. where growth takes place. Animals all over the body, producing a compact body + limbs Plants - only at growing tips and cambium (=meristems) producing a branching shoot (photosynthesis) & root system (anchorage; absorption of water & mineral ions) b. how long animals and plants can keep growing. Animals until they reach a finite, maximum (adult) size e.g. humans, 18 years Plants - continue growing in height & girth until they die c. ability of tissues and organs to regenerate from undifferentiated cells. Animals generally poor, but new research with stem cells shows that some can Plants - generally much better e.g. many plants will grow new roots from cuttings. d. which regions of plants and animals retain the ability to divide by mitosis. Plants divide ONLY at MERISTEMS - Specialised regions of plants which retain the ability to divide by mitosis throughout the life of the plant, giving rise to new tissues.e.g. root meristem in root tip, shoot meristem in shoot tip Animals most organs are capable of dividing throughout the life of the animal, although in some very specialised organs this is very limited e.g. brain & nervous system. 2. a. Find out what stem cells are. Use your textbook, support materials, or an internet search. Undifferentiated cells which are capable of dividing and differentiating into different cell types.e.g Bone marrow cells, umbilical cord cells, embryo - 9 -

10 cells. b. Explain how stem cells can be used to treat certain disorders, e.g. paralysis. Stem cells can be introduced into an area where cells are damaged e.g. motor nerve cells, and can differentiate into new cells to replace the damaged ones. If the damaged motor nerve cells are replaced, the paralysis might be cured. c. Discuss the advantages and possible ethical problems of using stem cells, especially those from human embryos, to replace damaged tissues. Advantages: 1. Stem cells are immortal keep dividing indefinitely 2. They can be used to repair many damaged tissues and organs e.g. spinal cord, pancreas, brain. 3. If they are taken from embryos cloned from a person s own cells, they will be genetically identical, and less likely to be rejected by that person s immune system. 4. This could reduce the need for transplant operations, with patients waiting for organs from donors (often dead ones). 5. They can be used to test drugs, so reducing the need to test them on animals. Disadvantages; moral/ethical problems (mainly of using embryo stem cells): 1. Stem cells can be obtained from human embryos. Is it morally right to kill these tiny balls of cells which could develop into human beings? 2. The S. Korean fraud shows that scientists are desperate to make breakthroughs in this field, because it could earn huge amounts of money for those who succeed, and this might become more important than the medical advantages. 3. Many egg cells are needed from young women. This is expensive & risky a needle is injected into their ovaries, and fertility drug treatment is needed. Is it worth the risk? 4. Research is still at a very early stage and not many trails have been successful yet. There could be unforeseen problems. 5. Other techniques, especially using adult stem cells, are available and highly promising so using embryo stem cells might not even be necessary. 6. Some research suggests that stem cells are more likely to mutate to form cancer cells which will produce tumours. 6 Boy or Girl Chromosomes are usually found in pairs in the nucleus of each body cell. These are called homologous pairs. Humans have 23 pairs (46) of chromosomes. Different species of plants and animals have different numbers of chromosomes e.g. cats have 19 pairs. In all mammals, males have one X and one Y chromosome, but females have two X chromosomes. The pictures (A and B) below show matched sets of chromosomes in the male and female of an organism. A B

11 a. What species of mammal have these 2 pictures come from? Explain your answer: Homo sapiens humans! 46 chromosomes = 23 pairs b. Which one is male and which is female? Explain your answer: A = male X and Y chromosome; B=female 2 X chromosomes Discuss with your teacher how the sex chromosomes can determine the sex of an individual. Think about which chromosomes, X or Y can be carried by the egg and sperm (remember a gamete can only carry ONE of a pair of chromosomes). Your teacher will explain to you about how to draw and use a PUNNETT SQUARE. c. Complete the diagram to show the probability of a couple having a boy or girl baby: PARENT: Male Female SEX CHROMOSOMES: (2) XY XX SEX CHROMOSOME (1) IN GAMETES: X or Y X PUNNETT SQUARE:

12 CHROMOSOME IN GAMETES X Y X XX XY X XX XY of a boy baby: 50% Probability Probability of a girl baby: 50% d. The sex (gender) of a baby is ultimately determined by the type of sex chromosome the sperm carries. Explain why. The egg can only carry an X chromosome. The sperm can carry either an X or a Y chromosome and it is this which determines the gender of the baby. e. Henry VIII either divorced or beheaded wives who were unable to give him a boy child. Was Henry correct in his assumption that his wives had failed him? Explain your answer clearly: No he was not! Only he could produce a boy baby by supplying a Y chromosome to the egg! 7 Genetics crosses a. Gregor Mendel Read the passage below and answer the questions: Gregor Mendel was an Austrian monk born in His work on pea plants in 1865 marked the start of modern genetics. He investigated the height of pea plants which were all either tall or dwarf. He started off by taking plants which was pure-breeding for tallness( this means when bred with itself or other tall plants they only produced tall plants)and plants which were pure-breeding for dwarfness (this means when bred with itself or others dwarf plants they only produced dwarf plants). He then cross-fertilized these two plants by taking pollen from each one. To Mendel s surprise he found that all the plants produced from the cross were tall. He concluded that When pure breeding plants with contrasting traits (e.g. tall and dwarf) are crossed, all the offspring will resemble one parent. He then crossed several of these tall plants which he had produced. Again to his surprise he found there was a 3:1 ratio of tall to dwarf plants. He concluded that for every characteristic each individual must have two determiners or factors (now known as genes)

13 a. What did Mendel investigate? The inheritance of height in pea plants He investigated the height of pea plants which were all either tall or dwarf. b. What are pure breeding plants? Plants which always produce the same characteristic as themselves when bred together c. When Mendel cross-pollinated pure breeding tall and dwarf plants what did he find? That all the plants were TALL d. When he cross-pollinated these tall plants which he had produced, what did he find? That TALL and DWARF pea plants were produced, in a 3:1 ratio. e. What did he conclude (find out) from this? He concluded that for every characteristic each individual must have two determiners or factors. Also that TALL genes (alleles) are DOMINANT over DWARF genes (these are called RECESSIVE). f. What are these determiners or factors now known as? Genes! Find out why Mendel s ideas were not recognised/accepted until He didn t publish his results widely enough. They were discovered by accident by British scientists after his death, & they realised how important they were. b. Key terminology Subjects/science/subject data/powerpoint presentations/ Boardworks/KS4 Biology/Inheritance. There are quite a few new words to learn in genetics. Discuss with your teacher/research what the following terms mean: Genotype A description of the pair of alleles present for a characteristic, e.g. BB, Tt, aa. Phenotype The physical expression of the alleles, e.g. tongue rolling, blue eyes, red petals. Allele (we have done this one!) A different form of a gene that codes for a different version of a characteristic. Dominant

14 An allele that will always be expressed even when there is only one of these alleles present, represented by a capital letter. Recessive An allele that will only be expressed when both alleles are of this type e.g. hh. Homozygous Pair of alleles that produce a characteristic are the same, e.g. HH. Heterozygous Pair of alleles that produce a characteristic are different, e.g. Hh. In genetics we use capital letters for dominant alleles and lower case for recessive alleles Complete the following table. The first one has been done for you Allele name & letter Tongue rolling Can roll = T Can t roll = t Eye colour Brown = B Blue = b Ear lobes Free = E Attached = e c. Monohybrid Inheritance Homozygous dominant genotype phenotype genotyp e Heterozygous Homozygous recessive phenotype genotype phenotype T T (can roll) T t (can roll) t t (can t roll) BB (brown eyes) Bb (brown eyes) bb (blue eyes) EE (free lobes) Ee (free lobes) ee (attached Lobes) When a characteristic is determined by just one pair of alleles we can do a simple genetics cross to investigate the mechanism of inheritance. This is called Monohybrid Inheritance. With the help of your teacher complete the following crosses involving eye colour: i. 2 Homozygous parents, one with brown eyes, one with blue eyes. Parents phenotype: brown eyes blue eyes Parents genotype: BB bb Parents gametes: B or B b or b (egg and sperm)

15 Offspring genotypes: ALL Bb All the offspring have BROWN eyes ii. 2 Heterozygous parents, both with brown eyes. Parents phenotype: brown eyes brown eyes Parents genotype: Bb Bb Parents gametes: B or b B or b (egg and sperm) Offspring genotypes: (Draw a Punnett square) B B B BB Bb b Bb bb The genetics diagram shows a 3:1 ratio of brown to blue eyes. This is called the MONOHYBRID RATIO iii. A heterozygous brown eyed parent and a blue eyed parent. Parents phenotype: brown eyes blue eyes Parents genotype: Bb bb Parents gametes B or b b or b (egg and sperm) Offspring genotypes: (Draw a Punnett square) B b b Bb bb b Bb bb The diagram shows a 1 : 1 ratio of Brown to Blue eyes. Genetics Practice Questions. The questions below get increasingly more difficult, have a go! Answer on separate paper All the answers below give only a summary make sure you answer in full to gain full credit! 1. The coat colour of mice is controlled by a single gene. This gene has 2 alleles B and

16 b. a. A mouse with black fur (genotype BB) was crossed with a mouse with white fur (genotype bb). All the baby mice were black. Use a genetics diagram to explain why all the mice had black fur. All baby mice were Bb and B allele is dominant b. A different mouse with black fur (Bb) was crossed with a mouse with white fur (bb). Use a genetics diagram to explain why some of the baby mice had black fur and some had white fur. Half of the baby mice were Bb (black) and half were bb (white). 2.Draw genetics diagrams to predict the possible outcome when two heterozygous tongue rolling people (Tt) have children. The tongue rolling allele is dominant to the non- rolling allele. There should be a 3:1 ratio of tongue rolling to nonrolling children (TT; Tt; Tt; tt only tt will be non-rollers). 3. A homozygous long haired cat is crossed with a homozygous short haired cat and produces a litter of 9 long haired kittens. Show the probable offspring which would be produced if two of these kittens were mated, including genotype and phenotype ratios. All of the long haired kittens would be Ll. If 2 of these were mated there should be a 3:1 ratio of longhaired to short haired kittens only ll would be short haired. 4. In humans, the allele for wavy hair is dominant to that for straight hair. (In your answer use H to represent the allele for wavy hair and h to represent the allele for straight hair) a. Explain the meaning of this statement: A person with wavy hair may be heterozygous or homozygous for this characteristic. Since the wavy hair allele is dominant, HH and Hh will produce wavy hair. b. Explain why a man with straight hair married to a woman with straight hair is unlikely to have any children with wavy hair. Both are homozygous hh. Since neither carries a H allele, NONE of their children should have wavy hair. c. By means of a genetic diagram explain why a man with wavy hair who marries a woman with wavy hair may have some children with straight hair. If both are heterozygous Hh, then each of them could supply a h allele in their egg/sperm, producing hh a straight haired child. 5. Can you draw the genetics diagrams for Mendel s experiment with pea plants? Use T to represent the allele for tallness and t to represent the allele for dwarfness. TT x tt all Tt heterozygous tall Tt x Tt ¼ TT; ½ Tt; ¼ tt 3:1 ratio of tall to dwarf plan ts. 8 Genetic disorders Discuss Genetic Disorders with your teacher. Watch the PowerPoint or read your textbook on Genetic Disorders. a. What is a Genetic Disorder? A body malfunction caused by one or faulty alleles inherited from one or both parents b. What is a mutation? A change in a gene, caused by a change in its DNA. It might or might not be harmful. c. Write down the name of 3 examples of Genetic Disorders. Cystic fibrosis; Down s syndrome; haemophilia; sickle-cell

17 anaemia; Huntington s syndrome The chance of children inheriting a genetic disorder from its parents depends on 2 things: i. Whether one or both parents have one or more alleles for the genetic disorder; ii. Whether the genetic disorder is caused by a dominant or recessive allele. d. In the disorder POLYDACTYLY, people have more fingers or toes than usual. The disorder is caused by a dominant allele, P. In the space below, carry out a genetics cross as indicated and find out the probability of children inheriting the disorder: i. A heterozygous man for Polydactyly has children with a normal-fingered woman. Parents phenotype: Polydactyl man Normal woman Parents genotype: Pp pp Parents gametes: P or p P or p (egg and sperm) Offspring genotypes: (Draw a Punnett square) P p p Pp pp p Pp pp The diagram shows a 1 : 1 ratio of Polydactyl and normal children. ii. How would the ratio of polydactyl to normal children differ if both parents were heterozygous for Polydactyly? There should be a 3 : 1 ratio of polydactyl to normal children as shown in the Punnett square only pp will be normal: P p P PP Pp p Pp pp e. Cystic Fibrosis Discuss Cystic Fibrosis with your teacher. Watch video clips which show cystic fibrosis (CF) or read your textbook. Answer the questions below: i. Describe the main symptoms of CF. Thick sticky mucus in lungs & intestine. Breathing difficulties. Problems digesting food. ii. What kind of allele causes CF? A faulty recessive allele. iii. People who are heterozygous for CF are described as SYMPTOMLESS CARRIERS. Explain what this means, & why they have no symptoms of CF. They are Nn they carry one normal dominant allele, N and one faulty recessive allele,n. The dominant allele prevents the expression of the recessive allele, so the carriers show no symptoms of CF

18 iv. Can 2 normal people or one normal person and one symptomless carrier have children with CF? Explain your answer in each case. NO! 2 Normal people (NN) can only produce normal children. A normal person (NN) & a symptomless carrier (Nn) can only produce normal children or symptomless carriers. v. Can 2 symptomless carriers have children with CF? YES! Complete a genetic cross & calculate the % probability that a child will be born with CF. (Label the dominant normal allele N and the recessive CF allele n) Parents phenotype: male symptomless female symptomless carrier carrier Parents genotype: Nn Nn Parents gametes: N or n N or n Offspring genotypes: (Draw a Punnett square) N n N NN Nn n Nn nn The diagram shows a 3 : 1 ratio of normal & symptomless carriers to CF children. vi. Embryo screening can be used to test for defective alleles including the CF allele. Explain what is meant by embryo screening. Embryo screening checks an embryo to see if it's carrying any alleles for a genetic disease (e.g. Huntington's disease or cystic fibrosis) so that the parents would know how likely it would be for their offspring to have a genetic disease. They could then decide whether to have a termination because of the genetic disorder. Read more: vii. (If you have finished. ) Approximately 1 in 25 people are symptomless carriers of CF. What is the likely incidence of CF in the population? Show your working. 2 symptomless carriers would need to have children, so the probability of any child being born is 1/25 x 1/25 = 1/625. However, only 1 in 4 children are likely to have CF, so the incidence is 1/625 x ¼ = 1/2500 which is about what the incidence is in the UK population. Well done! You have now completed the Cell division & Inheritance topic. Useful web links: The student room _Variation,_Mitosis_and_Meiosis#Mitosis s-cool revision world bbc bitesize

19 gcse-genetics purchon biology resources Glossary (muddled sort them out!) 1 Allele Contains the genetic code, Shaped like a double helix 3 2 Chromosome Controls your characteristics. It is a section of DNA molecule 5 3 DNA Found in the cell nucleus, X shaped, contains many genes. 2 4 Dominant The alternative form of a gene e.g. Alleles for eye colour brown and 1 blue 5 Gene Controls the development of a characteristic even when it is present 4 on only one of the chromosomes in a pair 6 Genetics Paired chromosomes; only come together at the start of meiosis 8 7 Genotype The chromosomes in a pair contain different alleles of a gene (Tt) 9 8 Homologous The chromosomes in a pair contain the same alleles of a gene (TT or 10 tt) 9 Heterozygous The type of genes an organism has e.g. TT, Tt, tt 7 10 Homozygous A study of the way parents pass characteristics on to their offspring 6 11 Meiosis Cell division, produces cells for growth and repair Mitosis Unspecialised cells, capable of differentiating into other cell types Phenotype Controls the development of a characteristic only if a dominant gene is 15 not present 14 Polydactyly Physical characteristics of an organism e.g. blue eyes, long tail Recessive Cell division, produces cells for sexual reproduction Stem Cells Condition of more than 5 toes or fingers, caused by a dominant allele 14 Questions

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24 AQA past paper question 3 In the 1850s, Gregor Mendel carried out breeding experiments using peas. (a) The importance of Mendel s work was not recognised until the early 1900s. Explain why (2 marks) (b) A student repeated one of Mendel s experiments. The flow chart shows her procedure

25 The diagram shows a representative sample of seeds produced by second generation plants. (i) Describe how the student could obtain a sample that is representative of seeds produced by the second generation (1 mark) (ii) What was the approximate ratio of yellow seeds to green seeds in the seeds produced by the second generation?... (1 mark) (iii) Seed colour in peas is controlled by a single gene which has two alleles. Use a genetic diagram to show why this ratio of yellow seeds to green seeds was produced by the second generation. Use the symbol A to represent the dominant allele, and a to represent the recessive allele. (4 marks) (H PAPER JAN 2008)

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