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1 BIOLOGY DNA, GENES, CHROMOSOMES p 55 DNA is the genetic code in the nucleus of cells A Chromosome is a strand of coiled up DNA DNA consists of sequences of bases: A, C, T & G A Gene is a section of DNA that codes for a particular trait (hair colour, eye colour etc.) MITOSIS p 59 A cell copies itself and splits in half, creating two identical daughter cells Occurs in the body cells (all except sex and blood cells). It is used for growth and repair MEIOSIS p 61 Meiosis only occurs in the sex cells Gamete male and female sex cells; the sperm and eggs Zygote The resulting one-celled organism, after a female egg is fertilized from a male sperm There are two stages of cell division resulting in four cells (gametes) that each have half the number of chromosomes The male and female gametes combine to give a zygote with the full number of chromosomes Pure Breeding When parents on both sides are homozygous dominant or homozygous recessive PUNNET SQUARES p 68 Punnet squares show the possible genotypes of offspring, based on the genotypes of the parents Phenotype ratio - compares the number of offspring with different physical traits Genotype ratio - compares the number of offspring with different alleles PEDIGREE DIAGRAMS p 73 Punnett square (genotype/phenotype) information can be shown on family trees This outlines affected people (usually homozygous recessive) BIOLOGY BIOLOGICAL IDEAS RELATING TO GENETIC VARIATION 4 CREDITS (90948) THE SKILLS YOU NEED TO KNOW: ALLELES p 57 An allele is the expression of a gene (e.g. either blue or brown allele for the eye colour gene) The dominant allele is the one that is always expressed (if it s present) The recessive allele will only be expressed if there are NO dominant alleles present Genotype is the genetic code an organism has for a particular trait (e.g. TT, Tt, tt) The phenotype is the physical appearance of that genotype (e.g. brown eyes, blue eyes etc.) Homozygous is when two alleles are the same (e.g. TT or tt) Heterozygous is when two alleles are different (e.g. Tt) GENETIC VARIATION p 64 Allows a population to survive by adapting to a changing environment. Variation can come from: Meiosis where four different cells are produced Crossing over where genes are swapped between chromosomes Mutations these are changes in DNA sequence and can occur before or after birth. Mutations can result from any number of factors Note: Problems may rely on knowledge from earlier biology sections PAGE 53

2 Exam Tip: Answering Questions A basic rule that s often broken: DON T WASTE TIME ON DIFFICULT QUESTIONS. By answering one question that you struggle with, you might be missing out on five questions that you can do. If it s hard, move on and come back to it later if you have time. PAGE 54

3 BIOLOGY DNA, GENES, CHROMOSOMES SUMMARY DNA is the genetic code in the nucleus of our cells. A Chromosome is a strand of DNA that has been coiled up DNA is made up of a sequence of bases; A, C, T and G A Gene is a section of DNA that codes for a particular trait (hair colour, eye colour etc.) An allele is the expression of a gene (e.g. either blue or brown allele for the eye colour gene) For a complete tutorial on this topic visit OLD NCEA QUESTIONS 1. Genes determine many of the features of organisms, such as the colour of the flowers on a plant. A gene is a part of a DNA molecule. The type of plant shown in the photograph below can have red or white flowers. Red flowers are due to a dominant allele and white flowers are due to a recessive allele. Discuss how information in DNA gives organisms (like the plant shown) their individual features, such as red and white flowers. In your answer, you should: Explain the difference between a gene and an allele Draw a labelled diagram to show the relationship between a gene and an allele and the structure of a DNA molecule Explain how the base sequence on DNA determines a particular feature (e.g. flower colour) and different forms (variations) of that feature (e.g. red and white flowers) PRACTICE QUESTIONS 2. What is a function of chromosomes? 3. Genes, chromosomes and DNA are important in inheritance. Describe is the relationship between genes, chromosomes and DNA. PAGE 55

4 ANSWERS NCEA 1. A gene is a section of DNA that codes for a particular feature (protein) while an allele is the forms the gene takes. PRACTICE 2. One of the following is required: Chromosomes carry genetic information or DNA Chromosomes control characteristics or traits of organisms. Chromosomes control activities of cells. Chromosomes are code for proteins. 3. A gene is a small part of a chromosome. A chromosome is made up of a long, coiled strand of DNA. Part of this DNA sequence, and therefore part of the chromosome, codes for a particular characteristic (e.g. eye colour). This section is called a gene. Along the DNA, base sequences provide the code for building different proteins, which then determine particular features. Specific sections of the DNA determine individual features such as flower colour. Variations in the feature (e.g. red or white flowers) are due to (slight) differences in the sequence of the bases making up the gene. The different variations of a particular gene are called alleles and they cause the variations in the phenotypes. (Merit) Study Tip: Last Minute Study If you are running out of time: Focus on topics that came up often in class Don t bother studying topics that haven t been covered in class. They probably won t be in the exam The LearnCoach summaries are a good place to start! PAGE 56

5 BIOLOGY ALLELES SUMMARY An allele is the expression of a gene (e.g. either blue or brown allele for the eye colour gene) The dominant allele is the one that is always expressed (if it s present). The recessive allele will only be expressed if there are NO dominant alleles present Genotype is the genetic code an organism has for a particular trait (e.g. TT, Tt, tt) The phenotype is the physical appearance of that genotype (e.g. brown eyes, blue eyes etc.) Homozygous is when two alleles are the same (e.g. TT or tt) Heterozygous is when two alleles are different (e.g. Tt) For a complete tutorial on this topic visit OLD NCEA QUESTIONS 1. One trait in squash plants is the colour of the fruit. White fruit are due to a dominant allele (F) and yellow fruit are due to a recessive allele (f). a. Explain the difference between a gene and an allele. b. The alleles for the colour of squash fruit combine to produce THREE different genotypes, but only TWO phenotypes. PRACTICE QUESTIONS 2. Describe the difference between the terms allele and gene. 3. Kevin was interested in breeding mice. Genes control the fur colour of these mice. They can have grey or white fur. Grey mice are homozygous recessive for this fur colour gene. a. Discuss the relationships between chromosomes, genes and alleles. b. Show the symbols for grey and white fur that could be used in a punnet square. 4. Yellow coated mice result from an allele N that is present in the mice genotype. The n represents the normal allele. The genotype NN is lethal so there are no live homozygous yellow coated mice. Jane crossed two mice with yellow coats and counted the live offspring. These are her results: Coat Phenotype Number of Offspring Yellow 40 White 20 a. What fraction of the mice have yellow coats? b. What is the genotype of a white coated mouse? 5. Define the term homozygous. Explain how the alleles combine to produce only two different squash colours white and yellow. In your answer you should: Define genotype and phenotype State the three different genotypes produced and the phenotype of each 6. When harvesting wheat it is easier if the plants have short stems. Tall stems (T) are dominant over short stems (s). What is the genotype of a plant with a short stem? 7. Define the term heterozygous. 8. A certain breed of dog can have either a long tail or no tail at all. The allele for the long tail is recessive and is given the symbol t. a. What is the phenotype of a dog with the alleles tt? b. What is the genotype of a dog which is heterozygous for no tail? 9. A variety of flower produces either purple or blue flowers. A botanist wants more blue flowers. The allele for blue flowers is dominant over the allele for purple flowers. a. Define the word dominant as it is used in genetics. b. Give the genotype for a purple flower if the blue flower is given the letter W. 10. In breeds of dog, there is a gene that produces an extra toe or dewclaw. This is a dominant trait. State the genotype of a dog that has the normal number of toes and explain why this genotype results in normal toes. In your answer you should use the terms allele and recessive. PAGE 57

6 ANSWERS NCEA 1. a. A gene is a selection of DNA that codes for a characteristic or trait such as hair or eye colour, an allele is a different form of a gene, with a different combination of bases; A, T, G and C. Each gene has more than one allele and the dominant one(s) will always be expressed if they are present, for example brown or blue eyes. b. Genotypes Ff and FF give the phenotype of white squash fruit. Genotype ff gives the phenotype of yellow squash fruit. The genotype describes which alleles are present and the phenotype is the expression of these alleles, i.e. What a trait looks like. In this case the fruit with the dominant allele (F) will be white, and the fruit with both recessive alleles (f) will be yellow. When the dominant allele (F) is inherited the fruit will always be white. This means homozygous dominant will be white. When one of each allele is inherited, (heterozygous genotype Ff or ff) the dominant allele will be observed in the phenotype. It isn t until both recessive alleles (ff) are present that the yellow phenotype will be observed. Hence, only two (white, yellow) phenotypes will result from three different genotypes. PRACTICE 2. Genes encode specific instructions for the characteristics of an organism. Alleles are a different expression of the same gene and therefore express different physical characteristics (phenotypes). one definition correct. (Merit) two definitions correct a. Chromosomes are lengths of DNA. Genes are specific sections of DNA, which carry instructions for the characteristics of an organism. Alleles are different expressions of the same gene. For example, the gene for fur colour in mice has two alleles: one for grey fur and one for white fur. (Merit) b. Grey fur = f White fur = F a. 40/60=2/3 b. Genotype = nn 5. Homozygous means having two identical alleles for a gene. 6. Genotype = ss 7. Heterozygous refers to a situation when two genes that an organism possesses for a trait are different, e.g. Ff(which represents a dominant and a recessive gene) a. No tail b. Genotype = Tt or tt a. A dominant allele produces the same phenotype whether it is with a homozygous or heterozygous allele. If the dominant allele is present, it is fully expressed phenotype (even in a heterozygote). b. Genotype = ww. 10. The genotype of a normal dog is tt. This results in a normal number of toes because the dog has only inherited the recessive alleles (t). There is no dominant allele (T) present to inhibit the recessive allele, so the trait of the extra toe cannot occur in the dog. PAGE 58

7 BIOLOGY MITOSIS SUMMARY A cell copies itself and splits in half, creating two identical daughter cells. Occurs in the body cells (all except sex and blood cells). It is used for growth and repair. For a complete tutorial on this topic visit PRACTICE QUESTIONS 1. The diagrams below show how a cell reproduces. 2. Mitosis is a type of cell division. Describe the purpose of mitosis. 3. After an embryo has been formed, either by cloning or by sexual reproduction, it grows by cell division. What is the name of this type of cell division? a. The order is incorrect. Please place them in the correct order. The first letter has been given to you. (1)..D.. (2)... (3)... (4)... (5)... b. Name the objects labelled X, shown in image E. c. In which part of a cow does this type of cell division occur?. Study Tip: Evaluation After Each Exam ask yourself: Where did most of the questions come from? Which parts ate up most of my time? Was I anxious during the exam? If so why? What could I do differently next time? PAGE 59

8 ANSWERS PRACTICE 1. a. (1)D (2)A (3)E (4)B (5)C (Merit) b. Chromosomes or chromatids c. This type of cell division occurs everywhere in the cow except for the ovaries or testes and blood. 2. The purpose of mitosis is to produce identical daughter cells. These cells are used for growth and repair in the body. 3. Mitosis Study Tip: Motivation When you study well for an exam: Treat yourself for giving it your best shot Watch a movie with friends or get some takeaways Small rewards motivate you to try your best in whatever you do. PAGE 60

9 BIOLOGY MEIOSIS (SEXUAL REPODUCTION) SUMMARY Meiosis only occurs in the sex cells There are two stages of cell division resulting in four cells (called gametes) that each have half the number of chromosomes The male and female gametes with half the number of chromosomes combine to give a zygote (fertilized egg) with the full number of chromosomes Gamete male and female sex cells; the sperm and eggs Zygote The resulting one-celled organism, after a female egg is fertilized from a male sperm Pure Breeding When parents on both sides are homozygous dominant or homozygous recessive For a complete tutorial on this topic visit OLD NCEA QUESTIONS 1. Meiosis is a particular form of cell division that produces male and female gametes. Describe what gametes are and explain why they are needed for sexual reproduction. PRACTICE QUESTIONS 2. There are 94 chromosomes in the body cell of a goldfish. How many are found in the gamete? 3. The normal chromosome number for toucans is 106. a. How many chromosomes would there be in a sperm cell of a male toucan? b. What is the name of the process that produces gametes? 4. Philip has two pedigree great Danes that he is breeding. A dog has 78 chromosomes. a. How many chromosomes are in each sperm cell and also in each cell of an embryo? b. What does the term pedigree mean as it is used above? 5. Fur colour in mice is determined by the allele B (grey fur) and the allele b (white fur). Barry is a pedigree white mouse. He escapes and breeds with a wild grey mouse. Barry s white fur b is recessive to the grey fur B of the wild mouse. Some of Barry s offspring were then collected. Explain whether the white offspring will ever show their wild ancestry through coat colour. 6. There are 23 pairs of chromosomes in each human body cell. One of these pairs of chromosomes is the sex chromosomes. Discuss why sex cells have 23 chromosomes, whereas body cells have 46. Refer to the cell division processes involved. 7. A mule, which is created when a horse and a donkey mate has 63 chromosomes. Explain why the mule would not be able to produce sex cells. 8. A kangaroo has 16 chromosomes in each of its cells. How many chromosomes will there be in its gamete? 9. A wolf has 39 chromosomes in each egg cell. How many chromosomes will this wolf have in each of its skin cells? 10. A scientist wanted to produce a hybrid crayfish by mating a fast growing crayfish with a different crayfish that had good flavour. He injected the sperm from a crayfish that had 202 chromosomes in each body cell into the egg of a crayfish which had 96 chromosomes in each egg cell. How many chromosomes would there be in a fertilized egg resulting from this technique? Show your working to explain your answer. 11. Why do cells produced by meiosis have fewer chromosomes than their parent cells 12. a. Name process A on the diagram above. b. Name the cell labelled B, which forms as a result of process A. c. What type of cell division occurs when cell B forms an embryo? d. Explain what happens to the number of chromosomes in each cell during cell division for growth. PAGE 61

10 NCEA 1. A gamete is a sex cell (e.g. an egg or sperm) which has half the normal number of chromosomes as body cells. It is required in sexual reproduction to ensure that when a sperm fuses with an egg, the resulting first cell of the new organism has the correct number of chromosomes. (Merit) PRACTICE 2. Half of 94 = 47. Therefore, there are 47 chromosomes in the gamete ANSWERS a. 53 chromosomes. b. Meiosis. a. 39 chromosomes. b. It means that the parents are known and are homozygous. 5. It won t because for the mouse to be white it needs to be homozygous recessive and so there is no grey gene left to bring out. (Merit) 6. Gametes have only 23 chromosomes so that when the male and female sex cells combine their 23 chromosomes, there is a full set of 46 chromosomes in the zygote. The 23 chromosomes are produced via meiosis. The parent cell is divided twice giving four resulting cells each with half the number of chromosomes. However, mitosis is used in the body for growth and repair, therefore, both the parent and daughter cells must contain the full 46 chromosomes to function. 7. The mule will not be able to produce sex cells because it has an odd number of chromosomes. Meiosis cannot occur in this case. (Merit) 8. 8 chromosomes chromosomes 10. The number of sperm will have half the number of body cells (202 / 2 = 101). The 101 chromosomes from the sperm + 96 chromosomes from the egg gives a total of 197 chromosomes. (Merit) 11. Gametes have fewer chromosomes because in meiosis a cell undergoes two cell divisions, producing four cells each with half the number of chromosomes as the parent cell. This allows male sex cells to be combined with female sex cells and produce offspring with the correct number of chromosomes. (Merit) 12. a. Fertilisation/conception b. Cell B is a zygote or fertilised egg c. Mitosis d. During growth mitosis occurs. Therefore, the total number of chromosomes remains the same. Study Tip: Pre-Exam Before an Important Exam: Don t: spend a lot of time talking with classmates who haven t studied. Do: Avoid negative vibes and focus on your own preparation and goals. PAGE 62

11 BIOLOGY Study Tip: Study Enhancement If you are spending long hours studying or working remember to: Drink fluids Eat well Sleep well Do regular exercise and move around occasionally while studying It s the basics that can make some of the biggest differences. PAGE 63

12 GENETIC VARIATION SUMMARY Genetic variation allows a population survive by adapting to a changing environment. Variation can come from: Meiosis where four different cells are produced, any of which could be passed on to different offspring Crossing over where genes are swapped between chromosomes (in the first stage of meiosis) Mutations these are changes in DNA sequence and can occur before or after birth. Mutations can result from any number of factors (some examples include: radiation, random errors in DNA replication, insertion, deletion, repeats, substitutions ) For a complete tutorial on this topic visit OLD NCEA QUESTIONS 1. Genetic variation is important in a population. a. Describe what is meant by the term genetic variation, and explain its importance to a population. b. One process that produces genetic variation is mutation. Explain what mutations are and how they contribute to genetic variation. In your answer you should include: What a mutation is The effect of mutations on genes, alleles and DNA Whether all mutations are passed on to the next generation 2. One of the parents of a baby is a teacher who developed deafness last year as a result of having noisy classes. Discuss the likelihood of this type of deafness being inherited by their new baby. In your answer you should consider: The reason for the parent s deafness What types of characteristics are inherited The effects of genetics and the environment on deafness in offspring 3. Meiosis contributes to genetic variation. Discuss how meiosis contributes to genetic variation, and why genetic variation is important in a population. In your answer, you should: Describe what is meant by genetic variation Explain how the process of meiosis leads to genetic variation Explain why genetic variation is of benefit to a population 4. Mutations can occur in DNA during cell division. a. Explain whether a mutation could be inherited if it occurred in a skin cell of an individual. In your answer, you should: Describe a mutation Explain what determines whether a mutation is able to be inherited or not b. A population of plants, species A, living in a certain area shows a lot of variation in its leaf size, from very small to very large as shown in the graph below. Leaf size affects the ability of a plant to absorb sunlight and make food. Plants with larger leaves can live in areas with lower light levels. A new plant, species B, starts growing in the same area as species A. Species B plants grow taller than species A plants, which reduces the light available to plants growing below species B. Discuss how variation in leaf size occurs in the starting population of species A and explain how this might help species A to survive when species B starts growing in the same area. In your answer, you should consider: What causes variation within a population The effect of reduced light on different individuals of plant species A PAGE 64

13 PRACTICE QUESTIONS 5. Maize has 20 chromosomes in each of its cells. Cells can divide in one of two ways: mitosis or meiosis. Study the information in the table below: Number of daughter cells produced Chromosome number in daughter cells Mitosis Meiosis Variation Nill Yes Purpose Growth Reproduction Discuss why meiosis is used for reproduction, rather than mitosis. In your answer, consider: The processes of mitosis and meiosis How variation occurs in meiosis The chromosome number in daughter cells 6. Originally dogs looked very different to the modern breeds we have today. How, with the help of humans, have different breeds of dog originated from a single ancestral dog type? Please discuss. 7. A common trait of fruit flies is curly wings. Even though it is dominant they don t occur often in nature. Why is this? 8. Cloning is where plants or animals are copied exactly. Cloning makes genetically identical plants or animals to their parents. Normally only cuttings of plants, or identical twins in animals, are identical. a. Explain why cloning only involves mitosis while sexual reproduction involves meiosis and mitosis? b. Plants and animals can be created through either sexual reproduction or cloning. Please compare the biological advantages and disadvantages of these two different methods of reproduction. BIOLOGY 9. A New Zealand kiwifruit marketer is developing a red kiwifruit. The vines, which have a particular genetic mutation, were bred for this trait. A whole orchard with the red fruit trait can be obtained by either selective breeding or by cloning/ taking cuttings. The genetic characteristics of the plants obtained via these two methods will be different. Discuss the reasons for the differences in the genetic characteristics of the plants produced by selective breeding and cloning/cuttings. Consider the type of cell division involved in each breeding technique. 10. In July 1996 the first successful cloning of an animal occurred, Dolly the sheep. Discuss the genetic advantages of producing a sheep by cloning, rather than from sexual reproduction. In your answer you should: Explain the cloning process, including the type of cell division involved Explain the sexual reproduction process, including the type of cell division involved Identify the differences in the offspring produced by cloning and sexual reproduction Discuss the advantage of cloning compared to sexual reproduction Study Tip: Cramming Cramming helps some people. But don t lose sleep. Getting your normal sleep will ensure you are your best physically and mentally for the exam. PAGE 65

14 ANSWERS NCEA 1. a. Genetic variation is important in populations to ensure the survival of a species when an environment changes. Genetic variation refers to the different DNA sequences of offspring, caused through mutations, crossing over and meiosis. These differences in DNA sequence can cause individuals to have a different phenotypes. This variation of phenotype means that some individuals are better equipped for survival than others. If these variations occurred in the gametes (sex cells) then individuals that are better equipped to survive would produce offspring that also have higher survival rates than others. In this way the population will slowly produce offspring that is increasingly suited to the new environment. b. A mutation is a change in genetic material / DNA / genes of an organism. When a mutation occurs, the base sequence of the gene changes; this results in completely new alleles. If mutations occur in the gametes, these new alleles have the possibility of being passed on to offspring. If mutation occurs in body cells, only the one individual will show variation the change will not be passed on. Mutations do not always result in variation, but when they do, the variation is often in the form of entirely new alleles. 2. The parent / teacher became deaf because of loud noises related to teaching. Deafness was caused by environment, not genetics. (The question makes no reference to him having inherited deafness, nor was he born with it). Only genetic characteristics can be inherited, not those acquired as a result of environment. It is unlikely any of his children will be born deaf, as it appears the deafness was caused by environment, not genetics. However, we cannot determine whether they will be deaf at any stage in their life, as deafness can be work- related and it depends on the job they have later in life. 3. Genetic variation refers to a variety of different genotypes (different DNA sequences) within a population. Meiosis produces gametes with half the number of chromosomes. This means that pairs of alleles are separated at meiosis. At fertilisation, the sperm which fertilises the egg is purely due to chance. This results in new combinations of alleles. The advantage of variation to a species is that it may enable some individuals to survive if a threatening event occurs. For example, if a new disease arrives, not all individuals will be wiped out, as some may randomly be immune due to genetic variation. 4. a. A mutation is a change to the base sequence of a gene along the DNA of an organism or a change in the genetic code. If the mutation occurs in a gamete, then it would be inherited. As a skin cell is not a gamete, a mutation in a skin cell cannot be inherited. b. The variation in plants of species A may be due to differences (mutations) in the sequence of bases in a particular gene and due to the process of meiosis. Those individuals of plant species A whose leaves are large will be better adapted to live under lower light levels. They will survive better and produce more offspring, increasing the number of larger leaves plants in the population. Plants with average or small leaves will not be able to absorb enough sunlight and make sufficient food. In time these plants might die off. (Merit - either a or b correct) (Excellence - a and b correct) PRACTICE 5. Meiosis is used for reproduction, whereas mitosis is used for growth and repair. This is because meiosis causes variation in the offspring. Meiosis halves the chromosome number so that when two gametes come together at fertilisation a full number of 20 are restored. Meiosis randomly causes one chromosome from each of the 10 pairs to pass into the daughter cell. As each member of a chromosome pair carries different alleles, each daughter cell will receive different combinations of alleles to pass on to their offspring. This means that offspring will show genetic variation from each other, and from their parents. Mitosis cannot be used for reproduction, as it produces two identical cells in the plants. The identical nature of the daughter cells means that mitosis cannot produce variation in offspring. Also, because mitosis doesn t halve the chromosome number to 10, there would be double the number when the two gametes met, producing maize with 40 chromosomes. 6. Different modern breeds of dog have all come from a single ancestral dog type through selective breeding. Selective breeding means that breeders choose from the natural variation present in the dogs. Different breeds have come about through selecting for certain desirable traits, such as hair colour, texture, or nose lengths. Dogs that share desirable traits are then mated together to produce similar offspring. If the mating is carried out over generations, then all undesirable traits can be bred out and a new dog breed will result. PAGE 66

15 BIOLOGY 7. If there are only a few curly winged flies, then curly wings must not be a favourable trait for survival. If curly winged flies cannot easily get food or escape predators, then they will die quickly without a chance to breed and pass on their dominant genes to their offspring. (Merit) 8. a. With cloning, the entire parent cell is used for the daughter cell and mitosis is then used for the reproduction of that cell. Sexual reproduction uses meiosis first to split up the chromosomes and then recombine them in fertilization. Mitosis can then be used for cell duplication once a zygote has been formed. (Merit) b. Cloning is an advantage if the parent has favourable traits but is not good if the parent has any genetic weaknesses. Cloning is preferable when a parent has traits that are desirable. A negative effect is that cloning reduces the diversity of a population and hence its ability to adapt to unexpected events such as climate change or an epidemic. If sexual reproduction is used, the traits of the offspring are unknown. This is good as it allows a population to adapt to changing environments. 9. Cloning/taking a cutting from a vine involves taking cells from the existing plant and creating new plants via mitosis. The plant will grow up having exactly the same genotype as the existing vine. Cloned plants are usually genetically identical to their parent, getting their entire DNA from that parent. Selective breeding involves pollinating the vine of the red kiwifruit with pollen from another vine. Meiosis will produce zygotes some of which will have the allele for red kiwifruit. The offspring will inherit a mixture of genes from both parent vines; getting their DNA from two different sources means that the offspring will have a unique set of genes. Vines that show the red kiwifruit phenotype will be saved for further breeding. By continually doing this eventually the whole orchard can be built up all producing red kiwifruit. 10. There are several advantages for a commercial farmer to clone sheep. The major advantage of using a cloned lamb is the selection of desirable traits - each clone would have exactly the same traits as its parent and these would be known. Desirable traits might include fast wool growth and a good colour. Similarly, undesirable traits can be avoided. Cloning means that offspring are produced via mitosis, where identical daughter cells are produced. This ensures that the clone is genetically identical to the parent. If the lamb was to be born through sexual reproduction, it would inherit genetic material from both parents. This would occur by meiosis, when the gametes (egg and sperm) join to restore the chromosome number. Once these have fused, the cell would replicate by mitosis as it grows into an embryo. This would give genetic variation in the offspring, rather than the identical offspring produced through cloning. There are some disadvantages to using cloning over sexual reproduction. Over time genetic variation in the flock decreases, which leaves the flock more vulnerable to disease or other adverse events that could be avoided with selective breeding. This is because selective breeding produces variation, which could allow the population to adapt to a changing environment. PAGE 67

16 PUNNETT SQUARES SUMMARY A punnet square shows the possible genotypes of offspring, based on the genotypes of the parents Phenotype ratio compares the ratio of offspring with different physical traits e.g. In the example to the right the ratio is 3:1 3 expresses B (BB, Bb and Bb) and 1 expresses b (bb) Genotype ratio compares the ratio of offspring with different alleles e.g. In the example to the right the ratio is 1:2:1 1 expresses BB, 2 express Bb, and one expresses bb For a complete tutorial on this topic visit OLD NCEA QUESTIONS 1. A couple are expecting their third child. They already have one boy and one girl. Discuss the likelihood of their third child being a girl. In your answer you should: Explain how sex is determined in humans Complete a Punnett square showing sex inheritance Explain the relevance of the couple already having children PRACTICE QUESTIONS 2. A variety of plant produces red or yellow flowers. The allele for yellow flowers is dominant over the allele for red flowers. a. Draw a Punnett square showing a cross between a pure-breeding, yellow flowered plant (Y) and a pure-breeding, red flowered plant (y). b. How could a plant with red flowers appear as a seedling from plants that had only produced seedlings with yellow flowers for several generations? Use Punnett squares. 3. Pea colour is determined by a single gene. Yellow peas (P) are dominant over green peas (p). A green pea plant is crossed with a yellow pea plant. There is a mix of both green and yellow pea plants as offspring. Jeremy buys some yellow peas from the local seed merchant. Explain what he could do to determine whether the peas were homozygous or heterozygous. You may use Punnett squares as part of your answer. 4. When harvesting wheat it is easier if the plants have short stems. Tall stems (T) are dominant over short stems (s). Complete a Punnett square using heterozygous parents for this trait. 5. A certain breed of dog can have either a long tail or no tail at all. The allele for the long tail is recessive, and is given the symbol t. A male dog is heterozygous. The dog is crossed with a female dog that has the same genotype. a. Complete a Punnett Square for the offspring of the cross. b. Give the phenotype ratio of the offspring of the cross. c. The cross resulted in eight puppies, two of which had no tail. Explain why this differs from the ratio in (b) above. d. Discuss how you could determine whether a no tailed dog was homozygous or heterozygous. You may use Punnett squares to help answer the question. PAGE 68

17 BIOLOGY 6. One of the genes in cats is responsible for tabby markings. Cats without this marking are homozygous recessive for this gene (tt). Two cats without markings are mated. a. Complete the diagram at the bottom of the page to show the possible genotypes of the offspring. b. A female offspring from the mating described in part (a) is crossed with a tabby male. Some of their kittens have tabby markings and some don t. Explain why 50% of the offspring produced from this cross would be expected to have tabby markings. A Punnett square could be included in your answer. 7. A guinea pig had a litter of six babies. The babies were all females. a. Complete a Punnett square to show the expected sex of the babies. b. If the guinea pig had another litter, what percentage of the babies would you expect to be male? Explain your answer. 8. Yellow coated mice result from an allele N that is present in the mice genotype. The n represents the normal allele. The genotype NN is lethal and the fetus does not develop so there are no live homozygous yellow coated mice. Jane crossed two mice with yellow coats and counted the offspring. These are her results: Coat Phenotype Number of offspring Yellow 40 White 20 a. i. Complete a Punnett square for the cross of the heterozygous parents. ii. In the Punnett square, what fraction of the mice carries at least one N allele? b. Explain why the fraction of yellow coated mice in Jane s experiment is different from the fraction in (a)(ii) above. Study Tip: Music Can enhance learning for some Be a distraction for others Find what works for you! PAGE 69

18 ANSWERS NCEA 1. Women are XX, so when they create eggs with half the number of chromosomes, both eggs will have an X chromosome. A male is XY so when they create sperm, half will have the X chromosome and half will have the Y chromosome. When the gametes come together (egg is fertilised), there is a 50% probability they will have a baby girl. The sex of the baby is determined by whether it is an X or a Y (sperm) that fertilises the egg. If it is X it will be female; if it is Y it will be male. The fact that they already have one girl and one boy has no effect on what the next baby will be. Fertilisation is random at each event, and previous fertilisations have no effect. PRACTICE 2. a. Y Y y Yy Yy y Yy Yy b. This could occur if two heterozygous yellow flowered plants were crossed. The parents of the crossed plants would both have yellow flowers, but they would not necessarily produce any red flowering plants. They could still pass on the recessive allele to the next generation. E.g. A homozygous dominant (YY) and a heterozygous dominant (Yy): 3. Jeremy could perform a cross with homozygous recessive (green) peas. If only yellow peas result then the yellow seeds are homozygous. If green also occurs then the seeds are heterozygous, as shown in the Punnett square below a. yellow pea p green pea p P Pp Pp p pp pp T T TT Tt t (Merit) t Tt tt T T TT Tt t Tt tt t b. 3:1 - three have no tail (TT, Tt and Tt), and one has a long tail (tt) c. Natural variation occurs and in such a small sample. 3:1 is the expected ratio, however, a different result can occur due to chance. d. Cross a known homozygous recessive dog (genotype tt) with the dog being tested. An analysis of the offspring of the test cross will confirm the genotype of the no tailed dog. The no tailed dog will be heterozygous if any of its offspring turn out to have long tails. This can be seen in the Punnett squares below. a. Homozygous TT x tt Heterozygous Tt x tt T T t t Tt t Tt tt (Merit) PAGE 70

19 BIOLOGY 7. b. The genotype of the female offspring (from a) must be tt. As some of the kittens don t have tabby markings, the genotype of the male must be Tt (if the male was TT the kittens would all have tabby markings). The Punnett square below shows why 50% of the kittens would be expected to have tabby markings. a. t T Tt Tt t tt tt X t Y X XX XY X XX XY 8. a. i. N n N NN Nn n Nn nn ii. 3/4 b. The fraction of yellow coated mice in the experiment is less than the Punnett square because ¼ of the offspring are NN yellow coated mice. These NN mice don t survive and are not counted in the statistics. Also, the expected ratio is only a theoretical expectation. In reality results may vary, particularly if the sample is small. (Merit - b correct) (Excellence - a and b correct) b. We would expect 50% of the babies to be male as can be seen in the Punnett square above. It is irrelevant that it is her second litter when determining outcomes as it doesn t rely on previous results. This shows a phenotype ratio of 2:2, meaning that we can expect 50% males. (Merit - b correct) (Excellence - a and b correct) Study Tip: Study Groups Can help with exchange of ideas and with motivation. More isn t always merrier. The larger the study group, the more likely you are to get distracted. Small study groups (up to 4) work better. PAGE 71

20 Study Tip: Understand NCEA You are sitting NCEA so: UNDERSTAND the marking system Each question is marked out of 8 so attempt all three questions in each topic. You can get up to 4 marks with Achieved parts. Merit parts give 5-6 marks and Excellence gives 7-8. If you are aiming for Excellence, do the Excellence parts of the question first. This gives you the full 7-8 marks straight away for each question. Easier parts of the question do not give you any extra marks (but they are a safety net if you ve made a mistake) PAGE 72

21 BIOLOGY PEDIGREE DIAGRAMS SUMMARY Punnett square (genotype/phenotype) information can be shown on a family tree This outlines affected people (usually homozygous recessive) Homozygous recessive parents ( affected ) will always pass on their genes, producing homozygous recessive or heterozygous (carrier) children For a complete tutorial on this topic visit OLD NCEA QUESTIONS 1. Gorillas show an inherited recessive condition called albinism. This results in white fur. The pedigree chart below shows the inheritance of albinism in a family of gorillas. Normal fur is the dominant allele (N), while albino fur is recessive (n). 2. The allele for a cleft chin (D) is dominant over the allele for a smooth chin (d). The pedigree diagram below shows the chin types in a family. a. Explain how the pedigree chart can be used to show that albinism is a recessive trait. In your answer you should: Define the terms dominant and recessive State the genotypes of albino and normal gorillas Complete a labelled Punnett square to support your answer Explain how your Punnett square shows that albinism is a recessive trait Note: Refer to the pedigree chart above. b. Explain whether the genotype of the 3 normal fur offspring in Generation IV can be determined, based on the evidence in the pedigree chart and your Punnett square from part (a) a. Explain how evidence in the pedigree diagram shows that the cleft chin allele (D) is dominant over the smooth chin allele (d). In your answer, you should: Explain what the term dominant allele means Draw a Punnett square(s) to show your reasoning b. Explain why the genotype of male A in generation 2 of the pedigree diagram shown above must have the genotype Dd. c. Individuals A and B, as shown on the pedigree diagram, decide to have another child. They draw a Punnett square to find what type of chin their child might have. Explain why the result predicted by the Punnett square may not accurately tell them what type of chin their child will have. In your answer, you should: Draw a Punnett square to show the cross between individual A and individual B Determine the probabilities of the child having a cleft chin and having a smooth chin Explain why the ratio of children born into the family with cleft and smooth chins may not match the probabilities PAGE 73

22 PRACTICE QUESTIONS 3. A variety of plant produces red or yellow flowers. The allele for yellow flowers is dominant over the allele for red flowers. The following is a pedigree chart showing the inheritance of flower colour in their descendants. 5. A pedigree chart for a particular trait of parrots is shown below. a. Which characteristic, yellow or red, is the recessive characteristic? Give a reason for your answer. b. Give the probable genotype of flower 1 and explain why. c. Give the probable genotype of flower 2 and explain why. 4. Dogs have been domesticated for thousands of years. During this time some characteristics have become popular and selected by dog breeders to form new breeds. A certain breed of dog can have either a long tail or no tail at all. The allele for no tail is dominant, and is given the symbol T. a. Two dogs were selected by a breeder to be mated. The male had a long tail and the female had no tail. They had four puppies between them, all with similar tails. Complete the pedigree chart below by showing the genotypes of the male and the puppies. b. The breeder chose two dogs, both heterozygous no tails to breed. Predict what the puppies from their mating would be, by making a Punnett square. c. What proportion (or percentage) would you expect to have a long tail? Determine the genotype of the parrots labelled A and B. Support your answer with: Reference to specific parrots from the pedigree diagram Reasons for which allele is dominant and which is recessive A Punnett square. 6. Jamie likes to breed goldfish. In one breeding cycle she mated two goldfish and had three resulting fish: two gold and one black. The gold colour allele G is dominant over the black. a. Complete the pedigree chart with the genotype of each fish. One gold coloured offspring has been completed for you. b. Jamie wants to breed black goldfish from the same parents. Complete a Punnett square to predict the genotypes and phenotypes of the offspring. c. What proportion (or percentage) of the offspring are expected to be black? d. Explain why the predicted results in part (c) are different to the actual results of the mating shown in (a). PAGE 74

23 BIOLOGY 7. The pedigree chart below shows the inheritance of a disease. White boxes show the creatures that don t have the disease. Use the chart to answer the following questions. 9. Hundreds of years ago before cloning was around farmers used selective breeding to make their herds better. In cows, the traits of milk volume, hide colour and meat quality have all been bred for. a. A farmer with a herd of brown cows observes two black calves. The parents are both brown. If B represents the gene or allele for brown coat colour which is dominant, complete the pedigree chart to show the genotypes of both parents and their two black offspring. a. Is the allele for this disease dominant or recessive? b. Explain your answer to (a) above. c. What is the expected genotype of the male with the asterisk. 8. Barry is a pedigree white mouse. He escapes and breeds with a wild grey mouse. Fur colour is determined by the alleles B and b. Barry s white fur b is recessive to the grey fur B. Give the genotypes of individuals A and B. b. The farmer next chooses two cows to breed that are both heterozygous for high milk production. High volume (l) is recessive to low volume (L). Predict the genotype of the calves from this cross by completing a Punnett square. c. What percentage of the calves from the cross in (b) would be expected to have a low volume of milk? Exam Tip: Proofreading Only takes a few minutes Finds the silly mistakes Gives you extra marks (especially if your teacher is supermeticulous) SO many people miss out on Extra Marks by not proof-reading PAGE 75

24 ANSWERS NCEA 1. a. Dominant means the trait will be expressed, even if only one allele is present in a pair (heterozygous). Recessive means the trait will be expressed only if two alleles are present (homozygous). It will be masked in the presence of one dominant allele (heterozygous). Albinism is a recessive trait. This can be established using Generation III and Generation IV. In Generation III, two normal individuals have three normal offspring and one albino offspring. The only way this is possible is for Generation III to both be Nn. When two n alleles come together, a homozygous recessive nn (albino) offspring forms. If normal was recessive, Gen III individuals would be nn. There is no way of forming an individual with N in its genotype. 2. b. All three Normal fur offspring are either Nn or NN. Because their parents must have both been Nn to produce albino offspring, there is a 25% chance they are NN and a 50% chance they are Nn. Their genotype cannot be established until they produce offspring. a. A dominant allele is the form of a gene that is always expressed whether the individual has one or two copies of that allele. A and B, who both have a cleft chin, produce a smooth chin child. If the cleft chin allele was recessive they would not be able to produce a child with a smooth allele as can be seen in the punnet square below, so the cleft chin allele must be dominant. d d d dd dd d dd dd b. As male A has a cleft chin this must mean that he carries at least one cleft chin allele (D). Male A and female B have a smooth chin boy, which means that A must have passed on a smooth chin allele (d) to the boy, to be smooth chinned the boy must have two recessive alleles, one from each parent. c. D d D DD Dd d Dd dd Probability of cleft chin = ¾ or 75% or 3 out of 4 Probability of smooth chin = ¼ or 25% or 1 out of 4 Random fertilisation of eggs by sperm means that number of offspring showing a particular variation will not always match the probability predicted by a Punnett square, unless the number of offspring is quite large. (Achieved - one of a, b or c) (Merit - two of a, b or c) (Excellence - all of a, b and c) PRACTICE 3. a. Red flowers are recessive. This is because two yellow flowers can have either yellow or red offspring, while two red flowers have only red flowers. b. Yy (where Y is yellow and y is red) because there are both yellow and red flowers in its offspring so it must have one of each (heterozygous). c. YY as even though it is reproducing with a homozygous recessive plant it produces only yellow flowers so it is likely to be homozygous dominant. (Achieved - one of a, b or c) (Merit - two of a, b or c) (Excellence - all of a, b and c) 4. a. b. T T TT Tt t Tt tt c. 25% would be expected to have a long tail. (Achieved - one of a, b or c) (Merit - two of a, b or c) (Excellence - all of a, b and c) t PAGE 76

25 BIOLOGY 5. A and B are heterozygous, or Pp. The trait is the recessive characteristic. This is because the A and B have offspring with the trait, even though they themselves don t have it. As the dominant trait appears in them they must have at least one dominant gene and because they produce offspring with the (recessive) trait, they must also have a recessive gene each. This is shown in the punnet square below: 6. a. P p P PP Pp p Pp pp 7. a. Dominant b. Parents that are both recessive will result in only recessive children, while parents with the dominant trait can end up with any combination of homozygous dominant, heterozygous and homozygous recessive depending on the genotype of the patents (DD or Dd). By looking at the second generation, it can be seen that two diseased mice mate and two without disease mate. The parents without disease only produce similar offspring, while the diseased parents produce both diseased and without disease offspring. This means that the diseased is dominant. c. Dd as it has both diseased and without disease offspring. (Achieved - one of a, b or c) (Merit - two of a, b or c) (Excellence - all of a, b and c) b. G G GG Gg g Gg gg c. 25% of the offspring would be expected to be black. d. Each fish has a 25% chance of being black. This probability doesn t depend on what colour the other siblings are as with sexual reproduction egg and sperm fertilisation is random. (Achieved - two of a, b, c or d) (Merit - two of a, b, c or d) (Excellence - all of a, b, c and d) g 8. A = bb (because he is white) B = Bb (there must be B for the dominant grey fur and a b from the white father). 9. a. b. L L LL Ll l Ll ll c. 75% would be expected to have a low volume of milk. (Achieved - one of a, b or c) (Merit - two of a, b or c) (Excellence - all of a, b and c) l Study Tip: Rewards Reward yourself for passing a tough year! Small achievements (e.g. topping a quiz) also deserve small rewards Maybe you could remind your parents about this one! PAGE 77