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Question 1
The diagram below shows the inheritance of cystic fibrosis in a family. Individual 7 has the cystic fibrosis phenotype. |
a) | What is meant by the term phenotype?
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b) | Explain one piece of evidence from the diagram above that cystic fibrosis is caused by a recessive allele.
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c) | Explain one piece of evidence from the diagram above that the allele for cystic fibrosis is not on the X chromosome.
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d) | Couple 3 and 4 want to have another child. Complete the genetic diagram to calculate the probability they will have a boy with cystic fibrosis. Use the symbols 'A' for the dominant allele and 'a' for the recessive allele.
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Question 2
Li-Fraumeni syndrome is a rare disorder that makes an individual much more likely to develop cancer. The diagram below shows the inheritance of Li-Fraumeni syndrome within a family. Li-Fraumeni syndrome is caused by a dominant allele.
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a) | Explain what is meant by a dominant allele.
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b) | Explain one piece of evidence from the diagram above which shows that the allele for Li-Fraumeni syndrome is dominant.
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c) | Explain one piece of evidence from the diagram above which shows that the allele for Li-Fraumeni syndrome is not sex-linked.
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d) | Individuals 5 and 6 decide to have another child. Calculate the probability they will have a child with Li-Fraumeni syndrome.
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Question 3
The body colour of fruit flies is determined by two alleles whereby the allele for a grey body is dominant to the allele for an ebony body. The wing shape of fruit flies is determined by two separate alleles whereby the allele for normal wings is dominant to the allele for vestigial wings. |
a) | Give the genotypes of a vestigial-winged fly, heterozygous for body colour and an ebony-bodied fly, heterozygous for wing shape. Use the letter G for body colour alleles and the letter N for wing shape alleles.
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b) | A vestigial-winged fly, heterozygous for body colour was crossed with an ebony-bodied fly, heterozygous for wing shape. Complete the diagram below to show the genotypes, phenotypes and ratio expected in this cross.
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c) | In genetic crosses, the observed phenotypic ratios produced in offspring are not usually the same as the expected ratios. Suggest two reasons for this.
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d) | A student wants to find out whether his results from this cross are significantly different from the results he expected. Which statistical test should he use?
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Question 4
In a species of flowering plant, Mirabilis jalapa, two alleles code for the colour of flowers. One allele (CR) codes for red flowers whereas the other allele (CW) codes for white flowers. When a true breeding red-flowered plant is crossed with a true-breeding white-flowered plant, all the offspring produce pink flowers. Another pair of alleles determine the height of plants whereby the allele for tall plants (T) is dominant to the allele for short plants (t). The alleles for flower colour display a phenomenon known as codominance. |
a) | What is codominance?
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b) | A heterozygous tall, pink-flowered plant was crossed with a short, white-flowered plant. Complete the genetic diagram below to show all possible offspring genotypes and the expected ratio of this cross.
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c) | A population of Mirabilis jalapa consisted of 600 plants. In this population, 180 plants had the genotype CRCR, 205 had the genotype CWCW and 215 had the genotype CRCW. Calculate the actual frequency of the CW allele in this population. Give your answer to 2 decimal places.
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d) | A mutation in another population of Mirabilis jalapa causes plants to produce yellow flowers. Give two other causes of genetic variation.
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Question 5
The gene controlling coat colour in rabbits has four alleles. The allele C gives brown fur, the allele cch gives chinchilla fur (black-tipped white fur), the ch allele gives Himalayan fur (white fur with black fur at the extremities) and the c allele gives albino (white) fur. • Allele C is dominant to alleles cch, ch, and c. • Allele cch is dominant to alleles ch and c. • Allele ch is dominant to allele c. |
a) | Complete the table below to show the phenotypes of rabbits with each of the genotypes shown.
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b) | A Himalayan male was crossed several times with a brown female. They produced: 10 brown kittens 3 himalayan kittens 5 white kittens Complete the genetic diagram below to show the results of this cross.
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c) | The breeder of these rabbits expected an equal number of Himalayan and white kittens. Explain why the actual numbers were different to those expected.
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d) | A breeder wants to produce a litter containing only chinchilla rabbits. Is this possible? Explain your answer.
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Question 6
The fur colour of cats is controlled by a sex-linked gene. The allele G gives ginger fur whereas the allele B gives black fur. Female cats with both the G and the B alleles have patches of ginger and black fur which is described as tortoiseshell. |
a) | The gene determining fur colour in cats is described as sex-linked. What does this mean?
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b) | Give the genotypes of cats with the following phenotypes: black female cat and ginger male cat.
Use XG to represent the allele G and XB to represent the allele B.
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c) | A male cat with black fur mates with a tortoiseshell female. Complete the genetic diagram below to show the genotypes and phenotypic ratio expected in this cross.
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d) | Polydactyly is a dominant condition that causes cats to have extra toes. The gene does not occur on either of the sex chromosomes. A breeder crosses a ginger male cat heterozygous for polydactyly with a tortoiseshell female cat without polydactyly. Calculate the probability that this cross will produce a black male kitten with polydactyly.
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Question 7
Fur colour in the Labrador dog breed is controlled by two genes, each with two alleles. Each gene is on a different chromosome. Gene A controls the presence of an enzyme that converts phaeomelanin (a yellow pigment) into eumelanin (a dark pigment), whereas gene B controls how much of the pigment eumelanin is deposited in the hairs. The diagram below shows how both of these genes interact to produce different fur colours.
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a) | What type of gene interaction is occurring between genes A and B? Explain your answer.
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b) | The presence of allele A results in an enzyme converting the pigment phaeomelanin into eumelanin, producing either brown or black fur. Dogs without the A allele do not make eumelanin and are therefore, yellow. The presence of allele B increases the amount of eumelanin deposited in the hairs. Dogs with both alleles A and B will have black fur. Using the information above, give all the possible genotypes of a dog with yellow fur and a dog with brown fur.
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c) | A dog with the genotype AaBb was crossed with a dog with the genotype aabb. Complete the genetic diagram below to calculate the phenotypic ratio of the expected offspring.
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d) | Use the information above to explain how the genotype AaBb produces dogs with black fur.
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Question 8
In fruit flies, the genes for body colour and wing length are linked. The allele G, for a grey body, is dominant to the allele g, for a black body. The allele N, for long wings, is dominant to the allele n, for short wings. A scientist carried out crosses between flies heterozygous for both genes and examined the offspring produced. His results are shown in the table below.
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a) | The genes determining body colour and wing length in fruit flies are described as being linked. Explain what this means.
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b) | Use the information above to explain the results shown in the table.
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c) | Complete the table below to show the expected numbers of offspring phenotypes if the genes for body colour and wing length were on different chromosomes.
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d) | Another gene found in fruit flies determines eye colour and is located on the X chromosome. The allele R codes for red eyes and is dominant to the allele r that codes for white eyes. Explain why male fruit flies are more likely than female fruit flies to have white eyes.
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Question 9
Scientists have studied the genes controlling tail shape and skin colour in domesticated pigs. The allele T, for a curly tail, is dominant to the allele t, for a straight tail. The allele D, for pink skin, is dominant to the allele d, for black skin. |
a) | List the gametes that would be produced by a pig heterozygous for both tail shape and skin colour.
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b) | List the genotypes and phenotypes of offspring you would expect when crossing a pig heterozygous for both traits with a straight-tailed pig with black skin.
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c) | A farmer carried out the cross described above. The results he obtained are shown in the diagram below
Model answer Complete the diagram to show the numbers of each phenotype the farmer would expect in this cross.
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d) | Use the information above and the formula below to calculate X2.
X2=ΣE(O−E)2 Give your answer to 2 decimal places.
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e) | The diagram below shows a X2 probability table.
What conclusion can be drawn from the chi-squared result calculated in part d?
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