16.2. The roles of genes in determining the phenotype
A subsection of Biology, 9700, through 16. Inheritance
Listing 10 of 116 questions
In a species of snail, shell colour is controlled by a gene with three alleles: • allele CB codes for a brown shell • allele CP codes for a pink shell • allele CY codes for a yellow shell. Allele CB is dominant to both CP and CY. Allele CP is dominant to CY. The shells of this snail may be banded (have dark stripes) or non-banded. The allele for non-banded, N, is dominant to the allele for banded, n. State what is meant by the terms dominant and allele. dominant allele A cross between a brown, non-banded snail and a pink, non-banded snail produces some offspring that are both yellow and banded. State the genotypes of both parents. brown, non-banded pink, non-banded List the parental gametes. brown, non-banded pink, non-banded State the genotype of the offspring that are both yellow and banded. Suggest the proportion of offspring expected to be both yellow and banded.
9700_m16_qp_42
THEORY
2016
Paper 4, Variant 2
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The foxglove, Digitalis purpurea, is a common plant in many parts of the world. shows a foxglove. Flower colour in foxgloves is controlled by two genes that interact with each other. • Dominant allele M codes for an enzyme involved in the production of a purple pigment. • Recessive allele m codes for a non-functioning enzyme so no purple pigment is produced, resulting in a white colour. • Dominant allele D interacts with allele M to produce dark purple flowers. • Recessive allele d does not interact with allele M. • Neither allele D nor allele d interact with allele m. A double homozygous foxglove with dark purple flowers was crossed with a double homozygous recessive foxglove with white flowers. All the offspring had dark purple flowers. Explain what is meant by the term homozygous. Using the symbols above, state the genotype of the offspring with dark purple flowers. Two of these offspring with dark purple flowers were crossed. This cross produced a mixture of plants with three different flower colours: • dark purple • purple • white. Draw a Punnett square to show the possible genotypes and phenotypes of the mixture of plants from this cross. Write down the expected ratio of plants with each flower colour. ratio
9700_m17_qp_42
THEORY
2017
Paper 4, Variant 2
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Flowers of the common morning-glory plant, Ipomoea purpurea, can have several different phenotypes. An example of these flowers is shown in . Flower colour in I. purpurea is controlled by two genes on different chromosomes. Gene R/r, which codes for a protein involved in pigment production, has 2 alleles: • the dominant allele, R, allows pigment production • the recessive allele, r, prevents pigment production of any colour, resulting in white flowers. Gene T/t, which determines the type of pigment produced, has two alleles: • the dominant allele, T, results in purple flowers • the recessive allele, t, results in red flowers. Define the term allele. Suggest ways in which the expression of allele R allows pigment production. Complete to show the results of a cross between two I. purpurea plants that are heterozygous at both loci. parental genotypes: parental gametes: parental gametes results of cross: phenotypic ratios: show parental gametes in first column and top row • show offspring genotypes and phenotypes in square boxes •
9700_m20_qp_42
THEORY
2020
Paper 4, Variant 2
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Use Sometimes a gene has more than two alleles, termed multiple alleles. The ABO blood group system in humans is controlled by a gene with three alleles, IA, IB and Io. Alleles IA and IB are codominant and Io is recessive to both. The blood group AB is the result of codominance. Explain what is meant by codominance. In humans, a gene that codes for the production of a protein, called factor VIII, is located on the X chromosome. The dominant allele for this gene produces factor VIII, but the recessive allele does not produce factor VIII. A person who is unable to make factor VIII has haemophilia in which the blood fails to clot properly. Explain why a man with haemophilia cannot pass haemophilia to his son but may pass haemophilia to his grandson. Examiner’s Use A gene for feather colour in chickens is carried on an autosome. This gene has two alleles, black (CB) and splashed-white (CW). When a male chicken with black feathers is mated with a female chicken with splashed-white feathers, all the offspring have blue feathers. This also occurs when a male chicken with splashed-white feathers is crossed with a female with black feathers. black feathers splashed-white feathers Another gene may cause stripes on feathers (barred feathers). This gene is carried on the X chromosome. The allele for barred feathers (XA) is dominant to the allele for non- barred feathers (Xa). In chickens the male is homogametic and has two X chromosomes while the female is heterogametic and has one X chromosome and one Y chromosome. barred feathers Examiner’s Use A male chicken with black, non-barred feathers was crossed with a female chicken with splashed-white, barred feathers. All the offspring had blue feathers, but the males were barred and the females were non-barred. Using the symbols given above draw a genetic diagram to show this cross. parents’ male, black, female, splashed-white, phenotype non-barred feathers. barred feathers. genotype gametes offspring genotypes phenotypes male, blue, barred feathers. female, blue, non-barred feathers. Explain how a farmer could use a breeding programme to find out the genotype of a male chicken with blue, barred feathers.
9700_s07_qp_4
THEORY
2007
Paper 4, Variant 0
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Colour blindness is a condition characterised by the inability of the brain to perceive certain colours accurately. • The most common form is termed red-green colour blindness (RGC). • RGC results from a recessive allele. • 0.6% of females worldwide have RGC. • 8.0% of males worldwide have RGC. shows the occurrence of RGC in one family. = male = female = male with RGC Explain the meaning of the terms allele and recessive. allele recessive Explain why females are less likely than males to have RGC. With reference to , and using the symbols R for the dominant allele and r for the recessive allele, state the genotypes of the individuals 1, 4, 6 and 7.
9700_s08_qp_4
THEORY
2008
Paper 4, Variant 0
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The inheritance of coat colour in horses is complex but all horses have one of two base colours, red or black. The base colour is controlled in a simple monohybrid way. • When chestnut stallions and mares are mated the foals are always chestnut. • When black stallions are mated with black mares, either black or chestnut foals may be produced. Draw a genetic diagram to show how two parents with black coat colour can produce a chestnut foal and the probability of such an event occurring. Choose a letter symbol to represent coat colour. Five other genes can modify the base coat colour. One of these genes is the C gene. There are two alleles of this gene, C and CCR. • C does not affect the base coat colour. • CCR may modify the base coat colour. • If a chestnut horse has at least one CCR allele its phenotype will be palomino, which is a light cream colour. • If a black horse has at least one CCR allele its effect will not be noticeable in the phenotype. Complete the genetic diagram below. parental genotype aaCCCR AaCC parental phenotype …………………… …………………… gametes …………… …………… …………… …………… offspring genotypes …………… …………… …………… …………… offspring phenotypes …………… …………… …………… ……………
9700_s09_qp_4
THEORY
2009
Paper 4, Variant 0
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In humans a rare, sex-linked, recessive allele results in a change in the shape of the iris in the eye. This condition is know as cleft iris (C. Explain what is meant by the term sex linkage. Using suitable symbols complete the genetic diagram below. Key to symbols recessive allele dominant allele parental phenotypes male with CI X normal female parental phenotypes gametes offspring genotypes offspring phenotypes A woman who is heterozygous for CI becomes pregnant by a man with a normal iris. State the probability that their child will have CI.
9700_s10_qp_42
THEORY
2010
Paper 4, Variant 2
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In humans a rare, sex-linked, recessive allele results in a change in the shape of the iris in the eye. This condition is know as cleft iris (C. Explain what is meant by the term sex linkage. Using suitable symbols complete the genetic diagram below. Key to symbols recessive allele dominant allele parental phenotypes male with CI X normal female parental phenotypes gametes offspring genotypes offspring phenotypes A woman who is heterozygous for CI becomes pregnant by a man with a normal iris. State the probability that their child will have CI.
9700_s10_qp_43
THEORY
2010
Paper 4, Variant 3
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The fruit fly, Drosophila melanogaster, has many phenotypic variations and has been used in experiments to demonstrate the principles of inheritance. The majority of fruit flies have red eyes but there is a variant with white eyes. shows the red-eyed and white-eyed variants of the fruit fly. The gene for eye colour is located on the X chromosome. Using suitable symbols, draw a genetic diagram to show the possible offspring of a cross between a heterozygous red-eyed female fruit fly with a white-eyed male fruit fly. key to symbols: parental phenotypes red-eyed female parental genotypes gametes offspring genotypes offspring phenotypes white-eyed male One of the genes controlling the clotting of blood in humans is also located on the X chromosome. A rare variation of the gene, a recessive allele for haemophilia, can lead to a condition where the blood fails to clot properly. State why a man who has haemophilia is unable to pass the condition on to his son. Queen Victoria of Great Britain in the 19th century was a carrier of haemophilia, but did not have the condition. State the term used to describe the genotype of a carrier. Neither of Queen Victoria’s parents carried the allele for haemophilia. Suggest how Queen Victoria could have become a carrier.
9700_s12_qp_41
THEORY
2012
Paper 4, Variant 1
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Explain what is meant by the term heterozygous genotype. heterozygous genotype The budgerigar, Melopsittacus undulatus, is a small type of parrot that is native to Australia. shows a budgerigar. A budgerigar can have blue, green, yellow or white feathers. Two genes, A/a and D/d, are involved in the inheritance of feather colour in budgerigars. • A bird which has at least one dominant allele A but is homozygous for d has blue feathers. • A bird which has at least one dominant allele D but is homozygous for a has yellow feathers. • A bird with at least one dominant A allele and one dominant D allele has green feathers. • A bird that is homozygous for a and d has white feathers. Two green-feathered budgerigars, heterozygous at both gene loci, were crossed. Draw a genetic diagram of this cross to show the probability of producing offspring with yellow feathers.
9700_s12_qp_42
THEORY
2012
Paper 4, Variant 2
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Questions Discovered
116