16. Inheritance
A section of Biology, 9700
Listing 10 of 199 questions
Coat colour in cats is determined by a sex-linked gene with two alleles coding for black and orange. When black cats are mated with orange cats: • the female offspring are always tortoiseshell (black and orange patches) • the male offspring are always the same colour as their mother. Explain what is meant by a sex-linked gene. sex-linked gene Using the symbols B for the allele for black coat and O for the allele for orange coat, complete the genetic diagram below. parental phenotypes tortoiseshell female parental genotypes gametes offspring genotypes offspring phenotypes black male Explain why a male cat cannot have a tortoiseshell coat.
9700_s13_qp_41
THEORY
2013
Paper 4, Variant 1
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Coat colour in cats is determined by a sex-linked gene with two alleles coding for black and orange. When black cats are mated with orange cats: • the female offspring are always tortoiseshell (black and orange patches) • the male offspring are always the same colour as their mother. Explain what is meant by a sex-linked gene. sex-linked gene Using the symbols B for the allele for black coat and O for the allele for orange coat, complete the genetic diagram below. parental phenotypes tortoiseshell female parental genotypes gametes offspring genotypes offspring phenotypes black male Explain why a male cat cannot have a tortoiseshell coat.
9700_s13_qp_43
THEORY
2013
Paper 4, Variant 3
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The stems of raspberry plants have spines. shows part of a raspberry plant. spines The colour of the spines is controlled by two genes, A/a and B/b. The two genes are on different pairs of chromosomes. • Allele A produces a pink anthocyanin pigment in the spines. • Allele B has no effect by itself, but increases the colour produced by allele A to give red spines. • Alleles a and b have no effect on colour. • In the absence of anthocyanin, the spines are green. State the colour of the spines of raspberry plants with the genotypes Aabb and aaBB. Aabb aaBB Plants with the genotype AaBb were crossed with plants with the genotype aabb. The resulting seeds were sown and the seedlings grown until their stems developed spines. Use a genetic diagram to show the outcome of this cross, including the ratio of offspring phenotypes. Suggest why the ratio you have given in your genetic diagram would be different if the genes A/a and B/b were on the same homologous pair of chromosomes.
9700_s17_qp_41
THEORY
2017
Paper 4, Variant 1
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The stems of raspberry plants have spines. shows part of a raspberry plant. spines The colour of the spines is controlled by two genes, A/a and B/b. The two genes are on different pairs of chromosomes. • Allele A produces a pink anthocyanin pigment in the spines. • Allele B has no effect by itself, but increases the colour produced by allele A to give red spines. • Alleles a and b have no effect on colour. • In the absence of anthocyanin, the spines are green. State the colour of the spines of raspberry plants with the genotypes Aabb and aaBB. Aabb aaBB Plants with the genotype AaBb were crossed with plants with the genotype aabb. The resulting seeds were sown and the seedlings grown until their stems developed spines. Use a genetic diagram to show the outcome of this cross, including the ratio of offspring phenotypes. Suggest why the ratio you have given in your genetic diagram would be different if the genes A/a and B/b were on the same homologous pair of chromosomes.
9700_s17_qp_43
THEORY
2017
Paper 4, Variant 3
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In the sweet pea plant, Lathyrus odoratus, one gene codes for flower colour and one gene codes for pollen grain shape. Flower colour is either purple or red. Pollen grain shape is either long or round. The inheritance of these genes is an example of autosomal linkage. • The allele F for purple flowers is dominant over the allele f for red flowers. • The allele G for long pollen grains is dominant over allele g for round pollen grains. Explain the meaning of the term autosomal linkage. A dihybrid cross was carried out between homozygous dominant and homozygous recessive sweet pea plant parents to produce the F1 generation. The offspring from the F1 generation were crossed to produce the F2 generation. Draw a genetic diagram to show a dihybrid cross between two offspring from the F1 generation. Assume that these genes are closely linked and that there are no crossing over events. The actual results of the dihybrid cross are shown in Table 2.1. Table 2.1 phenotypes of F2 offspring number of individuals purple flowers, long pollen grains purple flowers, round pollen grains red flowers, long pollen grains red flowers, round pollen grains State how the results support the fact that this is an example of autosomal linkage. In a test cross, an individual of known genotype is crossed with an individual that has a dominant phenotype but unknown genotype. State the genotype of the known individual in a test cross. A test cross was carried out with sweet pea plants known to be heterozygous for both flower colour and pollen grain shape. The results of the test cross are shown in Table 2.2. Table 2.2 phenotypes of offspring of test cross number of individuals purple flowers, long pollen grains purple flowers, round pollen grains red flowers, long pollen grains red flowers, round pollen grains The result of a test cross can be used to determine a crossover value (CO. A crossover value is the percentage of the total number of offspring showing recombination. The crossover value (COcan be calculated using the formula shown in . COV = number of recombinants total number of individuals # 100 Calculate the COV from the results shown in Table 2.2. COV = % Suggest what information about the relative distance between the linked genes can be gained from crossover values.
9700_s18_qp_42
THEORY
2018
Paper 4, Variant 2
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The tiger barb, Puntigrus tetrazona, is a South American fish that is popular worldwide as an aquarium fish. shows the appearance of a normal (wild-type) tiger barb. • Tiger barbs that show a wild-type phenotype are gold with black stripes. • Tiger barbs that show an albino phenotype are gold with white stripes. • In 2012, a fish breeder discovered a tiger barb with a new, transparent, phenotype. This fish had a transparent body and black stripes. The fish breeder crossed the tiger barb showing the new transparent phenotype with a tiger barb showing the albino phenotype. All the F1 offspring were wild-type. These F1 offspring were crossed with each other. Table 6.1 shows the phenotypes obtained in the F2 generation and the number of fish showing each phenotype. Table 6.1 F2 phenotype number of fish wild-type (gold with black stripes) albino (gold with white stripes) transparent with black stripes transparent with white stripes The fish breeder concluded that the new transparent phenotype had occurred because of a mutation. Explain how the results in Table 6.1 support this conclusion. State the approximate whole-number ratio shown by the results in Table 6.1. Explain what the results in Table 6.1 show about the genes and alleles that determine the wild-type, albino and the two different transparent phenotypes in tiger barbs. The F1 tiger barbs all looked the same but the F2 offspring showed variation. The F2 offspring showed four different phenotypes. Describe the processes that occurred during meiosis in the F1 fish that allowed this variation to occur.
9700_w23_qp_41
THEORY
2023
Paper 4, Variant 1
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The tiger barb, Puntigrus tetrazona, is a South American fish that is popular worldwide as an aquarium fish. shows the appearance of a normal (wild-type) tiger barb. • Tiger barbs that show a wild-type phenotype are gold with black stripes. • Tiger barbs that show an albino phenotype are gold with white stripes. • In 2012, a fish breeder discovered a tiger barb with a new, transparent, phenotype. This fish had a transparent body and black stripes. The fish breeder crossed the tiger barb showing the new transparent phenotype with a tiger barb showing the albino phenotype. All the F1 offspring were wild-type. These F1 offspring were crossed with each other. Table 6.1 shows the phenotypes obtained in the F2 generation and the number of fish showing each phenotype. Table 6.1 F2 phenotype number of fish wild-type (gold with black stripes) albino (gold with white stripes) transparent with black stripes transparent with white stripes The fish breeder concluded that the new transparent phenotype had occurred because of a mutation. Explain how the results in Table 6.1 support this conclusion. State the approximate whole-number ratio shown by the results in Table 6.1. Explain what the results in Table 6.1 show about the genes and alleles that determine the wild-type, albino and the two different transparent phenotypes in tiger barbs. The F1 tiger barbs all looked the same but the F2 offspring showed variation. The F2 offspring showed four different phenotypes. Describe the processes that occurred during meiosis in the F1 fish that allowed this variation to occur.
9700_w23_qp_43
THEORY
2023
Paper 4, Variant 3
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Genetic crosses can be used to investigate patterns of inheritance. A mutation in a gene involved in fruit colour in tomato plants, Solanum lycopersicum, results in the production of yellow fruits instead of red fruits. A genetic cross was carried out between a pure breeding plant with a red fruit and a pure breeding plant with a yellow fruit to produce the F1 generation. All offspring plants produced red fruits. The F1 plants were then crossed with each other and the seeds produced were planted to obtain the F2 generation. Construct a genetic diagram to show the cross of the F1 generation that produced this F2 generation. Use the symbols R and r for the alleles. A theoretical dihybrid cross involves two genes located on different autosomes. Each gene has two alleles, one dominant and one recessive. A parent, homozygous dominant for both genes, is crossed with a parent that is homozygous recessive for both genes. This produces F1 individuals that are then crossed to produce the F2 generation. State the phenotypic ratio of this dihybrid F2 generation and explain why some of these offspring phenotypes are different from the original parental phenotypes. ratio explanation
9700_w24_qp_42
THEORY
2024
Paper 4, Variant 2
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Use Outline the symptoms of cystic fibrosis (CF). CF is caused by a recessive mutation, b, on an autosome. Draw a genetic diagram to show, for parents with genotypes BbXX and BbXY, the probability of having a daughter who suffers from CF. In your genetic diagram, show the genotypes of the gametes and the genotypes and phenotypes of the offspring. genetic diagram parental genotypes BbXX x BbXY genotypes of gametes genotypes and phenotypes of offspring Examiner’s Use One of the many mutations for CF results in the amino acid arginine being replaced by histidine in the polypeptide encoded by the CF gene. Explain how a mutation may cause such a change in the amino acid sequence of a polypeptide. A genetic test was performed on two individuals, D and E, to find the base sequences of a small part of the CF gene. The different base sequences are shown diagramatically in . Individual E has CF. G T A individual D bases bases C G T A individual E C With reference to , state, how the base sequence of E differs from that of D Examiner’s Use the effect of this difference in the polypeptide produced by the two individuals.
9700_w07_qp_4
THEORY
2007
Paper 4, Variant 0
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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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Questions Discovered
199