16.2. The roles of genes in determining the phenotype
A subsection of Biology, 9700, through 16. Inheritance
Listing 10 of 116 questions
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_43
THEORY
2012
Paper 4, Variant 3
View
In mice, the intensity of pigmentation of the fur is controlled by multiple alleles of a single gene. The alleles are listed below in order of dominance, with C as the most dominant. • C = full colour • Cch = chinchilla • Ch = himalayan • Cp = platinum • Ca = albino Explain how multiple alleles arise. Eye colour in mice is controlled by two alleles of a single gene, B/b: • allele B codes for black eyes • allele b codes for red eyes. A mouse with full colour fur and black eyes was crossed with a mouse with himalayan fur and black eyes. One of the offspring was albino with red eyes. Using the symbols above, draw a genetic diagram to show the genotypes and phenotypes of the offspring of this cross.
9700_s15_qp_41
THEORY
2015
Paper 4, Variant 1
View
An autosomal gene controlling fur colour in cats has three alleles: • allele CB codes for black fur • allele CCH codes for chocolate fur • allele CCM codes for cinnamon (orange-brown) fur. Allele CB is dominant to both CCH and CCM. Allele CCH is dominant to CCM. State the name given to the position of a gene on a chromosome. A cinnamon-coloured cat must be homozygous. Explain what is meant by the term homozygous. Two black cats were crossed. Most of their offspring had black fur and a few had chocolate fur. When they were mature, one of the black offspring was crossed with one of the chocolate offspring. Some of the cats in the second generation had cinnamon fur. Draw a genetic diagram to show how cats with cinnamon-coloured fur were produced in the second generation.
9700_s15_qp_42
THEORY
2015
Paper 4, Variant 2
View
In mice, the intensity of pigmentation of the fur is controlled by multiple alleles of a single gene. The alleles are listed below in order of dominance, with C as the most dominant. • C = full colour • Cch = chinchilla • Ch = himalayan • Cp = platinum • Ca = albino Explain how multiple alleles arise. Eye colour in mice is controlled by two alleles of a single gene, B/b: • allele B codes for black eyes • allele b codes for red eyes. A mouse with full colour fur and black eyes was crossed with a mouse with himalayan fur and black eyes. One of the offspring was albino with red eyes. Using the symbols above, draw a genetic diagram to show the genotypes and phenotypes of the offspring of this cross.
9700_s15_qp_43
THEORY
2015
Paper 4, Variant 3
View
Cats with either black fur or white fur are common in Europe, whereas cats with brown fur are less common. A gene, coding for an enzyme involved in pigment production, has two alleles. • The dominant allele, B, results in black fur. • The recessive allele, b, results in brown fur. A second gene can affect fur colour. • The dominant allele, A, prevents pigment production, resulting in a cat with white fur. • The recessive allele, a, has no effect on fur colour. The two genes are on different pairs of autosomes. Use a genetic diagram to show how a cross between two cats, heterozygous at both loci, can produce offspring with three different colours: white, black and brown. State the expected ratio of the different coloured offspring. Suggest how the presence of allele A prevents pigment production.
9700_s17_qp_42
THEORY
2017
Paper 4, Variant 2
View
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
View
In the fruit fly, Drosophila melanogaster, eye colour and wing shape are controlled by genes. • E/e are alleles of a gene involved in eye colour. • E results in red eyes, e results in purple eyes. • N/n are alleles of a gene involved in wing shape. • N results in normal wings, n results in vestigial (short, non-functional) wings. These genes are both autosomal. A dihybrid cross was carried out between a fly with red eyes and normal wings and a fly with purple eyes and vestigial wings. Both parents were homozygous for both genes. The offspring from the F1 generation were crossed to obtain the F2 offspring. The results are shown in Table 2.1. Table 2.1 F2 offspring phenotype expected number of individuals expected F2 ratio observed number of individuals observed F2 ratio red eye, normal wing 22.1 red eye, vestigial wing 1.0 purple eye, normal wing 1.3 purple eye, vestigial wing 6.4 Complete the missing expected number of individuals and the expected F2 ratio in Table 2.1. A chi-squared test showed that the results for the F2 generation in Table 2.1 were significantly different from those expected. To investigate this, test crosses were carried out using flies taken from the F1 generation and flies that were homozygous recessive for both genes. The investigator assumed that the genes were unlinked and expected a ratio of 1:1:1:1. Draw a genetic diagram of this test cross. Use the symbols E/e and N/n. The results of the test crosses described in are shown in Table 2.2. Table 2.2 offspring phenotype number of individuals red eye, normal wing red eye, vestigial wing purple eye, normal wing purple eye, vestigial wing Flies with red eye, vestigial wing and flies with purple eye, normal wing phenotypes are described as recombinant. Name the stage of meiosis when these recombinants are produced and state how this occurs. Explain why the results in Table 2.2 are different from the expected ratio of 1:1:1:1.
9700_s18_qp_43
THEORY
2018
Paper 4, Variant 3
View
People vary in which molecules are present on the surface of their red blood cells. For example, in the ABO blood group system, people may be categorised into one of four blood groups: A, B, AB and O. A single gene, I, with three alleles, determines these blood groups. Gene I is an example of a gene with multiple alleles. • Alleles IA and IB are codominant. • Allele Io is recessive to alleles IA and IB. Explain what is meant by codominant and recessive. codominant recessive Table 7.1 shows a cross between a person with blood group A and a person with blood group B. Complete Table 7.1. Table 7.1 parental phenotype (blood group) A B parental genotype IA Io IB Io gametes offspring genotype offspring phenotype (blood group) Haemophilia is a blood condition where bleeding continues for a long period of time after a blood vessel has been damaged. A person with haemophilia has a mutation in a gene on the X chromosome, which results in the lack of a blood clotting factor. Explain why a man who has haemophilia cannot pass the condition to his male offspring.
9700_s21_qp_41
THEORY
2021
Paper 4, Variant 1
View
The fruit fly, Drosophila melanogaster, feeds on sugars found in damaged fruits. A fruit fly with normal features is described as wild type. It has red eyes and its wings are longer than its abdomen. There are mutant variations such as purple eyes or short wings. shows a wild type fruit fly and a mutant fruit fly with purple eyes and vestigial wings. wild type mutant red eyes long wing purple eyes vestigial wing • The genes coding for eye colour and wing length are located on the same chromosome. • Allele R for red eyes is dominant to allele r for purple eyes. • Allele N for long wings is dominant to allele n for vestigial wings. Explain what is meant by the terms allele and dominant. allele dominant A wild type fruit fly, heterozygous for both genes, was crossed with a fruit fly that was homozygous recessive for both genes. Table 7.1 is a summary of the cross. Complete Table 7.1. Table 7.1 wild type parent double homozygous recessive parent parental phenotype x parental genotype x offspring offspring phenotype offspring genotype number of offspring Explain why the four offspring phenotypes are not present in a 1:1:1:1 ratio.
9700_s21_qp_42
THEORY
2021
Paper 4, Variant 2
View
Questions Discovered
116