16.2. The roles of genes in determining the phenotype
A subsection of Biology, 9700, through 16. Inheritance
Listing 10 of 116 questions
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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Domestic goats, Capra hircus, show a wide range of coat patterns and colours. One gene involved in coat colour and pattern has multiple alleles. Four of these alleles are: • A, the allele for white, is dominant to all others • Ab , the allele for badgerface (stripes on face) and Ag, the allele for grey, are codominant • a, the allele for black, is recessive to all others. State all the possible genotypes of: a grey badgerface goat a white goat. A cross between a black goat and a white goat produced a white goat. This white offspring was crossed with a grey goat. The genotype of the grey goat was not known. Use genetic diagrams to show all the possible offspring genotypes and phenotypes that could result from the cross between the white offspring and the grey goat. When goat skin cells produce black melanin pigment these events occur: • Melanocyte stimulating hormone (MSH) is secreted into the bloodstream. • MSH binds to the melanocortin 1 receptor on the cell surface membranes of skin cells. • This causes a rise in cyclic AMP within the cell. • The enzyme tyrosinase is activated and begins melanin production. The white allele A codes for a protein that binds irreversibly to the melanocortin 1 receptor. No rise in cyclic AMP occurs, even when MSH is present in the blood. Explain the appearance of goats with genotype aa.
9700_s18_qp_41
THEORY
2018
Paper 4, Variant 1
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The interpupillary distance (IPD) is the distance in millimetres between the centres of the pupils of the eyes. shows how IPD is measured. Interpupillary distance IPD is one example of a characteristic of human facial structure that shows variation. shows the pattern of variation in IPD in a large sample of adults. interpupillary distance (IPD) / mm number of people Name the type of variation shown in . Suggest and explain how genes and the environment contribute to variation in IPD in humans. Individuals with an IPD of 70 mm or more have a mutation in the PAX3 gene that results in less PAX3 protein being made. The normal role of the PAX3 protein is to increase the expression of many other genes involved in embryonic development. These genes affect a range of phenotypic features such as facial structure, hearing and eye colour. State the term that is used to describe a gene, such as PAX3, that controls the expression of other genes and suggest how the PAX3 protein controls the expression of other genes. Describe how microarray analysis could be used to identify the genes switched on by PAX3 in embryonic cells. The chimpanzee, Pan troglodytes, has DNA that is 98.5% similar to humans, including possession of the PAX3 gene. Investigations show that chimpanzees express higher levels of the PAX3 protein during embryonic development than humans. shows a chimpanzee, Pan troglodytes. Suggest how knowledge of the PAX3 gene helps scientists explain how humans and chimpanzees are very different in facial structure, even though they have very similar DNA.
9700_s19_qp_41
THEORY
2019
Paper 4, Variant 1
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Some neurones in the brain produce a neurotransmitter known as dopamine. Parkinson’s disease occurs when the neurones that produce dopamine die. A person with the disease may experience difficulty in coordinating movement, often seen as tremors in different parts of the body. Parkinson’s disease typically occurs in people older than 55 years. Younger people with these symptoms are said to have early onset Parkinson’s disease (EOPD). Recessive mutations in a gene known as PINK1, located on chromosome 1, an autosome, are believed to be one cause of EOPD. A person with this form of EOPD has a homozygous recessive genotype. Draw a genetic diagram of a cross between two individuals who are heterozygous at the PINK1 gene locus. key to symbols used for alleles parental genotypes gametes offspring genotypes ratio of offspring phenotypes PINK1 codes for a protein kinase enzyme that is important in the functioning of mitochondria in neurones. Most recessive PINK1 mutations are base substitutions which lead to the production of a non-functioning protein kinase enzyme. Explain how a base substitution mutation can lead to the production of a non-functioning protein kinase. One rare, dominant mutation of the PINK1 gene codes for a product that inhibits the normal protein kinase. Explain how this mutation causes EOPD in a heterozygote.
9700_s19_qp_41
THEORY
2019
Paper 4, Variant 1
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Steelhead trout, Oncorhynchus mykiss, are fish that live in streams in North America. To increase the number of steelhead trout, captive breeding has occurred since 1992. Fish eggs and sperm are mixed and the young fish grow in large tanks of aerated water for the first year of their lives. Most are then released into the wild, however a few male and female fish are kept to become the parents of the next generation of captive-bred fish. Each tank may hold up to 50 000 fish. The young captive fish are fed processed food. Some young fish are unable to survive these conditions and a proportion die. Death is usually the result of poor wound-healing after accidents due to overcrowding and due to the spread of diseases. Name the expected pattern of variation in wound-healing ability in a population of fish. Name the process that results in improved survival of captive fish in second, third and subsequent generations of captive-bred fish. Suggest and explain three ways in which the tank environment may make the phenotype of a captive fish different from a wild fish. Two groups of fish were bred and grown in the same environment and were then compared to see if they showed differences in gene expression. The two groups of fish came from: • eggs and sperm from parent fish that had always lived in the wild • eggs and sperm from parent fish that had been bred in captivity. Results from microarray analysis showed that the offspring of the wild and captive-bred fish differed in the expression of over 700 genes. Describe how microarray analysis can detect differences in the expression of many genes when comparing two samples, such as the offspring of wild and captive-bred fish. Explain how gene expression is controlled in eukaryotes such as fish. Many of the differences in gene expression between the offspring of the wild and captive-bred fish were in genes coding for proteins involved in the immune response and in genes coding for proteins involved in wound healing. The fish from captive-bred parents expressed these genes to a greater degree. The researchers concluded that the differences were inherited and adaptive. Explain why the researchers concluded that the differences in gene expression between the two groups of fish were: inherited adaptive.
9700_s19_qp_42
THEORY
2019
Paper 4, Variant 2
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Oculocutaneous albinism (OCA) is a type of albinism. There are many different forms of OCA. OCA1A is one form of OCA, caused by a recessive mutation in the autosomal gene, TYR, coding for the enzyme tyrosinase. This enzyme is involved in the biosynthetic pathway that results in the production of melanin, the pigment responsible for the colour of hair, skin and eyes. A person with OCA1A has white hair, very pale skin and pink eye colour. Draw a genetic diagram to show the probability of a child having OCA1A, if both parents are carriers. Use the symbols A and a for the alleles. parental genotypes gametes offspring genotypes offspring phenotypes probability shows the biosynthetic pathway involving tyrosinase. tyrosinase tyrosinase tyrosine DOPA dopaquinone melanin There are a number of different mutations of the TYR gene that can result in an absence of melanin and cause OCA1A. These include: • a missense mutation, caused by a base substitution, is most common • a nonsense mutation, caused by a base substitution, is less common • an insertion mutation, which is extremely rare. A missense mutation results in a complete polypeptide chain that does not fold properly to form the functioning enzyme. A nonsense mutation results in a shortened polypeptide. Explain why a missense mutation results in a different product from a nonsense mutation. Explain how an insertion mutation in TYR can lead to a lack of melanin in a person with OCA1A. Worldwide, 1 in 17 000 people are born with OCA. This compares with 1 in 165 people among the Guna people of Panama. The Guna people of Panama have a small population and mostly live on many small islands off the coast of Panama. Suggest reasons why the Guna population of Panama has a relatively high number of cases of OCA.
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THEORY
2019
Paper 4, Variant 2
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Albinism is a condition that results from disruption of a biosynthetic pathway involving the enzyme tyrosinase. It is estimated that 1 in 17 000 people in the world has albinism. A recessive mutation in the TYR gene, which codes for the enzyme tyrosinase, is one cause of albinism. Individuals with this form of albinism are homozygous recessive. Describe the phenotype of a person with albinism. A recessive mutation in a different gene causes a type of albinism that mainly affects the eyes (ocular albinism). A person with this condition has reduced clarity of vision and involuntary eye movements. shows the pattern of inheritance of ocular albinism in one family. The pattern indicates sex-linked inheritance. Key: = unaffected female = carrier female = unaffected male = male with ocular albinism Explain why supports sex-linked inheritance of ocular albinism. Draw a genetic diagram to show how individuals 1 and 2 cannot have a child with ocular albinism. key to symbols parental genotypes gametes offspring genotypes offspring phenotypes Ocular albinism may be caused by a base deletion mutation. This mutation results in a non-functional protein. Explain how a base deletion mutation can result in a non-functional protein. Ocular albinism is a non-progressive disorder and clarity of vision remains stable throughout life. A female has a family history of ocular albinism but she does not have any symptoms. A test to find out if she has the mutant allele is available. Suggest one reason for taking this test and one reason against taking this test. for against
9700_s19_qp_43
THEORY
2019
Paper 4, Variant 3
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The patty pan squash plant, Cucurbita pepo, produces edible fruits that vary in colour. The colour of the fruits is controlled by two genes, A/a and B/b, that occur on different chromosomes. • Allele A produces a white fruit colour. • Allele a does not produce a colour by itself but allows the colours coded by gene B/b to show in the phenotype. • Allele B produces a yellow fruit colour. • Allele b produces a green fruit colour. In a dihybrid cross, an AABB plant was crossed with an aabb plant. All the resulting F1 plants produced white fruits. The F1 plants were then crossed with each other to obtain the F2 generation. Complete with the gametes produced by the F1 parents, the F2 genotypes and the F2 phenotypes. State the ratio of fruit colours in the F2 offspring. F1 parents: AaBb × AaBb ratio of fruit colours in F2 offspring Test crosses were carried out on two white-fruited plants, P and Q, from the F2 generation. Each of these plants had its female flowers pollinated with pollen from a green-fruited plant. For plant P, half of the offspring were white and half were yellow. For plant Q, half of the offspring were white and half were green. Deduce the genotypes of plants P and Q. plant P plant Q Plants P and Q show genetic variation with respect to fruit colour alleles. Identify the process that occurred during meiosis in the F1 parents that produced this variation and the stage of meiosis at which it occurred. process stage of meiosis In 1994, two new varieties of squash were grown in the USA. Both varieties showed resistance to two viral diseases of squash plants. • The variety ‘Tigress’ was developed by selective breeding. • The variety ‘Freedom II’ was developed by adding genes for viral coat proteins to the squash genome. Identify, with reasons, the variety of squash that can be described as recombinant. Outline one social advantage and one social disadvantage of the recombinant squash variety compared to non-resistant squash varieties. advantage disadvantage
9700_s20_qp_41
THEORY
2020
Paper 4, Variant 1
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The summer squash plant, Cucurbita pepo, produces edible fruits that vary in shape. shows the fruits of three different varieties of squash plants. Alfresco Di Nizza Patty pan (disc-shaped) Fruit shape in squashes is controlled by two genes, A/a and B/b, that are located on different chromosomes. • A disc-shaped fruit is produced when both dominant alleles, A and B, are present. • A spherical fruit is produced when either allele A or allele B is present, but not if both A and B are present. • A long fruit is produced when both allele A and allele B are absent. Table 2.1 shows the possible genotypes of the Patty pan and Alfresco varieties. Complete Table 2.1 to show the possible genotypes of the Di Nizza variety. Table 2.1 variety possible genotypes Patty pan (disc-shaped) AABB AaBB AABb AaBb Di Nizza Alfresco aabb A gardener used pollen from a male flower of Alfresco to pollinate a female flower of Di Nizza. The gardener grew the seeds produced from this cross and found that half the offspring produced spherical fruits and half produced long fruits. Draw one genetic diagram to explain this result. parent genotypes gametes offspring genotypes offspring phenotypes The offspring show genetic variation with respect to fruit shape alleles. Name the process that occurred during meiosis in the parents that produced this variation and state the stage of meiosis at which it occurred. process stage of meiosis Genetically modified (GM) summer squash plants with resistance to viral diseases have been grown in the USA since 1995. Scientists have been concerned that viral resistance genes pass easily from GM squash plants to their wild relative, the Texas gourd, Cucurbita texana. Explain why the possibility of gene flow from GM squash plants to the Texas gourd is a cause of social and environmental concern. social environmental A study compared the survival of two different types of hybrids that were formed by cross-pollination between GM virus-resistant squash plants and wild Texas gourd plants: • virus-resistant hybrids that had inherited the viral resistance gene • non-resistant hybrids that had not inherited the viral resistance gene. Viral disease outbreaks caused many of the non-resistant hybrids to die. Leaf-eating beetles then moved in larger numbers to the surviving healthy virus-resistant hybrids. The beetles carried a pathogenic bacterium Erwinia which was capable of killing the plants. compares infection with Erwinia in the virus-resistant hybrids and the non-resistant hybrids. percentage of plants infected with Erwinia July August Key: non-resistant hybrids virus-resistant hybrids Discuss whether these results provide support for the use of genetically modified organisms (GMOs) in food production. Suggest why the GM virus-resistant squashes grown by farmers rarely suffer infection by Erwinia.
9700_s20_qp_42
THEORY
2020
Paper 4, Variant 2
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Questions Discovered
116