16.2. The roles of genes in determining the phenotype
A subsection of Biology, 9700, through 16. Inheritance
Listing 10 of 116 questions
Mitochondrial complex I is a large enzyme complex that forms part of the electron transport chain. The enzyme is composed of many different polypeptides. The genes coding for these polypeptides are located either in mitochondrial DNA (mtDNA) or in nuclear DNA. Mutations in these genes can lead to the production of an enzyme that does not function efficiently. This results in a disease known as mitochondrial complex I deficiency. If severe, this can lead to death in early childhood. Explain why people with mitochondrial complex I deficiency may have muscle weakness and difficulty with nervous coordination of movement. When mitochondrial complex I deficiency is caused by mutation in mtDNA: • a cell in an ovary produces gametes with different proportions of normal mitochondria and mitochondria that contain the mtDNA mutation (mutant mitochondria) • a person has disease symptoms when the proportion of mutant mitochondria in their cells exceeds a certain threshold • the severity of disease symptoms, and the age at which they appear, can vary greatly in the children of one woman. In a family with a history of mitochondrial complex I deficiency that is caused by a mutation in a nuclear gene, the probability of a child inheriting the mutation can be predicted. Suggest why, in families where mitochondrial complex I deficiency is caused by mtDNA mutation, it is not possible to predict the probability of a child inheriting the mutation. Genetic screening can be carried out on people with symptoms of mitochondrial complex I deficiency. Previously 7 mtDNA genes and 37 nuclear genes were sequenced. Some of the people tested did not have mutations in any of these genes. As a result, another gene was sequenced in these people and was found to be mutated. This led to the suggestion that genetic screening should sequence a larger proportion of the genome for people suspected of having this disease. Discuss the ethical reasons for and against sequencing a larger proportion of the genome for people suspected of having mitochondrial complex I deficiency. One mutation linked to mitochondrial complex I deficiency is a base substitution. It causes the amino acid glycine to be replaced by the amino acid valine in a region of α helix in a protein that is important for the formation of mitochondrial complex I. Glycine is a small amino acid with an R group of one hydrogen atom whereas valine has a larger and branched R group. Predict how the change in amino acids would affect the structure of the protein.
9700_s20_qp_41
THEORY
2020
Paper 4, Variant 1
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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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In sickle cell anaemia the recessive allele HbS replaces the normal allele HbA. • The frequency of HbS is much higher in West Africa than in most parts of the world. • The frequency of HbS corresponds with the distribution of malaria. Explain what is meant by the term allele. State whether the likely life expectancy is high or low in West Africa for individuals with the following genotypes. In each case give a reason for your answer. HbAHbA HbAHbS HbSHbS Explain why populations of West African descent living in the USA have a decreased frequency of the HbS allele compared to West African populations.
9700_w10_qp_41
THEORY
2010
Paper 4, Variant 1
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In sickle cell anaemia the recessive allele HbS replaces the normal allele HbA. • The frequency of HbS is much higher in West Africa than in most parts of the world. • The frequency of HbS corresponds with the distribution of malaria. Explain what is meant by the term allele. State whether the likely life expectancy is high or low in West Africa for individuals with the following genotypes. In each case give a reason for your answer. HbAHbA HbAHbS HbSHbS Explain why populations of West African descent living in the USA have a decreased frequency of the HbS allele compared to West African populations.
9700_w10_qp_42
THEORY
2010
Paper 4, Variant 2
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Galactosaemia is a rare genetic disease in which the build-up of the monosaccharide galactose can result in an enlarged liver, kidney failure and brain damage. Galactose is produced in the body from the digestion of the sugar lactose, found in milk. Galactosaemia is caused by a recessive mutation of the GALT gene. The normal dominant allele codes for an enzyme which converts galactose to glucose. Suggest how a person with galactosaemia can minimise damage to the liver, kidney and brain. Explain how a mutation in the GALT gene could result in a change in the enzyme responsible for the metabolism of galactose. If the phenotypes of parents are known, the probabilities of having a child with galactosaemia, an unaffected child (healthy, not a carrier) or a child who is a carrier can be calculated. Complete Table 7.1 to show the results of these calculations. Table 7.1 parent 1 parent 2 percentage probability of having a child with galactosaemia percentage probability of having an unaffected child percentage probability of having a child who is a carrier unaffected carrier carrier carrier unaffected has galactosaemia carrier has galactosaemia Testing for galactosaemia is usually carried out on newborn babies, although it is possible to carry out the test on a foetus. Explain how the presence of galactosaemia in a foetus may be determined.
9700_w17_qp_41
THEORY
2017
Paper 4, Variant 1
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Galactosaemia is a rare genetic disease in which the build-up of the monosaccharide galactose can result in an enlarged liver, kidney failure and brain damage. Galactose is produced in the body from the digestion of the sugar lactose, found in milk. Galactosaemia is caused by a recessive mutation of the GALT gene. The normal dominant allele codes for an enzyme which converts galactose to glucose. Suggest how a person with galactosaemia can minimise damage to the liver, kidney and brain. Explain how a mutation in the GALT gene could result in a change in the enzyme responsible for the metabolism of galactose. If the phenotypes of parents are known, the probabilities of having a child with galactosaemia, an unaffected child (healthy, not a carrier) or a child who is a carrier can be calculated. Complete Table 7.1 to show the results of these calculations. Table 7.1 parent 1 parent 2 percentage probability of having a child with galactosaemia percentage probability of having an unaffected child percentage probability of having a child who is a carrier unaffected carrier carrier carrier unaffected has galactosaemia carrier has galactosaemia Testing for galactosaemia is usually carried out on newborn babies, although it is possible to carry out the test on a foetus. Explain how the presence of galactosaemia in a foetus may be determined.
9700_w17_qp_43
THEORY
2017
Paper 4, Variant 3
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During one cardiac cycle: • blood enters the heart from the lungs and from the rest of the body • blood leaves the heart to be transported to the lungs and to the rest of the body. Name the blood vessels entering the heart that bring blood from the rest of the body. One phase of the cardiac cycle is ventricular diastole (ventricular relaxation). A number of events occur in the heart during this phase. Outline and explain the events that occur in the heart during ventricular diastole. Blood arriving in the lungs from the heart is oxygenated as it passes through the pulmonary capillaries. Sickle‑shaped red blood cells are present in a person with sickle cell anaemia. These cells have a very high quantity of abnormal (sickle cell) haemoglobin and take up and transport less oxygen than red blood cells containing normal haemoglobin. The cause of the differences between sickle cell haemoglobin and normal haemoglobin is a mutation in the gene that codes for one of the two types of polypeptide found in a haemoglobin molecule. This mutation leads to a change in the mRNA produced during transcription, causing a change in the primary structure of the polypeptide formed. shows some of the changes that occur as a result of this gene mutation. normal haemoglobin P glu triplet in DNA template strand (strand that is transcribed) triplet in DNA non-template strand mRNA codon formed amino acid carried by tRNA sickle cell haemoglobin G A G G A G C A C Q R val With reference to , state: • the base sequence of DNA triplet P • the base sequence of DNA triplet Q • the base sequence of mRNA codon R. Name the type of polypeptide in a haemoglobin molecule that is different in sickle cell haemoglobin compared to normal haemoglobin. shows the oxygen dissociation curve for adult haemoglobin in a person who does not have sickle cell anaemia. partial pressure of oxygen / kPa percentage oxygen saturation of haemoglobin Compared to , the oxygen dissociation curve for adult haemoglobin in a person with sickle cell anaemia is shifted to the right. The uptake of oxygen by haemoglobin in the lungs and the release of oxygen by haemoglobin in respiring tissues is different in a person with sickle cell anaemia compared with a person who does not have the disease. With reference to , state and explain these differences. uptake of oxygen release of oxygen
9700_m20_qp_22
THEORY
2020
Paper 2, Variant 2
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Use Sickle cell anaemia is a genetic disorder that is caused by the presence of two recessive alleles. It is common amongst people of African origin. Malaria is a major cause of death in sub-Saharan Africa where 90% of the world’s cases occur. shows the distribution of sickle cell anaemia and malaria in Africa. sickle cell anaemia malaria Explain why malaria is found in the areas shown but not in areas such as northern Europe and South Africa. Examiner’s Use With reference to , explain the relationship between the distribution of sickle cell anaemia and malaria.
9700_w07_qp_4
THEORY
2007
Paper 4, Variant 0
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Huntington’s disease (HD) is an inherited disease of the central nervous system. The symptoms of HD usually develop in adulthood and include uncontrollable muscular movements, short-term memory loss and changes in mood. HD is caused by a dominant allele of the huntingtin gene on chromosome 4. Explain what is meant by the terms allele and dominant. allele dominant The dominant allele of the huntingtin gene contains many repeats of a triplet sequence of nucleotides, CAG. The age at which symptoms of HD first appear is linked with the number of CAG repeats. This is shown in . number of CAG triplet repeats age at which symptoms of HD first appear / years Describe the pattern shown in . A blood test to detect the dominant allele is available for people at risk of HD. Suggest why some people at risk of HD may decide not to take the blood test.
9700_w13_qp_41
THEORY
2013
Paper 4, Variant 1
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Huntington’s disease (HD) is an inherited disease of the central nervous system. The symptoms of HD usually develop in adulthood and include uncontrollable muscular movements, short-term memory loss and changes in mood. HD is caused by a dominant allele of the huntingtin gene on chromosome 4. Explain what is meant by the terms allele and dominant. allele dominant The dominant allele of the huntingtin gene contains many repeats of a triplet sequence of nucleotides, CAG. The age at which symptoms of HD first appear is linked with the number of CAG repeats. This is shown in . number of CAG triplet repeats age at which symptoms of HD first appear / years Describe the pattern shown in . A blood test to detect the dominant allele is available for people at risk of HD. Suggest why some people at risk of HD may decide not to take the blood test.
9700_w13_qp_42
THEORY
2013
Paper 4, Variant 2
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