16. Inheritance
A section of Biology, 9700
Listing 10 of 199 questions
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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There are a number of mutations affecting the production of fetal haemoglobin, HbF, and normal adult haemoglobin, HbA. • The HbA allele codes for the normal β-globin polypeptide of haemoglobin. • The HbS allele, caused by a base substitution mutation, codes for an abnormal β-globin polypeptide. • The base substitution results in the amino acid glutamine, which has a polar R group, to be replaced by valine, which has a non-polar R group, in the polypeptide. The abnormal haemoglobin molecules (HbS) form fibres in low partial pressures of oxygen (pO2). The fibres cause red blood cells to become sickle shaped and the cells can block blood capillaries. Individuals with adult haemoglobin molecules that are all abnormal (HbS) have sickle cell anaemia. This is a painful chronic condition that can be life-threatening. Explain why this mutation causes the HbS to form fibres. Fetal haemoglobin, HbF, is produced by the fetus until just before birth, when adult haemoglobin begins to be made. By the age of six months, adult haemoglobin has replaced most of the HbF. This change occurs when the genes coding for HbF are switched off and the genes coding for adult haemoglobin are switched on. • A base substitution, British-198, causes fetal haemoglobin to continue to be produced. • Normally by the age of six months, the concentration of HbF reduces to less than 1% of total haemoglobin. • With the British-198 mutation, the concentration of HbF may be as high as 20% of total haemoglobin in an adult. • HbF has a higher affinity for oxygen at low pO2 than adult haemoglobin. Individuals who have both sickle cell anaemia and British-198 mutation have reduced symptoms of sickle cell anaemia. Suggest why having the British-198 mutation reduces the symptoms of sickle cell anaemia. In adults with the British-198 mutation, the gene coding for a fetal haemoglobin polypeptide remains switched on. This is due to the presence of a protein that controls gene expression. State the term that is used to describe a protein that controls gene expression. Gel electrophoresis can be carried out to test individuals for the different versions of haemoglobin: HbA, HbS and HbF. • A buffer with alkaline pH is used to make all haemoglobin molecules negatively charged. • HbS molecules have an additional positive charge compared to HbA. Describe and explain how gel electrophoresis is used to diagnose sickle cell anaemia. Four individuals had their haemoglobin analysed by gel electrophoresis. One of the individuals was heterozygous for the HbA and HbS alleles and had a condition known as sickle cell trait (SCT). Some of the results are shown in . In , lane 1 and lane 5 are complete. lane 1 individual with normal phenotype one-month- old baby with normal phenotype individual with SCT individual with sickle cell anaemia reference lane 2 lane 3 lane 4 lane 5 HbA HbF HbS Predict the results for the individuals analysed, by adding bands to lanes 2, 3 and 4 on .
9700_w20_qp_41
THEORY
2020
Paper 4, Variant 1
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Questions Discovered
199