16.2. The roles of genes in determining the phenotype
A subsection of Biology, 9700, through 16. Inheritance
Listing 10 of 116 questions
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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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_43
THEORY
2020
Paper 4, Variant 3
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The red poppy, Papaver rhoeas, and several species of daisy of the family Compositae often co-exist as weeds of wheat fields. shows changes in the percentage frequency of red poppies and daisies in an area of wheat fields over a six year period from 1998 to 2003. From 1985, the herbicide metsulfuron-methyl was used to control weeds in this area of wheat fields. This practice continued throughout the six year period. 1998 showed the first occurrence of a red poppy known as biotype X. This red poppy had a specific mutation not present in normal red poppies. year percentage frequency daisies normal red poppy biotype X red poppy Describe how the percentage frequencies of daisies and red poppies changed over the six year period. Metsulfuron-methyl acts by inhibiting an enzyme called acetolactate synthetase, which is needed for the daisies and red poppies to synthesise three amino acids essential for growth. The specific mutation carried by the red poppies of biotype X occurred within the gene coding for this enzyme. The mutation changed amino acid 197 of acetolactate synthetase from proline to leucine. Suggest the effect of this mutation on the structure and activity of the acetolactate synthetase enzyme of biotype X red poppies. Suggest the effect of this mutation on the biotype X red poppies in the presence of metsulfuron-methyl. With reference to , predict and explain the effect of biotype X red poppies on the relative proportions of weeds and wheat in the area of wheat fields in 2003 compared to 1998. . . Suggest how stopping the use of the herbicide metsulfuron-methyl and replacing it with a herbicide that inhibits a different target enzyme in weeds would affect the abundance of red poppies of biotype X.
9700_s17_qp_41
THEORY
2017
Paper 4, Variant 1
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The red poppy, Papaver rhoeas, and several species of daisy of the family Compositae often co-exist as weeds of wheat fields. shows changes in the percentage frequency of red poppies and daisies in an area of wheat fields over a six year period from 1998 to 2003. From 1985, the herbicide metsulfuron-methyl was used to control weeds in this area of wheat fields. This practice continued throughout the six year period. 1998 showed the first occurrence of a red poppy known as biotype X. This red poppy had a specific mutation not present in normal red poppies. year percentage frequency daisies normal red poppy biotype X red poppy Describe how the percentage frequencies of daisies and red poppies changed over the six year period. Metsulfuron-methyl acts by inhibiting an enzyme called acetolactate synthetase, which is needed for the daisies and red poppies to synthesise three amino acids essential for growth. The specific mutation carried by the red poppies of biotype X occurred within the gene coding for this enzyme. The mutation changed amino acid 197 of acetolactate synthetase from proline to leucine. Suggest the effect of this mutation on the structure and activity of the acetolactate synthetase enzyme of biotype X red poppies. Suggest the effect of this mutation on the biotype X red poppies in the presence of metsulfuron-methyl. With reference to , predict and explain the effect of biotype X red poppies on the relative proportions of weeds and wheat in the area of wheat fields in 2003 compared to 1998. . . Suggest how stopping the use of the herbicide metsulfuron-methyl and replacing it with a herbicide that inhibits a different target enzyme in weeds would affect the abundance of red poppies of biotype X.
9700_s17_qp_43
THEORY
2017
Paper 4, Variant 3
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