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Q1. Some genetic disorders are caused by alleles inherited from the parents. (a) What are alleles? ........................................................................................................................ ........................................................................................................................ (1) (c) Polydactyly is a genetic disorder that leads to extra fingers or toes. Polydactyly is caused by a dominant allele, D. A man has polydactyly. His wife does not have polydactyly. This couple‘s children have a 50% chance of having polydactyly. Draw a genetic diagram to explain why. (3) (d) Cystic fibrosis is another genetic disorder. It is caused by a recessive allele. The diagram shows the inheritance of cystic fibrosis in one family.
Woman 5 is pregnant with her fourth child. What is the probability that this child will have cystic fibrosis? Draw a genetic diagram to explain your answer. Use the following symbols. N = allele for normal health n = allele for cystic fibrosis (4) (Total 12 marks) Q2. DNA is the genetic material of human cells. Figure 1 shows the structure of part of a DNA molecule. (a) (i) Describe where DNA is found in a human cell. ............................................................................................................... ............................................................................................................... ............................................................................................................... (2) (ii) When a cell divides by mitosis the new cells are genetically identical.
(a) (i) What is an allele? ............................................................................................................... ............................................................................................................... (1) (ii) What does recessive mean? ............................................................................................................... ............................................................................................................... (1) (iii) Give evidence from the diagram that CRAM is caused by a recessive allele. ............................................................................................................... ............................................................................................................... (1) (b) (i) Person 2 is homozygous for CRAM. What does homozygous mean? ............................................................................................................... ............................................................................................................... (1) (ii) None of person 2 ’s children have CRAM. Explain why. ............................................................................................................... ............................................................................................................... ............................................................................................................... (2) (c) Persons 7 and 8 want to have another child. (i) What is the probability that this child will have CRAM? Draw a genetic diagram to explain your answer. Probability = ........................................................... (4)
(ii) To avoid having another child with CRAM, persons 7 and 8 may decide to use embryo screening. Two ways of doing this are:
- PGD (pre-implantation genetic diagnosis)
- CVS (chorionic villus sampling). PGD involves IVF (in vitro fertilisation) of a few eggs, then taking a cell from each embryo when it is 3 days old. The image below shows how the cell is removed. © Rtimages/iStock/Thinkstock The DNA in the cell can then be tested. An unaffected embryo can be implanted in the woman’s uterus. The possibility of a false positive result is around 1 in 6. The procedure costs about £6000. Affected embryos would be discarded. Extra unaffected embryos might be frozen and kept for later implantation. Alternatively, the extra embryos might be used in scientific research. CVS involves taking a sample of blood from the placenta a few weeks into pregnancy. DNA from white blood cells can then be tested. If an affected embryo is detected, the parents then have to decide whether to terminate the pregnancy or allow it to continue. CVS has a 1 percent chance of giving an incorrect result and a 0.9 percent chance of causing a miscarriage. CVS costs about £600.
M1. (a) (different / alternative) forms of a gene do not accept types of genes 1 (c) genotypes of parents and gametes correct (Man D and d , Wife d and d ) allow half-size genetic diagram with only one d from wife 1 offspring genotypes correct (½ = Dd and ½ = dd ) allow ecf if parental genotypes are wrong 1 offspring phenotypes correctly assigned to genotypes 1 (d) genotypes of parents and gametes correct ( N and n ) allow ecf if parental genotypes are wrong 1 offspring genotypes correct ( NN , 2 × Nn , and nn ) 1 offspring phenotypes correctly assigned to genotypes; 1 correct probability = 0.25 / ¼ / 25% / 1 in 4 / 1:3, only;
do not allow ‘3:1 ߣ / ‘1:4 ߣ
1 [12] M2. (a) (i) in the chromosome(s) ignore genes / alleles 1 in the nucleus allow nuclei allow mitochondria 1 (ii) the DNA / chromosomes / genes are replicated / copied / multiplied / doubled / duplicated allow DNA is cloned ignore same DNA / chromosomes / genes if unqualified
1 (b) (i) 1 / one 1 (ii) 2 / two 1 M3. (a) (i) alternative / different / one form of a gene or a mutation of a gene do not allow a type of gene (For info: CRAM = Childhood Recurrent Acute Myoglobinuria) 1 (ii) not expressed if dominant / other allele is present or it is heterozygous or only expressed if dominant allele not present / no other allele present or it is homozygous need two copies to be expressed / not expressed if only one copy allow ‘gene’ for allele 1 (iii) unaffected parents have an affected child allow 7 and 8 have 10 allow skips a generation 1 (b) (i) has two alleles that are the same accept (person is) nn / NN or has two recessive / dominant alleles 1 (ii) (all) inherit N / normal / dominant allele from 1 / from father ignore they are carriers
pro CVS: PGD may destroy some embryos ethical implications of research on embryos (with PGD) lower incidence of false positives / false results low(er) financial cost conclusion: must relate to candidate’s argument must have at least one point from each technique for max marks 1 [15]
