7.03 Genetics: Solutions to Problem Set 7 (2001)

7.03 PS #7 Solutions 1. a) A B a c A B Fall 2001 c a These are the alleles that mom passed to her kids. SSR95 A A B B B SSR96 a a c a c cb cb cb + + + A a cb B c + B a cb B c + A a cb b) As we can see from the table above, there was recombination in 2 of the children between SSR95 and SSR96. LOD0.1 = log10 ½(0.45)8(0.05)2 + ½(0.05)8(0.45)2 = 0.343 (0.25)10 c) Recombination in 2 out of 10 children suggests a recombination fraction of 1/5. θ = 0.2 LOD0.2 = log10 ½(0.4)8(0.1)2 + ½(0.1)8(0.4)2 = 0.536 (0.25)10 d) A B cb + A B + cb e) Recombination between SSR95 and color blindness only happened in one of the children. LOD0.1 = log10 ½(0.45)9(0.05) + ½(0.05)9(0.45) = 1.297 (0.25)10 2. a) Disease susceptibility modified by sex hormones is the more likely explanation. This is because an X-linked recessive trait would show square the incidence in females than in males. If frequency in boys = q = 0.005, then frequency in girls would be q2 = 0.000025, which is considerably less than what is observed. b) No, this data is not enough to make any sort of conclusion about genetic components. We need more information about how being related to an affected individual affects a person’s probability of being affected themselves. Studies such as twin studies would give us this information c) Yes, there is a genetic component. We know this because MZ > DZ. The ratio of the two is 11:1, so this indicates multiple genes. Yes, there is an environmental component. We know this because MZ<100%. d) The most common explanation is that these results are indicative of multiple genes and some disease susceptibility modified by sex hormones. Since males are automatically more susceptible to this condition, then they would not need as many mutant genes as women would to cause the disease. Thus, an affected mother will have more genes to pass on to her offspring than an affected father. A second possibility that the development of the fetus is affected by the environment in which it develops, namely the womb. If the mother is affected, there may be a greater chance that the fetus is affected. Maternal affect is another possibility. Since a sperm and an egg contribute unequally to the zygote’s cytoplasm, there might be some cytoplasmic factor mutated in an affected mother that predisposes the offspring to the condition. 3. a) Nondisjunction occurred in the mother. b) Since the daughter is heterozygous at the SSRs that are closely linked to the centromere, this shows that nondisjunction occurred in meiosis I. c) A A A B A A A B C C B B B B C C Meiosis I nondisjunction First polar body No X chromosome A A A B A A A C C C B B B B B C Meiosis II Second polar body Egg A A A B A A A C C B B B C B B C Note: This is only one possibility for phase of the alleles. However, we know that crossing over occurred between SSR3 and SSR4, and we know which alleles are in the egg. d) If the daughter got a Y chromosome from her father that lacked SRY, then she could develop as a female. e) The presence of SSR1 from the father suggests that there might have been crossing over between the X and the Y chromosomes in the father. Consequently, the Y lost SRY and gained the SSR1 from the X chromosome.