Review Session 2 Key

Review Session 2 Key - This question that appeared on an...

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1 This question that appeared on an exam a few years ago puts together non-disjunction with properties of mutations: 1. Autosomal monosomic human embryos, resulting from fusion of a normal haploid gamete with an aneuploid gamete, don’t survive gestation but trisomics sometimes do. For example, individuals having 3 copies of chromosome 21 (trisomy 21) have Down’s Syndrome (a relatively mild impairment), whereas monosomy 21 individuals do not survive. Which of the following could help to explain the difference between the fate of monosomy 21 and trisomy 21 individuals? Circle all that apply: a. Haploinsufficient genes on chromsome 21 b. Polyploidy c. Gain-of-function mutations on chromosome 21 d. Synapsis e. Recessive lethal mutations on chromosome 21 f. Mendel’s Law of Segregation 2. The parents of a child with Down’s Syndrome are both heterozygous for a recessive mutation on chromosome 21. What is the probability that the child will have the recessive trait? Zero. Down’s Syndrome results from the presence of 3 copies of chromosome 21 (usually two from the mother and one from the father). If the mother is heterozygous for the recessive trait, then the child would have to inherit a wild type allele (and a mutant allele) from the mother. Same is true if extra copy of chromosome 21 was inherited from the father. Polyploidy Question: 3. In problem set 2, you were asked the following question: “Mendelian ratios are modified in crosses involving autotetraploids. Assume that one plant expresses the dominant trait brown seeds and has the genotype WWWW. This plant is crossed to another with the recessive trait white seeds whose genotype is wwww. If only one dominant allele is sufficient to produce brown seeds, what proportion of F2 progeny seeds (obtained by selfing F1 plants) do you predict will be white? Assume that synapsis between any two chromosomes carrying a W or w allele is equally likely.” Having answered that question, now answer this one: Among the brown seeds, what proportion will grow into a plant that would breed true for brown seed color when selfed? Answering this question is almost the same as answering the question of what proportion of the seeds will be white. The solution to this
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Review Session 2 Key - This question that appeared on an...

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