Three loci,mitochondrial malate dehydrogenase that forms a and b MDHa,MDHb ,glucuronidase that forms 1 and 2 GUS1,GUS2 ,and a histone gene that forms + and - H⁺,H^- ,are located on chromosome #7 in humans.Assume that the MDH locus is at position 35,GUS at position 45,and H at position 75.A female whose mother was homozygous for MDHa,GUS2,and H⁺and whose father was homozygous for MDHb,GUS1,and H^- produces a sample of 1000 egg cells.Give the expected numbers of the MDHa GUS2 H^-cells she would produce.Assume no chromosomal interference. A)15 B)235 C)315 D)135 E)0

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Quizplus · Accepted Answer

Given the loci positions on chromosome #7 (MDH at 35, GUS at 45, and H at 75), we can calculate the recombination frequencies between these loci. The distance between MDH and GUS is 10 map units, and between GUS and H is 30 map units. Without chromosomal interference, the expected proportion of recombinants between MDH and GUS is 10% (0.1), and between GUS and H is 30% (0.3). The female is heterozygous at all three loci, so to get MDHa GUS2 H^-, recombination needs to occur between GUS and H but not between MDH and GUS. The probability of no recombination between MDH and GUS is 90% (0.9), and the probability of recombination between GUS and H is 30% (0.3). Multiplying these probabilities gives 0.9 * 0.3 = 0.27 or 27% of the gametes. Since she produces 1000 egg cells, 27% of 1000 is 270. However, since recombination can happen on either chromosome, only half of these recombinants will have the desired combination, leading to 270 / 2 = 135 egg cells with the MDHa GUS2 H^- combination.

Three Loci,mitochondrial Malate Dehydrogenase That Forms a and B MDHa,MDHb