You hybridize 2 closely related species whose only difference is the order of ge

Question

You hybridize 2 closely related species whose only difference is the order of genes on their fourth chromosome as shown below. A single crossover takes place between genes B and C on non-sister chromatids in the F1 hybrid. Draw a diagram showing the configuration you would expect to see during prophase I and anaphase I of the first meiotic division. Indicate the positions of the centromeres and the 5 genes in each case. How would your answer to part (a) effect the fertility of the hybrids? map is as follows A homozygous stock of wild type flies was crossed with a stock homozygous for recessive mutants a, b, c, d and e. FI females (wild type phenotype) were backcrossed to the mutant parental stock. The only phenotypes of the back cross progeny were: Calculate the pairwise recombination frequencies as observed by this cross. Why are these results unexpected? Describe two possible changes in chromosome structure that could cause such discrepancies in recombination frequencies. Why is one of the possibilities is unlikely in this example? For the more likely case, draw the pair of chromosome homologues of the FI female at prophase I. Show the gametic products of a crossover occurring between the d and e loci. Which of the gametes produced may be inviable and why? Three closely related diploid species A, B and C each possessing two chromosomes per gamete are crossed to produce hybrids. The order of the eight chromosomal regions, a, b, c, d, u, v, w, and x, are shown below for the gametes of each of these three species. Each chromosomal region is ten mop units in length: Gamate from Species A: Gamate from Species B: Gamate from Species C: Diagram meiosis in the A-B hybrid. Will the A-B hybrid have the same fertility as its parents? What are the effects on fertility, if any, of a crossover in the w-x region? Briefly explain your answer. Diagram meiosis in the A-C hybrid. Will the A-C hybrid have the same fertility as its parents? What are the effects on fertility, if any, of a crossover in the w-x region? Briefly explain your answer. Diagram meiosis in the B-C hybrid. Will the B-C hybrid have the same fertility as its parents? What are the effects on fertility, if any, of a crossover in the w-x region'* Briefly explain your answer. Regions of DNA that contain transposable elements are more prone to duplications, deletions, and inversions. Describe two mechanisms that could account for tins observation.

Explanation / Answer

Part 2

A homozygous stock of wild type flies was crossed with a homozygous for recessive mutants a,b,c,d and e.

F1 females (wild type phenotype) were backcrossed to the mutant parental stock.

a) We have to calculate the pairwise recombination frequencies.

Recombination Frequency, RF = (Recombinants/ Total Number of Offsprings) x 100%

Thus, RF (A to B) = (4 / 4 +12 + 8 + 6) x 100%

= (4 / 20) x 100% = 1/5 x 100% = 20%

RF (B to C) = (12 / 4 +12 + 8 + 6) x 100%

= (12 / 20) x 100% = 3/5 x 100% = 60%

RF (C to D) = (8 /  4 +12 + 8 + 6) x 100%

= (8/20) x 100% = 2/5 x 100% = 40%

RF (D to E) = 6/20 x 100%

= 30%

Thus, Pairwise Recombination Frequencies, a = 0.2/ 0.44 x 100% = 45.45%

ab = 0.3/ 0.44 x 100% = 68.18%

b) The two possible changes in chromosomal structure that may lead to discrepancies in the recombination frequencies are chromosomal mutations and deletion of genes in a chromosome.

  

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