Case 5: In theory, the mechanism for crossing over (recombination) during meiosi

Question

Case 5: In theory, the mechanism for crossing over (recombination) during meiosis I occurs when homologous chromosomes that line up adjacent to each other during metaphase and exchange sections of chromosomes or genes. This generates new and unique combinations of traits that are observed when the gametes containing these unique combination of genes combine to form a zygote and develop into off-spring. Single birth of human off-spring reveal one of the four potential new combinations of genes from the single mitotic event but cannot confirm the cross-over mechanism because we cannot observe the genetic combination in the other three gametes (after fertilization). To investigate the mechanism, A Neurospora crassa cross was made between one strain of genotype c+ and another strain of genotype+ d. Four hundred linear octads were isolated, and they fell in the classes shown below. For simplicity, products of the final mitotic division are not shown. c+ c+ cd +dc+ cd c+ c+ c+ cd cd c+d c+ +d ++ C+ cd cd cd + 127 125 100 36 2 4 5 1 a. Draw a map of these genes, including the centromere(s). Indicate all map distances. Show your work. b. What event occurred to give rise to class 8? Briefly explain.

Explanation / Answer

a)

Parental ditypes are group 1, and 4.

There total progenies are 127 +36 = 163.

Non parental ditypes are group 3 and 6, that have total numbers 100+4 =104.

remaining others are tetrads type = 125+ 2+5+1 =133

Gene distance between c and d=

Map distance = [ {NPD +1/2 TT} / Total tetrads ] x 100

= [{104 + 1/2 (133) }/400] x100

=[304/400] x100

= 42.6 cM

b) Class 8 comes through the mitotic nondisjunction.

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