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

Question

Case 5: In theory, the mechanism for crossing over (recombination) during melosis occurs when homologous chromosomes that line up adjacent to each other during met aphase 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 Single birth of human off-spring reveal one of the four potential new combinations of genes from the single because we cannot observe the genetic combination develop into off-spring. mitotic event but cannot confirm the cross-over mechanism ). To investigate the mechanism, A Neurospora crassa cross was made in the other three gametes (after one strain of genotype c+ and another strain of genotype + d. Four hundred near 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 +dctcdc+ +d c+ cd cd cd ++ 127 125 100 36 24 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 87 Briefly explain. c. On the next page, diagram a meiosis that could give rise to class 7. Be sure that your drawing accounts for the linear order of the spores. Draw the meiosis giving rise to Class 7 from previous page d. Define and draw i. heteroduplex formation i. mismatch repair

Explanation / Answer

We can see that

Parental ditypes are group 1, and 4. There total progenies are 127 +36 = 163.

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

Others are tetrads type = 125+ 2+5+1 =133

Now 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

Class 8 comes through the mitotic nondisjunction.

Class 7 is the non cross over between chromosome 1 and 2.

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