please provide clear, step by step answers. Thank you!! Genetic Mapping Problems

Question

please provide clear, step by step answers. Thank you!!

Genetic Mapping Problems 1. A three-point test cross is done to determine the order & map distances for three mutant phenotypes in Drosophila: v for vestigial wings(versus wild-type long wings), b for black body (versus wild-type gray body), and p for purple eyes (vs. wild-type red eyes). Assume all mutant phenotypes are recessive. Pure-breeding parental lines are crossed: long wings, black body, purple eyes X vestigial wings, gray body, red eyes. The heterozygous F1 female offspring are crossed to males with vestigial wings, black body and purple eyes to produce 1448 flies. The number of offspring with various genotypes from this second cross are listed below. Note: only the allele contributed by the female parent is listed, as the male in this cross will always provide a recessive allele at each of these three loci i.e., v, b, p). b+ 580 592 45 40 89 94 V4 b+ b+ b+ Vt A. Are these loci linked? Why or why not? B. If they are linked, what is the order of these loci on the Drosophila chromosome and what are the map distances between these loci?

Explanation / Answer

A- If the genes would not have been linked and independently assorted then, in that case, we would have got all the progenies in 1:1:1:1:1:1:1:1 ratio. But when we look at the cross result we will find that the number of progenies is not at all near to the test cross ratio.

So we can say that the genes are linked.

v b+ p+ 580

v+ b p 592

v b p+45

v+ b+ p 40

v b p 89

v+ b+ p+ 90

v b+ p 3

v+ b p+ 5

Total number of progenies = 1448

In any double cross, the maximum number of progeny is the progenies which have the same genotype as that of the parents and the progeny which have the least number is the result of a double crossover. So looking at the progenies we can easily conclude that v b+ p+ and v+ b p are the original parent type and v vb+ p and v+ b p +are the double recombinant.

This also gives us the idea about the gene which is present in the middle as only the middle gene is transferred to the other chromosome during the double crossover. So the middle gene is p or purple eye

The distance between vestigial wing and purple eye

If we want to find the distance between the v and p then we need to find out the crossover frequency and divide it by the total number of progeny

v  b p 89

v+ b+ p+ 90

v b+ p 3

v+ b p+ 5

Linkage distance = total number of recombinant * 100 / total number of progeny

187* 100/ 1448

= 12.9

So, map distance between vestigial wing and purple eye is 12.9 map units or 12.9cM

The distance between the purple eye and black body.

If we want to find the distance between the p and b and v then we need to find out the crossover frequency and divide it by the total number of progeny

v b p+45

v+ b+ p 40

v b+ p 3

v+ b p+ 5

Linkage distance = total number of recombinant * 100 / total number of progeny

93*100/ 1448

= 6.4

So, map distance between purple eye and black body 6.4 map units or 6.4 cM

Allelic arrangement is the first parent is v p+ b+

Allelic arrangement is the second parent is v+ p b

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