Question

By analyzing the gametes from a trihybrid fruit fly that is heterozygous for three linked genes (Xx Yx Zz), you obtain the following data:

http://tinypic.com/r/33vn5g9/9

a. Organize the 8 haploid genotypes into pairs that correspond to parentals, double crossovers (DCO) and each of the two types of single crossovers (SCO). [No written explanation necessary]

Parentals:

DCO:

SCO:

SCO:

Draw a map showing the order of the three genes on the chromosome. Label the map to indicate the distances (in centimorgans or map-units) between the gene in the middle and the genes on either side of it. [No written explanation necessary but show calculations used to obtain map distances.]

Is there evidence of chromosomal interference in this region? Briefly explain why or why not? Show calculations to justify your answer.

Answer 1

Hi,

Parentals: XYz [842]; xyZ [825]

DCO: XYZ [2]; xyz[0]

Gene in the middle = comparing parental and DCOs.

Z is in the middle; so new gene order = XZY / xzy

SCO: XZY[42]; xZY[39]

SCO: XZy [65]; xzY[60]

Recombination frequency between X and Z = 42+39+2+0 / 1875

=0.0442

= 4.4 mu..

Recombination frequency between Z and Y = 2+0+65+60 / 1875

=0.0677

= 6.7 mu.

Map: X--------4.4---------Z------------6.7mu--------------Y.

Expected DCO = [recombination frequency in region 1 (map units / 100)] X [recombination frequency in region 2 (map units / 100)]

Expected DCO = 0.0442 x 0.0667 = 0.0029

Observed DCO = 2+0/1875 = 0.00106

Coeficient of coincidence = observed DCO / expected DCO

COC = 0.00106 / 0.0029 = 0.367

Interference = 1 – COC

I = 1 – 0.367 = 0.633

There is chromosomal interference.