Fine spines (s), smooth fruit (tu), and uniform fruit color (u) are three recess

Question

Fine spines (s), smooth fruit (tu), and uniform fruit color (u) are three recessive traits in cucumbers, the genes of which are linked on the same chromosome. A cucumber plant heterozygous for all three traits is used in a testcross, and the following progeny are produced for this testcross:

a. Determine the order of these genes on the chromosome.

b. Calculate the map distance between the genes.

c. Determine the coefficient of coincidence and the interference among these genes.

d. List the genes found on each chromosome in the parents used in the testcross.

S U Tu 2 s u Tu 70 S u Tu 21 s u tu 4 S U tu 82 s U tu 21 s U Tu 13 S u tu 17 Total 230

Explanation / Answer

First let us determine the parental genotypes. The genotypes which are observed in large numbers are the parental genotypes. Here the parental genotypes are SUtu (82) and suTu (71).

The double crossover genotypes are those which are found with the least frequency. Here the double cross over genotypes are SUTu (2) and sutu (4).

a. To determine the order of the gene

by observing parental and double cross over genotypes it is clear that allele Tu/tu must be in the middle. Therefore the gene order will be S-tu-U and s-Tu-u

b. calculation of map distance

The distance between two genes is calculated by dividing total number of recombinant gamets by total number of gamets.

The distance between s and tu gene is (21+21+4+2)/230=0.28x100= 20.86cM

The distance between tu and u genei s (17+13+4+2)/230=36/230=0.156x100=15.65cM

The double cross over events should also be taken in consideration for calculating the gene distance using trihybrid cross. So number of these genotypes (4 and 2) are added in both calculations.

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