Trihybrid F1 tomatoes carrying mutations for hairy branches (h), tangerine fruit

Question

Trihybrid F1 tomatoes carrying mutations for hairy branches (h), tangerine fruits (t) and uniformly coloured fruits (u) are test crossed. In the F2 different phenotypes are found in the numbers given in question 39. What is the distance between H and U loci in map units?

Smooth branches and uniformly coloured, non-tangerine fruits: 10 individuals

Hairy branches and uniformly coloured, tangerine fruits: 25 individuals

Hairy branches and uniformly coloured, non-tangerine fruits: 502 individuals

Smooth branches and uniformly coloured, tangerine fruits: 148 individuals

Hairy branches and green-based, non tangerine fruits: 142 individuals

Smooth branches and green based, tangerine fruits: 498 individuals

Smooth branches and green based, non-tangerine fruits: 27 individuals

Hairy branches and green-based, tangerine fruits: 12 individuals

Which gene is in the middle on the chromosome?

Explanation / Answer

Hairy branches and uniformly coloured, tangerine fruits: 25 individuals

Smooth branches and green based, non-tangerine fruits: 27 individuals

Smooth branches and uniformly coloured, non-tangerine fruits: 10 individuals

Hairy branches and green-based, tangerine fruits: 12 individuals

Smooth branches and uniformly coloured, tangerine fruits: 148 individuals

Hairy branches and green-based, non tangerine fruits: 142 individuals

Hairy branches and uniformly coloured, non-tangerine fruits: 502 individuals

Smooth branches and green based, tangerine fruits: 498 individuals

Total population = 1364

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 are the least number is the progeny are the result of a double crossover. So looking at the progenies we can easily conclude that Hairy branches and uniformly coloured, non-tangerine fruits and Smooth branches and green based, tangerine fruits are the original parent type and Hairy branches and green-based, tangerine fruits and Smooth branches, uniformly coloured, non-tangerine fruits 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 Hairy/smooth branch

The distance between smooth hair and tangerine fruits

If we want to find the distance between the smooth hair and tangerine fruits then we need to find out the crossover frequency and divide it by the total number of progeny

Hairy branches and uniformly coloured, tangerine fruits: 25 individuals

Smooth branches and green based, non-tangerine fruits: 27 individuals

Smooth branches and uniformly coloured, non-tangerine fruits: 10 individuals

Hairy branches and green-based, tangerine fruits: 12 individuals

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

74 * 100/1364

= 5.42

So, map distance between smooth hair and tangerine fruits is 5.42 map units or 5.42cM

The distance between smooth and green fruit.

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

Smooth branches and uniformly coloured, non-tangerine fruits: 10 individuals

Hairy branches and green-based, tangerine fruits: 12 individuals

Smooth branches and uniformly coloured, tangerine fruits: 148 individuals

Hairy branches and green-based, non tangerine fruits: 142 individuals

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

312 *100/ 1364

= 22.8

So, map distance between smooth and green fruit is 22.8 map units or 22.8cM

Map distance between Hairy H and uniform is 22.8cM

The middle gene is Hairy branches

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