One homozygous fruit variety produces fruit averaging 20g in weight, while anoth

Question

One homozygous fruit variety produces fruit averaging 20g in weight, while another homozygous fruit variety produces fruit averaging 8g. The F_1 obtained by crossing these two plants has fruit weight ranging from 12 to 16g (shown below). The F_2 produces plants with fruit weight ranging from 8 to 20g. a. How many genes likely contribute to fruit weight in this species? b. Which generation shows the greatest amount of variation, the F_1 or F_2? c. How much does each contributing allele add to the mass of the fruit in grams (assume that every allele is equally contributing ie. each allele adds the same mass)?

Explanation / Answer

a. How many genes likely contribute to fruit weight in this species:

For this we have to determine the number of loci (allelic pairs)that affect the trait.

The relationship that gives the proportion of homozygous recessive individuals in the F2 generation for any n-hybrid cross is important. In the case of quantitative traits this proportion is the frequency with which we expect the extreme phenotypes to occur. This proportion is given by

((1/2n))2
where n = number of loci involved.

In this case one of the extreme phenotypes, is 8 g variety in the F2. where 80 is total no .of individuals in f2

8/80 = ((1/2^n))^2 and solve for n and Finally n= 2 (exactly 1.7 rounding off to 2)

Thus, there are two loci (allelic pairs) that affect fruit weight in this fruit variety.

b. The greatest amount of variation is seen in F2 generation because F2 fruit weights varying from 8-20g where as in F1 12-16g only

c. To determine how much each contributing allele adds to mass of the fruit in grams, the formula is

Finally To determine how much each contributing allele adds to mass of the fruit in grams, substituing the given values in the formula

Ans is 3g per allele

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