A true-breeding strain of Virginia tobacco has dominant alleles determining leaf

Question

A true-breeding strain of Virginia tobacco has dominant alleles determining leaf morphology (M), leaf color (C), and leaf size (S). A Carolina strain is homozygous for the recessive alleles of these three genes. These genes are found on the same chromosome as follows: 6 m.u An F1 hybrid between the two strains is now backcrossed to the Carolina strain. Assuming no interference: a. What proportion of the backcross progeny will resemble the Virginia strain for all three traits? b. What proportion of the backcross progeny will resemble the Carolina strain for all three traits? c. What proportion of the backcross progeny will have the leaf morphology and leaf size of the Virginia strain but the leaf color of the Carolina strain? d. What proportion of the backcross progeny will have the leaf morphology and leaf color of the Virginia strain but the leaf size of the Carolina strain?

Explanation / Answer

Virginia tobacco has dominant allele for leaf morphology (M), color (C), and size (S) whereas the Carolina strain is homozygous recessive for all three alleles.

Virginia: MMCCSS

Carolina: mmccss

Parents: MMCCSS x mmccss

F1:        MmCcSc

The F1 (MmCcSs) is backcrossed with Carolina strain (mmccss) and the following progeny are obtained.

The distance between M and C is 6 m.u. so the recombination frequency will be 6% (0.06) between these two. Therefore, the chance of gamete formation for

“McS” = 3%

“mCS”= 3%

Similarly, there is 17 m.u distance between C and S so the 17% (0.17) recombination will occur between them. Therefore, the chance of gamete formation for

“MCs”= 8.5%

“mcS” = 8.5%

Then there will be double crossover recombinants between M and S and by applying product rule the frequency of expected Double crossovers can be calculated as following:

(0.06 * 0.17) = 1.02%

So, the chance of gametes with

“McS” = 0.51%

“mCs” = 0.51%

The proportion of total recombination = 6 + 17+ 1.02 = 24.02%

So, the total proportion of parental offsprings = 75.98% (MCS and mcs)

a. The proportion of backcross progeny resembling Virginia strain (MCS) for all three traits will be 75.98/2 = 37.99%

b. The proportion of backcross progeny resembling Carolina strain (mcs) for all three traits will be 37.99%

c. The proportion of backcross progeny that will have leaf morphology and size of Virginia but resemble the leaf color of Carolina will be:

The progeny with Double Crossover (McS) = 0.51%

d. The proportion of backcross progeny that will have leaf morphology and leafcolor of Virginia but resemble the leaf size of Carolina will be:

The progeny with Double crossover (mcs) = 8.5%

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