Two different purebred lines of squash have mean fruit sizes of 3 kg and 1 kg re

Question

Two different purebred lines of squash have mean fruit sizes of 3 kg and 1 kg respectively. A cross between these two plants will produce an F1 hybrid that has a mean fruit size of 2 kg.

Progeny from a self of the F1 hybrid grown in strictly controlled environmental conditions will lead to F2 progeny with fruit sizes that range from 1 to 3 kg. From two hundred F2 progeny only three are found to weigh about 1 kg and only three are found to weigh about 3 kg.

In a field trial a breeder selected many individuals from a similar group of F2 progeny (mean fruit size of 2.5 kg). These were used as parents for a new breeding program. The breeder found that the average fruit size in the next generation was 2.3 kg.

a) (2 points) Estimate the number of genes that are involved for controlling this trait in the two varieties. Show your calculations.

b) (2 points) Estimate how much weight does a single additive allele of these genes contribute to the phenotypes observed.

c) (3 points) How can two squash plants from the F2 progeny, both with fruit of moderate weight, produce progeny with fruit that are much larger than either of them when crossed? (Assume there is a negligible difference in the environment for this part of the question).

Explanation / Answer

a) Three genes are involved

AABBCC (3kg) x AAbbcc(1kg)

F1: AABbCc (2kg)

AABbCc x AABbCc

F2:

Here only one representative is shown which makes 16 phenotypes (4x4)

But there are 64 phenotypes for three genes (6 alleles) involved which will have 8 rows and 8 columns where each is repeated twice. And that is mentioned in bracket as 2.

Total number of fruits is 200.

AABBCC : 1/64 x 200 = 3 fruits with 3kg

AAbbcc : 1/64 x 200 = 3 fruits with 1 kg

b) Each allele contribute 0.5 kg so six alleles (AABBCC) contribute 3kg

two alleles contribute 1kg (AAbbcc)

four alleles contribute 2kg (AABbCc)

c) Genes play an important role in determining the fruit weight even when there is difference in environmental conditions.

ABC (2) Abc (2) ABc (2) AbC (2) ABC (2) AABBCC AABbCc AABBCc AABbCC Abc (2) AABbCc AAbbcc AABbcc AAbbCc ABc (2) AABBCc AABbcc AABBcc AABbCc AbC (2) AABbCC AAbbCc AABbCc AAbbCC

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