An inbred strain of plants has a mean height of 20cm. Another inbred strain from

Question

An inbred strain of plants has a mean height of 20cm. Another inbred strain from a different geographical region also has a mean height of 44cm. When the two strains are crossed, all of the F_1 also have a mean height of 32cm. However, when the F_1 progeny are crossed to form an F_2, a wide range of heights is produced. The most common height of progeny is approximately 32cm. The least common height classes are 20cm. (5 individuals out of 1040) and 44cm. (3 individuals out of 1040). Assuming that plant height is determined in an additive manner, How many gene loci are involved? On average, how much does each additive allele contribute to plant height? Indicate one set of possible genotypes for the original parental strains and their F_1 progeny that could explain these results. List three possible genotypes for an F_2 plant of 29cm. height.

Explanation / Answer

a.

The mode of inheritance seen in this example is polygenic inheritance. From the data, we know that 5 out of 1040 individuals have least height. This ratio (5/1040) is the key to determine the number of gene pairs. This ratio can be reduced to 1/208, which is very close to 1/256. Using the formula 1/4n = 1/256, n = 4. Therefore, four gene pairs (eight gene loci) are involved.

b.  

The variation between the two extreme heights is 44-20 = 24 cm.

As there are eight (8) potential additive alleles (AABBCCDD), each contributes 3 cm (24/8 = 3 cm) to the base height of 20 cm, which results when there are no additive alleles (aabbccdd) in the genotype.

c.  

P1 – AABBCCDD – 44 cm

P2—aabbccdd – 20 cm

F1 – AaBbCcDd – 32 cm [20 + 12 = 32 cm] (4 alleles * 3 cm = 12 cm added).

d.  

20 + 9 = 29 cm. (3 alleles * 3 cm = 9 cm added)

aaBbCcDd, AabbCcDd, AaBbccDd, AaBbCcdd, etc are the possible genotypes for an F2 plant of 29 cm.

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