Question

An inbred strain of plants has a mean height of 48 cm. A second strain of the same species has a mean height of 57 cm. When plants from the two strains are crossed together, all the plants in the F1 generation is 52.5 cm tall. On the other hand, crossing two F1 plants provides a wide range of different heights in the F2 generation: Most of the plants are the same height as the F1 generation plants, while fewest plants are in the most extreme height categories (respectively 30 cm and 75 cm). In F2 approximately 1 out of 1000 plants are 30 cm tall. Simultaneously about 1 out of 1000 F2 plants are 75cm tall.

a) How many loci controls the trait?

b) How much does each allele contribute to plant height?

c) Specify a possible set of genotypes for the original P1 and F1 plants that can account for these results.

d) Is it possible to observe F2 plants that are 50.5 cm or 43.5 cm high? If so, what genotype do these have?

Answer 1

1. A single loci seems to be controlling the trait (in plants, only one gene controls tallness).
2. 50% is contibuted by each allele.
3. P1 since are inbred, must be pure i.e. homozygous recessive and dominant. For ex. P1 plants can be given HH(homozygous dominant) and hh(homozygous recessive). F1 plants will be heterozygous i.e. Hh.
4. Intermediate heights and extreme heights are all possible for F2 generation. Given below is the cross which can help explain this phenomenon:

   	H	h
   H	HH- Tall	Hh- Intermediate
   h	Hh- Intermediate	hh- Dwarf