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

a)      One locus controls the trait basically and another locus might be involved in the gene expression of extreme heights in F2 generation. Or environmental influences also might be involved in the extreme cases.

b)      A recessive allele contributes 50% of a heterozygous trait for height. A dominant allele contributes 100% of a homozygous trait for height.

c)       HH for 48cms height in one strain (inbred) and Hh for 57cms height in another strain.

P1 - HH X Hh

F1 – HH, Hh, HH, Hh

Here, the homozygous trait would give rise to the same height, while a few of the heterozygous ones are with extreme height. These fewer genotypes might be influenced by another locus or any other environmental factor.

d)      The genotype of the plants having the heights of 43.5 or 50.5 might have homozygous or heterozygous genotypes (HH or Hh) if they are of the inbred strain and have heterozygous genotype (Hh), if they are of another strain. The decrease in the heights or a slight increase in the height depends either on epigenetic effect or environmental effect.