A true-breeding strain of tall soybean plants is crossed with a true-breeding strain of short soybean plants. All the offspring are short. These offspring are then crossed with each other to create an F2 generation. If the height of soybean plants is controlled by a single gene, then of 400 F2 plants should be tall.

100

Explanation:

This cross involves a gene that codes for height in soybean plants. According to the cross between a purebreeding or homozygous tall soybean and a purebreeding or homozygous short soybean, all the F1 offsprings were short phenotypically. This implies that the allele for short phenotype (S) is dominant over the allele for tall phenotype (s). The F1 offsprings are heterozygous for the short trait (Ss), because the short allele is masking the expression of the tall allele in the gene.

If the F1 offsprings (Ss) are crossed, a F2 offsprings with the genotypes: SS, Ss, Ss, ss will be produced.

SS, Ss and Ss are all dominant (short) phenotypes while ss is a recessive (tall) phenotype. This illustrates a phenotypic ratio of 3 tall : 1 short.

Based on this phenotypic ratio of 3:1, if a total of 400 F2 soybean plants are produced, 1/4 × 400 = 100, will have a tall phenotype.

Out of 400, 100 of these will be tall plants: 1/4 of 400= 100

Explanation:

to produce an offspring of short soybean plants from a cross between a true breeding strain of tall soybean plant and a true breeding strain of short soybean plants, it means the allele coding for short plant is dominant to that of the tall plant. Thus, let S represent the dominanat allele coding for short plant.

short plant       tall plant

  SS           x           ss

F1                Ss

             all short offspring

  Ss          x           Ss

F2   SS      Ss      ss

3/4 short plants and 1/4 tall plants

Out of 400, 100 of these will be tall plants: 1/4 of 400= 100