Question

in a plant species, a tall variety was crossed with a dwarf variety. all f1 plants were tall. when f1 × f1 plants were interbred, 9/16 of the f2 were tall and 7/16 were dwarf. list the genotypes of the 2 parents and the f1 (select your own gene symbols). use punnett square or genotypic categories to correctly explain what happened.

Explanation:

Step1: Determine dominant - recessive

Since all F1 plants are tall when tall and dwarf are crossed, tall is dominant. Let the allele for tall be T and for dwarf be t.

Step2: Genotype of parents

The tall parent must be homozygous dominant (TT) and the dwarf parent must be homozygous recessive (tt) because when they cross, all F1 are tall. The cross is TT × tt.

Step3: Genotype of F1

The F1 generation from the cross TT × tt will all have the genotype Tt.

Step4: F1 - F1 cross

When Tt × Tt (F1 × F1), using a Punnett - square:

	T	t
t	Tt	tt

The genotypic ratio is 1 TT : 2 Tt : 1 tt. The phenotypic ratio is 3 tall (TT + Tt + Tt) : 1 dwarf (tt). But we have 9/16 tall and 7/16 dwarf, which indicates a case of epistasis. Let's assume two genes A and B are involved in determining plant height. The tall parent has genotype AABB and the dwarf has aabb. The F1 has genotype AaBb. When AaBb × AaBb, the phenotypic ratio of 9:7 is a modified 9:3:3:1 ratio. The 9/16 tall plants have at least one dominant allele at both gene loci (A - B -), and the 7/16 dwarf plants have either aa or bb or both (aaB -+A - bb + aabb).

Answer:

Parent 1 genotype: AABB (or TT if single - gene inheritance); Parent 2 genotype: aabb (or tt if single - gene inheritance); F1 genotype: AaBb (or Tt if single - gene inheritance)