Question

This diagram shows a dihybrid cross between a homozygous dominant yellow (I) round (R) pea plant with a homozygous recessive green (i) wrinkled (r) pea plant. The offspring produced in the F1 generation are all heterozygous IiRr - yellow round pea plants.a. Show the f2 generation Punnet square, when two offspring from the f1 generation cross. b. What is the phenotypic ratio for the outcome above? Put full phenotype descriptions with the ratio. c. This ratio is regarded as the key clue which led Mendel to discover which law? d. From the ratio, how many homozygous pure breed offspring were they eventually after the final cross? Give genotypes of all.

Answer 1

a. The Punnet square for the crossing between two members of the offspring (RrIi x RrIi) is:

b. The phenotypic ratio, then, will be

9 yellow and round pea plants:

1 RRII (homozygous dominant for both traits)

2 RRIi (homozygous dominant for the shape - round -, and heterozygous for the color - yellow)

2 RrII (heterozygous for the shape and homozygous dominant for the color)

4 RrIi (heterozygous for both traits)

3 yellow and wrinkled pea plants:

1 rrII (homozygous recessive for the shape and homozygous dominant for the color)

2 rrIi (homozygous reccessive for the shape and heterozygous for the color)

3 green and round pea plants:

1 RRii (homozygous dominant for the shape and homozygous recessive for the color)

2 Rrii (heterozygous for the shape and gomozygous recessive for the color)

1 green and wrinkle plant:

1 rrii (homozygous recessive for both traits)

c. This is the key for Mendel's law of independent assortment. This law states that the alleles of two or more different genes will be sorted into the gametes independently.

d. The homozygous pure breed can be considered the pea plants that are homozygous for both traits (RRII or rrii), like the original parents that generated the f1 offspring.

Therefore, we have only one homozygous for recessive traits (rrii) and only one homozygous for dominant traits (RRII).