Question

Describe the cross between a homozygous tall, round-seeded pea plant and a dwarf, wrinkled-seeded pea plant. What will be the types of progeny in the F₂ generation of this cross and in what proportion will it be?
Name and state the law which is explained by this example.

Answer 1

Let the gene for the tall habit of the pea plant be represented by ‘T’ and the dwarf habit be represented by ‘t’.

Let the gene for round seed be represented by ‘R’ and that of wrinkled seed be represented by gene ‘r’.

Then, the genotypes of the parents would be:

1. Homozygous tall, round seeded − TTRR
2. Homozygous dwarf, wrinkled seeded – ttrr 

F₂ progeny obtained → Tall Round; Tall Wrinkled; Dwarf Round; Dwarf Wrinkled

Phenotypic ratio → 9 : 3 : 3 : 1 

Genotypic ratio→ 

1 :	2 :	2 :	4 :	1 :	2 :	1 :	2 :	1
TTRR	TTRr	TtRR	TtRr	ttRR	ttRr	TTrr	Ttrr	ttrr

The above example of a dihybrid cross between a homozygous tall, round-seeded pea plant and dwarf, wrinkled-seeded pea plant explains the ‘Law of Independent Assortment’. 

The law of independent assortment states that “when two parents differing from each other in two or more pairs of contrasting characters are crossed, then the inheritance of one pair of characters is independent of the other pair of characters.”