Question

In a plant tallness is dominant over dwarfness and red flower is dominant over white. Starting with the parents, work out a dihybrid cross. What is the standard dihybrid ratio? Do you think the values would deviate if the two genes in question are interacting with each other?

Answer 1

Dihybrid cross

A cross involving two pairs of contrasting characters is called a dihybrid cross.

F${}_1$ generation of dihybrid cross

Let T and t be the alleles responsible for tallness and dwarfness respectively and R and r be the alleles responsible for the production of red and white flowers respectively.
Dihybrid cross between a tall, red flowered plant and a dwarf, white flowered plant is shown below.

Genotype of parents: TTRR (male) and ttrr (female)
Gametes: TR and tr
F${}_1$ generation : TtRr (All tall, red flowered plants)



F${}_2$ generation of a dihybrid cross

Self cross between heterozygous tall red flowered plants TtRr x TtRr:

Phenotypes (4)	Tall, red flowered plant	Tall, white flowered plant	Dwarf, red flowered plant	Dwarf, white flowered plant
Ratio	9	3	3	1

Genotypes(9)	TTRR	TTRr	TTrr	TtRR	TtRr	Ttrr	ttRR	ttRr	ttrr
ratio	1	2	1	2	4	2	1	2	1

Interaction of genes in dihybrid cross

If the two genes interact with each other, then the phenotypic ratio of dihybird cross (9:3:3:1) will vary depending on the type of interaction. So the possible phenotypic ratios can be:

• For complementary genes, 9:3:3:1 will be modified into 9:7
• For duplicate genes, the ratio will be changed into 15:1
• In recessive epistasis, the ratio modifies to form 9:3:4
• In dominant epistasis, the ratio modifies into 12:3:1