Question

What phenotypic ratio would you expect as a result of a test cross between two individuals where 9) one that is homozygous recessive for alleles at two independent loci?

Answer 1

What phenotypic ratio would you expect as a result of a test cross between a dihybrid organism and one that is homozygous recessive for alleles at two independent loci?

a. 3:1

b. 9:3:3:1

c. 1:1:1:1

d. 1:2:1

e. 9:4:2:1

Answer:

c. 1:1:1:1

Explanation:

When a heterozygous individual for two genes is test crossed with a double homozygous recessive individual, the progeny is obtained in 1:1:1:1 phenotypic ratio. This occurs as the heterozygous dominant individual forms four types of gametes in 1:1:1:1 ratio while the homozygous recessive individual would form only one type of gamete having one recessive allele for each gene.  

For example, a test cross between TtRr (tall and red) and ttrr (short and white) would produce a progeny in following ratio=

1 tall, red: 1 tall, white: 1 short, red: 1 short, white

Here, T= tall, t= short, R= red, r= white