Question

A typical dihybrid and trihybrid test cross ratio are respectively:
A) 1:1 and 1:1:1:1
B) 1:1:1:1:1:1:1:1 and 1:1:1:1
C) 1:1:1:1 and 1:1
D) 1:1:1:1 and 1:1:1:1:1:1:1:1

Answer 1

A back cross is when an individual from the F1 generation is crossed with a parent or another individual genetically identical to the parent. This is generally used in horticulture and animal breeding to produce “perfect” offspring for the desired traits.

Complete Answer:
- In a test cross organisms with an unknown genotype, but expressing the dominant phenotype, are crossed with one that is homozygous recessive for the particular traits.
- A dihybrid test cross involves two dominant traits; while a trihybrid test cross involves three dominant traits. The purpose of a test cross is to determine the phenotype of the unknown individual: whether it is homozygous or heterozygous for the dominant trait.
- In a typical dihybrid test cross, the resulting offspring will be in the ratio 1:1:1:1, whereas for a trihybrid, the ratio will be 1:1:1:1:1:1:1:1. An example for a trihybrid cross is pea plants with the traits green/yellow, smooth/wrinkled, tall/short.
- The ratio of 1:1 is typically found in a monohybrid Mendelian cross. That is, take a plant expressing the trait Tallness, and cross it with a short plant. Your crosses could be either Tt x tt, or TT x tt. Because the dominant phenotype of tallness will be expressed even in the heterozygote, the genotype won't be obvious.

The answer is therefore option (D).

Note: Another way to put it, a test cross is between the F1 generation involving dominant phenotypes and the recessive parent. These techniques work best when there is an independent assortment of genes.