Question

Explain test cross with suitable example and state its ratios.

Answer 1

1. A test cross is used to find out the genotype of any plant with dominant expression when it is not known whether it is homozygous (pure) or heterozygous for that trait.
2. For example, a pea plant having purple flowers (F1 hybrid with unknown genotype) is crossed with a pea plant with white flowers (recessive).
3. If all flowers produced are purple, we can conclude that plant is pure or homozygous and if we get purple and white flowers in a 1:1 ratio, conclude that plant is heterozygous.
4. Following is the graphical representation for test cross:

1. Assume that purple flower is Homozygous dominant (PP).
   All flowers are purple. Thus, the F1 hybrid is homozygous dominant (PP).
2. Assume that purple flower is Heterozygous dominant (Pp).
   
   Half of the flowers are purple and half are white. Thus, the F1 hybrid is heterozygous dominant (Pp).