Question

How would you find genotype of a tall pea plant bearing white flowers? Explain with the help of a cross. Name the type of cross you would use.

Answer 1

To know the genotype of the plant, a test cross is performed. In the test cross, pea plant showing the dominant phenotype is crossed with the pea plant that is homozygous recessive for the same trait. Results of the test cross are then interpreted to know the exact genotype of the given plant. Tall pea plant with white flowers could have two possible genotypes: TTpp and Ttpp (white, being a recessive trait, must be pp in either case).
Case 1: Plant with genotype TTpp is crossed with a plant recessive for both the traits (ttpp). In this case, all the plants in the first generation will be tall with white flowers.
Case 2: Plant with genotype Ttpp is crossed with a plant recessive for both the traits (ttpp). In this case, tall plants with white flowers and dwarf plants with white flowers will be produced in the ratio 1 : 1.

Accordingly, the cross can be represented as follows:

Case 1
Parents	TTpp   x      ttpp
Gamets	Tp	tp
Offsprings	Ttpp	Ttpp

Case 2
Parents	Tp,tp   x      ttpp
Gamets	Tp, tp	tp
Offsprings	Ttpp	Ttpp

Hence, the appearance of dwarf plants with white flowers in the progeny indicates that the parent is heterozygous.