Question

Explain the statement – “Test cross is back cross but back cross is not necessarily a

test cross”.​

Answer 1

Answer:

(1) In a monohybrid cross between pure tall and pure dwarf, all

hybrids will be tall with the genotype Tt. <br> (2) In order to verify the genotype, when they are crossed with their recessive parent, the progeny will 50% tall and 50% dwarf. This is because the recessive parent will produce only one type of gamete, i.e. t and the hybrid tall will produce two types of gametes, T and t. Therefore half the progeny will have the genotype Tt and half will have tt. Thus dominant and recessive plants will be in the ratio of 1 : 1. This is called a test cross ratio. <br> (3) The cross between

hybrid and homozygous tall plant is also a back cross, but this cross will produce all tall plants, 50% pure tall and 50% hybrid tall. From this it is obvious that back cross with the dominant parent cannot be used for testing the genotype. <br> (4) Thus, a test cross is a back cross but back cross is not necessarily a test cross.