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Question : Test cross is a back cross but cross is not necessarily a test cross. |
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Answer» Solution :(1) In a monohybrid cross between pure tall and pure dwarf, all `F _(1)` hybrids will be tall with the GENOTYPE Tt. (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. (3) The cross between `F_(1)` 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. (4) Thus, a test cross is a back cross but back cross is not NECESSARILY a test cross. |
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