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Question : Describe test cross. |
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Answer» Solution :Though the genotypic ratios can be calculated using mathematical probability but it is not POSSIBLE to know the genotypic composition by looking at the phenotype of dominant trait. = That is for example whether a tall plant from `F_1 or F_22` has TT or Tt composition cannot be predicted. =Therefore, to solve this problem Mendel devised test cross. =Back cross is a cross of `F_1` PROGENY back to one of their parents. = A special back cross to the recessive parent is known as test cross. = This METHOD was devised by Mendel to determine whether the dominant phenotype is homozygous or heterozygous. =e.g. In a monohybrid cross, between violet COLOUR flower (W) and white colour flower (w). the `F_1` hybrid was a violet colour flower. = If all the `F_1` - progeny are of violet colour, then the dominant flower is homozygous and if the progenies are in 1:1 RATIO, then the dominant flower is heterozygous. =In case of double heterozygous i.e. heterozygous violet and axial flower (WwAa) crossed with double recessive i.e. recessive white and terminal flower [wwaa] the ratio will be 1 : 1:1:1. 
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