Define and design a test cross.

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Test Cross. It is a cross between an organism of an unknown genotype and a homozygous recessive organism.

Results of A Test Cross. If the test cross yields offspring of which 50%

â–² Fig. 1.3. Genetics of a test cross.

show the dominant character and 50% show the recessive character, i.e. F1 ratio is 1 : 1, the individual under test is heterozygous (Fig. 1.1). This is so because the individual showing the recessive trait (say white coat colour in guinea pig, dwarf size in pea plant) must have received one recessive allele (b in guinea pig, t in pea plant) from each parent.

If all the offspring of the test cross show the dominant trait, the individual being tested in homozygous dominant with genotype BB for guinea pig and TT for pea plant (Fig. 5.3).