Which of the following represents a testcross A- t? x tt
B- XX x XY
C-T? x Tt
D- T? x TT

The option should be like this:

A- T? x tt

Explanation:

In an inheritance pattern in which a particular allele is exhibiting doiminance over the other, the dominant phenotype can either be homozygous or heterozygous. Hence, it is impossible to tell the genotype of a dominant phenotype by merely observing it. A cross called TEST CROSS is used.

A test cross is a cross used to determine the actual genotype of a dominant phenotype i.e. whether it is homozygous or heterozygous by crossing tye unknown genotype with a homozygous recessive parent. If all of the offspring of this cross have a dominant phenotype, it means the dominant parent is HOMOZOGOUS but if offsprings with an equal ratio of dominant and recessive phenotypes are produced, it means the dominant parent is HETEROZYGOUS.

In this case, the dominant phenotype whose genotype is unknown is (T*) while the recessive parent will be (tt).