Question

How many of the given crosses indicate a possible test cross?

I. TT X tt
II. Tt X tt
III. TT X Tt
IV. Tt X Tt

• A. 3
• B. 2
• C. 1
• D. None

Answer 1

The correct option is B 2

A test cross is usually carried out to find out the homozygosity or heterozygosity of a dominant trait for a given character. It is because the dominant trait can be expressed even if only one copy of the trait (allele) is present (heterozygous condition). In the homozygous condition, the dominant and recessive alleles express the respective dominant and recessive traits. The confusion lies with the dominant trait only. A test cross is a cross between the dominant trait expressing plant (in this case TT or Tt) and a plant exhibiting homozygous recessive condition for the same character (in this case tt). So we can see options I and II are possible test crosses.
In the question, it has been asked ‘possible test cross’. The word ‘possible’ is given for a reason. Not all crosses that involve a homozygous recessive trait are test crosses. Some are just parental crosses. As in option I, it could very well be considered a cross between purelines. Usually in a test cross, the allele composition (identical or non identical) of one of the parents is unknown. The conclusion is reached by observing the progeny of the cross. But here we can see in the options I and II, the allele composition of both the parents are given. This could very well eliminate the possibility of a test cross for both the cases. So, the word ‘possible’ has been used to indicate that the question is discussing the probability of a test cross.
So, options I and II indicate possible test crosses. III and IV don’t indicate test cross at all because no homozygous recessive parent is involved in both cases.