In genetics, test cross is used to

Step 1: Analyze the question The question asks about the purpose of a test cross in genetics.

Step 2: Explain the concept A test cross is a cross between an individual showing the dominant phenotype (whose genotype is unknown) and a homozygous recessive individual. The dominant phenotype could come from either a homozygous dominant genotype (AA) or a heterozygous genotype (Aa), and a test cross reveals which one it is.

Step 3: Work through the solution Let the dominant allele be T (tall) and the recessive allele be t (dwarf). Cross the unknown tall plant (T?) with a dwarf plant (tt).

• If the unknown is homozygous (TT): TT × tt → all offspring Tt (all tall)

• If the unknown is heterozygous (Tt): Tt × tt → 1 Tt : 1 tt (50% tall, 50% dwarf, a 1:1 ratio)

If any recessive offspring appear, the unknown parent must carry the recessive allele, so it is heterozygous.

Step 4: Check the options

• A. Counting offspring in F1 is not the purpose of a test cross.
• B. It does not count the gametes in the parents.
• C. Testing for heterozygosity is correct. It shows whether the dominant-phenotype individual is homozygous or heterozygous.
• D. Sex chromosomes (X and Y) are not what a test cross tests for.
• E. The phenotypic ratio of the offspring is used as evidence, but it is not the aim of the cross. The aim is to determine the unknown genotype.

Correct answer: C. test for heterozygosity

Exam tip: Remember "test cross = unknown × recessive (tt)". If you see a 1:1 ratio in the offspring, the parent is heterozygous. If all offspring show the dominant trait, the parent is homozygous dominant.