The genotype of a species with respect to a particular trait can be determined by carrying out

Analyze the question: It asks how we can find out the genotype of an organism that shows a particular trait, usually the dominant phenotype, whose genotype could be homozygous (e.g., TT) or heterozygous (Tt).

Concept: The testcross (back cross) A testcross crosses the organism of unknown genotype with a homozygous recessive individual (e.g., tt). The recessive parent can only give the recessive allele, so the offspring's phenotypes reveal the unknown parent's genotype.

Working:

• If the unknown is TT: TT × tt gives all Tt, so all offspring show the dominant trait.
• If the unknown is Tt: Tt × tt gives 1 Tt : 1 tt, so about half the offspring show the dominant trait and half show the recessive trait (a 1:1 ratio).

If any recessive offspring appear, the unknown parent must be heterozygous. If none appear, it is most likely homozygous dominant.

Checking the options:

• A. A back cross with a pure (homozygous) recessive stock is the standard testcross. ✔
• B. Crossing two heterozygous F1 individuals gives a 3:1 ratio, but this does not identify the genotype of an unknown individual.
• C. Crossing with a pure dominant stock gives offspring that all show the dominant trait whether the unknown is TT or Tt, so it cannot tell the genotypes apart.
• D. Inbreeding does not give a clear test of genotype.
• E. Outbreeding with an exotic variety is used to introduce new traits, not to determine genotype.

Answer: A.

Exam tip: Remember "Test = Recessive." A testcross always uses the homozygous recessive parent, because it hides nothing. Whatever alleles show up in the offspring must have come from the unknown parent.