Test Cross

The problem it solves

An organism with a dominant phenotype could be homozygous dominant (TT) or heterozygous (Tt), the two genotypes look identical. A test cross is designed to reveal which one it actually is, by crossing it with a homozygous recessive individual (tt), whose genotype is already certain because the recessive phenotype only ever comes from tt.

Case 1: unknown parent is TT

Case 2: unknown parent is Tt

Interpreting the offspring

• If all offspring show the dominant phenotype, the unknown parent is homozygous dominant (TT).
• If about half the offspring show the recessive phenotype, the unknown parent is heterozygous (Tt).
• The more offspring produced, the more reliable the conclusion, since ratios are only exact on average over large numbers.

Worked problem 1, easy: all offspring dominant

Question. In mice, black coat (B) is dominant to brown (b). A black mouse of unknown genotype is test-crossed with a brown mouse, and the whole litter is black.

Deduce the genotype of the black parent and explain.

Model answer. The brown tester is bb and forms only b gametes. Since every offspring is black, none received a b allele from the black parent, so the black parent forms only B gametes; its genotype is homozygous, BB.

If it had been Bb, about half the litter would have been brown.

Worked problem 2, medium: a 1 : 1 result

Question. A tall pea plant (T dominant to dwarf t) of unknown genotype is test-crossed with a dwarf plant. The offspring are 52 tall and 48 dwarf.

What is the genotype of the tall parent, and what does the ratio show?

Model answer. A count of roughly equal tall and dwarf is a 1 : 1 ratio. The dwarf tester (tt) forms only t gametes, so the dwarf offspring must have received a t allele from the tall parent.

The tall parent therefore carries both T and t and is heterozygous, Tt. The reappearance of the recessive dwarf phenotype confirms the hidden t allele.

Worked problem 3, SPM-level: a dihybrid test cross

Question. A purple, tall pea plant (purple P dominant to white p; tall T dominant to dwarf t) is test-crossed. The offspring are one quarter purple tall, one quarter purple dwarf, one quarter white tall and one quarter white dwarf.

Work out the genotype of the purple, tall parent.

Model answer. A test cross uses a fully recessive tester, which here is pptt, forming only pt gametes. Four equally common offspring types (a 1 : 1 : 1 : 1 ratio) mean the unknown parent formed four gamete types in equal numbers: PT, Pt, pT and pt.

Producing four gamete types shows it is heterozygous for both genes, so the purple, tall parent is PpTt.

Practice problems

• 1. A tall pea plant is test-crossed and every offspring is tall. State the genotype of the tall parent.
• 2. A coloured-seed plant (colour dominant) is test-crossed and gives about half coloured and half colourless seeds. State the genotype of the coloured parent.
• 3. Explain why a homozygous recessive parent, rather than a heterozygote, is used for a test cross.

Practice answers

How it is examined

SPM questions often describe a dominant-phenotype organism of unknown genotype and ask you to design a test cross, draw the Punnett square for both possible outcomes, and explain how the offspring ratio would distinguish TT from Tt.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions