Question

Briefly explain the test cross.

Hint:

Recall crossing the unknown dominant genotype with a full recessive (aa) partner.

Answer 1

Understanding the Question:
A test cross is used to find out whether an organism showing a dominant trait is homozygous or heterozygous for that trait.

Detailed Explanation:
The individual of unknown genotype (showing the dominant phenotype, genotype either AA or Aa) is crossed with a homozygous recessive individual (aa).
If the unknown parent is homozygous dominant (AA), all offspring of this cross show the dominant phenotype.
If the unknown parent is heterozygous (Aa), the offspring appear in a 1 to 1 ratio of dominant to recessive phenotype, because the recessive parent contributes only recessive alleles, so the ratio directly reflects the gametes made by the unknown parent.

Final Answer:
A test cross, crossing the unknown dominant individual with a homozygous recessive one, reveals the hidden genotype from the ratio of offspring phenotypes.