Question

Explain test cross and back cross.

Hint:

Test cross always uses a homozygous recessive partner; back cross can go to either parent.

Answer 1

Step 1: Understanding the Concept:
When an organism shows a dominant trait, you cannot tell just by looking at it whether it is homozygous dominant or heterozygous, since both genotypes look identical. A special cross is needed to find out which one it is.

Step 2: Define test cross.
A test cross is a cross between an individual with an unknown genotype (showing the dominant phenotype) and an individual that is homozygous recessive for that trait. If all offspring show the dominant trait, the unknown parent is homozygous dominant; if about half the offspring show the recessive trait, the unknown parent is heterozygous.

Step 3: Define back cross.
A back cross is a cross between an F1 hybrid and either one of its two parents (or an individual genetically similar to a parent).

Step 4: Connect the two terms.
When the F1 hybrid is crossed back to the homozygous recessive parent specifically, that particular back cross is also, by definition, a test cross, since it fits both descriptions at once.

Final Answer:
A test cross reveals an unknown genotype by crossing with a homozygous recessive individual, while a back cross is any cross of the F1 with either parent; the back cross to the recessive parent is itself a special case of a test cross.