Which of these is a testcross?
a. A? x A?
b. A? x Aa
c. A? x AA
d. aa x aa
e. A? x aa
The Correct Answer and Explanation is :
The correct answer is e. A? x aa.
Explanation of Testcross
A testcross is a genetic cross performed to determine the genotype of an individual with a dominant phenotype (represented here as A?) by mating it with an individual that is homozygous recessive for the trait in question (in this case, aa). The purpose of a testcross is to reveal whether the dominant phenotype individual is homozygous dominant (AA) or heterozygous (Aa) for the trait.
In the given options:
- a. A? x A?: This cross will not provide information about the genotype of the dominant individual, as both parents could be either homozygous or heterozygous.
- b. A? x Aa: This cross will also not definitively reveal the genotype of the dominant individual, as the offspring could be AA, Aa, or aa.
- c. A? x AA: This cross will only yield dominant phenotypes (AA or Aa) and does not provide information about the genotype of the first parent.
- d. aa x aa: This cross involves two homozygous recessive individuals and will only produce offspring that are also homozygous recessive (aa), providing no insight into dominant phenotypes.
- e. A? x aa: This is the correct testcross. When a dominant phenotype individual (A?) is crossed with a homozygous recessive individual (aa), the offspring will exhibit a 1:1 ratio of phenotypes: half will express the dominant phenotype (Aa) and half will express the recessive phenotype (aa). This allows us to determine the genotype of the dominant individual: if any offspring show the recessive phenotype, we know the A? parent must be heterozygous (Aa). If all offspring display the dominant phenotype, the A? parent must be homozygous dominant (AA).
In summary, a testcross is a valuable tool in genetics for determining unknown genotypes, particularly in breeding experiments and genetic research.