Explain a cross in which phenotype and genotype ratio are same also give an example
Answers
Many genes have dominant and recessive alleles, where the dominant allele controls the phenotype in heterozygotes. An example in humans is that if one parent has blood type Rh+ (Rh+ Rh+) and the other parent has blood type Rh- (Rh- Rh-) all offspring will be Rh+/Rh- and have the Rh+ phenotype
A “test cross” for a single gene locus is where a parent with the dominant trait (either Rh+/Rh+ or Rh+/Rh-) is crossed with a parent with the recessive trait (Rh- Rh-). The purpose of the cross is to learn the genotype of the parent with the dominant trait. If that parent’s genotype is Rh+/Rh+, all offspring will be Rh+/Rh- and phenotypically Rh-positive. If the parent’s genotype is Rh+/Rh-, the offspring will be 50:50 Rh+/Rh- and Rh-/Rh-.
This test can be extended to learn the genotype of a parent with dominant traits at multiple gene loci. For all loci, the phenotypes of the offspring (either 100% dominant or 50:50 dominant:recessive) will reveal whether the parent is homozygous dominant or heterozygous at that locus.
In this example, I used a human gene (Rh). However, of course, humans cannot be forced to mate the way that fruit flies can be—and in any case, large numbers of offspring must be scored for the results to be statistically convincing. (Also, a “test cross” is sometimes referred to as a “back cross.”)