How do geneticists normally tell whether an organism exhibiting a dominant phenotype is homozygous or heterozygous?

Answer 1

They perform a test cross. A test cross takes the unknown genotype and crosses it with a known homozygous recessive. If the F1 generation is all dominant, then they know the organism was a homozygous recessive. If recessive offspring appear, then the organism was a heterozygote. As an example, consider a gene with two alleles, A and a, with Adominant. Now consider the test cross. The unknown genotype can only be one of two possibilities: AA (homozgous dominant) Aa (heterozygous) In a test cross,the unknown genotype is crossed with a known homozygous recessive. Since there are only two possible unknown genotypes, there can be only two possible results. First, consider the case of the unknown genotype being a homozygous dominant. The cross looks like this: AA X aa Remember that a homozygote for an allele can only produce one kind of gamete. In this case the homozygous dominant can only produce gametes with the allele A in them, while the homozygous recessive can only produce gametes with the recessive allele a in them. This means the F1 offspring can only be ONE genotype; Aa. Therefore, all of the offspring would have the dominant phenotype. Now consider the other possible cross, where the unknown genotype is heterozygous: AaX aa Remember that a heterozygote can produce two types of gametes. In this case, the unknown would produce gametes with the dominant allele A or the recessive allele a. The homozygous recessive would still only produce one kind gamete, with the recessive a allele. Therefore, we expect to see only two genotypes in the F1, Aa and aa, in equal proportions. In either case, only one test cross is needed to tell one immediately the nature of the unknown genotype. If all of the F1 are of the dominant phenotype, then the unknown genotype must be homozygous dominant; if a mixture of phenotypes appears in equal proportion, then the unknown genotype must be a heterozygote.

Answer 2

the genotype will be uniformly heterozygous. If we track one gene, call it A, and the organisms are diploid, it will be like AA x aa. You cannot get anything else from this than Aa = heterozygote

Answer 3

A test cross - which breeds the organism with a known homozygous recessive individual.

If all of the offspring show the dominant phenotype, the unknown individual is homozygous dominant.

If half of the offspring show recessive, and half show dominant, then the individual was heterzygous.