By phenotype, it is not always possible to determine what genotype a given individual has. The genotype may not be known with complete dominance: the dominant homozygote and the heterozygote have the same traits. For example, yellow-seeded peas can have genotypes AA and Aa.
Analyzer is the crossing of an individual with an indeterminate genotype (AA or Aa) with a recessive homozygous individual (aa).
An individual with recessive alleles forms gametes of only one type (a). Against the background of recessive alleles, the alleles of the analyzed organism become “visible”, since splitting by phenotype coincides with splitting by genotype. This allows an unknown genotype to be determined.
Consider the course of the analyzing crossing. There is an individual with a dominant phenotype and an unknown genotype A_ (instead of the “_” sign, there can be allele a or A). When crossed with a recessive aa, two cases are possible.
1. If all hybrids of the first generation have a dominant phenotype, then the studied specimen has genotype AA, since it forms gametes of the same type – with the dominant allele A. With the recessive allele a of the analyzer, only genotypes Aa are formed.
2. If half of the first generation hybrids have a dominant trait, and half have a recessive trait, then the parent’s genotype is Aa, since it forms gametes of two types in equal amounts – with allele A and allele a. When combined with the recessive allele of the analyzer, genotypes Aa and aa are formed, which have a different phenotype.
If no splitting is observed in the offspring from the analyzing cross, then the studied individual is homozygous (AA).
If the offspring of two phenotypic classes is formed in a 1: 1 ratio, then the studied individual is heterozygous (Aa).