Question

1. When Mendel crossed pea plants that were true breeding for purple flowers (PP) with pea plants with white flowers (pp), all of the offspring had purple flowers.  When he let these offspring self-fertilize, he got purple and white second generation (F2) offspring in a phenotypic ratio of 3:1. But, we know that the genotypic ratio is really 1:2:1.  Why? (1 point)

Answer 1

That happens because in an autosomal dominant system we don't have one phenotype for each genotype, the heterozygous and dominant homozygous genotypes share the same phenotype. Don't let yourself get confused between genotype and phenotype, let us analyze that:

- Genotype ratio 1:2:1. That means that 1 out of four possibilities will bring the PP genotype, 1 the pp genotye and 2 the Pp genotype. Now let us translate that to phenotype:

- Phenotype ratio 3:1. That means that 3 out of four possibilities will bring a purple flower, but these 3 are either heterozygous or dominant homozygous, actually 2 of those are heterozygous and 1 the dominant homozygous. The recessive homozygous actually has an exclusive phenotype only for itself.