In shepherd's purse, seeds are ovoid only when both genes are homozygous recessive. What ratio comes from crossing AaBb with AaBb?

1. Seeds are ovoid only for the genotype aabb, and every other genotype gives triangular seeds. 2. At the A gene, the chance of aa is 1/4, and at the B gene the chance of bb is 1/4. 3. The two genes assort independently, so the chance of aa and bb together is 1/4 times 1/4. 4. That is 1/16, so one sixteenth of the offspring have ovoid seeds. 5. Everything else, which is 15/16, has triangular seeds. 6. The ratio is therefore 15 triangular to 1 ovoid. 7. This is reciprocal epistasis, since a dominant allele at either gene alone is enough to give triangular seeds. 8. The parts again total 16, the signature of two interacting genes. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 12 "Mendel's Experiments and Heredity", section 12.3 Laws of Inheritance_