The inheritance of linked genes. Linked genes segregate together because they are on the same chromosome—that is, they are part of the same DNA molecule. In this hypothetical example, the dominant and recessive genes and their phenotypes are: A, round; a, square; B, violet; b, yellow. The cross between homozygous dominant and homozygous recessive parents produces F₁ AaBb progeny with linked alleles A, B and a, b. A testcross with a double-recessive homozygous individual (aabb) reveals the genotypes of the gametes produced by the F₁ progeny. The Punnett square shows the expected results for completely linked genes. The F₁ generation can produce only AB and ab gametes, and thus only two types of F₂ progeny are observed; they have the same phenotype as the original P generation.