These are used to show the inheritance of two different genes on two different chromosomes. Consider two pea plants - one with round, yellow seeds and one with green, wrinkled seeds. The allele for yellow seeds is dominant whilst the allele for green seeds is recessive. The allele for round seeds is dominant and the allele for wrinkled seeds is recessive. If a true-breeding, homozygous pea plant with yellow, round seeds is crossed with a true-breeding homozygous plant with green, wrinkled seeds, the parental genotypes are YYRR and yyrr. All of the offspring in the F1 generation are YyRr heterozygotes.