Principles of genetics

Mendel’s hybridization experiments demonstrate the difference between phenotype and genotype. When true-breeding plants (P1), in which one parent had yellow pods and one had green pods were cross-fertilized, all of the first generation (F1) hybrid offspring had yellow pods. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow pods. However, we know that the allele donated by the parent with green pods was not simply lost, because it reappeared in some of the second generation (F2) offspring. Therefore, the F1 plants must have been genotypically different from the parent with yellow pods.

The P1 plants that Mendel used in his experiments were each homozygous for the trait he was studying. Diploid organisms that are homozygous at a given gene, or locus, have two identical alleles for that gene on their homologous chromosomes. Mendel’s parental pea plants always bred true because both of the gametes carried the same trait. When P1 plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous for the contrasting trait, meaning that their genotype reflected that they had different alleles for the gene being examined.