Monohybrid Crosses

A monohybrid cross combines two true-breeding organisms that differ by a single trait. Mendel used pea plants because they were easy to grow, had easily observable traits like pod color, and could self-fertilize or be cross-pollinated. His experiments with pea plants established the fundamental principles of inheritance that form the basis of modern genetics.

All F1 offspring display the same phenotype because they inherit one dominant allele from each parent, making them heterozygotes. The dominant trait masks the recessive trait in these offspring. This consistent result across all F1 individuals demonstrates Mendel's Principle of Uniformity, showing that heterozygotes for a single gene trait always display the dominant phenotype.

When F1 heterozygotes self-fertilize, parental alleles randomly distribute to gametes. The four possible combinations produce three offspring with the dominant phenotype and one with the recessive phenotype. This 3:1 ratio occurs because three genotypes (GG and two Gg) express the dominant trait, while only one (gg) expresses the recessive trait, confirming Mendel's law of segregation.

A dominant trait is expressed when at least one dominant allele is present, while a recessive trait only appears when an organism has two recessive alleles. In Mendel's pea plants, green pods are dominant and yellow pods are recessive. The recessive trait can be inherited but hidden in heterozygous offspring, only reappearing in the F2 generation when two recessive alleles combine.

True-breeding organisms consistently produce offspring with the same trait when self-fertilized or crossed with other true-breeding individuals of the same type. They are homozygous for the trait being studied. Mendel used true-breeding pea plants as parents because their genetic uniformity ensured predictable inheritance patterns, allowing him to clearly observe how single traits pass between generations.

Dominant traits are not necessarily common because human selection and environmental pressures influence which traits persist in populations. For example, green peas are recessive but common because farmers prefer them and selectively breed for green peas, excluding yellow peas from crops. This demonstrates that trait frequency depends on selection practices and preferences, not on whether a trait is dominant or recessive.

The law of segregation states that organisms distribute one of their two gene copies to each gamete randomly. In a heterozygote, gametes are equally likely to carry the dominant or recessive allele. When heterozygotes self-fertilize, this random distribution of alleles to gametes produces the predictable 3:1 phenotypic ratio observed in F2 generations across all seven traits Mendel studied.