Gregor Mendel conducted his foundational work on heredity in the mid-19th century by cross-breeding common garden pea plants (Pisum sativum). His choice of the pea plant was instrumental because they are easy to cultivate, have a short generation time, and produce many offspring, providing large sample sizes for statistical analysis. Furthermore, pea plants exhibit clearly defined, contrasting traits (e.g., tall versus short stems or yellow versus green seeds), which allowed him to track inheritance patterns without the confusion of intermediate blending.
The Foundation of Inheritance: Unit Factors and Trait Expression
Mendel proposed that an organism’s traits are governed by discrete, inherited elements, which he termed “unit factors” (now known as genes). These factors control distinct characteristics like flower color or seed shape. He established that unit factors exist in pairs within an individual plant, with one factor inherited from each parent.
When two contrasting factors were present, one form of the trait appeared (the dominant characteristic) while the other remained hidden (the recessive characteristic). For instance, crossing a plant that produced yellow seeds with one that produced green seeds resulted in all offspring having yellow seeds, indicating that the factor for yellow seed color was dominant.
This distinction introduced the idea that an organism’s physical appearance, or phenotype, is not always a perfect reflection of its underlying hereditary makeup, or genotype (the specific combination of unit factors). A plant could possess one factor for the dominant trait and one for the recessive trait, yet still display only the dominant phenotype.
The Principle of Segregation
To understand how unit factors passed between generations, Mendel performed monohybrid crosses, tracking the inheritance of a single trait. He crossed two “true-breeding” parent (P) plants with contrasting traits (e.g., tall and short). The resulting first generation (F1) uniformly displayed only the dominant trait; all offspring were tall plants.
When Mendel allowed the F1 tall plants to self-pollinate, the recessive trait reappeared in the next generation (F2) alongside the dominant trait. He observed that approximately three-quarters of the F2 plants were tall, and one-quarter were short, establishing a consistent 3:1 phenotypic ratio.
Mendel explained this phenomenon with the Principle of Segregation, which states that the two unit factors for a trait must separate from each other during the formation of specialized reproductive cells, or gametes. When fertilization occurs, the single factors from each parent randomly combine to restore the paired condition in the offspring.
The Principle of Independent Assortment
Mendel then expanded his research to examine the inheritance of two different traits simultaneously, conducting dihybrid crosses. For example, he crossed a true-breeding plant that produced round, yellow seeds with one that produced wrinkled, green seeds. The F1 generation plants all exhibited both dominant traits, producing only round, yellow seeds.
When these F1 plants were allowed to self-pollinate, the resulting F2 generation showed four distinct combinations of traits. Mendel observed a consistent phenotypic ratio of 9:3:3:1 in the F2 offspring: nine parts showed both dominant traits (round, yellow), three parts showed one dominant and one recessive trait (round, green), three parts showed the opposite combination (wrinkled, yellow), and one part showed both recessive traits (wrinkled, green).
This distinct ratio led to the Principle of Independent Assortment, which holds that factors for different characteristics are inherited independently of one another. This independent shuffling allows for the creation of new combinations of traits that were not present in the original P generation parents, such as plants with round, green seeds or wrinkled, yellow seeds. This provides a mechanism for the vast genetic diversity seen in sexually reproducing organisms.