Gregor Mendel explains how he discovered that genes come in pairs by studying pea plants.

Peas have distinctive traits that are inherited in predictable ways. Each visible trait is called a phenotype. Let me show you the traits I worked with. . . These are stem and flower traits. Flower Position: Top, Side Stem Length (height): Tall, Short These are pod traits. Pod Shape: Puffed, Pinched Pod Color: Green, Yellow These are seed traits. Seed Shape: Round, Wrinkled Seed Color: Green, Yellow Seed Coat Color: Colored, White Here are the seven traits all together. Each trait has two phenotypes. Let's focus on one of these traits. I purchased pure-bred seeds for my experiments. Each harvest, farmers would keep some of their pea crop to use as seeds for the following year. These seeds came from the pea plants' regular habit of self-fertilization. This is how the "pure-bred" strains came about. In the case of seed color, one strain had only green seeds and the other, only yellow seeds. Green Phenotype Yellow Phenotype I reasoned that traits like seed color are controlled by one gene, which has a "green" form and a "yellow" form. Each form is called an allele. The pair of alleles is called the genotype. Y = yellow allele y = green allele Pure-bred peas have two copies of the same allele. y y Y Y GENOTYPE

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