Gregor Mendel explains the rules of inheritance.

I crossed pure-bred green with pure-bred yellow peas. Since yellow is dominant to green, all of the offspring were yellow. The green phenotype had disappeared. I allowed the yellow pea plants to self- fertilize to see what the second generation would look like. Some of the second generation offspring were green! After performing many experiments, I figured out the basic rules of inheritance and why the green color reappeared. The results of these experiments showed that pea color is controlled by one gene, which has a "green" form and a "yellow" form. Each form is called an allele. If we suppose that a pure-bred green seed plant has two "green" alleles (y) and the pure-bred yellow seed plant has two "yellow" alleles (Y), we can diagram these plants like this. When a pure-bred green plant is crossed with a pure-bred yellow plant, the offspring inherit one allele of the color gene from each parent. Plants with two different alleles of a gene are heterozygous. Since all the heterozygous offspring are yellow, then the yellow allele must be dominant over the recessive green allele. The green allele didn't disappear; its effect is not seen in heterozygotes. Peas are yellow when they have either two yellow alleles . . . . . . or one yellow allele and one green allele. Green peas must have two copies of the recessive green allele. Now, let's see how the genes sort out when we cross two heterozygous plants. When two heterozygous plants are crossed, the resulting progeny mostly show the dominant phenotype. But, the recessive phenotype reappears.

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