Figure 1. Mendel's demonstration of the particulate nature of the gene. Two true-breeding strains of peas, one tall and the other short, were crossed to one another. All of their offspring were tall. At the level of their genes, the tall plants were TT homozygotes, and the short plants were tt. When a tall plant is crossed to another tall plant, the gametes are all T, and the offspring produced are all TT homozygotes. Like the parents, they are tall. Similarly, tt homozygotes, when crossed to one another, produce only short, tt homozygotes. A cross of a tall plant with a short one produces Tt heterozygotes, which are tall because the T allele of the tallness gene is dominant to the t allele. When these Tt heterozygotes make gametes, half of the gametes are T and half are t. In a cross between two Tt heterozygotes (or a self-cross, which is genetically the same) T and t gametes join at random to form zygotes. A T egg is equally likely to be fertilized by a T pollen nucleus or a t pollen nucleus. Similarly, a t egg is equally likely to be fertilized by a T pollen nucleus or a t pollen nucleus. Thus, the progeny of a cross between two heterozygotes is 1/4 TT, 1/2 Tt and 1/4 tt. The homozygous TT offspring are tall, and so are the heterozygous Tt offspring (because T is the dominant allele). However, the tt homozygotes are short. This experiment shows that the t allele remains unchanged in the Tt heterozygote and reappears in the offspring of the Tt heterozygote when an appropriate cross is done.

Figure 2. The genetic code. RNA contains four bases, adenine (A), guanine (G), cytosine (C) and uracil (U). One difference between DNA and RNA is that RNA contains U instead of T. These four bases can be assembled into 64 possible triplet codons, every one of which has a meaning in the genetic code. The codons are traditionally written in their mRNA form. The amino acid translations of the codons are shown in italics. The abbreviations stand for phenylalanine, leucine, isoleucine, methionine, valine, serine, proline, threonine, alanine, tyrosine, histidine, glutamine, asparagine, lysine, aspartate, glutamate, cysteine, tryptophan, arginine and glycine. The three ‘stop’ codons, UAA, UAG and UGA are given the whimsical names ochre, amber and opal, respectively.

Figure 3. The cis-trans complementation test. A wild-type gene (+) makes a functional protein, which is shown in the figure as a purple line. A mutant gene (m) makes a nonfunctional protein, which is shown in the figure as a blue line. If the two mutations, m₁ and m₂, affect different genes (top frames), a heterozygous cell that contains both mutations and their wild-type (+) counterparts will contain some functional protein product from each gene. This will be true whether the mutations are in the cis configuration or the trans configuration. If the two mutations affect the same gene (bottom frames), then the cis heterozygote will make some functional protein and will have the wild-type phenotype, but the trans heterozygote will make only nonfunctional protein and will have the mutant phenotype.