Mendel’s Laws

photo of Gregor Mendel in 1880
Gregor Mendel
(1822 - 1884)This photo was taken in 1880. Original file here.

The emergence of modern genetic theory is arguably the most important development in biology since Darwin’s time, but the science of genetics had humble beginnings. While Darwin was rising to fame, an Augustinian friar named Gregor Mendel was conducting experiments in the garden of St. Thomas’s Abbey, Brno (located in what is now the Czech Republic). The modest friar would receive little recognition for his work during his lifetime. Darwin never heard of him,For an interesting discussion of what might have happened if he had, see this. and Mendel’s discoveries were largely ignored even by the few scientists who were aware of his work. Eventually, however, his theory of genetics would be hailed as the momentous breakthrough it truly was.

Mendel wanted to know how traits (physical characteristics) are passed from parents to offspring, so he devised a clever series of experiments to study patterns of heredity with successive generations of pea plants in the monastery gardens. He began with true-breeding pea plants—peas that always produced offspring with a certain set of traits (e.g. long stems, yellow pods, and shriveled seeds) generation after generation. Using several varieties of true-breeding pea plants, Mendel tried cross-pollinating plants with differing traits. For example, he would fertilize the flowers of tall pea plants with pollen from the flowers of dwarfed plants, and vice versa, to produce hybrid offspring. After repeated experiments, he discovered some remarkable patterns.Here’s an English translation of Mendel’s 1865 paper, “Experiments in Plant Hybridization.”

When Mendel bred tall (long-stemmed) plants and short (dwarfed) plants together, all of the offspring were tall. But when he bred those tall hybrids together, about ¼ of their offspring were short! In other words, the first generation of hybrids were all tall; yet in the second generation of hybrids, some were dwarfed. Roughly ¾ of the total number of pea plants in that second generation of hybrids were tall (like their hybrid parents), but ¼ were short.Mendel (1865), “Experiments in Plant Hybridization,” p. 11: “Out of 1,064 plants, in 787 cases the stem was long, and in 277 short.”

Mendel repeated the experiment with other pairs of traits, testing a total of seven types of characteristics, including pod color (green vs. yellow), pod texture (smooth vs. wrinkled), seed shape (round vs. shriveled), and a few other variable features. He performed each of these experiments with hundreds of plants, sometimes more than a thousand. In each case, Mendel observed the same pattern. The first generation of hybrid plants always had one particular trait; but in the second generation of hybrids, about ¾ of the plants had that same trait and ¼ had the alternate trait.

To explain these findings, Mendel proposed three laws of heredity, which describe how hereditary traits are determined by units of heredity that we call genes. Mendel called them “factors.” The word “gene” was later coined by Wilhelm Johannsen.See here for more information. Each type of physical characteristic (stem length, seed shape, pod texture, etc.) corresponds to one or more genes. Moreover, each gene comes in multiple forms, called alleles, which correspond to various possible traits. For example, there are at least two alleles of the gene for stem length: an allele for tall stems and another for short stems. There may be other alleles of this gene too, assuming there are other varieties of peas besides the tall plants and dwarfed plants Mendel used in his experiment. Similarly, there are different alleles of the gene that determines the color of pea pods: one allele produces green pods, another produces yellow pods, and so on.

With the concepts of genes and alleles in view, here are the three laws of heredity known as Mendel’s Laws:

  1. An organism has two alleles of each gene. In sexual reproduction, the child inherits one allele of each gene from each parent.

    In the case of Mendel’s peas, each plant inherits two alleles of the gene that determines stem length: one from “Mom” and one from “Dad”. (For peas and other flowering plants, the pollen-producing stamen is the male part, and a structure called the pistil is the female part.) The child may inherit the “tall” allele from Mom and the “short” allele from Dad, or vice versa; or it may inherit “tall” from both parents, or “short” from both parents.

  2. Alleles of separate genes are inherited independently.

    The gene for height and the gene for pod color are separate genes, so inheriting the “tall” allele from Mom doesn’t make it any more or less likely that the “yellow pods” allele will also be inherited from Mom.

  3. Some alleles are dominant and others are recessive. If an organism has both a dominant and a recessive allele of a given gene, it will display the dominant trait (i.e., the trait corresponding to the dominant allele).

    If the child receives the “tall” allele from Mom and the “short” allele from Dad, the child will be tall like Mom, since “tall” is the dominant trait (at least for Mendel’s peas).
A Punnett Square
A Punnett Square

Possible combinations of alleles can be illustrated with a diagram like this, called a Punnett square.

Mendel’s laws explain the patterns he observed in the pea plant experiments. Consider his experiment with tall and short plants. The true-breeding plants with which he started had two copies of the same allele for stem length: the true-breeding tall plants had two “tall” alleles, and the true-breeding dwarfed plants had two “short” alleles. When those tall and short plants were bred together, the offspring inherited one allele from each parent, so the children each had one “tall” allele and one “short” allele. But the “tall” allele is dominant, so all of the first-generation hybrids were tall.

Now consider what happened when Mendel bred those hybrid plants together. Since each of the hybrid parents had one “tall” and one “short” allele, there are four possible combinations of alleles that their second-generation hybrid children might inherit: tall from Mom and short from Dad, short from Mom and tall from Dad, tall from both parents, or short from both parents. Of these four combinations, which are equally likely, only the last one (short from both parents) will yield short offspring. The other three possibilities each contain at least one “tall” allele, which is dominant, so they will produce tall offspring. That is why three out of four plants in the second generation of hybrids were tall, and ¼ were short.