4.3.2 Gregor Mendel and the Laws of Heredity

The true nature of inheritance was not really understood until the beginning of the 20th century, when the 19th-century work of Gregor Mendel, a Catholic priest from Slovakia, was rediscovered. While in college, Gregor Mendel was introduced to cell theory, which states that all organisms are composed of cells and that cells are the fundamental unit of all living things. Cell theory raised many questions in Mendel’s mind, including whether both parents contribute equally to the cells in their offspring. In 1854, Mendel began a series of experiments with pea plants to help resolve this question and better understand how traits are inherited from generation to generation.

The first stage of Mendel’s experiments was identifying plants that breed true, meaning that each parent only produces one kind of offspring when self-crossed. A self-cross is essentially a self-mating; some plants, such as peas, have both male and female parts and can self-fertilize. Not all self-crosses are the same as the parent plant, however. For example, self-crossed pea plants that have yellow pods sometimes produce offspring with yellow pods and sometimes produce offspring with green pods. Mendel continued to selectively breed only those pea plants that produced offspring that were the same as the parents. He called them purebreds and referred to them as the P1 generation. It took him more than two years to establish plants that always bred true.

Then Mendel selected seven traits of his pea plants that each had two distinct phenotypes, or observable expressions of the trait. For example, seed shape can be either round or wrinkled, while pod color can be either yellow or green. Over the next eight years, Mendel studied the mating and resulting traits of more than 28,000 plants. Mendel’s first round of experiments used his purebred pea plants to create what is known as a monohybrid cross. A monohybrid cross is a mating between two purebred individuals who differ in a single characteristic. In Mendel’s monohybrid crosses, the parent pea plants differed from one another in terms of whether the pods of the parental pea plants were yellow or green or whether the seeds of the parental pea plant were wrinkled or round.

In his first monohybrid crosses, Mendel mated a purebred yellow pea plant with a purebred green pea plant. He found that all the offspring resulting from this monohybrid cross were yellow, even though when the green peas self-crossed, all their offspring were green. In other words, all the hybrid offspring were yellow in color. A hybrid plant is one in whose parents differ in a term of a specific characteristic, such as pod color or seed shape. The trait that was expressed (yellow) Mendel referred to as dominant, and the trait that disappeared (green) he referred to as recessive. Mendel’s next set of experiments involved mating two hybrid plants—in other words, those that resulted from the monohybrid cross. In these experiments, he found that the recessive traits reappeared in a ratio of three dominant to one recessive.

Mendel’s experiments suggested two very important facts. First, Mendel noted that various expressions of a trait (such as pea color) were controlled by discrete units that occur in pairs and that offspring inherited one unit of each pair from each parent. This observation became Mendel’s first law of inheritance, the law of segregation, which states that the two alleles for each trait segregate, or separate, during the formation of gametes (eggs and sperm) and that during the reproductive process, the alleles combine at random with other alleles. Today, we know that the process of meiosis—division of sex cells—explains Mendel’s law of segregation. Each of the seven traits identified by Mendel is controlled by a pair of genes in the plant, one on each chromosome. During the reproductive cycle, the chromosomes separate from one another so that each gamete has only one allele for each trait. During fertilization, the alleles combine, and the two-gene state is restored.

After Mendel established his first law of inheritance, he extended his studies to more complex situations. He began performing experiments with two set of traits, using dihybrid crosses. A dihybrid cross is a cross between individuals who differ with respect to two gene pairs—for example, a cross between a plant with a round yellow pea and a plant with a wrinkled green pea. Because yellow and round are both dominant traits and wrinkled and green are both recessive, all the offspring resulting from the first-generation mating were 100% yellow and round. The green color and the wrinkled pea shape had disappeared. However, these recessive traits reappeared in a ratio of three dominant to one recessive when two round yellow individuals from the first-generation dihybrid cross were mated. The green color and the wrinkled pea shape had not truly disappeared. In the second generation of the dihybrid cross, Mendel found that 9/16 of the offspring were round and yellow, 3/16 were wrinkled and yellow, 3/16 were round and green, and 1/16 were wrinkled and green. The results of these dihybrid crosses indicate that the two characteristics—pea color and pea shape—segregate independently. The expression of one trait is not influenced by the expression of the other trait. This is known as the law of independent assortment, which is Mendel’s second law of inheritance. There is nothing to dictate that round peas will be yellow or that wrinkled peas will be green. The alleles that code for different traits sort independently of one another during sex cell division (meiosis).