Classical Hybrid Genetics

Common garden peas (Pisum sativum) are wonderful when eaten raw. Gregor Mendel (1822-1884) no doubt ate his share. Today he is recognized for using peas to establish the science of genetics.

Mendel investigated hereditary patterns of hybrids. Hybrids are off-spring from two organisms that are of different breeds, varieties, or species. Hybrids create new cultivars, from new apple varieties to tangelos to hybrid corn. Some mammals produce hybrids; a mule is the progeny of a horse and a donkey.

Mendel was interested in new flower varieties and absorbed by what hybrids reveal about inheritance. Nineteenth-century scientists wanted to know how organisms created a vast diversity of forms while faithfully maintaining distinct sets of characteristics. Constancy was a known quality of life. Then, as now, people had no trouble recognizing the difference between a housefly and a bee. But what prevented people from suddenly sprouting flowers or losing their human features? Genes, DNA, meiosis, and chromosomes were all unknown. Hybrid genetics was a means to discover answers to fundamental biological questions.

Mendel's aim was to discover the mathematical rules behind the reappearing patterns he saw in hybrids. After testing many varieties of peas, he decided to study seven specific traits: shape of the ripe seeds, seed color, seed coat color, form of the ripe pods, color of the unripe pods, position of the flowers, and length of the stems.

Initially, Mendel determined the inheritance patterns for one trait at a time, ignoring other traits. He crossed two varieties that differed sharply in a specific trait. For instance, for the trait of seed shape, he used one variety whose seeds were wrinkled and another whose seeds were smooth. The seedshape trait was represented by two specific inherited features, smooth and wrinkled, that were alleles.

Mendel also crossed plant varieties, each with specific combinations of alleles, in order to follow the fates of two or even three traits at the same time. For instance, he crossed a tall plant with wrinkled, green seeds and a short plant with round, yellow seeds. (The underlined words represent alleles for three traits: height, seed shape, and seed color.) To cross two varieties, Mendel first had to make sure the flowers did not pollinate themselves. To do that, he cut off their anthers, their pollen-bearing parts. Then he used the anthers from one variety to pollinate another.

Mendel was the first person to follow specific alleles (which he called "factors") as they were passed from parental varieties through several generations of offspring. He selected plant varieties that were true-breeding: Each generation of plants looked like the antecedent generations, with regard to the studied traits. Mendel used artificial pollination (described above) to perform an initial outcross, a mating between individuals differing in their alleles for at least one trait. The outcross created hybrids, or, more precisely, heterozygotes for the chosen traits. Next, Mendel let the heterozygotes self-pollinate, or intercross (i.e., the mating of inbred animals or self-pollinating plants), that are heterozygous for one or more traits. He then let two subsequent generations of progeny self-pollinate.

Mendel discovered that hybrids (heterozygotes) resembled only one parental variety, despite clearly identified input from both parents. Regarding the stem-length trait for example, one parental variety had tall stems, whereas the other variety had very short (dwarf) stems, yet all their hybrid offspring had only tall stems. Mendel named the displayed feature "dominant," because the hybrid's appearance of a tall stem (phenotype) reflected input from only one parent. He identified the dominant factor or allele for each of the seven traits (seed shape, seed color, length of stem, etc.). The other hidden feature (named "recessive" by Mendel) resurfaced in the next generation, when both hybrid (heterozygous) parents donated their hidden (recessive) allele to a descendent. For example, dwarf stems appeared again in progeny, instead of the conspicuous tall stems seen in the heterozygous parents. A descendent with dwarf stems had a "homozygous recessive" genotype for this trait. An individual's genotype described the two alleles received for a trait, whether hidden or displayed. If a plant had a dominant phenotype for one or several traits, but the genotype was unknown, a method called a test cross was used to reveal the presence of a recessive allele for each of the traits in question. The dominant phenotype to be tested might carry one dominant and one recessive allele, or two dominant alleles (see Mendel's laws, below), depending on what its parents had donated. The dominant phenotype was crossed with a plant known to be homozygous recessive for a particular trait. If half the off-spring showed the dominant phenotype and the other half the recessive Punnett squares are used to track the inheritance of traits. The upper and lowercase letters represent dominant and recessive alleles, respectively. P₁ is the parental generation, and F₁ is the first filial, or offspring, generation. A test cross is performed to determine the genotype of an organism showing the dominant trait, but whose genotype is unknown. form, then the parent with the dominant phenotype had to carry one recessive and one dominant allele for that trait. In contrast, if all its offspring showed only the dominant form of the trait, the plant with the dominant phenotype must have contained only dominant alleles.

By categorizing and counting offspring of several generations of plants, Mendel discovered two laws of inheritance, described as segregation and independent assortment. Segregation meant that a gamete or reproductive cell received only one allele out of a choice of two alleles carried for each trait (gene) within a parent cell. The gamete had an equal chance of receiving either allele. Independent assortment indicated that traits entered into a gamete independently of each other. This is only true for gene or trait collections located on long strings of hereditary material (chromosomes) that are now termed "nonhomologous" (not alike). Nonhomologous chromosomes carry unique collections of specific genes or traits, whereas two homologous chromosomes carry the same collection of genes, but may carry two distinct alleles for a specific trait or gene.

Armed with his laws, Mendel was able to predict the frequency at which various alleles for several traits would co-appear in descendents of hybrids of the common garden pea. Mendel's principles were not appreciated for about thirty years, until Walter Sutton (1877-1916), an American physician and geneticist, described chromosome movements during meiosis and identified chromosomes as the carriers of genes and heredity.

Back crosses, which are based on Mendel's principles, are used to develop commercially useful plant or animal varieties. In a back cross, a heterozygous plant and its offspring are crossed repeatedly to one of their parents to develop a line of plants that mostly resemble one parent but that have an allele of interest from the other. For instance, a rare flower color is integrated into a line of nematode-resistant stock of roses, joining beauty and disease-resistance.