FIG. 16.14

How are single-gene traits inherited?

BACKGROUND Gregor Mendel’s experiments, carried out in the years 1856–1863, are among the most important in all of biology.

EXPERIMENTS Mendel set out to improve upon previous research in heredity. He writes that “among all the numerous experiments made, not one has been carried out to such an extent and in such a way as to make it possible to determine the number of different forms under which the offspring of the hybrids appear, or to arrange these forms with certainty according to their separate generations, or definitely to ascertain their statistical relations.” By studying simple traits across several generations of crosses, Mendel observed how these traits were inherited.

RESULTS Mendel concluded that the “statistical relations” were very clear. Crosses between plants that were hybrids of a single trait displayed two phenotypes in a ratio of 3:1. Crosses between plants that were hybrids of two traits displayed four different phenotypes in a ratio of 9:3:3:1.

HYPOTHESIS From observing these ratios among several different traits, Mendel made two key hypotheses about the inheritance of traits, now called Mendel’s laws:

1. The principle of segregation states that individuals inherit two copies (alleles) of each gene, one from the mother and one from the father, and when the individual forms reproductive cells, the two copies separate (segregate) equally in the eggs or sperm.

2. The principle of independent assortment states that the two copies of each gene segregate into gametes independently of the two copies of another gene.

ANALYSIS Mendel found support for his two hypotheses in the statistical analysis of the results of his meticulous crosses.

1. The principle of segregation: A prediction of the hypothesis of segregation is that, among seeds with the dominant phenotype, the ratio of homozygous to heterozygous genotypes should be 1:2. Mendel tested this prediction in several ways, one of which was simply to allow plants grown from F₂ seeds to self-fertilize. Any that were true breeding for the dominant phenotype he classified as homozygous, and any that segregated to yield both dominant and recessive phenotypes he classified as heterozygous. In the experimental test, the observed numbers fit the expected values within the margins that would be expected by chance.

2. The principle of independent assortment: Although the Punnett square for two pairs of alleles has 16 squares, there are only 9 genotypes, and the hypothesis of independent assortment predicts that these genotypes should appear in the ratio of 1 AA BB:2 AA Bb:1 AA bb:2 Aa BB; 4 Aa Bb:2 Aa bb:1 aa BB:2 aa Bb:1 aa bb. As before, Mendel tested this hypothesis by self-fertilization of plants grown from the F₂ seeds, classifying any that bred true for either trait as homozygous and any that segregated to yield both dominant and recessive phenotypes as heterozygous. In the experimental test, once again the observed numbers fit the expected values within the margins that would be expected by chance.

FOLLOW-UP WORK Mendel’s work was ignored during his lifetime, and its importance was not recognized until 1900, 16 years after his death. The rediscovery marks the beginning of the modern science of genetics.

SOURCE Mendel’s paper in English is available at http://www.mendelweb.org/Mendel.html.