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His work was ignored by the larger scientific community, in part because it was not published in a widely read journal, and in part because it tackled a problem, the physical basis of heredity, that few other scientists were thinking deeply about at that time.

That changed shortly afterward, when microscopic studies of cells revealed that chromosomes divided when cells divided, provoking speculation that they might be involved in inheritance. Mendel's studies were rediscovered in 1900, sixteen years after his death, by three biologists studying similar phenomena. The importance of his theory of inheritance was immediately recognized and widely accepted, and became the starting point for further investigations of the nature of inheritance that were carried out by Thomas Hunt Morgan, Alfred Sturtevant, and other twentieth-century geneticists. Mendelism, as the theory was called, was merged with Darwinism in the 1930s to form the "New Synthesis," which explained evolutionary theory in modern genetic terms. see also Chromosomal Theory of Inheritance, History; Inheritance Patterns; Mendelian Genetics; Nature of the Gene, History; Probability.

Richard Robinson

Bibliography

Henig, Robin Marantz. The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics. Boston: Houghton Mifflin, 2000.

two different types

Mendelian Genetics

Gregor Mendel (1822-1884), an Austrian monk and botanist, was curious and loved nature. He grew plants with diverse flower colors, and he cross-hybrids combinations of pollinated plant varieties to create hybrids. Mendel's fascination with "the striking regularity with which the same hybrid forms always reappeared," broadened his quest into discovering laws for inheriting any trait, not just flower color, from one generation to the next.

Mendel designed a series of experiments to learn the statistical rules governing the features that appeared in hybrids and in their offspring. Mendel identified plant varieties that exhibited the same features over many generations when the plants were allowed to self-pollinate or cross-pollinate with plants from the same variety. He chose hybrids that were fertile, so that their inherited characteristics, or traits, could be passed on to their offspring. He also made sure to exclude foreign pollen, so that outside plants did not get mixed up in his breeding experiments. Mendel chose peas as an ideal plant that had these characteristics.

Mendel obtained thirty-four varieties of peas from seedsmen, and, after two years of preparative work, he selected for study seven traits exhibited by the peas. The seven traits were: color of the seed coats (white or non-white); form of the ripe seeds (round or wrinkled); color of the seeds (yellowish orange or green); form of the ripe pods (inflated or constricted); color of the unripe pods (dark green or vivid yellow); position of the flowers (axial or terminal); and length of the stems (long or short).

Mendel carefully avoided choosing any traits, such as size and form of leaves, length of flower stalk, or size of pods, that would have generated a

maternal gametes

paternal

AB

Ab

aB

ab

gametes

AB

AABB

AABb

AaBB

AaBb

Ab

AABb

AAbb

AaBb

Aabb

aB

AaBB

AaBb

aaBB

aaBb

ab

AaBb

Aabb

aaBb

aabb

Figure 1. This Punnett square shows the possible outcomes, in genotypes, of a dihybrid cross (AaBb x AaBb). Reginald Punnett designed this layout to illustrate how alleles will assort according to Mendel's laws (segregation and independent assortment). The two parents are heterozygous for two traits, A and B, on non-homologous chromosomes and carry the genotype AaBb. The four gamete types from the male and female are shown in boldface. Lowercase letters represent recessive alleles; uppercase letters represent dominant alleles.

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