ABSTRACT
Gregor Mendel’s vision The concept of equal segregation is central to the genetics of eukaryotes. Gregor Mendel developed this idea in the mid-nineteenth century using the results of garden pea crosses. According to this principle the members of a gene pair or, using standard terminology, alleles have equal chance to enter a gamete. It means that approximately 50% of gametes produced by a heterozygote Aa, will carry allele A and the rest will carry allele a. Mendel’s discovery became widely known at the beginning of the twentieth century and has been a subject of fascination ever since. Regardless of what Mendel’s preconceived ideas were, one thing is clear: he found a highly organized source of randomness. As was confirmed later this source of randomness indeed exists in eukaryotic species with meiotic production of gametes. At the time Mendel wrote his major papers nothing was known about meiosis, chromosomes, and genes. Meiosis was discovered by the German biologist Oscar Hertwig in 1876, ten years after the publication of Mendel’s major work. The term chromosome was introduced in 1888 by another German biologist Heinrich von Waldeyer. The role of meiosis was first recognized by the outstanding German biologist August Weismann in 1890, and his contribution to the development of genetics and the theory of evolution was exceptional. The formulation of chromosome theory, which connected chromosome behavior in meiosis and the formation and fertilization of gametes, was achieved independently by Theodor Boveri in Germany and Walter Sutton in the United States in 1902 (Box 5.1 and Box 5.2; see Figure 5.1 and Figure 5.2). Finally starting from 1911 Thomas Morgan (see his biography in Chapter 3, Box 3.1) developed the chromosome theory of heredity which fused together cytological and genetic data. Interestingly enough the first reaction of Morgan to
BOX 5.1 THEODOR BOVERI
Theodor Boveri was born in 1862 in Bamberg, Germany. In 1881 he entered the University of Munich as a student of anatomy and biology. He received his Ph.D. in 1885 and was fortunate to continue research at the University of Munich in the laboratory of Richard Hertwig, who a decade earlier discovered meiosis and fertilization in sea urchin. In 1891, Hertwig invited Boveri to take a position of assistant professor. Two years later Boveri was appointed professor of zoology and comparative anatomy at the University of Würzburg and held this chair until he died on 1915. Studying meiosis and early cleavage in horse nematodes he realized that despite numerous morphological changes individual chromosomes maintain the integrity of their structures during cell cycle and division. It was the hypothesis of chromosome individuality (1887), which built his reputation as a prominent cell researcher.
