Biology Reference
In-Depth Information
that the alleles for the two different traits are distributed
independently of each other in the formation of gametes. This idea is
called the Principle of Independent Assortment or Mendel's Second Law.
More information about Mendel's experiments can be found in
traditional genetics texts, such as Russell (2006).
Mendel's work was an early application of quantitation to biology. His
appreciation of the value of large sample sizes, which anticipated modern
statistics, prompted him to obtain thousands of peas in his experiments
and allowed him to formulate the explanation that alleles occurred in
pairs and that each sperm or egg delivered one allele to the offspring. It is
remarkable that he formed this model without any knowledge of the
underlying physical and chemical processes. He never observed cell
division and never saw a chromosome—the physical carriers of genetic
information, where his particulate factors were to be found.
In 1865, Mendel presented a paper on his experiments and his
interpretation of the results to the Natural Science Society in Br¨ nn,
and the following year published ''Versuche ¨ ber Pflanzen-hybriden''
(''Experiments in Plant Hybridization'') in the Society's journal.
Unfortunately, the scientific community paid little attention to Mendel's
work, and when he was named head of the monastery, his experimental
career ended. Mendel died in 1884.
It was not until 1900 that Mendel's unparalleled achievements were
independently resurrected by Hugo De Vries (1848-1935), Eric von
Tschermak (1871-1962), and Carl Correns (1864-1933), and Mendel was
recognized as the father of genetics. This new appreciation of Mendel's
work was the result of advances in the science of cytology that made it
possible to stain cell nuclei and observe chromosomes directly.
It also became possible to examine the behavior of chromosomes during
mitosis (cell division) and meiosis (the reduction division that gives rise to
gametes). In 1903, working independently, the German Theodor Boveri
(1862-1915) and the American Walter Sutton (1877-1916) came to the
conclusion that the behavior of chromosomes strongly resembled the
behavior of genes. This realization led them to put forth the Chromosome
Theory of Inheritance, which states that genes are found on chromosomes.
It soon became apparent that Mendel's work was insufficient to explain
all inheritance patterns and that more complex genetic behavior was
possible. For example, Thomas Hunt Morgan (1866-1945) used the
white-eye color mutant of Drosophila melanogaster to show that some
genes behave in a sex-linked manner. In 1909, Hermann Nilsson-Ehle
(1873-1949) reported experiments in which multiple genes were
involved in producing a single trait. We investigate the behavior of such
polygenic traits later in the chapter.
Unfortunately, the laws of genetics have sometimes been purposefully
misinterpreted and abused. What Mendel and other scientists had
