Complexity_ A Guided Tour - Melanie Mitchell [39]
A major issue not explained by Darwin’s theory was exactly how traits are passed on from parent to offspring, and how variation in those traits—upon which natural selection acts—comes about. The discovery that DNA is the carrier of hereditary information did not take place until the 1940s. Many theories of heredity were proposed in the 1800s, but none was widely accepted until the “rediscovery” in 1900 of the work of Gregor Mendel.
Mendel was an Austrian monk and physics teacher with a strong interest in nature. Having studied the theories of Lamarck on the inheritance of acquired traits, Mendel performed a sequence of experiments, over a period of eight years, on generations of related pea plants to see whether he could verify Lamarck’s claims. His results not only disconfirmed Lamarck’s speculations but also revealed some surprising facts about the nature of heredity.
Mendel looked at several different traits of pea plants: smoothness and color of seeds; shape of pea pod; color of pods and flowers; locations of flowers on the plants; and height of stems. Each of these traits (or “characters”) could have one of two distinct forms (e.g., the pod color could be green or yellow; the stem height could be tall or dwarf).
Mendel’s long years of experiments revealed several things that are still considered roughly valid in modern-day genetics. First, he found that the plants’ offspring did not take on any traits that were acquired by the parents during their lifetimes. Thus, Lamarckian inheritance did not take place.
Gregor Mendel, 1822–1884 (From the National Library of Medicine) [http://wwwils.nlm.nih.gov/visibleproofs/galleries/
technologies/dna.html].
Second, he found that heredity took place via discrete “factors” that are contributed by the parents, one factor being contributed by each parent for each trait (e.g., each parent contributes either a factor for tall stems or dwarf stems). These factors roughly correspond to what we would call genes. Thus, the medium of inheritance, whatever it was, seemed to be discrete, not continuous as was proposed by Darwin and others. (Note that pea plants reproduce via either self-pollination or cross-pollination with other pea plants.)
For each trait he was studying, Mendel found that each plant has a pair of genes responsible for the trait. (For simplicity, I am using more modern terminology; the term “gene” was not used in Mendel’s time.) Each gene of the pair encodes a “value” for the trait—for example, tall vs. dwarf. This value is called an allele. For stem height there are three possibilities for the allele pairs encoded by these two genes: both alleles the same (tall/tall or dwarf/dwarf) or different (tall/dwarf, which is equivalent to dwarf/tall).
Moreover, Mendel found that, for each trait, one allele is dominant (e.g., tall is dominant for stem height) and the other recessive (e.g., dwarf is recessive for stem height). A tall/tall individual will always be tall. A tall/dwarf individual will also be tall since tall is dominant; only one copy of the dominant allele is needed. Only a dwarf/dwarf individual—with two copies of the recessive allele—will be dwarf.
As an example, suppose you have two tall/dwarf individuals that cross-pollinate. Both the parents are tall, but there is a 25% chance that both will pass on their dwarf gene to the child, making it dwarf/dwarf.
Mendel used such reasoning and the laws of probability to predict, very successfully, how many plants in a given generation will display the dominant or recessive version of a given trait, respectively. Mendel’s experiments contradicted the widely believed notion of “blending inheritance”—that the offspring’s traits typically will be an average of the parents’ traits.
Mendel’s work was the first to explain and quantitatively predict the results of inheritance, even though Mendel did not know what substance his “factors” were made out of, or how they recombined as a result of mating. Unfortunately, his 1865 paper, “Experiments in Plant Hybridization,” was published in a