Mendel’s laws were predictively successful for a wide range of hereditary traits, but his theory of genetics was limited in several ways. For one, his laws of heredity apply only to sexual reproduction; they don’t apply to asexual reproduction—for example, the reproduction of bacteria and other single-celled organisms that propagate by dividing themselves in two. Secondly, experiments by other scientists showed that some hereditary traits do not follow Mendel’s laws but exhibit more complicated patterns, a phenomenon which came to be known as “non-Mendelian inheritance.” Thirdly, his theory says nothing about the physical mechanisms involved in storing, transmitting, and utilizing genetic information. Although some core tenets of Mendel’s theory were vindicated as scientists came to understand the physical mechanisms of heredity, other aspects of his theory needed refinement.
Several key discoveries came with the aid of improved microscope optics. In the late 1800s, scientists
observed tiny paired structures, now known as chromosomes, inside the nuclei of living cells. They
noticed that chromosomes behave differently during two types of cell division. Normally, when a cell divides
into two new cells, the chromosomes duplicate so that each of the two new cells receives a complete set of chromosomes like the original set. This normal process of cell division is called mitosis. Mitosis occurs during the asexual reproduction of single-celled organisms, and it is also the process by which multicellular organisms like plants and animals produce new cells in order to grow or to heal wounds.
However, a unique type of cell division occurs during meiosis—the process that produces sex cells, called gametes, such as male sperm and female egg cells. Meiosis is a complex, multi-step process in which a parental cell divides twice to produce four gametes. The parent’s chromosomes are copied just once during this process, so each gamete ends up with half the normal number of chromosomes: one chromosome from each pair of the parent’s chromosomes. Thus, when male and female gametes join in fertilization to form a new organism, the child inherits a complete set of paired chromosomes. One chromosome in each pair is inherited from the mother, the other from the father.
In 1900, thirty-five years after Mendel had published his research on pea plants, several biologists took notice of his work and connected his findings with their knowledge of meiosis and fertilization. They surmised that chromosomes are the physical structures which contain genes, thus explaining Mendel’s first law of heredity: in each pair of the child’s chromosomes, one chromosome contains a set of alleles from the father and the other contains a corresponding set of alleles from the mother.Hugo de Vries, Carl Correns, and Erich von Tschermak independently rediscovered Mendel’s work in 1900 and confirmed his findings with their own experiments. Prior to this, some biologists had speculated that chromosomes might be the carriers of genetic information. The simple explanation of Mendel’s first law provided additional evidence for that hypothesis. The subsequent explanation of Mendel’s second law by Thomas Hunt Morgan (see next footnote) removed any doubt. For further discussion, see Edward J. Larson, Evolution: The Remarkable History of a Scientific Theory (New York: Random House, 2004), 158-172.Mendel’s second law was easily understood as well. During meiosis, the parental chromosome pairs are separated and distributed so that each gamete receives exactly one chromosome from each pair, randomly selected from either the maternal or the paternal copy. This explains why alleles of separate genes are inherited independently, provided the genes are located on different chromosomes.
Mendel’s second law doesn’t always hold for genes located in the same chromosome. Nevertheless, even genes in the same chromosome sometimes are inherited independently, and further observations of meiosis revealed how that is possible. Before the parent cell divides into gametes, the two chromosomes in each pair become intertwined and exchange segments with each other in a process called chromosomal crossover. This process of swapping chromosome segments allows genes from the same chromosome to be inherited independently, unless both genes are located within the same segment of the chromosome. This also explains why exceptions to Mendel’s second law sometimes occur: genes that lie in proximity to each other on the same chromosome tend to be correlated rather than independent, because they are not as likely to be separated during chromosomal crossover.Franz Janssens first observed chromosomal crossover in 1909, but its relation to Mendel’s second law wasn’t understood until a few years later, when Thomas Hunt Morgan recognized its role in explaining why some genes are more strongly correlated than others. See Edward J. Larson, Evolution: The Remarkable History of a Scientific Theory (New York: Random House, 2004), 172.
Later discoveries revealed that Mendel’s third law also required modest emendations. For example, some alleles exhibit more complex relations like incomplete dominance and co-dominance rather than being simply dominant or recessive. Moreover, some traits depend on multiple genes, which explains why many heritable characteristics seem to vary across a continuum rather than exhibiting just a few discrete varieties. For example, adult human beings have many different heights, varying smoothly across a wide range, whereas Mendel’s pea plants grew in distinctly “tall” and “short” varieties. Human beings exhibit a greater diversity of heights because many different genes (along with some non-genetic factors) are involved in controlling our vertical growth. Nevertheless, with suitable amendments, Mendel’s laws can explain even the complex patterns of heredity associated with so-called “non-Mendelian” traits, and the basic principles he discovered are still employed by geneticists today.
Despite the successes of Mendel’s theory, however, his laws still couldn’t explain how new genetic information might be introduced. Mendel’s laws only describe how pre-existing alleles can be combined in various ways to produce different traits—a process known as recombination. Recombination is the result of chromosomal crossover, but chromosomal crossover and recombination can only explain the emergence of traits corresponding to alleles (or combinations of alleles) that were already present in an organism’s ancestors. They don’t introduce new genes or alleles. This shortcoming of Mendel’s theory posed a serious problem for evolutionary biology, because it implied that the possible range of genetic variation is limited.
Granted, recombination can produce a remarkable variety of new traits. Consider, for example, the following vegetables: broccoli, cauliflower, cabbage, Brussels sprouts, kale, and collard greens. All come from the very same species of plant! The species Brassica oleracea—known in its native form as “wild cabbage”—has been cultivated for thousands of years and selectively bred to produce the delightful variety of veggies we all hated when we were kids. Nevertheless, there are limits to how far selective breeding can go. No combination of dominant and recessive cabbage alleles will yield a baby squirrel, for instance. For the same reason, mere genetic recombination doesn’t explain how major evolutionary transitions between radically different species could occur. It doesn’t explain how reptiles could evolve into birds, for example, unless all of the genes required for growing feathers were already present (as recessive alleles) in the reptile population! So, genetic recombination cannot produce new genes or alleles. Where do new genes come from, then? An answer to that question eventually would arrive with a deeper understanding of the inner workings of chromosomes, as we’ll see on the next page.