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Extensions and Modifications of Basic Principles

At the start of the twentieth century, Mendel’s work on inheritance in pea plants became widely known (see Chapter 3), and a number of biologists set out to verify his conclusions by conducting crosses with other organisms. Biologists quickly confirmed that Mendel’s principles applied not just to peas, but also to corn, beans, mice, guinea pigs, chickens, humans, and many other organisms. At the same time, biologists began to discover exceptions—traits whose inheritance was more complex than the simple dominant and recessive traits that Mendel had observed. One of these exceptions involved the spiral of a snail’s shell.

The direction of coiling in snail shells is called chirality. Most snail shells spiral downward in a clockwise or right-handed direction; these are termed dextral shells. A few snails have shells that coil in the opposite direction, spiraling downward in a counterclockwise or left-handed direction; these are termed sinistral shells. The shells of most snail species are all dextral or all sinistral; only in a few rare instances do both dextral and sinistral shells coexist in the same species.

In the 1920s and 1930s, Arthur Boycott of the University of London investigated the genetics of shell coiling in Lymnaea peregra, a common pond snail in Britain. In this species, most snails are dextral, but a few sinistral snails occur in some populations. Boycott learned from amateur naturalists of a pond near Leeds, England, where an abnormally high number of sinistral snails could be found. He obtained four sinistral snails from this location and began to investigate the genetics of shell chirality.

Boycott’s research was complicated by the fact that these snails are hermaphroditic, meaning that a snail can self-fertilize, or self (mate with itself). If a suitable partner is available, the snails are also capable of outcrossing—mating with another individual. Boycott found that if he isolated a newly hatched snail and reared it alone, it would eventually produce offspring, so he knew that it had selfed. But when he placed two snails together and one produced offspring, he had no way of knowing whether it had mated with itself or with the other snail. Boycott’s research required rearing large numbers of snails in isolation and in pairs, raising their offspring, and determining the direction of shell coiling for each of the offspring. To facilitate the work, he enlisted the aid of several amateur scientists. One of his assistants was Captain C. Diver, a friend who worked as an assistant for the British Parliament. Since Parliament met for only part of the year, Diver had time on his hands and eagerly enlisted to assist with the research. Together, Boycott, Diver, and other assistants carried out numerous breeding experiments, selfing and crossing snails and raising the progeny in jam jars. They eventually raised more than 6000 broods and determined the direction of coiling in a million snails.

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Initially, their results were puzzling: shell coiling did not appear to conform to Mendel’s principles of heredity. They eventually realized that dextral was dominant to sinistral, but with a peculiar twist: the phenotype of a snail was determined not by its own genotype, but by the genotype of its mother. This phenomenon—a phenotype that is influenced by the genotype of the mother—is called a genetic maternal effect. Genetic maternal effects often arise because the maternal parent produces a substance, encoded by her own genotype, that is deposited in the cytoplasm of the egg and that influences early development of the offspring.

The substance that determines the direction of shell coiling in snails has never been isolated. However, in 2009, Reiko Kuroda and her colleagues demonstrated that the direction of coiling in Lymnaea snails is determined by the orientation of cells when the embryo is at an early developmental stage—specifically, the eight-cell stage. By gently pushing on the cells of eight-cell embryos, they were able to induce offspring whose mother’s genotype was dextral to develop as sinistral snails; similarly, they induced the offspring of mothers whose genotype was sinistral to develop as dextral snails by pushing on the cells in the opposite direction.

Boycott’s research on the direction of coiling in snails demonstrated that not all characteristics are inherited as simple dominant and recessive traits like the shapes and colors of peas that Mendel described. This demonstration doesn’t mean that Mendel was wrong; rather, it indicates that Mendel’s principles are not, by themselves, sufficient to explain the inheritance of all genetic characteristics. Our modern understanding of genetics has been greatly enriched by the discovery of a number of modifications and extensions of Mendel’s basic principles, which are the focus of this chapter.