IT SKIPS A GENERATION

Long before they understood why the strategy worked, farmers knew how to crossbreed plants to obtain more desirable traits. Even today, a farmer who knows nothing about genetics can tell you that when a blue type of corn crosses with a yellow one, the offspring are blue. However, the farmer might add, if you cross a corn plant with small ears with a large-eared one, the offspring will have ears that are intermediate in size. Without any knowledge of genetics, the farmer has just told you a great deal about how the genes for blue color and for ear size work.

Gregor Mendel, an Austrian monk often described as the “father of genetics,” worked with pea plants in the 1860s to understand how traits are passed from one generation to the next. Mendel made his discoveries by making crosses between true-breeding pea plant populations with different characteristics and keeping careful track of the characteristics of their offspring. Sometimes, when he transferred pollen from one tall plant to another tall plant (like in the cross shown in the F1 generation of Figure 1.1), some of the offspring were tall but some also were short. Where was this shortness coming from, if not from the parental populations?

“It skips a generation”—the shortness was coming from the grand-parental populations. Shortness, the recessive trait, was masked by the tall dominant traitin the “hybrid” or F1 generation. In essence, the shortness was hidden because of sexual recombination. Each offspring receives one copy of a gene from its mother and one from its father.

In this way, gene combinations are shuffled with every generation and new types may appear. Many of the early discoveries in genetics occurred in plants. Plants have a few special characteristics that make them ideal for studying genetics. From one known cross, many genetically similar “siblings” are produced. Pea pods, like the ones Mendel worked with, produce about five peas, and a cucumber has hundreds of seeds. Furthermore, some plants (but not all) have the remarkable capability of being able to fertilize their own flowers. This means that the same plant can be both the male and female parent of a seed. Therefore, scientists can easily and naturally create whole populations of genetically identical individuals.

The cross in Figure 1.1 resulted from two true-breeding individuals. The F1 generation would have contained 5–10 seeds that were genetically identical to one another for the alleles that determine height (all had the Tt alleles). To make the F2 generation, Mendel had two options: He could self-pollinate the plants, or he could cross two different individuals of the F1 generation. Regardless of which method he used, in the F2 generation, the individuals would not all be genetically identical!