7.4: Segregation: you’ve got two copies of each gene but put only one copy in each sperm or egg.

One odd and recurring result spurred Mendel to figure out the mechanism by which traits could be passed from parent to offspring. Just as brown-eyed parents can have blue-eyed children, sometimes traits that weren’t present in either parent pea plant would show up in their offspring. When plants with purple flowers were fertilized by pollen from other plants with purple flowers, they produced mostly purple-flowered offspring, but sometimes they produced plants with white flowers.

How was it possible to produce white flowers from a purple cross? Where did the whiteness come from? Here’s where Mendel’s meticulous and methodical experiments paid off. Let’s follow his process. First, he started with some true-breeding white-flowered plants. Then he got some true-breeding purple-flowered plants. He wondered: which color wins out when a white-flowered plant is crossed with a purple-flowered plant? The answer was definitive: purple wins (FIGURE 7-9).

Figure 7.9: White or purple? By careful and repeated crosses among pea plants, Mendel determined that there were “dominant” and “recessive” traits.

All of the offspring from these crosses were purple, every time. For this reason, Mendel called the purple-flower trait dominant, and he considered the white-flower trait to be the recessive trait. In general, a dominant trait masks the effect of a recessive trait when an individual carries both the dominant and the recessive versions of the instructions for the trait.

Things got a bit more interesting when Mendel took the purple-flowered plants that came from the cross between true-breeding purple- and white-flowered plants and bred these purple offspring with each other. He found that these mixed-parentage plants were no longer true-breeding. Occasionally, they would produce white-flowered offspring. (To be exact, of the 929 offspring plants Mendel examined, 705 had purple flowers, and 224 had white flowers.) Apparently, the directions for building white flowers—last seen in one of their grandparents—were still lurking inside their purple-flowered parent plants. The existence of traits that could disappear for a generation and then show up again was quite perplexing.

Mendel devised a simple hypothesis to explain this pattern of inheritance. It incorporated three ideas that helped him (and now help us) make predictions about crosses (FIGURE 7-10).

Figure 7.10: Segregation of alleles in meiosis. Organisms have two copies of each gene but place only one copy into each gamete during the process of meiosis (as described in Chapter 6).

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“There were no questions.”

—Entry in meeting notes following Mendel’s first public presentation of his ideas on how heredity works. (Tragically, no one in the audience of scientists had any idea what he was talking about. Not until about 40 years later did the world understand his discoveries.)

When an individual reproduces, it contributes just one of its two copies of a gene to its offspring. The other parent will contribute the other allele. So, when sperm and eggs are made, each sex cell gets only one copy of a gene—as opposed to the two copies present in every other cell in the body. For a male who is heterozygous for a particular gene, for example, it means that half of the sperm he produces will have one of the alleles and half will have the other. The idea that, of the two copies of each gene everyone carries, only one of the two alleles gets put into each gamete is so significant and important that it is called Mendel’s law of segregation.

TAKE-HOME MESSAGE 7.4

Each parent puts a single set of instructions for building a particular trait into every sperm or egg he or she makes. This instruction set is called a gene. The trait observed in an individual depends on the two copies (alleles) of the gene it inherits from its parents.

How did Gregor Mendel explain that some purple-flowered pea plants could produce white-flowered offspring?

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