How do Mendel’s theories explain the proportions of traits seen in the F₁ and F₂ generations of his monohybrid crosses? Mendel’s first law—the law of segregation—states that when any individual produces gametes, the two copies of a gene separate, so that each gamete receives only one copy. Thus gametes from a parent with the RR genotype will all carry the R allele; gametes from an rr parent will all carry the r allele. Half of the gametes in an Rr individual will carry the R allele and the other half will carry r. What genotypes are produced when these gametes fuse to form the next generation? The allele combinations that will result from a cross can be predicted using a Punnett square. This device ensures that you consider all possible combinations of gametes when calculating expected genotype frequencies of the resulting offspring. A Punnett square looks like this:

It is a simple grid with all possible male gamete (haploid sperm) genotypes shown along the top and all possible female gamete (haploid egg) genotypes along the left side. The grid is completed by filling in each square with the diploid genotype that can be generated from each combination of gametes.

Let’s use a Punnett square to derive the progeny of an RR × rr cross (Figure 12.1). To determine the genotypes in the F₁ generation, into each square we put an R from the male gamete (a sperm cell from the pollen tube) and r from the female gamete (the egg cell). All the F₁ offspring of this cross (the F₁ generation) have the Rr genotype, which produces seeds with a round phenotype. What happens when the F₁ generation are crossed among themselves to produce the F₂ generation? Once the Punnett square is filled in, we readily see that there are four possible combinations of alleles in the F₂ generation: RR, Rr, rR, and rr (see Figure 12.1). Since R is dominant, there are three ways to get round seeds in the F₂ generation (genotype RR, Rr, or rR), but only one way to get wrinkled seeds (genotype rr). Therefore we predict a 3:1 ratio of the round and wrinkled phenotypes in the F₂ generation, remarkably close to the ratios Mendel found experimentally for all the traits he compared (see Table 12.1).

Mendel did not live to see his theories placed on a sound physical footing with the discoveries of chromosomes and DNA. Genes are now known to be sequences of DNA found on the much longer DNA molecules that make up chromosomes. With your knowledge of *meiosis, you can envision the different alleles of a gene segregating as chromosomes separate during meiosis I (Focus: Key Figure 12.2).

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We know now that genes determine phenotypes mostly by producing proteins with particular functions, such as enzymes. In many cases a dominant gene is expressed (transcribed and translated) to produce a functional protein, while a recessive gene is mutated so that it is no longer expressed, or it encodes a mutant protein that is nonfunctional. For example, the wrinkled seed phenotype of rr peas is caused by the absence of an enzyme called starch branching enzyme 1 (SBE1), which is essential for starch synthesis. With less starch, the developing seed has more sucrose and this causes an inflow of water by osmosis. When the seed matures and dries out, the water is lost, leaving a shrunken seed. A single copy of the R allele produces enough functional SBE1 to prevent the wrinkled phenotype, which accounts for the dominance of R over r.