Dominating issues

Consider the snapdragons. Snapdragons are diploid, just like you are, though they make ovules and pollen rather than eggs or sperm. At a particular place in their genome is a locus that's responsible for flower color. Two possible alleles appear at this locus; call these alleles W and R. (Alleles are often referred to by letters.) Plants with two copies of the W allele are white; plants with two copies of the R allele are red. How about plants that have one copy of each allele? In the case of snapdragons, the plants with one W allele and one R allele are pink, which seems to make sense.

But it doesn't always work that way. Sometimes one of the alleles is dominant, explained in the earlier section "Dominant, recessive, or passive aggressive." If a plant has only one copy of that allele, that allele determines the organism's characteristics. In the pea plants that the famous geneticist Gregor Mendel worked with, there are two possible alleles at the locus responsible for flower color: one that codes for purple flowers (P) and one that codes for white flowers (w). In Mendel's peas, the purple allele is dominant (hence the capital rather than lowercase letter). If a plant has two alleles that code for purple, then it has purple flowers. If it has two alleles that code for white, then it has white flowers. But because the purple allele is dominant, a plant with one purple and one white allele turns out just as purple as a plant with two purple alleles.

In this case, knowing the color of the organism doesn't give you perfect information about the underlying genetics. If the pea flour is white, you know that it has two of white alleles (ww). But if the pea flour is purple, it may have two purple alleles (PP) or one purple allele and one white allele (Pw). This situation leads to the topic of genotype and phenotype, which very conveniently comes next.

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