Of alleles and loci

When most people think about heritable traits, they think about genes. Unfortunately, the term gene is a little too general for a discussion of evolution. Instead, you need to know a little more about how the DNA strand is put together. As I state earlier in this chapter, DNA is simply long strings of nucleotides, abbreviated A, C, T, and G. That's good for starters, but you need to know a bit more.

The word gene is commonly used in a couple of different but related ways:

¡^ To refer to a specific place in an organism's genome: An organism's DNA contains a lot of information, including all the instructions for building and maintaining that particular life form. The different instructions for different genes are located in different places along the DNA sequence. A specific location of a gene in an organism's DNA is called a locus (plural loci, pronounced low-sigh).

¡^ To refer to the exact sequence of the DNA at a specific place (or locus) in the genome: The sequence of nucleotides at a specific locus can differ among organisms. These different sequences are called alleles. Take the locus that stores the information to make the blood type protein. When scientists examine this locus in several individuals, they find variations; some alleles code for type O blood, some for type A blood, and so on. The exact sequence of the ACTG alphabet is different, and this is why people have different blood types.

Using the first, you might refer to a gene for "blood type," but using the second, you'd refer to a gene for "type O blood" — the specific DNA sequence that results in a particular blood type. Both definitions are workable as long as you keep track of which one you're using. In this book, when I talk about genes, I'm referring in most instances to the loci. And when I'm referring to alleles, I let you know.

At any given place (locus) on the DNA strand, the sequence of nucleotides (the alleles) that appears can be the same or different, with each particular sequence representing a different manifestation of a particular trait. At the locus where the eye-color trait resides, for example, you find alleles representing the different colors: blue, green, brown, and so on.

Imagine a single locus with two alleles, A and a, each representing a different manifestation of a particular trait. Because the locus has only two options, you can figure out pretty easily what individual combinations you may find in the population. In this case, some people would have AA (having received A from both parents), Aa (having received A from one parent and a from the other), and aa (having received a from both parents).

Here are a couple of other facts about alleles to keep in mind:

1 As is the case with eye color (or blood type or a number of other traits, for that matter), you can have more than two alleles at a given locus in the population, but any particular individual can have at most only two alleles: one that came from the mother and one that came from the father.

i When two alleles at a particular locus are the same, they're said to be homozygous. When the alleles are different, they're heterozygous. I don't throw these terms in just for kicks. Whether the alleles are different or the same is an important factor in how, when, or even whether the heritable trait manifests itself, as the following sections explain.

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