Genetics Chapter 3: Alleles

Previous - Chapter 2: Genetic variation

In the last chapter, we learned that there can be different variations of a gene, called alleles. These alleles contain information for the same "dish", with a different recipe. But if a horse can have two different recipes for a certain dish in its DNA, what does the final dish look like?

Combinations of alleles

Since horses have two copies of each gene (one from the sire and one from the dam), they can inherit either two identical alleles or two different ones. When both alleles (or recipes) are the same, we call this homozygous. When the two alleles differ, we call it heterozygous.

When a horse is heterozygous, the allele that will be expressed (= show up in the horse's traits) depends on whether one allele is dominant over the other or if both share influence.

Dominance

Not all alleles have equal influence. Different types of interactions between alleles determine how traits are expressed.

In many cases, one allele can “mask” the effect of another.

• A dominant allele will be expressed even if only one copy is present.
• A recessive allele is only expressed when two copies are present, one from each parent. 

Imagine we have two recipes for a cake: one of which is correct and one of which misses sugar. The sugar-containing recipe will still produce plenty of delicious cake, which will make up for the cake that has to be thrown away. Only when we have two sugar-missing recipes would we end up without any cake. In this example, the sugar recipe would be dominant, and the non-sugar recipe recessive.

Other types of interactions also exist.

• Incomplete dominance happens when the heterozygous phenotype is between the two homozygous phenotypes.

Imagine we also have two recipes for the icing on the cake: one recipe that uses red food dye, and another that calls for blue dye. The result might be a purple-coloured icing, a mix between the two. The red and blue icing recipes are incomplete dominant.

• In co-dominance, both alleles are expressed equally.

Once again, imagine we have two cake recipes: one for a vanilla cake, one for a chocolate cake. This time, the result is a marble cake showing both vanilla and chocolate layers in equal amounts. The vanilla and chocolate cake recipes would be co-dominant.

Carriers and hidden alleles

Just because a hose doesn't show a certain trait, that doesn't mean it can't carry it. Recessive alleles, for example, can stay hidden for many generations until the right combination appears. These horses are called carriers: they hold the genetic information for a trait without visibly expressing it.

Writing down genes and alleles

To write down a horse's genotype clearly and quickly, we use a letter-based system.

Dominant alleles are represented with capital letters (e.g. "E"), recessive alleles with lowercase letters ("e"). When writing a horse’s genetic combination for a specific gene, we write the two alleles together.

• E/E = homozygous dominant
• E/e = heterozygous
• e/e = homozygous recessive

You might also see a letter used to indicate the gene, and a superscript to specify the allele (E^E/ E^e instead of E/e).

These letters are usually an abbreviation of the gene name or the trait they cause (such as in the example above, the extension gene).

Some genes can have more than two alleles. In this case, you'll usually see "n" used to indicate the "normal" (wild type) allele (e.g. CR/prl, CR/n, prl/n).