Genetic Alleles

Many times you may hear the word "allele", but not really know what it means. Fortunately, it's actually fairly easy to understand.

An allele is one of two (or sometimes more) variants of a gene, causing different traits to occur.

When there are only two possible allele options in a gene (or locus- a group of genes), it's generally easiest to think of the gene as a switch. If the switch is "off", the trait-producing allele is nonactive, and replaced by the non-trait producing allele. If the switch is "on", the trait-producing allele is either dominating or in homozygous form (sometimes both).

Sometimes there are more than two options for alleles at a specific gene, though. At this point, it's easiest to think of it as a three-(or more)-way switch.

Alleles are either homozygous or heterozygous, and recessive or dominant.

• Homozygous: The horse has two copies of this allele, and will always pass on one of the copies to its offspring.
• Heterozygous: The horse has one copy of the allele, and will pass on the allele to its offspring about 50% of the time. Sometimes you won't be able to tell by looking at the horse if he is heterozygous for a recessive allele.
• Recessive: A recessive allele is one that the horse must be homozygous for in order for the trait to show in the coat color (or phenotype). If a horse only has one copy of a recessive allele, it will not show in the phenotype.
• Dominant: Dominant alleles are really great, because if a horse has just one copy of them, you'll be able to tell. However, dominant alleles can sometimes mask a recessive allele.
• An example of this would be a Bay horse carrying one copy of the Extension allele. Because the Extension (e) allele is recessive, the dominant Bay (A-) allele would take control, and the horse would be bay.

Also, there's one awesome way to tell if an allele is dominant or recessive in genetic shorthand. As an example, genetic shorthand for a Linebacked Palomino would look like this:

E^eE^e/C^Cr/Dn^Dn

First of all, the normal-sized letters are the shorthand abbreviations for the name of the gene. The superscript ones are the abbreviations of the alleles. Now, note how the first letter of some of the alleles are capitalized, and some aren't?

If the first letter of the allele is capitalized, the allele is dominant. If the first letter is in small caps, the allele is recessive.

And, it's that easy. ;) Ask if you have any questions!