Simple Dominant & Recessive Alleles
by C. Williams

Simple Dominance is when there are two alleles at one locus... in which one dominates, or masks, the expression of the other. The allele that "covers up" the other, is known as the Dominant allele. The allele that is "covered up" or "hidden", is called a Recessive allele.

Simple Dominant gene action can be described as on/off switches. If a dominant allele is present, then the switch is set to on. If no dominant allele is present, then the switch can be considered off.

Since only one dominant allele needs to be present to turn a switch "on", it can "hide" the fact that the other allele may be a recessive "off switch" . Therefore, a horse can carry a recessive allele, and not show it.

The Grey coat color in horses, is an excellent example of this Simple Dominant gene action.

Let's begin the discussion, by crossing a grey horse, and a non-grey horse.

The grey horse in this example, is Homozygous for the Dominant Grey gene. Both alleles are the dominant allele, known as G. The G alleles turn the "switch" for the grey color to on.

This horse's Genotype can be shown in text as GG.

The Phenotype of this horse... that is, what the horse looks like (the physical "expression" of the grey trait or characteristic) is a grey-colored horse.

GG
Homozygous Grey

The chestnut horse in this example, is Homozygous for the recessive allele g, which would turn the "switch" for the grey color to off.

The Genotype for this horse is gg. Since both alleles are set to off, the horse is not grey.

The Phenotype (or physical appearance) of this horse is not grey since both alleles indicate that grey coloration is turned off.

gg
Homozygous non-grey (recessive)

Now, for our edification and amusement, let's cross these two individuals. The chart shows all four possible combinations of the two alleles that each parent carries... this is what we could expect to see....

F1 Generation Offspring		Sire
		Allele #1 G	Allele #2 G
Dam	Allele #1 g	Gg Grey	Gg Grey
	Allele #2 g	Gg Grey	Gg Grey

Hey! They're all grey!

This is because the Dominant G allele that produces grey color, overrides or covers up the non-grey color (in this case, a chestnut). It only takes one "on" switch to turn the horse grey. You can't tell by looking at the horses, but all of these offspring in the F1 Generation are carrying a hidden non-grey "off" switch.

Since each of the possible offspring carries one G and one g allele, they are known as heterozygotes.

Now, let's see what happens when we cross two of these offspring together . . .

F2 Generation Offspring		Sire
		Allele #1 G	Allele #2 g
Dam	Allele #1 G	GG Grey	Gg Grey
	Allele #2 g	Gg Grey	gg Chestnut

Hey! There's an odd one in the group!

Notice the genotypes (the letter designations, of GG, Gg and gg.) Three of the four offspring are grey, carrying at least one G allele that turns the grey switch to on. However, one foal received two gg "off" switches... and with no G "on" switch to dominate, or override it... the resulting offspring came out non-grey. (In this case, chestnut.)

Based on the chart, we can establish relative "probabilities" to obtain grey color from this type of mating.

• 25% Homozygous Grey (GG)
• 50% Heterozygous Grey (Gg)
• 25% Homozygous Non-Grey (gg)

It should be noted that the grey gene in horses, is considered to be dominant to all other colors. If the horse possesses a dominant G allele at the "G" locus, the horse will be grey. The underlying color doesn't matter, and grey can occur on chestnut, bay, dun, palomino, pinto, appaloosa and other colors. Of course, where there might be white markings on the horse, you won't be able to tell the horse is grey in those places.

Now, just for fun, let's cross a Heterozygous Grey (Gg) to a Homozygous Non-Grey (gg).

Offspring		Sire
		Allele #1 G	Allele #2 g
Dam	Allele #1 g	GG Grey	gg Chestnut
	Allele #2 g	Gg Grey	gg Chestnut

Did you guess that this might happen?

Based on the chart, the following probabilities can be established:

• 50% Heterozygous Grey (Gg)
• 50% Homozygous Non-Grey (gg)

Based on this information, you should be able to easily chart out the probable colors of offspring of any single Simple Dominant gene.

All simple dominant genes express (show) themselves when present. If a horse has a dominant gene for grey color, the horse will be grey. It also follows, that for a foal to be grey, it must have at least one grey parent.

Colors that are the result of simple dominant gene action are easy to select for, or against. By choosing individuals who do not show the physical effects of the dominant allele, you can eliminate that trait from a population.

It should also be kept in mind a few things:

• You can never be absolutely sure you've eliminated a recessive allele from a population. It will only show itself, if two parents that happen to have that hidden recessive are crossed... and even then, there is only a 1 in 4 chance.
  
  
• You can't prove a negative. When it comes to probabilities, the percentages you should obtain are going to refer to rather large averages. These probabilities may not hold true for a much smaller sample (such as the number of foals a mare may have in her lifetime). Just because a horse does not produce a foal that displays a recessive trait, doesn't mean the parent is not carrying the gene!
  
  
• If you cross individuals who are homozygous for a recessive trait (such as non-grey color), you will only get more homozygotes for the recessive trait in the offspring. If there is no dominant allele present to "override" the recessive alleles... then you will have only horses with the recessive alleles.

---

Views expressed herein are those of the writers and compilers of the various information. Reference sources are cited where applicable. Copyrights are the property of the respective authors.