GeneticsChapterI | salvatellastud

Genetics Chapter One

Whether you are creating a horse, or deciding to put two horses together for a foal, the genetic factors take an important role. Genetics can be exciting as they explain why two black horses can produce a chestnut foal, or why a seemingly solid colored mare seems to only produce frame overo offspring. Genetics, or more specifically the genotype and phenotype can be infuriatingly confusing, as they are extremely complex as well as diverse. We are here to help break it down to the basics and help you up the realism in your Sims stables.

For the sake of simplicity, and because this is a game, I’m only going to be covering the basics of understanding and/or writing a genotype. Additionally, as this is all just in play, I won’t be covering incomplete dominance of genes or any of the complexities of the many ways characteristics or traits can be passed to offspring, or be expressed.

An Introduction

So What is a Genotype?

The genotype is essentially the genetic code of any particular traits that an organism (ie. A horse) has. A horse can have a genetic code for flaxen hair, or the silver gene but not express it. This is what we call carriers. For example, the agouti (the bay gene) uses the letter A, while the extension, or black/red factor gene uses the letter E. The combination of these two genes can be written as EE/aa, Ee/aa, EE/AA, EE/Aa, Ee/AA, Ee/Aa, ee/AA, ee/Aa, or ee/aa. We will get more into what each of these mean, and how they affect each other later.

Dominant vs. Recessive

To simplify, Dominant genes are going to express themselves over any present Recessive genes. They are written with a capital letter. Recessive genes, are therefore, written with a lower case letter. This is not to say that Dominant genes are more common, however.

Some genes are only expressed when there are two recessive copies. The most common example of this is probably the extension gene, or red/black factor gene. A horse will only be chestnut if it carries (ee) in its genotype.

Homozygous vs. Heterozygous

Homozygous is basically any two copies of an either dominant, or recessive allele. Heterozygous is any one dominant copy and one recessive copy of an allele. For example, you might have heard the term “homozygous black”. This means that a horse’s genotype would be EE/aa. In this case, homozygous refers to the dominant pair of alleles, as a recessive homozygous pair (ee) would result in a chestnut horse.

Base Colours

All coat colors are derived from only two base pigments: red or black. The extension gene, or red/black factor gene, controls this base pigment. It is represented with the letter ‘E’, for extension. All colors we see, from white to roan begin with this allele, regardless of their physical appearance. There are, however, many dilutions and modifiers that affect this base pigment.

Black

The black variation of the extension gene is dominant. This means that it will affect the coat color in both the homozygous (EE) and heterozygous (Ee) variations. A homozygous black horse (EE) will only be able to give a dominant copy of the black gene, resulting in only black or bay foals. A heterozygous horse (Ee) will pass either black (E) or red (e) to its offspring. This explains how two black horses are able to produce chestnut foals.

Since each foal receives one copy of the gene from its parents, two black horses have a 25% chance of producing chestnut offspring. Remember though, this is only possible if both parents are heterozygous black (Ee).

Coats with black base: Smoky Black, Smoky Cream, Pearl Cream Pearl, Grulla, Classic Champagne, Silver Dapple, and Blue Roan.

Chestnut

The red variation of the extension gene is recessive. This means it is only expressed if a horse carriers two copies of the recessive extension gene (e). When using a genotype calculator, the recessive expression of the extension gene is sometimes referred to as the red factor because of this. Interestingly, the red factor only affects coat color. A chestnut horse will still have black or dark grey skin.

Chestnut horses come in a variety of shades: light chestnut, liver chestnut, red chestnut, and standard. The cause of these variations is not known, but it is theorized that it is caused by the Agouti gene, and the Agouti gene’s modifiers. We will cover this in a later chapter.

Coats with chestnut/red base: Palomino, Cremello, Apricot, Red Dun, Gold Champagne, Flaxen, and Strawberry/Red Roan.

Bay (Agouti)

The bay gene, or more commonly known as the Agouti gene is dominant. This means that it will express itself in both the homozygous (AA) and heterozygous (Aa) variations. The Agouti gene controls the distribution of black pigment. It can be uniform, or simply distributed to the “points” on a horse. For those who don’t know, the “points” refers to the ear tips or rims, lower legs, mane and tail.

The Agouti gene actually has its own specific modifiers, but we will go over those in a later chapter. For now, we are just covering the basics.

Note: A chestnut with either “AA” or “Aa” can produce a bay foal if bred to another carrier or to a bay horse.

Coats with bay base: Buckskin, Perlino, Classic Dun, Brown Dun, Amber Champagne, Silver Dapple Bay, and Bay Roan.

Going back to the genotypes we had written earlier (EE/aa, Ee/aa, EE/AA, EE/Aa, Ee/AA, Ee/Aa, ee/AA, ee/Aa, or ee/aa.), can you name all the coat colors each genotype is for?

Note: All horses will have copies of these two genes: the extension gene (E) and the Agouti gene (A).

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