Genetics Basics

I am going to make this as easy as possible for anyone to understand, even if you do not have a scientific understanding of genetics. Animals get two genes for all their inherited traits, one from their mother, one from their father. There are many, many, genes involved in controlling colors. So while reading this remember these are only two sets of genes that a horse would have that help determine color.

To help explain how genetics work I will reference one of the best known genes, the XY genes. These are the ones that determine gender in all animals. Females are XX, Males are XY. When reproduction takes place, each parent gives one of each of its’ gene pairs. Females can only give the X gene, males can give the X or the Y gene, it is a 50/50 chance which they give, therefore a 50/50 chance if the baby is going to be male or female. The mothers unfertilized egg would carry only one X gene for gender, the males sperm would have either one X or Y, when they unite the egg now has two genes for that pairing and the gender is determined by which pairing it was. Eggs and sperm also have each of every other gene sequence, for now we will talk about the roan and the grey gene, these are two separate genes.

Roan

This is a roan horse, he was born this color and, other than slight seasonal changes, his color will remain the same for his lifetime. Roans have a mixture of white and colored hairs on their body, with fewer white hairs on their head and legs.  Roan horses always carry the gene combination of Rr. Roan horses can give either the R or r gene to their foals. There are no rr horses, because such a combination of genes is letal. All non-roan horses are RR. So following what we know about how genes work, we know each parent can only give one its’ genes (for each trait) to its young.  This is my photo. 

This chart demonstrates what would happen if you bred two Roan horses together. How these charts work is one parent, it does not matter which, is represented by the gene pair across the top. The other is represented down the side. Since each is Rr, there is one R on the top, one r on the top (to represent one parent) one R on the side, one r on the side (for the other parent). The letters are carried down or across to fill in the chart. The four squares represent the possible combinations. One in four will be RR, non-roan. Two in four will be Rr, roan. One if four will be rr, since this is a lethal combination, one in four matings of this pair would not result in a live foal. If you were to make another chart for breeding a roan to a non-roan, you would see the changes for roan and non-roan are equal. Non roan bred together will never produce a roan.

Grey

This is a grey horse, she was born a solid color, like her foal, however over the years white hairs have filled in and she has “turned grey”. Her photo is from Wikimedia. Grey horses are always born colored, and turn grey over time, some faster than others. Grey is a dominant gene, meaning if a horse even only has one grey gene it will be grey. Therefore a grey horse could be GG or Gg. If you can look at its’ pedigree you might be able to determine what it is. If one parent is grey and one is not, then the horse is Gg.  If it produces a foal that does not turn grey, you know that it is Gg, as otherwise it could only produce grey foals if it were GG. A non-grey horse is always gg.

This chart demonstrates what would happen if you bred a grey horse who is Gg with a non-grey horse. The grey horse is represented on the top as Gg, the non-grey is on the side. The letters are carried forward into the boxes, and we see the resulting combination is that half the time this pairing will result in a grey, Gg foal, and half will result in a non-grey, gg, foal. If you do the same with breeding a horse who has two grey genes, GG to a horse that is non grey, gg, you will notice you will always get a grey offspring, Gg. Two non-greys can never produce a grey when bred together.

Note:

The color grey in donkey’s is different, they do not change over time, a grey donkey is born a solid tone of grey and remains that way for his or her life. Their genetics are entirely different.  This is my photo. 

Click here to read about the genetics of Cremello, Palomino, and Chestnut.