The Basics of Equine Genetics

Equine genetics touches many of the important health and disease processes in our horses. In this article, we brought together some basic technology of genetics to help you better understand the terminology.

Anatomy of a Cell

Each cell of every living organism (animal or plant) has a center, known as the nucleus. The nucleus contains chromosomes, which carry genetic information about that particular organism. Chromosomes can be seen with a high-powered microscope.

Chromosomes come in twos, called chromosome pairs or diploid chromosomes (diploid means double or two). The genetic makeup of any individual organism contains one chromosome from each of its parents, and this makes up the chromosome pair.

Every kind of organism has a certain number of chromosome pairs. For example, horses have 32 chromosome pairs. Humans have 23.

A chromosome is made up of genes. The genes are arranged like beads on a necklace along the chromosome.

Equine Genetics: The Basics of Genes

Genes are so small that they cannot be seen, even with a microscope. They are tiny bits of information contained in the cells of every living organism that make the organism what it is. Genes are passed from one generation to the next.

Genes are made up of complex molecules of DNA (deoxyribonucleic acid). DNA is a molecule composed of nucleic acids, arranged in two thread-like strands that are twisted together like a coil. This coil-like structure is called a “double helix.” Between the two twisted strands are bonds that hold the DNA together, somewhat like the rungs of a ladder.

Equine Genetics: Passing Chromosomes

Chromosome pairs split during one phase of the reproduction process. When this happens, half of the genetic information of each parent is passed on to the next generation. This process is called meiosis.

Meiosis occurs only in reproductive cells and produces gametes, or sex cells, in preparation for the rest of the reproductive process. The chromosome now has what is called the haploid number of chromosomes—haploid means “half.” The process of meiosis occurs in the reproductive organs (the ovaries of mares and the testes of stallions).

Equine Genetics: When Cells Split

Body cells also split in half; however, body cells are not involved in the reproductive process. When body cells split in half—this process is called mitosis—they produce identical copies of the original cell.

These copies contain the diploid number of chromosomes.

Different equine species have different numbers of chromosomes:

NOTE: The different ass/donkey species and zebra species each have their own numbers of chromosomes, which can vary greatly from one species to the next.

Ass/donkey—from 50 to 56 chromosomes. (Asiatic wild asses have been reported to have 50-63 chromosomes).
Zebra—from 32 to 46 chromosomes.

A chromosome example:

The horse has 64 diploid chromosomes or 32 chromosome pairs (haploid number).
Each of the 64 chromosomes of the horse contains different genetic information.
Each body cell in the horse contains 64 chromosomes.
A sex cell that splits in half has only 32 chromosomes.

Modern genome research has developed ways to decode and take an “inside look” at genes on the molecular level. They call this whole genome sequencing. The first horse was sequenced in 2006.

By understanding the complicated science of genetics, we can make predictions about the genetic makeup of the next generation, based on genetic information about the mare and stallion being mated.

The process of passing genetic material to the next generation is known as inheritance. Having a good understanding of inheritance traits is very important to the future of the horse industry. It gives breeders a basis of information to consider when deciding whether to breed a particular mare to a particular stallion.

Genetic selection is crucial in preserving breed purity and strongly influences the offspring. Some heritable traits include performance, conformation, gaits, coat color, white-spotting patterns, predisposition for disease, potential genetic problems, and more.

Parentage testing is now readily available and has become a common practice in many breed associations.

Genetic Terms

Following is a list of terms that might come in handy as you are learning about genetics.

Locus: The position or location of a gene on a chromosome (singular = locus; plural = loci).
Alleles: Genes that are in a chromosome pair and responsible for a particular genetic trait; alleles are not identical but are at the same location or locus. alleles are alternative forms of genes.
Dominant: “Simple genetic inheritance” works by allele pairs; if a gene is “dominant,” it has the ability to mask the genetic makeup of the other gene in the allele pair. When present, dominant genes have an effect.
Ee: The two alleles are not the same, and the individual is heterozygous for that locus.
Recessive: “Simple genetic inheritance” works by allele pairs; if one gene cannot mask the genetic makeup of the other gene in the allele pair, it is “recessive.” Recessive alleles have an effect only in the absence of dominant alleles.
ee: Neither “e” is dominant over the other “e”; both are “recessive.”
Homozygous: Refers to an organism whose chromosomes carry two identical members of a given allele pair.
EE or ee: The two genes are the same, or are “homozygous.”
Heterozygous: Refers to an organism whose chromosomes do not carry identical members of a given allele pair.
Ee: The two genes are not the same, or are “heterozygous.”
Polygenic traits: Genetic traits that are controlled by multiple genes.
Phenotype: What an individual “looks like,” based on the genetic interaction of the alleles present.
Genotype: What the genetic makeup of an individual really is; what alleles are actually present. The genotype identifies the alleles that a horse might pass to its offspring. For example, breeders are interested in knowing if their Tobiano stallion is homozygous or heterozygous for Tobiano. In the first case, 50% of the offspring inherit the trait while in heterozygous, 100% inherit an allele for Tobiano.

Equine Genetics: Sex Inheritance

The sex of a foal is determined by the sire not the dam. No other genetic trait currently known is determined solely by the sire. The pair of chromosomes carrying the genes that determine the sex of the offspring are called sex chromosomes.

In the mare, these chromosomes are identical; she carries two X chromosomes. In the stallion, they are not identical; he carries an X and a Y chromosome in the chromosome pair.

Female or Mare = XX
Male or Stallion = XY

So when the chromosome pairs split during the process of meiosis (forming two gametes) in preparation for the reproductive process:

A mare would produce: X X
A stallion would produce: X Y

Because the process of sex inheritance is a “simple genetic inheritance” trait, a Punnett square can be used to determine the expected outcome of the mating. Geneticists use the Punnett square to determine expected outcomes and can determine the percentage of expected offspring that should show a particular trait. The Punnett square can only be used when simple inheritance patterns apply.