Basic Genetics for Horse Breeding
R. Hodgman in the journal of Animal Science stated that most breeders are ignorant of elementary genetics and breed merely by mating the best to the best, hoping for the best.
Phil Bull from Timeform said “if we don’t understand the rudiments of mendelian heredity then we have no hope of breeding better horses.”
Pedigree analysis and genetic inheritance are intrinsically linked and extensively used by geneticists and breeders in the breeding of all animals.
Genes are units of heredity made up of DNA which determine how traits and abilities are passed on and expressed in living organisms.
The horse genome project has shown that up to 80 000 genes can be found on the 64 chromosomes in the horse.
Each horse has 32 sets of chromosomes with 31 known as autosomes, and the sex chromosomes – XX in females, XY in males.
Each cell in the body has 2 complete sets of chromosomes making 64 gene pairs. A horse’s genetic makeup is called its genotype.
Genes are sections of DNA that contribute to certain traits and functions by a process of coding proteins.
ALLELES
Have you ever wondered why the same mating combination between a mare and a stallion does not result in identical foals each time?
A gene is made up of 2 different alleles – one allele is inherited from each parent to form a pair for each gene.
A trait may not be seen in a horse but can still be passed onto the next generation. An allele can be dominant or recessive.
If the alleles are dissimilar – one dominant and one recessive allele – then the horse is said to be heterozygous for that gene.
Two copies of a recessive allele are needed for a recessive gene to be displayed. Recessive alleles are hidden in heterozygous individuals.
You cannot tell by looking at a horse if the second allele is dominant or recessive – dominant alleles mask recessive ones.
When a trait is displayed then for sure there is one dominant allele in the genotype.
Just because a horse shows a trait such as a certain shoulder angle – does not mean it will pass it on. It depends on whether it is homozygous or heterozygous for that trait.
A horse needs double dominant genes to display the trait and have 100% chance of passing it on to the next generation.
The alleles are the primary determinants of the horse’s phenotype (conformation) – from genes at specific locations on the chromosomes.
It is the genotype – what cannot be seen – that is responsible for creating the phenotype – that which can be seen.
The parent’s genes determine what the horse will look like – what the parents actually look like has no bearing on this. This is why breeding ‘type to type’ does not work reliably.
MENDELS LAWS OF INHERITANCE
Gregor Mendel’s work on pea plants in the 18th century was the foundation for the modern study of genetics.
If a horse has a dominant and a recessive allele the dominant allele will always be expressed in the phenotype.
Recessive genes are much more of a problem and there are hundreds of these genes floating around in a horse’s genome.
Undesirable recessive genes won’t do any harm in a heterozygous state – but if they are homozygous they can cause problems.
Most horses inherit and exhibit recessive genes in their phenotype – they can bring about undesirable traits.
These are inherited from parents who are not affected by the particular trait themselves. The similar the parents’ genetics, the more likely recessive genes are to double up.
Similarly for recessive genetic disorders such as WFFS – if two carriers are bred the foal has a 25% chance of being affected, a 50% chance of being a carrier, and a 25% chance of being clear.
The goal is not to stop breeding horses that are carriers, but avoid breeding carrier to carrier.
Many carriers have super performance genes with desirable traits that are valuable for breeding sport horses.
Recessive genes cannot be weakened or diluted, they are either present or not.
Mother nature likes variety, diversity and deviations – these are the drivers of evolution – and they exist largely through the mechanisms of recessive genes.
Mother nature uses a hidden reservoir of recessive genes as the primary tool to encourage modification and evolution.
This contrasts with human goals to mould the phenotype to better allow the horse to excel in sport.
This is why inbreeding has taken place in all livestock breeding including show dogs, cattle, sheep, racehorses and warmbloods – to try to increase the chance of doubling up on desired genes.
The horse is actually one of the least inbred animals of all – especially in hybrids like warmbloods. Some horses are dominant in passing on their characteristics – these are prepotent animals – but they are quite rare.
Mitochondrial DNA is another form of genetic inheritance. Mitochondria are located within the cell but outside the nucleus. There are also organelles within the cell which provide the mitochondria with the energy required for the cells to work the entire machinery of the horse.
MDNA is only passed by mares on the X chromosome, and it is not subject to the mendelian laws of inheritance.
Continue reading about Complex Genetic Traits, MtDNA, Sex Linked Traits, Inheritance, Coefficient of Inbreeding, and Gender Imprinting in the Education Portal (members only).