Inheritance and DNA
What is a genotype?
A horse always carries two copies of each gene, one that is inherited from the sire and one that is inherited from the dam. Because of genetic changes (mutations) most genes show some genetic variation. When we refer to the “genotype” then we describe how the two gene copies look like (i.e. inherited from each parent respectively).
The tables below show all the combinations that exist for a mare and a stallion and the probability of each genotype in the offspring.
Table for inheritance of SynchroGait®
The genetic variation that we examine using the SynchroGait® test is a C or an A in the certain position within the gene DMRT3. A horse can thus have the genotype AA, CA or CC.
AA x AA = 100% of the offspring will be AA
CA x AA = 50% will be AA and 50% will be CA
CA x CA = 25% will be AA, 50% will be CA, 25% will be CC
CA x CC = 50% will be CC and 50% will be CA
AA x CC = 100% will be CA
CC x CC = 100% will be CC
Table for inheritance of Skeletal Atavism TestTM
The genetic variation that we examine using the Skeletal Atavism TestTM test is that a part of the DNA is missing in some horses and this “deletion” affects at least two different genes. A horse can have the genotype N/N, N/Del or Del/Del. There are two different deletions partly overlapping the same genetic region. These are called Del1 and Del2 respectively.
N/N x N/N = 100% of the offspring will be N/N
N/Del x N/N = 50% will be N/N and 50% will be N/Del
N/Del x N/Del = 25% will be N/N, 50% will be N/Del and 25 % will be Del/Del
N/Del x Del/Del = 50% will be N/Del and 50% will be Del/Del
N/N x Del/Del = 100% of the offspring will be N/Del
Del/Del x Del/Del = 100% of the offspring will be Del/Del
Different kinds of inheritance – genes and environment
- Are caused by just one gene and the environment has no, or very minor effect.
- There is (usually) a simple correlation between genetics and the trait.
- There are different kinds of inheritance patterns within monogenic traits for example recessive and dominant (read more here: (Wikipedia link)).
- It is much easier for researchers to find the genetic cause of monogenic disorders than complex traits and a range of genetic tests in many different species have been developed. If used to its full potential they can be very valuable in breeding and eradicate disease-causing variants quickly.
Example of monogenic diseases: Skeletal atavism, MCOA syndrome and SCID
Example of monogenic traits: coat colours such as greying with age, chestnut and dun
- Depends on the combined effect of many genes together with environmental factors such as feeding and training.
- A horse may carry in principle all “risk” gene variants but still stay healthy, or just a few and develop the disease.
- It is often difficult for researchers to pinpoint these risk genes because they usually have such a small effect each. The value of a genetic test for these kinds of trait depends on how large effect the identified gene variant actually have.
Examples of complex diseases: summer eczema, subluxation of the patella and laminitis.
Examples of complex traits: conformation, performance and temperament
Skeletal Atavism TestTM
The defect Skeletal atavism has a monogenic (recessive) inheritance.
The identified gene variant has a large effect on the pattern of limb movement in horses and the effect of the gene variant varies depending on the gait and breed under investigation.
When the capacity for flying pace in Icelandic horses is studied the inheritance is getting close to monogenic recessive. However, there is a number of AA Icelandic horses that are not reported to perform flying pace. It still unknown how much of this that depend on other genes and how much it depend on lack of training.
If the competition result of horses used for harness racing (in trot) is investigated then the DMRT3 gene is important for performance and good training is also of major importance. Performance in harness racing is a classical example of a complex trait and the gene tested for by the SynchroGait® test is one of several important factors.
What is DNA?
The horse’s genome has 32 pairs of chromosomes. A chromosome is a very long DNA molecule that is shaped like a ladder, or more accurately like a ‘spiral’ ladder. The sides of this ladder represent the structure and do not show very much variation. But there are four different kinds of ‘rungs’ on this ladder, which are molecules that are called ‘bases’: A, C, G or T. The sequence or order of these bases contain the genetic information. Each rung on the ladder actually contains two bases that are in pairs, but we only look at one side at a time. If you know the sequence of the one side, you can predict exactly what the sequence of the other one will be.
The positions on the genome that code for proteins are called genes. These proteins are the building material for the body’s cells and are also the actual machinery that builds up and maintains cells and all tissue. Examples of proteins include different kinds of hormones, muscle proteins, enzymes that break down food and extract energy and haemoglobin that transports oxygen. A total of 21 different small molecules (known as ‘amino acids’) are needed to build a protein. They are put together, almost like pearls on a pearl necklace, that are then folded into a 3D structure, making the final protein. A protein can contain several thousand amino acids. The blueprint for this, which dictates the order of the amino acids, is therefore the genes. As there are more amino acids (21) than bases (4), the bases are read three at a time. For example the three bases CAC mean that the amino acid histidine has to be added. If this is followed by AGA, the amino acid arginine is added to the protein, etc. The final triplet is always TAG, TAA or TGA. This means STOP and shows that the protein is complete.
Almost 2% of the horse’s genome are genes that code for proteins. There is a total of just over 20,000 genes. The horse’s genome has around 2,700,000,000 bases (‘rungs’) divided between 32 pairs of chromosomes. A horse has a total of 64 chromosomes (31 pairs + 2 sex chromosomes XX or XY). The reason why they are called ‘pairs’ is that horses, just like human beings, always have two virtually identical chromosomes of each kind: one they inherit from their mother and one from their father. They code for the same genes and are on average 99.9% identical. This is why the few differences that do exist are so interesting and these are the differences you want to identify when genotyping your horse.