To understand the importance of DNA testing it is essential to have some basic knowledge of genetics. All hereditary information about exterior and functioning of humans or animals is captured on the DNA. DNA is a structure of double strands, also called chromosomes. A horse has 32 pairs of chromosomes. Within animal species by far the biggest part of the DNA looks the same, with only minimal differences . Still, these relatively small differences in DNA form the basis for similarities and variation of characteristics between animal species, between breeds and between individuals of the same breed.
Egg Cell and Sperm
Every egg cell and sperm contains single chromosomes and with that represents half of the parent’s hereditary blueprint. It is sheer coincidence which chromosomes of the different pairs end up in a sperm or egg cell. During conception egg cell and sperm melt together. The matching bits of information on the DNA know how to find each other and this way new strands of chromosome pairs are created in which each pair contains one half from the father and one half from the mother.
In all body cells specific bits of DNA on a chromosome are transcribed into the production of special proteins that direct body functions in the animal or define exterior characteristics. Those bits of DNA that are transcribed into proteins with a specific function, are called genes.
Because the shape of the produced proteins is determined by the gene that the individual animal carries, animals with different hereditary blueprints (different genes) can be different from each other. When genes have alternative forms these are called alleles.
Mutations
When a small mutation occurs during the duplication of a gene this can cause a change in how its related functional protein operates. Such a change can sometimes result in an undesirable, anomalous body function or characteristic (hereditary defect). For some hereditary defects it has been established that they are caused by one mutation. This is, for example, the case with the defect hydrocephaly. Fortunately however, Nature is so clever that when a horse inherits the wrong gene from one parent and the healthy gene from the other, the wrong gene is eliminated. But the unfortunate side effect is that this wrong gene can still be passed on to its offspring by the affected horse! There is a 50% chance that an unaffected carrier of a gene which contains one wrong allele passes this wrong allele on to the next generation.
Gene Test
DNA tests have been developed to determine whether or not a horse is a carrier of the dwarfism- or hydrocephaly gene. The DNA test for hydrocephaly is a so-called marker test with a 100% reliability rate. To establish whether or not a horse is a carrier of the dwarfism gene a marker was applied which allowed a narrow window of false readings. In the intervening time a gene test has become available for dwarfism which also has a 100% reliability rate.
If a mare or stallion is a carrier of the gene for hydrocephaly or dwarfism it does not automatically follow that these animals will produce offspring with an anomaly. This is only the case when both dam and sire are carriers of the same gene and when this gene is passed on to the offspring by both parents. By pairing two carriers you have a 25% risk to end up with a foal with either dwarfism or hydrocephaly. A risky match therefore is defined as a pairing between a stallion and mare who are both carrier of the same defect. If only one of the parents is a carrier this will never lead to dwarfism or hydrocephalus in the foal. So risky matches can easily be avoided when all horses used in breeding are tested for carrier status with the help of the available DNA tests.
Testing of Horses
Applications for DNA tests for dwarfism or hydrocephaly can be sent in via the KFPS website. This can be done under ‘My Horse’, category Member Services: ‘Application DNA Tests’. Click on the name of the horse you wish to enter for DNA testing. Next the application form will open. The test requires a hair sample of the horse in question. If at some point in the past the KFPS received a hair sample of the mare to be tested then click the option ‘Use DNA sample from the KFPS data bank’. If this is not the case you are requested to send a hair sample to the Studbook office with about ten hairs from the mane. Costs for DNA testing are € 49.15 per horse. For a surcharge of € 20.25 a paternity test can be carried out as well.
The KFPS has contracted VHL Genetics in Wageningen to carry out the DNA tests. Testing for the chestnut factor is carried out at the same time. As soon as the results have come in the applicant will receive a letter containing the Analysis Certificate which lists all test results as well as a complementary explanation.
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