Quick Summary
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Phenotype: In male horses, external genitalia fails to develop and is coupled with associated sterility.
Mode of Inheritance: X-linked recessive
Alleles: N = Normal; AR1, AR2, AR3, AR4, AR5 = Androgen receptor variants
Breeds appropriate for testing: Quarter Horse (AR1), Tennessee Walking Horse (AR4), Thoroughbred (AR2 and AR5), Warmbloods (AR3)
Explanation of results:
Males only have one X chromosome whereas females have two, therefore possible genotypes will differ by sex.
- Female horses with the N/N genotype do not have any of the genetic variants that result in Androgen Insensitivity Syndrome and cannot transmit these AIS-associated variants to any of their offspring.
- Female horses with the N/AR(1-5) genotype carry one copy of a genetic variant that results in Androgen Insensitivity Syndrome. They are predicted to transmit the variant to 50% of their offspring. All male offspring inheriting the affected allele will be sterile and will appear phenotypically female.
- Female horses cannot be homozygous for the AR variants.
- Males horses with the N genotype do not have any of the genetic variants that result in Androgen Insensitivity Syndrome.
- Male horses with the AR1, 2, 3, 4 or 5 genotype have one copy of a genetic variant that results in Androgen Insensitivity Syndrome. Since males only have one copy of the gene, they will be sterile and will appear phenotypically female.
Sample Collection
Horse DNA tests are carried out using cells from the roots of a hair sample (roughly 20-30 hairs).
1. Grab about 10 hairs at the base.
2. Wrap the hairs around your finger and give it a quick pull.
3. Check the ends to make sure the pulled hairs have roots.
4. Repeat the process until you have collected about 20-30 hairs with intact roots.
5. You can choose different places on the mane or tail. NOTE: For foals, we recommend pulling all hairs from the tail only.
6. Tape the hairs to the submission form and fold the form along the dotted line to protect the sample. Do not use ziploc bags as they can cause condensation that allows mold to grow on the hair.
7. Place the folded form containing the sample in a paper envelope and mail it to the laboratory.
Sexual differentiation in mammals results from the presence and temporal expression of specific genes during development. In mammals, females are the homogametic sex, possessing two X chromosomes, while males are heterogametic, possessing one X and one Y chromosome. While most genes controlling sex determination are found on the sex chromosomes, not all genes controlling sexual differentiation are restricted to the X and Y chromosomes.
In development, the presence of genes on the Y chromosome triggers undifferentiated embryonic reproductive tissue to develop as testes. The developing testes subsequently produce hormones to complete male reproductive development. These circulating hormones bind to androgen receptors on the external surfaces of cells, resulting in development of additional essential male reproductive tissues. The androgen receptor gene (AR) is located on the X chromosome. Sequence variants in the androgen receptor gene (AR) may result in an inability of the circulating hormones to bind, thus preventing further induction of male development. Thus, horses with variants in AR are insensitive to the effects of androgen. This androgen insensitivity results in the remaining tissues failing to develop as male specific organs and can result in the presence of external female genitalia.
Five breed-specific variants in the AR gene are known to result in androgen insensitivity in the domestic horse: c.1A>G (AR1): Quarter Horse (Revay, et al 2012), c.2042G>C (AR2): Thoroughbred (Bolzon et al. 2016), c.1630_1654del (AR3): Warmbloods (Welsford et al. 2017), c.183delT (AR4): Tennessee Walking Horse (Villagomez et al. 2020), and c.2132C>T (AR5): Thoroughbred (Villagomez et al. 2020).
Since males, as the heterogametic sex, only have one X chromosome and AR is located on the X chromosome, only males are affected by AR1-5. The presence of any of the known androgen receptor variants will result in androgen insensitivity in a male foal. As females do not have testes-derived circulating hormones, non-functional androgen receptors have no developmental impact on them. However, since males with the variant are sterile, it is not possible for males to pass an affected AR allele to offspring and thus females cannot be homozygous for the AR variants.
The VGL offers a test for all five known variants, allowing for the detection of mares carrying the androgen insensitivity alleles and providing owners the opportunity to avoid producing AIS foals. This test also allows for confirmation of sterile males.
