An update of chromosomal abnormalities in mares.

The research paper discusses the detection of chromosomal abnormality in 98 of 180 mares that showed signs of gonadal dysgenesis. The most common abnormalities were X monosomy and male horse-like karyotype. Moreover, two mares displayed a new X chromosome structural abnormality which had not been previously observed. One of these mares managed to give birth to a filly who inherited the same chromosomal configuration, despite instances of fertility being rare in mares with gonadal dysgenesis and chromosomal abnormality.

Chromosomal Abnormalities in Mares

• The study analyzed 180 mares aged three and above with gonadal dysgenesis, a condition characterized by the underdevelopment of the gonads. In 98 of these mares, chromosomal abnormalities were observed.
• The most common abnormality found was X monosomy (63,X), which is a condition in which a mare is missing an X chromosome. In humans, X monosomy results in Turner syndrome but the effects on horses are not clearly defined.
• The second most frequent abnormality was a karyotype, or chromosomal arrangement, resembling that of a male horse (64,XY), which is unusual for a female horse.

New Observed Abnormalities

• Two of the studied mares had a structural abnormality in one of their X chromosomes [64,X,del(Xp)]. This is a new observation that has not been documented in equine chromosome studies before.
• One of these two mares, despite the chromosomal abnormality and gonadal dysgenesis, managed to give birth to a filly that had the same chromosomal arrangement. Parentage was verified through blood typing. This is rare because mares with these conditions usually face fertility issues.

Yearling Fillies with Chromosomal Abnormalities

• The researchers also examined four yearling fillies solely for their small size, poor physical conformation, and lack of vitality. Chromosomal abnormalities were found in each of these fillies.
• The abnormalities included X monosomy, a mix of X monosomy and normal female karyotype, the abnormal X chromosome, and an additional autosomal trisomy which presents a second new abnormal karyotype for horses.