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The XY sex-reversal syndrome occurs when a phenotypic mare is born that has the karyotype of a stallion. The syndrome is manifested by both genotypic and phenotypic heterogeneity. The sex-reversed genetic condition occurs frequently within certain pedigrees where XY females have been found and can be readily detected by chromosome karyotyping. The phenotypic spectrum ranges from the feminine mare with a reproductive tract that is within normal limits to the greatly masculinized mare. Pedigree analysis suggests that there are two modes of inheritance: (1) an X-linked recessive or autosomal sex-limited dominant transmitted through the female and (2) an autosomal sex-limited dominant or a Y chromosomal mutation with variable expression transmitted through the male.

An inherited genetic disorder causes XY embryos of the horse to develop as mares. On the basis of our study of 38 such mares, we have identified four grades or classes of XY sex reversal according to this scheme: class I, nearly normal female, of which some are fertile; class II, female with gonadal dysgenesis, normal mullerian development; class III, intersex mare with gonadal dysgenesis, abnormal mullerian development, enlarged clitoris; class IV, virilized intersex characterized by high levels of testosterone. In general, class I and class II mares were typed H-Y antigen-negative whereas class III and class IV mares were typed H-Y antigen-positive.

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Lymhocytes from clinically healthy Holstein animals (8 males and 23 females) were cultured in media containing 5-bromodeoxyuridine (10 microgram/ml) for two cycles of deoxyribonucleic acid replication (48 h). The exchange of sister chromatids per cell varied from 1 to 16 with a mean of 5.4 and a standard deviation of 2.1 in 603 differentially stained metaphase chromosome spreads. The major fraction of exchanges in the X chromosome were located in the region of the subcentromeric G negative bands q1 and q2. When the cells were pulse labeled with tritium labeled thymidine (1 muCi/ml) for the last 6 h of the cell cycle, the rate of exchange was higher in the late replicating X chromosome (facultative heterochromatin) in comparison to its homologue (active X). This study characterizes the yield of sister chromatid exchange in lymphocytes exposed to a given amount of 5-bromodeoxyuridine in a population of normal individuals so that it may be used as a standard for 1) a diagnostic tool in pathological conditions and 2) as an assay of chromosome stability in relation to environmental hazards.

A type of blindness due to lack of rods and cones in the retina was found to be controlled by a single autosomal recessive gene, re. The mutation was first identified in the second generation decendents of a male carrying an ethyl methanesulfonate (EMS) induced chromosome translocation involving one arm of the Z sex chromosome and the long (q) arm of chromosome 3. A linkage test between the locus causing the blindness and the translocation break-point on chromosome 3 was not significant. There is no proof that the mutation was EMS induced. (

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