Higher Incidence of Small Y Chromosome in Humans with Trisomy 21 (Down Syndrome)

Summary: The length of the Y chromosome was measured in 42 black patients with trisomy 21 (47,XY,+21) and a similar number of normal individuals of American black ancestry. The length of the Y was expressed as a function of Y/F ratio and arbitrarily classified into five groups using subjectively defined criteria as follows: very small, small, average, large, and very large. Thirty-eight % of the trisomy 21 patients had small or very small Ys compared to 2.38% of the controls (P < 0.01). In both populations the size of the Y was not normally distributed. In the normals it was skewed to the left, whereas in the Downs the distribution was flat (platykurtic). A significantly higher incidence of Y length heteromorphisms was noted in the Down as compared to the normal black population. In the light of our current understanding that about one-third of all trisomy 21 patients are due to paternal nondisjunction, it may be tempting to speculate that males with small Y are at an increased risk for nondisjunction of the 21 chromosome.Speculation: The present study suggests that one-third of trisomy 21 cases have small Y chromosome. Presently, it is believed that approximately one-third of all cases of Down Syndrome are due to “paternal” nondisjunction. Therefore, it is speculated that fathers with small Y chromosome might be at higher risk for nondisjunction for chromosome 21.