The hows and Ys of genome integrity.

Much of the Y chromosome consists of large palindromic arrays harboring genes that are critical for spermatogenesis. In this issue, Lange et al. (2009) show that although gene conversion within these arrays maintains their integrity, it also permits rare unequal sister chromatid-exchange events within palindromes that create unstable dicentric chromosomes, resulting in infertility, sex reversal, and Turner syndrome.

The origin of trisomy 13.

Trisomy 13 is one of the most common trisomies in clinically recognized pregnancies and one of the few trisomies identified in liveborns, yet relatively little is known about the errors that lead to trisomy 13. Accordingly, we initiated studies to investigate the origin of the extra chromosome in 78 cases of trisomy 13. Our results indicate that the majority of cases (>91%) are maternal in origin and, similar to other autosomal trisomies, the extra chromosome is typically due to errors in meiosis I. Surprisingly, however, a large number of errors also occur during maternal meiosis II ( approximately 37%), distinguishing trisomy 13 from other acrocentric and most nonacrocentric chromosomes. As with other trisomies, failure to recombine is an important contributor to nondisjunction of chromosome 13.

The origin of trisomy 22: evidence for acrocentric chromosome-specific patterns of nondisjunction.

Trisomy 22 is one of the most common trisomies in clinically recognized pregnancies, yet relatively little is known about the origin of nondisjunction for chromosome 22. Accordingly, we initiated studies to investigate the origin of the extra chromosome in 130 trisomy 22 cases. Our results indicate that the majority of trisomy 22 errors (>96%) arise during oogenesis with most of these errors ( approximately 90%) occurring during the first meiotic division. As with other trisomies, failure to recombine contributed to nondisjunction of chromosome 22. Taken together with data available for other trisomies, our results suggest patterns of nondisjunction that are shared among the acrocentric, but not all nonacrocentric, chromosomes.