Reduced Recombination Rate on Chromosomes 21 That Have Undergone Nondisjunction
- S.E. Antonarakis*,
- A. Chakravarti†,
- A.C. Warren*,
- S.A. Slaugenhaupt†,
- C. Wong*,
- S.L. Halloran†, and
- C. Metaxotou‡
- *Genetics Unit, Department of Pediatrics, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
- †Human Genetics Program, Department of Biostatistics, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
- ‡Cytogenetics Unit First Department of Pediatrics, Athens University Medical School, Athens, Greece
This extract was created in the absence of an abstract.
Excerpt
Down syndrome (trisomy 21) is the most common known genetic cause of mental retardation, with an incidence of 1.0-1.3 per 1000 live births, or about 0.45% of all clinically recognized pregnancies (Hassold and Jacobs 1984). The cytogenetic mechanism leading to trisomy 21 is meiotic nondisjunction, which could occur either at the first or the second meiotic division (Polani 1981). Several authors have suggested that homologous chromosomes must be linked by chiasmata in diakinesis in order to segregate at the first meiotic metaphase (Darlington 1929; Dobzhansky 1933; Mather 1938). It was hypothesized that at least one chiasma per bivalent was necessary for normal segregation (Mather 1938). Experimental studies in the mouse suggest that meiotic nondisjunction due to failure of separation of homologs at anaphase is rare and is usually due to aberrant segregation of univalents (Henderson and Edwards 1968). These univalents may be produced by two processes, namely, asynapsis (failure of...
