engleski

Unbalanced chromosome segregation of constitutional t(11:22) due to crossover followed by 3:1 disjunction at first meiosis

A reciprocal translocation t(11 ; 22)(q23 ; q11) is of particular interest because the unbalanced offsprings of the translocation carriers usually presents a normal chromosome complement plus an extra copy of derived 22. This unbalanced karyotype is the result of 3:1 chromosome segregation during meiosis. Lockwood et al. (1989) and Simi et al. (1992) presented an unusual segregation of translocation t(11 ; 22) and additional derived chromosome 22. Two mechanisms leading to such unusual karyotype were proposed: nondisjunction in parental meiosis II and postzygotic non disjunction. In this report we present a third child who acquired both chromosmes involved in reciprocal translocation and an additional copy of der(22). The aim of our investigation was to determine the mechanism leading to this unusual chromosome complement. Cytogenetic analysis was performed on slides obtained by peripheral blood cultures of the proband and her parents. Slides were stained after GTG, and RBG banding methods. In order to demonstrate any heteromorphism associated with two der(22) we used different selective staining methods. The proband's father carried an apparently balanced translocation with a 46, XY, t(11 ; 22)(q23 ; q11) chromosome complement. The girl's karyotype was 47, XX, -11, -22, +der(11), +der(22)t(11 ; 22)(23 ; q11)pat. There are different mechanisms leading to such unusual karyotype. The presence of three derived chromosomes may be the consequence of alternate, adjacent 1 and 3:1 segregation patterns. In this study we used chromosome polymorphism analysis to distinguish different segregation patterns and determine the mechanism leading to the observed chromosome constitution in our patient. In this child we suggested 3:1 segregation after the crossing over involving the derived and normal 22 with or without crossing over involving derived and normal chromosome 11.

chromosome polymorphism; chromosome segregation; rare unbalanced t(11; 22); t(11; 22)

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