Genetic risks for familial reciprocal translocations with special emphasis on those leading to 9p, 10p and 12p trisomies.

An analysis of genetic risks for reciprocal translocations is given. Data from translocation families, ascertained through unbalanced offspring with trisomy 9p, 10p and 12p, were taken from the literature. The translocations were specified according to (1) the type of trisomy, (2) the degree of resulting chromosomal imbalance (partial short arm trisomies, complete short arm trisomies, complete short arm trisomies including long arm segments) and (3) the type of disjunction (2:2 or 3:1) and segregation (adjacent-1, adjacent-2; tertiary trisomy, interchange trisomy). The risks for unbalanced liveborn offspring were high for translocations leading to partial short arm trisomies through 2:2 disjunction and adjacent-1 segregation (25-29%). They were lower for translocations leading to complete short arm trisomies through the same disjunction/segregation mechanism (5-17%). Low risks were obtained for translocations, leading through 3:1 disjunction to unbalanced offspring (about 2%). For 2:2 disjunction and adjacent-2 segregation and for 3:1 disjunction the risk is significantly lower for male than for female carriers.--The frequency of balanced karyotypes compared with normal karyotypes deviated among the phenotypically normal offspring of parental carriers from the theoretical 1:1 ratio, both for translocations ascertained through trisomy 12p and trisomy 10p.--It was demonstrated that the genetic risk for reciprocal translocations depends exclusively on (1) the degree of possibly resulting genetic imbalance and (2) the probability of the disjunction/segregation mechanism leading to this type of imbalance. Both factors can be predicted from the position of breakpoints.--The precision of breakpoint localizations and its impact on the risk estimation are also considered.--Finally, general rules for genetic counselling of families with reciprocal translocations are indicated.