[Genetic risk in cases of reciprocal translocation (author's transl)].

The analysis of 206 families with reciprocal translocations revealed thanks to a patient who was a carrier of an unbalanced translocation demonstrated the risk of a child suffering from the disease at about 16.4%, in the case of a meiotic segregation 2 : 2. In the case of translocation with meiotic segregation 3 : 1, the risk is much less, i.e. 7.6% for female carriers and significantly less than 3/83 for male carriers. It is extremely important to take this difference into account when evaluating the risks during genetic counselling. On the other hand, the freqeuncy of spontaneous abortions is practically the same for carriers of translocations with these two types of segregation and rises to nearly 23%; i.e. the postzygotic selection influences, to the same degree, zygotes with one chromosome too many, one two few, and zygotes with 46 chromosomes, but of which one has a defect or an excess of genetic material. Such selection depends rather on an interaction between the abnormal genome of the zygote (embryo) and the genome of the mother, whilst the prezygotic selection operates against the gametes having one chromosome too many or one too few. A large asymmetry in the length of the exchanged fragments could be one of the causes of the 3 : 1 segregation. This is proved by a more frequent participation (48/60) of the acrocentric chromosomes in the translocations with segregation 3 : 1. On the other hand, in translocations with segregation 2 : 2, acrocentrics are found much more rarely (66/146).