Noncomplementation phenomena and their bearing on nondisjunctional effects.

In the mouse, unbalanced gametes with major gains and/or losses of chromosomal material seem just as capable of forming a zygote as normal, fully balanced gametes. This is shown by the results of intercrossing genetically marked translocation heterozygotes, in which complementary unbalanced gametes usually fuse to form fully viable zygotes. However, there are some notable exceptions to this. Studies on a number of reciprocal translocations have shown that gametes with maternal duplication of particular chromosome regions may fail to complement those with a corresponding paternal deficiency, but produce lethal zygotes instead, whereas the reciprocal combination of a paternal duplication with a maternal deficiency produces fully viable offspring. For a particular distal region on chromosome 7 the reverse situation holds. More recent studies on genetic methods of detecting nondisjunction with Robertsonian translocations have revealed the same phenomenon. Mouse chromosomes affected include numbers 2, 6, 7, and 8. There is also defective complementation on chromosome 11 and related phenomena on chromosome 17. These findings help to explain why diploid embryos with 2 male or 2 female pronuclei fail to come to term and may be connected with genetic imprinting of gametes. It seems probable that the same phenomenon occurs in homologous regions of human chromosomes and may mean that the severity of a trisomic effect will depend sometimes on the parental source of the extra chromosome. The phenomenon also affects the efficiency of certain genetic tests for nondisjunction which depend on full complementation.