Risks of unbalanced progeny at amniocentesis to carriers of chromosome rearrangements: Data from United States and Canadian laboratories

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Abstract
Data on 1,237 prenatal (amniocyte) diagnoses in cases of familial chromosome rearrangements were collated from 79 American and Canadian laboratories. These were added to European data (Daniel et al: Prenatal Diagn 6:315–350, 1986) on 596 reciprocal translocations (rcp) from 71 collaborative laboratories. The total data set was examined for relationships between balanced or unbalanced result and mode of ascertainment, sex of carrier parent, chromosomes involved, and (in cases of reciprocal translocations and pericentric inversions) for potential or actual chromosome imbalance size (% haploid autosome length). Risk rates for unbalanced segregants were markedly dissimilar. These ranged from approximately 50% down to essentially a negligible risk. The risk was approximately 50% for carriers of the following: complex chromosome rearrangements (ccr); insertions (ins); and for 2:2 segregating rcp ascertained by mode 1 (term unbalanced proband) with small imbalance segments. Pooled carriers (either sex) of 2:2 segregating rcp of mode 1 had a risk of 20–25% whereas female Robertsonian (rob) translocation (D;21) carriers and pericentric inversion (pii) carriers of pii with small distal segments had a risk of 10–15%. Pooled 2:2 segregating rcp carriers ascertained by mode 2 (a couple with recurrent miscarriages) and male carriers of rob (D;21) had a risk of 1.5–5%. The risk of unbalanced segregants was 1–2% (in this data) for male and female rob (13; 14) carriers and for pooled pericentric inversion carriers. However, for carriers of most “type” (recurrent breakpoints) pii, for all paracentric inversions, and (as expected) for rob not involving 13 or 21, there were no term unbalanced progeny. For 2:2 segregating reciprocal translocations plots were prepared that could be used to determine broad risk groups for carriers of such rcp. In 3:1 segregating rcp there were 3.3 times fewer male than female carriers, whereas there were 1.3 times fewer male carriers in 2:2 segregating rcp. In 2:2 segregating rcp there is little effect on the fertility of male carriers and risks of unbalanced progeny were found to be equal to those for female carriers, whereas in the 3:1 segregating rcp, risks were much less for male as compared to female carriers. This indicates that 3:1 segregating rcp are more similar to Robertsonian translocations in their greater effect on the fertility of male carriers.