Abstract
A large family in which a t(21q22q) chromosome is segregating is described. Segregation analysis using data from this family and previously published families shows the risk to a female carrier of having a child with translocation Down's syndrome to be 0·0684±0·0270. The risk to a male carrier is probably much smaller with an upper limit of 0·0286. The segregation ratio for phenotypically normal offspring of carrier parents of both sexes is 0·5053±0·0524.