Segregation analysis of a large t(21q22q) family.

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 transloca-tion Down's syndrome to be O-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. Familial t(21q22q) Down's syndrome (DS) is the least common type of Robertsonian translocation Down's syndrome (TDS). In the combined data accumulated by Mikkelsen (1971) and Matsunaga and Tonomura (1972/73), 5% of a total of 4330 DS cases had a Robertsonian translocation. The (Dq2lq) translocation was slightly the more common , occurring in 55-3% of all TDS cases; of these 45 0% were known to be familial. (Gq2lq) trans-locations were present in 44.7% of the TDS cases, but only 8-2% of these were familial. Stene (1970) has published a segregation analysis of the offspring of t(21q22q) carriers from the data he was able to accumulate at the time. Information was available from only eight families, some of them small. We have recently investigated a large New Zealand family in which a t(21q22q) chromosome is segregating. We have added the data from this family to those assembled by Stene to produce a more precise segregation analysis. The Family The family (Fig. 1) was ascertained through two pro-positi. A preliminary report is appearing as part of a survey of Down's syndrome in New Zealand (Gardner et al, 1973). The first propositus (V.172) was born in 1964 in Auckland and chromosome analysis was done at seven months of age because of uncertainty of the clinical diagnosis of Down's syndrome. The mother, the maternal grandmother, and a sister were shown to be t(21q22q) carriers at that time. The second propositus (V.16) was born in 1954, and was karyotyped in 1968 during a survey of all clinically diagnosed cases of Down's syndrome in institutions for the retarded in the Nelson area. Subsequently , it was discovered that a first cousin was recorded in the hospital notes as having Down's syndrome. The mother was shown to be a t(21q22q) carrier at that time. A full investigation of these 'two' families was begun in 1971. The 82-year-old great-aunt (III.35) of the first propositus, reputed to be a reliable repository of …