Table 3

 Summary of the three groups

GroupsType of transmitted chromosome anomaly
Transmitted UBCAs (n = 130)Euchromatic variants (EVs) (n = 70)
Groups 1 to 3Copy number not variable in the normal population. Chromosomal segments of several megabases in size; copy number change usually plus or minus one. Most have phenotypic consequences.Copy number variable in the normal population. Pseudogene or gene casettes of limited extent; relatively high copy number changes needed for cytogenetic visibility. None has established phenotypic consequences.
Group 1: normal offspring with normal parentsn = 23 (18%). Most group 1 families ascertained at prenatal diagnosis. Unknown whether post-natally ascertained cases would also be free of phenotypic effect. Homozygous imbalances of the same type unlikely to be equally free of phenotypic consequences.n = 38 (54%). Most group 1 families ascertained at prenatal diagnosis. Assumed that postnatally ascertained cases also free of phenotypic effect. Homozygous copy number variants unlikely to have significant phenotypic consequences.
Group 2: affected offspring with normal parentsn = 30 (23%). Most group 2 families ascertained via phenotype of offspring. Some likely to be coincidental to phenotype, some causal and some of uncertain significance.n = 30 (43%). Most group 2 families ascertained via phenotype of offspring. Phenotype of probands assumed to reflect ascertainment bias in all cases.
Group 3: affected offspring with affected parentsn = 77 (59%). Common co-segregation of group 3 imbalance and mild phenotype common and likely to be causal in the great majority of families.n = 2 (3%). Rare co-segregation of group 3 variant and mild phenotype regarded as coincidental in both families.