chromosomal abnormalities

In liveborn infants chromosomal abnormalities occur in
about 9 per 1000 births. The incidence of unbalanced
abnormalities affecting autosomes and sex chromosomes is
about the same. The effect on the child depends on the type of
abnormality. Balanced rearrangements usually have no
phenotypic effect. Aneuploidy affecting the sex chromosomes is
fairly frequent and the effect much less severe than in
autosomal abnormalities. Unbalanced autosomal abnormalities
cause disorders with multiple congenital malformations, almost
invariably associated with mental retardation.