A boy with 46, X, del, Y, due to a de nove mutation.

A newborn male referred for genetic investigation because of a large sized head and dysplastic ears, but with apparently normal male genitalia was found to have a deletion of all of the brightly fluorescent part of the long are of chromosome Y and absence of the Y fluorescent body on buccal smear. His father and his two brothers had normal Y chromosomes. Social and family history as well as marker investigation make illegitimacy most unlikely and leaves an occurrence of a new chromosomal mutation in the father the most probably interpretation. Follow-up of the infant to the age of 9 months revealed a large baby with normal development.