THE XXYY VARIANT OF KLINEFELTER'S SYNDROME.

Three males with an XXYY sex chromosome complex are described. These patients, together with five XXYY subjects recorded in the literature, show the clinical features of Klinefelter's syndrome. Taking into consideration the findings in XYY and XXXYY individuals, it appears that the addition of a Y chromosome to XY, XXY and XXXY complexes has a variable and as yet not clearly delineated harmful effect. For example, a 44 + XXYY complement of chromosomes may prove to have significant manifestations in skeletal maturation and predispose to vascular and cutaneous abnormalities of the lower extremities in older patients. But when two Y chromosomes are present, the phenotype does not differ markedly from that resulting from the presence of a single Y chromosome in the sex chromosome complex. This finding is compatible with the view that the Y chromosome of man is relatively inert, compared with the autosomes, except for genes that function in male sex determination.