The cytogenetic finding 47, XYY refers to the presence of an extra Y chromosome in the fetal cells. The extra Y chromosome is always paternal in origin and results from nondisjunction in the second meiotic division (84% of cases) or a postfertilization mitotic error (16%) (Robinson and Jacobs, 1999). XYY is one of the diagnoses included in the group of disorders known as sex chromosome abnormalities. Anomalies involving the sex chromosomes are the most common abnormal findings in prenatal cytogenetic diagnosis, accounting for 25% of abnormal amniocentesis results, yet their existence is rarely discussed in preprocedural genetic counseling. Thus, while prospective parents are reasonably prepared for a prenatal diagnosis of autosomal trisomy, the diagnosis of XYY usually comes as a surprise.

Much of the confusion regarding prognosis for males with XYY stems from studies performed in the 1960s that were highly biased by ascertainment of patients in maximum security hospitals or prisons (Price and Whatmore, 1967). Fortunately, prospective longitudinal studies of newborns identified at birth do not support an increased incidence of aggression or criminality (Ratcliffe et al., 1990). As with all of the sex chromosome abnormalities, the developmental phe-notype is variable and subject to many other influences, such as parental IQ, socioeconomic status, and family stability.

The incidence of 47, XYY is 1.45 in 1000 livebirths (Autio-Harmainen et al., 1980) and 1 in 1000 amniocenteses for advanced maternal age (Robinson et al., 1992a). The incidence is not increased among offspring of parents with advanced age. As opposed to the other sex chromosome abnormalities, the XYY karyotype does not carry an increased chance of in utero mortality (Ferguson-Smith and Yates, 1984). As many as 85% of cases are believed to be undiagnosed (Ratcliffe, 1999).

There are no characteristic sonographic findings because the fetal phenotype in this condition is generally normal (Autio-Harmainen et al., 1980). In one study, the fetal nuchal translucency measurement was above the 95th percentile in 40% of the fetuses with 47, XXX, 47, XYY, or 47, XXY (Sebire et al., 1998). More recently, Maymon et al. (2002) reported on two fetuses with 47, XYY that had brain anomalies detected on prenatal sonographic examination. One had complete agenesis of the corpus callosum and one had a Dandy–Walker malformation. Both were terminated, so long-term developmental outcome is unknown.

47, XYY is a definitive diagnosis obtained by prenatal karyotype (Figure 137-1). Occasionally, cases of 46, XY/47, XYY mosaicism are detected, resulting from mitotic nondisjunc-tion after fertilization. These patients are expected to have milder manifestations of the developmental abnormalities seen in this condition.

The antenatal natural history for these fetuses does not differ from normal fetuses. A few patients with 47, XYY have been identified through abnormally increased maternal serum α-fetoprotein levels (Robinson et al., 1992a). It is unclear whether a true relationship exists between abnormalities in serum analytes and this particular chromosomal abnormality.