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Phenotypic sex is determined by a sequential process that involves the establishment of chromosomal and genetic sex in the zygote at conception, gonadogenesis and the determination of gonadal sex (sex determination), and sexual differentiation of the genital tract and external genitalia as programmed by gonadal sex.1 Divergent differentiation from the sex chromosome constitution is possible at each level of sexual organization. Table 538-1 considers the hierarchal ontogeny of sex from chromosomal sex to gender identity.

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Table 538-1. Ontogeny of Sex
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Recent advances allow more accurate prenatal diagnosis of sex. This is of particular importance when embryo gender determination is desired for gender determinations of X-linked disorders. Despite preimplantation sex determination, in about 1% of cases of mistakes occur in preimplantation sex assignment. Ultrasound or amniocentesis allows sex determination in the second trimester, and chorionic villus sampling allows determination and molecular analysis of X-linked genetic disorders during the first trimester, but it is associated with a risk of fetal loss. Newer techniques that measure maternal plasma cell-free fetal nucleic acids (DNA and RNA), such as sex determining region (SRY) gene amplification, allow reliable determination of sex early in the first trimester.2

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In placental mammals, the male is the heterogametic sex, XY, and the female is homogametic, XX. The Y chromosome functions as a dominant male determinant. The XX and XY are the chromosomal basis of sex determination.1,3-5 The sex chromosomes and the autosomes harbor genes that regulate sex determination and, as a consequence, gonadogenesis. Although the presence of only a single X chromosome (as in 45,X gonadal dysgenesis) can lead to the beginning of ovarian organogenesis, with rare exceptions 2 X chromosomes are required for survival of oocytes in utero and the development of a normal ovary at birth. Two X chromosomes (in the absence of a Y chromosome or translocation of its testis-determining gene to an X-chromosome) lead to ovarian differentiation.

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Sex Chromatin Pattern (Barr Body)

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The 2 X chromosomes in female diploid somatic cells exhibit striking morphologic and functional differences. One X chromosome is in a highly condensed (heteropyknotic) state in interphase and is visible as sex chromatin (Barr body).6 It completes DNA synthesis later than any other chromosome, and the action of genes located on the precociously condensed segments is suppressed (silent X) by transcriptional inactivation.7-9 The other X chromosome, the single X chromosome in male somatic cells, is in a highly extended (isopyknotic) state during interphase. It completes DNA replication with most of the complement and is genetically active. This discordant behavior of the 2 homologous ...

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