Disorders of sexual differentiation raise questions involving the multiple components of sexuality. Various related terms and definitions have become part of medical vocabulary.

Biological sex refers to an individual’s intrinsic biological status as male or female based on karyotype, internal reproductive organs, and external genitalia. Often, the first question that expectant parents want to discuss is whether their child will be a boy or a girl. Biological sex disorders can present in utero. If so, they are usually detected when there is a discrepancy between karyotype and observed external genitalia. One subset of sex disorders will present at birth with ambiguous genitalia, and some represent true emergencies, such as salt-wasting congenital adrenal hyperplasia. Another subset of sex differentiation disorders are not diagnosed until adulthood. Both sex-linked and autosomal genes govern sexual differentiation.

It is important to understand that biological sex does influence gender and gender identity. Gender refers to the attitudes, feelings, and behaviors that a given culture associates with a person’s biological sex, while gender identity refers to whether one identifies as being male, female, or transgender. A conflict between gender identity and biological sex may lead the individual to identify as transsexual.

Sexual orientation refers to the sex of those to whom one is sexually and romantically attracted. Gay men and lesbians are attracted to members of their own biological sex, heterosexuals are attracted to members of the other sex, and bisexuals are attracted to members of both sexes. Although we see sexual orientation in black and white terms, it can also be part of a continuum, and may differ at various stages of one’s life. For example, some people may experience bisexual relationships for a time and then commit to either a homosexual or heterosexual orientation.

Biological sex is closely intertwined with the biology and genesis of male and female gonads. Experiments in rabbits have shown that removal of either ovaries or testes will lead to the development of external genitalia consistent with female sex.¹ These experiments have been used to argue that ovarian development happens by default, while testes development requires the activation of genes that will lead to a male phenotype. This misconception has led to much research effort on male sex determination, but little is known regarding the pathways that lead to female sexual differentiation (Figure 13-1). Many genes are involved in the genesis of mammalian gonads and sex determination. It is of interest that, in some organisms, such as alligators and clownfish, environmental temperature and behavior play a dominant role in determining sex.²

Gonadal development is dependent on germ cells that are set aside early in the embryo. The germ cells are initially called primordial germ cells and migrate to the gonadal ridge where they divide by mitosis. The gonadal ridge will eventually become the gonad, either male or female. In the female, oocytes enter meiosis I, and arrest in utero in the diplotene stage, while male germ cells, called spermatogonia, do not enter meiosis until the onset of puberty.

Historically, the emphasis has been to elucidate the role of the Y chromosome in gonadal development and sex determination, as males are 46, XY and females are 46,XX. Since random X inactivation in females renders one X inactive in each cell, the Y chromosome must carry the determinant for testis development. It has been known since 1959 that the Y chromosome carries a determinant for testis development, and subsequent studies have shown that the SRY (sex-determining region of chromosome Y, also known as testis-determining factor or TDF) gene is essential for testis development.³ However, the X chromosome is rich in genes necessary for testis development, and genetic defects on the X will cause abnormal male gonadal development. Both X chromosomes are essential in females for normal ovarian differentiation, and individuals with monosomy X (Turner syndrome) fail to develop normal ovaries.

Although emphasis was initially placed on the role of the sex chromosomes in sex determination, we now know that a number of autosomal genes are essential drivers of sexual differentiation. These include WT1 (Wilms tumor 1), NROB1 (also known as steroidogenic factor 1), WNT4 (wingless-related MMTV integration site 4), SOX9, and DMRT1, among others. We also know that other, yet-to-be-discovered, autosomal genes for sex determination exist. The use of animal models such as transgenic mice has given us a good sense of the hierarchy of genetic control of gonadal and sexual development² (Figure 13-1). Disorders in any of these genes can lead to ambiguous genitalia and sex reversal.

Figures 13-2 and 13-3 give a simplified outline of the progression from the bipotential gonad to the development of a functional ovary and testis and the resultant internal and external genitalia. As will be outlined in this chapter, many genes contribute to gonadal and sexual development, but understanding this basic framework helps in predicting the expected phenotype of a specific genetic mutation.

46,XY Gonadal Dysgenesis

In mammals, the gonads in both sexes have the potential to develop into either ovaries or testes. Normal male sexual differentiation in 46,XY individuals depends on the proper function and complex interaction of numerous testis-determining genes, including SRY, SOX9, NR5A1/SF1, NR0B1, AR, DHH, and CBX2.⁴ Failure in the normal male sex differentiation process can cause complete or partial 46,XY gonadal dysgenesis.

Partial 46,XY gonadal dysgenesis is characterized by impaired testicular development and ambiguous external genitalia, including mild to severe penoscrotal hypospadias, dysgenetic (abnormally developed) testes, and reduced sperm production or none at all. Mullerian structures (uterus and fallopian tubes) may or may not be present.

Individuals with complete 46,XY gonadal dysgenesis, or Swyer syndrome, have normal female external genitalia and internal organs, but also have bilateral streak gonads.^5,6 Swyer syndrome has been estimated to occur in approximately 1 out of 30,000 individuals. Affected individuals are typically tall, lack secondary sexual characteristics, may have mild clitoromegaly, and are infertile. This condition commonly remains undiagnosed until adolescence, when puberty fails to occur. These females with a 46,XY karyotype have an increased risk of developing gonadoblastoma or dysgerminoma; therefore, the streak gonads are usually removed shortly after diagnosis. Women with Swyer syndrome cannot produce eggs; however, successful spregnancies have been achieved in some patients using donated eggs or embryos.

Complete 46,XY gonadal dysgenesis is a heterogeneous condition that may result from either chromosomal abnormalities (deletions, duplications, structural rearrangements) or point mutations in genes implicated in sexual differentiation. Despite considerable advances in understanding the genetic factors involved in gonadal determination and differentiation, a molecular diagnosis is made only in approximately 25% of cases with complete 46,XY gonadal dysgenesis. Mutations and deletions of the SRY gene are the cause of complete 46,XY gonadal dysgenesis in approximately 10% to 15% of patients with Swyer syndrome. The SRY gene is located on the short arm of the Y chromosome, and structural Y chromosome rearrangements resulting in the loss of the SRY gene include Yp deletions, dicentric Y isochromosomes composed of the long arms only, ring Y chromosomes, and Y-autosome translocations. Structurally abnormal Y chromosomes can be detected by conventional G-band chromosome analysis in some cases. However, molecular cytogenetic studies, such as fluorescence in situ hybridization (FISH) and array comparative genomic hybridization (aCGH) analyses, are essential for accurate diagnosis.⁵