Core idea Sex determination establishes the developmental pathway of the gonads, internal ducts and external genitalia; sex differentiation is the subsequent process by which those tissues develop. Chromosomal sex, gonadal sex, hormonal environment and phenotypic sex are related but not identical. Disorders of sex development (DSD) occur when chromosomal, gonadal or anatomical development does not follow the usual pathway.
Learning outcomes
Explain chromosomal and molecular sex determination; trace SRY–SOX9 and ovarian pathways; compare X-linked, Y-linked, pseudoautosomal, sex-limited and sex-influenced inheritance; calculate recurrence risks; and approach a newborn or patient with atypical genital development safely and respectfully.
1. Chromosomal and gonadal foundations
Most individuals with a 46,XX karyotype develop ovaries and most with 46,XY develop testes, but this is a probability rather than a complete definition of biological development. The Y chromosome may carry SRY, which initiates a testis pathway in the supporting cells of the undifferentiated gonad. In the absence of an effective testis pathway, ovarian development is supported by genes including WNT4, RSPO1 and FOXL2.
Testis differentiation produces Sertoli cells, anti-Müllerian hormone and Leydig cells, which produce testosterone. Anti-Müllerian hormone causes Müllerian duct regression; testosterone supports Wolffian duct development; conversion of testosterone to dihydrotestosterone by 5-alpha-reductase supports prostate and external male genital development. Without these signals, Müllerian structures can develop and external genital development follows the female-typical pathway.
2. Sex-determining pathways and clinical disorders
| Pathway or condition | Mechanism | Clinical consequence |
|---|---|---|
| SRY or SOX9 activation | Supports testis development and Sertoli-cell differentiation. | Loss or dysfunction may cause 46,XY gonadal dysgenesis; duplication or dysregulation may produce testicular development in an XX context. |
| Androgen insensitivity | 46,XY cells cannot respond fully to androgen through the androgen receptor. | Complete forms may present with female external phenotype and absent uterus; partial forms have variable genital development. |
| 5-alpha-reductase deficiency | Reduced conversion of testosterone to dihydrotestosterone. | Variable undervirilisation at birth with possible virilisation at puberty. |
| Congenital adrenal hyperplasia | Excess adrenal androgen in a 46,XX fetus, often from 21-hydroxylase deficiency. | Variable virilisation, salt-wasting risk and need for urgent endocrine care. |
| 45,X or mosaic Turner pattern | Absent or abnormal second sex chromosome affects gonadal development. | Short stature, ovarian insufficiency and cardiovascular or renal anomalies. |
3. X-linked inheritance
Genes on the X chromosome follow a characteristic pattern because males usually have one X chromosome while females have two. A male transmits his X chromosome to all daughters and his Y chromosome to all sons; therefore an X-linked trait cannot pass directly from father to son. A heterozygous carrier mother may transmit the variant to half of her sons and half of her daughters, although penetrance and X-inactivation can alter expression.
X-linked recessive disorders include haemophilia A, haemophilia B, Duchenne muscular dystrophy and red–green colour vision deficiency. Affected males are often more severely affected because they lack a second allele. Females may be unaffected, mildly affected or clinically significant because of skewed X-inactivation, Turner karyotype, homozygosity or a variant with substantial effect.
4. X-linked dominant inheritance
An X-linked dominant condition can affect females and males, sometimes with greater severity in males. An affected father transmits the variant to all daughters and no sons. An affected heterozygous mother has a 50% chance of transmitting it to each child. Some conditions are male-lethal, producing an apparent excess of affected females and recurrent miscarriages.
5. Y-linked inheritance
Y-linked genes pass from father to son only. Affected fathers transmit the variant to all biological sons and to no daughters. Y-linked infertility genes in the AZF regions can be lost or deleted, and a man may pass the deletion to sons conceived with assisted reproduction. A Y-linked pattern requires strict male-to-male transmission and should not be assigned from a single affected father and son without molecular evidence.
6. Pseudoautosomal and sex-influenced traits
Pseudoautosomal genes lie in regions shared by X and Y chromosomes and can recombine during male meiosis. Their inheritance resembles autosomal inheritance because both sexes carry a copy, but copy number and sex-chromosome context still matter. Sex-limited traits occur in only one sex because anatomy or hormones are required, while sex-influenced traits are expressed differently in males and females. Male-pattern baldness is a teaching example, but its biology is polygenic and should not be reduced to a single-gene rule.
7. Pedigree reasoning
Ask whether affected males are related through mothers, whether father-to-son transmission exists, whether all daughters of an affected father are affected, and whether both sexes are affected. Record consanguinity, miscarriages, infertility, neonatal deaths and variable expression. The pedigree suggests an inheritance pattern; molecular testing confirms or refines it.
Risk examples
- A carrier mother for an X-linked recessive disorder has a 25% chance of an affected child overall, or a 50% chance among sons.
- An affected father with an X-linked disorder transmits the variant to all daughters and no sons.
- An affected father with a true Y-linked variant transmits it to all biological sons.
- An affected heterozygous mother for an X-linked dominant condition has a 50% chance of transmission to each child.
8. Approach to atypical genital development
Do not assign blame or rush a cosmetic procedure. Stabilise the newborn first, especially check for hypoglycaemia, dehydration, vomiting, shock and salt-wasting congenital adrenal hyperplasia. Take a pregnancy, family, medication and consanguinity history; examine carefully; obtain electrolytes, glucose, 17-hydroxyprogesterone and other endocrine tests as indicated; and request karyotype or rapid chromosome testing with specialist input.
Use respectful language and protect privacy. A multidisciplinary team may include paediatrics, endocrinology, genetics, urology or gynaecology, psychology and ethics. Explain uncertainty honestly, avoid unnecessary photography or repeated examinations, and involve parents in decisions while considering the child’s future autonomy.
9. Clinical relevance of sex-linked disease
When a patient has an X-linked disorder, identify carrier relatives, offer molecular confirmation, discuss reproductive options and plan condition-specific surveillance. In Duchenne muscular dystrophy, a maternal carrier may need cardiac assessment and genetic counselling; in haemophilia, invasive procedures and bleeding risk require coordinated haematology care. Inherited Y-chromosome deletions mainly affect fertility and may have implications for sons conceived through assisted reproduction.
10. Examination summary
SRY initiates a testis pathway; Sertoli cells produce anti-Müllerian hormone and Leydig cells produce testosterone; dihydrotestosterone supports external male genital development. X-linked traits show no father-to-son transmission; Y-linked traits show father-to-all-son transmission; pseudoautosomal traits are shared between X and Y; sex-limited and sex-influenced traits depend on anatomy or hormonal context. Atypical genital development is a medical and psychosocial situation requiring urgent exclusion of adrenal crisis, careful testing and respectful multidisciplinary care.
