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Sex Chromosome DSDs
Typical genotype-phenotype relationships:
46 XX, female-typical phenotype
- Primordial germ cells in the gonadal ridge induce ovarian differentiation, in approximately week 8.
- Genes from the Wnt family and others transform the paramesonephric ducts to produce the uterine tubes, uterus, and the vagina.
- Estrogen from the fetal ovaries and placenta guide formation of the vulva, aka, the female external genitalia.
- The SRY gene in the somatic cells of the gonadal ridge induces testes differentiation.
- Subsequently, the testes produce androgens and anti-Mullerian hormone (AMH), which facilitate transformation of the mesonephric ducts to produce the tubules of the testes, the epididymis, and the ductus deferens.
- Androgens also guide formation of the male external genitalia, the penis and scrotum.
DSDs caused by atypical sex chromosome number:
DSD 45 X_, aka, Turner Syndrome
- The genotype tells us that there are 45 total chromosomes, with one X chromosome; thus, one of the sex chromosomes is missing.
- Gonads do not fully differentiate, and typically present as fibrous, non-functional streak ovaries.
- As a result, most 45 X_ individuals do not pass through puberty, and typically have impaired fertility.
- Other notable characteristics include short stature, which becomes noticeable during childhood; broad chest; and a "webbed" neck, with extra skin folds. Be aware that these features are not present in every individual, but, when present, may be indicative of an atypical genotype.
- Clinical concerns include cardiac and renal defect, and lymphedema.
- Extra X chromosome in each cell.
- The extra X chromosome impairs testes development, resulting in small testicles that produce less testosterone.
- Thus, puberty is often delayed or incomplete, and fertility is impaired.
- Individuals tend to experience gynecomastia (enlargement of the breast tissue); reduced body and facial hair; and, perhaps most obviously, tall stature with long extremities.
- Clinical concerns are increased breast cancer risk (relative to typical male rates) and, in some cases, learning disabilities.
- A type of mosaicism; in these individuals, some cells have the 45 X genotype, others have the 46 XY genotype.
- Streak ovaries and testis, which is likely to be undescended; puberty is likely to be incomplete, and fertility is impaired.
- Clinical implications are short stature and cardiac and renal defects (similar to Turner Syndrome).
- Mosaic
- Both testicular and ovarian tissues are present, either symmetrically or asymmetrically.
- The resulting phenotype depends on the functionality of the gonadal tissues and the hormones they produce.
- Clinical concerns include hypospadias (displaced urethral opening) and cryptorchidism (undescended testes), both of which are likely the result of deficient androgen influence.