Sex Chromosome Abnormalities
Sex chromosome abnormalities are constitutional changes in the number or structure of the X and Y chromosomes. As a group they are among the most common chromosomal conditions identified at birth, and they span a wide clinical spectrum, from the often subtle and underdiagnosed sex chromosome trisomies to the well-characterised Turner and Klinefelter syndromes. This area orients the reader to the category and links to its principal topics.
Definition
Sex chromosome abnormalities are deviations from the normal 46,XX or 46,XY complement involving the gonosomes (the X and Y chromosomes), comprising gains or losses of whole chromosomes (aneuploidy), structural rearrangements, and mosaic combinations of cell lines.
Scope
The entry surveys numerical changes (aneuploidies such as 45,X, 47,XXY, 47,XXX and 47,XYY), structural changes of the X or Y chromosome, and mosaicism, as encountered through cytogenetic and genomic testing. It is a navigational overview of a category within cytogenetics; detailed clinical descriptions belong to the topic entries, and the material is reference-educational rather than a basis for individual care.
Sub-topics
Key concepts
- Gonosomes (sex chromosomes X and Y)
- Aneuploidy of the sex chromosomes
- Mosaicism
- X-chromosome inactivation (lyonisation)
- Pseudoautosomal regions and SHOX
- Nondisjunction in meiosis
- Phenotypic variability and underdiagnosis
Mechanisms
Most numerical sex chromosome abnormalities arise from nondisjunction during meiosis or from anaphase lag in early mitosis, producing extra or missing gonosomes; mosaicism results when an error occurs after fertilisation, leaving more than one cell line. X-chromosome inactivation partly buffers the dosage effect of supernumerary X chromosomes, which contributes to the comparatively mild and variable phenotypes seen in many of these conditions, while genes that escape inactivation (such as SHOX in the pseudoautosomal region) help explain features such as short stature in 45,X. The clinical consequences therefore depend on which chromosome is involved, on dosage, and on the proportion and distribution of any mosaic cell lines.
Clinical relevance
Sex chromosome abnormalities are encountered across prenatal screening, paediatric, endocrine, and fertility settings, and many remain undiagnosed because their features can be subtle. Understanding the category supports accurate interpretation of cytogenetic and genomic reports and informed appraisal of the literature; this overview describes the category and is not a substitute for specialist clinical assessment or management decisions.
Epidemiology
Sex chromosome abnormalities are collectively the most frequent chromosomal findings in newborn surveys. Hamerton and colleagues' classic survey of 14,069 newborns documented their incidence and helped establish that gonosomal aneuploidies, taken together, occur in roughly 1 in 400 to 1 in 500 births, with individual conditions such as 47,XXY, 47,XXX, 47,XYY, and 45,X each contributing. Many cases, particularly the trisomies, are never clinically ascertained.
History
The category emerged with clinical cytogenetics in the late 1950s, after the human chromosome number was established and karyotyping became feasible: 45,X was identified as the basis of Turner syndrome and 47,XXY as the basis of Klinefelter syndrome in 1959, transforming previously descriptive clinical syndromes into defined chromosomal disorders. Newborn cytogenetic surveys in the 1960s and 1970s then mapped the population frequency of these and related abnormalities.
Related topics
Seminal works
- hamerton-1975
- gravholt-2018
- gravholt-2017
Frequently asked questions
- How common are sex chromosome abnormalities?
- Taken together they are the most common chromosomal abnormalities found in newborn surveys, occurring in roughly 1 in 400 to 1 in 500 births, though many individual cases are mild and never diagnosed.
- Are sex chromosome abnormalities usually inherited?
- Most are not inherited; they typically arise sporadically from errors of chromosome separation (nondisjunction) during the formation of eggs or sperm, or shortly after fertilisation.