Sex Chromosome Differences
What Are Sex Chromosomes?
Every cell in the human body typically has 23 pairs of chromosomes. The first 22 pairs are the same in males and females. The 23rd pair is our sex chromosomes, where males typically have one X and one Y chromosome and females typically have two X chromosomes.
How Often Do Sex Chromosome Differences Happen?
- Variations can occur with our sex chromosomes, resulting in individuals with extra or missing sex chromosomes. Generally, these types of genetic differences are quite common, occurring in about 1 in 500 to 1 in 1000 individuals.
- These genetic differences may actually be more common than we think, since it is possible for individuals to have a sex chromosome difference and not be aware of them.
- With the use of prenatal screening and diagnostic testing, these sex chromosome differences are found in pregnancy more frequently and often as a finding that was was not related to the reason for testing. These sex chromosome differences likely would not have otherwise presented themselves in the pregnancy, or even long after birth.
Features of Sex Chromosome Differences
The features of these chromosome differences are variable, and depend on which chromosome is extra or missing. Some features may be mild (like tall or short stature), or may lead to differences in development such as delayed speech or learning challenges. Some individuals with chromosome differences have health issues involving puberty or the heart.
Common Sex Chromosome Differences
45, X (Turner Syndrome)
Illustration adapted from Genetic Counseling Aids, 7th Edition, Copyright 2020, permission for use granted by Greenwood Genetic Center
Turner syndrome (also known as Monosomy X) is a chromosome difference that occurs in females and is caused by a female having one X chromosome instead of two X chromosomes. This chromosome difference occurs in about 1/2500 newborn girls and in 1-1.5% of pregnancies. Turner syndrome is found more frequently in pregnancy because many babies with Turner syndrome will miscarry before birth. Most cases of Turner syndrome are not inherited.
A missing X chromosome can affect the development of each baby differently.
During the pregnancy, some girls with Turner syndrome will have increased fluid at the back of the neck (known as increased nuchal translucency or cystic hygroma). Some common issues after birth include heart defects, short stature, delayed puberty without the use of hormone replacement therapy, infertility and learning disabilities.
In general, girls with Turner syndrome do not have an intellectual disability.
47,XXX (Trisomy X Syndrome)
Illustration adapted from Genetic Counseling Aids, 7th Edition, Copyright 2020, permission for use granted by Greenwood Genetic Center
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47,XXX (also called Trisomy X or Triple X syndrome) is a chromosome difference that occurs in females and is caused by having an extra copy of the X chromosome.
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Girls with 47,XXX typically have no physical differences with the exception that they may be taller than average.
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Many girls with 47,XXX may have no challenges while some girls may experience learning disabilities and delayed language and speech.
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Most cases of 47,XXX are not inherited.
47, XXY (Klinefelter Syndrome)
Illustration adapted from Genetic Counseling Aids, 7th Edition, Copyright 2020, permission for use granted by Greenwood Genetic Center
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47, XXY (also called Klinefelter syndrome) is a chromosome difference that occurs in males and is caused by having an extra copy of the X chromosome.
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Unless identified during the pregnancy through diagnostic testing, boys with Klinefelter syndrome are typically not identified until puberty as their symptoms are so mild.
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Physical features associated with this condition may include tall stature, development of breast tissue, reduced body and facial hair, infertility and small testes that produce a reduced amount of testosterone.
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Some of these features are due to this reduced testosterone. Treatment is available for some of these symptoms.
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Some individuals with Klinefelter syndrome may experience learning disabilities and delayed language and speech. Autism spectrum disorder, attention deficit hyperactivity disorder (ADHD) and mental health concerns have been reported to be associated with the condition.
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Most cases of Klinefelter syndrome are not inherited.
47,XYY
Illustration adapted from Genetic Counseling Aids, 7th Edition, Copyright 2020, permission for use granted by Greenwood Genetic Center
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47, XYY is a chromosome difference that occurs in males and is caused by having an extra copy of the Y chromosome.
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Boys with 47,XYY typically have no physical differences with the exception that they may be taller than average.
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47,XYY is associated with an increased chance of behavioural, learning, communication and social interaction difficulties.
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Most cases of 47,XYY are not inherited.
Next Steps
You may be hearing about sex chromosome differences because your prenatal screening test was positive or high risk for having a baby with a sex chromosome difference. If this is the case, your health care provider should offer to refer you for genetic counselling to review your test results and your options for additional testing.
Additional Resources
Sources for This Page
Hook EB, Warburton D Turner syndrome revisited: review of new data supports the hypothesis that all viable 45,X cases are cryptic mosaics with a rescue cell line, implying an origin by mitotic loss. Hum Genet. 2014;133(4):417-424.
Linden MG and Bender MG. Fifty-one prenatally diagnosed children and adolescents with sex chromosome abnormalities. Am J Med Genet 2002; 110(1):11-8.
U.S. National Library of Medicine. Genetics home reference [Internet]. Bethesda (MD): U.S. National Library of Medicine; 2017
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