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.
What are Sex Chromosome Differences?
Sometimes people are born with extra or missing sex chromosomes, which means that they have a sex chromosome difference. This happens in about 1 in 500 to 1 in 1000 individuals and it may actually be more common than we think, since it is possible for someone to have a sex chromosome difference and not be aware of it.
Can Sex Chromosome Differences be Detected During Pregnancy?
With the use of Non-Invasive Prenatal Testing (NIPT) and diagnostic testing, sex chromosome differences are found in pregnancy more frequently now than in the past. Most types of sex chromosome differences do not have an impact on the health and development of the pregnancy. However, 45,X (Turner syndrome) in particular can have implications for the baby's health even before birth.
What if there is a "High Risk" Prenatal Screening Result?
You may be hearing about sex chromosome differences because your NIPT result was "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.
What Will a Sex Chromosome Difference Mean for the Baby?
The features of chromosome differences partly depend on which sex chromosome is extra or missing. However, even individuals with the exact same sex chromosome difference may experience variable degrees of health and developmental challenges. Some individuals will have a completely typical development and no health concerns. Others may have differences in height (taller or shorter stature), health issues involving puberty, heart or other organs, and differences in development (such as delayed speech or learning challenges).
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.
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