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Inherited Disorders and Birth Defects

Mosaic Chromosome



We are a couple with an AS (for maternal age) result 45,X[13]/46,XY[27]. The US at 21 weeks gestation shows normal male genitalia (no hypospadias, no micropenis). We searched for the current literature. We would appreciate it if you coul comment on this particular issue. Thank you so much. Best wishes.


The chromosomal defect you ask about is known as 45,X/46, XY  mosaicism.  Mosaicism occurs when some tissues of the body have one set number of chromosomes such as 46 and other tissues in the body have another set number of chromosomes - such as 45.  In the case you describe - it is the Y chromosome that is missing in some cells of the body.  Most likely, the Y chromosome was lost either at conception or at some time very early in fetal development.

The problems associated with this type of chromosomal finding - 45,X/46,XY mosaicism - can range from totally normal to an infant having ambiguous genitalia (abnormalities of the genitalia).   When 45,X/46, XY mosaicism is found on prenatal diagnosis, such as in your situation, if there are no other problems seen on ultrasound in the fetus, the literature states that about 90-95% of the time, these pregnancies result in the birth of a normal male infant. 

Problems that have been reported in the literature in some cases are the possibility of the testes developing a cancer or infertility.  Some males with 45X/46XY mosaicism have been found to be infertile.  However, fertility cannot be evaluated until puberty. 

Hopefully you have had the opportunity to discuss the findings with a team of people - your doctor, the specialist doing the ultrasound, the geneticist and/or genetic counselor.  They will have the most up-to-date and detailed information.  If you have not talked to a genetic counselor or geneticist, you might want to discuss these issues with them as well.  The National Society of Genetic Counselors Resource Link (see website below) can help you can locate a genetics center near you.

Related Resources:

Find a Genetic Counselor - National Society of Genetic Counselors

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Go to the Inherited Disorders and Birth Defects health topic, where you can:

Response by:

Anne   Matthews, RN, PhD Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University