What are chromosome conditions?

To the Point- Chromosome conditions are the result of an individual having a whole extra (also called trisomy) or missing (also called monosomy) chromosome.  As humans, we typically have a total of 46 chromosomes, 23 inherited from our mom and 23 from our dad. An extra or missing chromosome may potentially cause physical birth defects or intellectual disabilities.  That being said, there is much variability depending on what chromosome is involved.  For example, individuals with X & Y chromosome variations may have very mild symptoms and may not even know they have a chromosome condition.  Conversely, babies with an extra 13 or 18 chromosome typically do not survive very long after birth, and if they do they have severe physical birth defects and intellectual disability.  

Genetics Review of Chromosome Conditions

To the Point: Most of the chromosome conditions identified during pregnancy involve an extra chromosome and are called trisomies, such as Down syndrome. Trisomies are the result of a “chromosome packaging error” in the maturing egg or sperm cell, which is not your fault.

Typically, there is nothing you can do to cause or prevent Down syndrome or other trisomies (Trisomy 13, Trisomy 18, X & Y chromosome variations). At this time, there are no known environmental or behavioral causes; it just happens.

Now that we have that covered, let’s discuss how something like Down syndrome chromosome conditions involving an extra chromosome happen.

As was discussed in Genetics 101, mom’s egg and dad’s sperm each contribute 23 chromosomes to result in the baby having a total of 46 chromosomes or 23 pairs. But, mom’s egg and dad’s sperm don’t start with 23 chromosomes. They start with 46, and it isn’t until they go through a process of “maturing” that they end up with just the 23 chromosomes.

So, picture mom’s egg with the 46 (23 pairs). Prior to ovulation (release of the egg from the ovary), the 23 pairs of chromosomes essentially line up along the middle of the egg and are pulled apart, resulting in 23 of each chromosome in the mature egg. What occasionally can happen is that this process of the pairs being pulled apart does not go according to plan and a pair can essentially stick together, and the result can be the mature egg ending up with 2 of one particular chromosome instead of one as usual. So, when the sperm enters the picture and contributes its copy of that chromosome, the result is an embryo with 3 copies of one particular chromosome; 2 from mom and 1 from dad. This is a trisomy. This “chromosome packaging error” can happen in the egg or the sperm cell.

Information about Specific Chromosome Conditions

Down syndrome

Trisomy 18

Trisomy 13

Turner Syndrome

X & Y Chromosome Variations

Jacob’s syndrome (47,XYY)