Medical Genetics: How Chromosome Abnormalities Happen
Chromosomes are stick-shaped structures in the middle (nucleus) of each cell in the
body. Each cell has 46 chromosomes grouped in 23 pairs. When a chromosome is abnormal,
it can cause health problems in the body. Abnormal chromosomes most often happen as
a result of an error during cell division. Chromosome abnormalities often happen due
to 1 or more of these:
Other causes of birth defects can include exposures of many types of substances (teratogens)
to the developing baby.
Gendered terms are used here to talk about anatomy and health risk. Please use this
information in a way that works best for you and your provider as you talk about your
Errors during dividing of sex cells (meiosis)
Meiosis (my-OH-sis) is the process in which sex cells divide and create new sex cells
with half the number of chromosomes. Sperm and eggs are sex cells. Meiosis is the
start of the process of how a baby grows. Normally, meiosis causes each parent to
give 23 chromosomes to a pregnancy. When a sperm fertilizes an egg, the union leads
to a baby with 46 chromosomes.
But if meiosis doesn’t happen normally, a baby may have an extra chromosome (trisomy),
or have a missing chromosome (monosomy). These problems can cause pregnancy loss or
health problems in a child.
A woman age 35 years or older is at higher risk of having a baby with a chromosomal
abnormality. This is because errors in meiosis may be more likely to happen as a result
of the aging process. Women are born with all of their eggs already in their ovaries.
The eggs begin to mature during puberty. If a woman is 35 years old, the eggs in the
ovaries are also 35 years old. You may be referred for genetic counseling or testing
if you’re age 35 or older when you are pregnant. Men make new sperm ongoing. So age
doesn’t increase the risk for chromosome abnormalities for older fathers. But some
newer studies suggest that rare abnormalities do occur.
Errors during dividing of other cells (mitosis)
Mitosis (my-TOH-sis) is the dividing of all other cells in the body. It’s how a baby
in the womb grows. Mitosis causes the number of chromosomes to double to 92, and then
split in half back to 46. This process repeats constantly in the cells as the baby
grows. Mitosis continues throughout your lifetime. It replaces skin cells, blood cells,
and other types of cells that are damaged or naturally die.
During pregnancy, an error in mitosis can occur. If the chromosomes don’t split into
equal halves, the new cells can have an extra chromosome (47 total) or have a missing
chromosome (45 total).
Substances that cause birth defects (teratogens)
A teratogen (ter-AT-uh-jen) is something that can cause or raise the risk for a birth
defect in a baby. They are things that a birth parent may be exposed to during pregnancy.
Researchers have 2 ways of finding out if a substance is a teratogen:
Animal studies. Animal studies are the main way to find out if a medicine or other substance is safe
during human pregnancy.
Observations from human exposure. These are the reports of problems about a substance over time.