Medical Genetics: How Chromosome Abnormalities Happen

Medical Genetics: How Chromosome Abnormalities Happen

Chromosomes are stick-shaped structures in the middle 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 one or more of these:

  • Errors during dividing of sex cells (meiosis)

  • Errors during dividing of other cells (mitosis)

  • Exposure to substances that cause birth defects (teratogens)

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
they
can cause 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 a lot. But 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 mother may be exposed to during her pregnancy.
Teratogens include:

  • Some medicines

  • Street drugs

  • Alcohol

  • Tobacco

  • Toxic chemicals

  • Some viruses and bacteria

  • Some kinds of radiation

  • Certain health conditions, such as uncontrolled diabetes

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.