How do genes and chromosomes affect birth defects?

How do genes and chromosomes affect birth defects?

About 20% of birth defects are caused by genetic or hereditary factors. Genetic causes of birth defects fall into three general categories:

• Chromosomal abnormalities.
• Single-gene defects.
• Multifactorial.

Every human body cell contains 46 chromosomes, and each chromosome contains thousands of genes. Each gene contains a blueprint that controls development or function of a particular body part. People who have either too many or too few chromosomes will therefore receive a scrambled message regarding anatomic development and function.

Down syndrome is an example of a condition caused by too many chromosomes. Because of an accident during cell division, individuals with Down syndrome have an extra copy of a particular chromosome (chromosome 21). This extra chromosome can cause a typical constellation of birth defects. Characteristic features of Down syndrome can include developmental delay, muscle weakness, downward slant of the eyes, low-set and malformed ears, an abnormal crease in the palm of the hand and birth defects of the heart and intestines.

With Turner syndrome, a female lacks part or all of one X chromosome. In the affected women, this can cause short stature, learning disabilities and absence of ovaries.

Trisomy 13 (Patau Syndrome) and Trisomy 18 (Edwards Syndrome) are caused by inheriting extra copies of the 13th or 18th chromosome, respectively. These are rarer, more serious conditions which cause severe birth defects that are incompatible with survival after birth.

In addition to inheriting an extra or absent chromosome, deletions or duplications of single genes can also cause developmental disorders and birth defects. One example is cystic fibrosis (a disorder that causes progressive damage of the lungs and pancreas).

Defective genes can also be caused by accidental damage, a condition called “spontaneous mutation.” Most cases of achondroplasia (a condition that causes extreme short stature and malformed bones) are caused by new damage to the controlling gene. In addition, recombination errors can cause translocations of chromosomes which may lead to complex birth defects.