A chromosome disorder is an abnormality that results when an embryo develops with either too many or too few chromosomes. The normal human being has 46 chromosomes. An egg and sperm each contribute 23 chromosomes when they unite to form an embryo. Sometimes the egg or sperm will contain the wrong number of chromosomes. When this happens, the baby will be born with a chromosome disorder.
Sometimes an embryo will have the correct number of chromosomes, but one of the chromosomes will contain an abnormality. A piece may be missing, inverted, duplicated, or switched with another chromosome. When this happens, the person will have a structural chromosome disorder.
Usually a fetus with a chromosomal disorder doesn’t survive, and the woman has a miscarriage. It is estimated that more than half of miscarriages that occur during the first trimester are due to a chromosome disorder in the embryo. When a birth does take place, the baby is often born with birth defects.
Down syndrome is one of the most common chromosome disorders. Children born with Down syndrome generally display distinctive facial features and have learning disabilities that vary in severity. Many of these children also have heart defects and other physical problems. Older women have an increased risk of giving birth to a child with Down syndrome or another chromosome disorder. This risk increases with age, so tests are often recommended to determine whether or not the fetus is normal when an older woman becomes pregnant.
Turner syndrome is a chromosomal disorder that affects only girls. A normal girl possesses two X chromosomes. Turner syndrome results when a girl only has one X chromosome, and the other X chromosome is either damaged or missing. Girls with this disorder usually aren’t fertile and are incapable of going through puberty without medical intervention.
Triple X is another chromosome disorder that affects only girls. Females with this disorder have an extra X chromosome. They are often tall and have no apparent physical or mental defects. The parents of a girl born with this chromosome disorder may not even know their daughter has it unless prenatal testing is done.
Some chromosome disorders only affect boys. One of these is called XYY. A normal male has two Y chromosomes, while a boy with XYY has extra Y chromosomes. These individuals display normal development, and their parents may not know their son has a chromosome disorder unless it shows up during prenatal testing.