A chromosome disorder is an abnormality that occurs when an embryo develops with too many or too few chromosomes. The normal human being has 46 chromosomes. An egg and a sperm contribute 23 chromosomes when they come together 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, reversed, duplicated, or switched with another chromosome. When this happens, the person will have a structural chromosome disorder.
Usually, a fetus with a chromosomal disorder does not survive and the woman miscarries. It is estimated that more than half of spontaneous abortions that occur during the first trimester are due to a chromosomal disorder in the embryo. When labor does occur, the baby is often born with birth defects.
Down syndrome is one of the most common chromosomal 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 are at higher risk of giving birth to a child with Down syndrome or another chromosomal disorder. This risk increases with age, so tests to determine whether or not the fetus is normal are often recommended when an older woman becomes pregnant.
Turner syndrome is a chromosomal disorder that affects only girls. A normal girl has two X chromosomes. Turner syndrome occurs when a girl has only one X chromosome, and the other X chromosome is damaged or missing. Girls with this disorder are usually not fertile and are unable to go through puberty without medical intervention.
Triple X is another chromosomal 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. Parents of a girl born with this chromosomal disorder may not even know that her 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 people show normal development, and their parents may not know their child has a chromosomal disorder unless it shows up during prenatal testing.