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Chromosomal disorders occur when an embryo has too many or too few chromosomes, or when a chromosome has an abnormality. Miscarriages and birth defects often result, with Down syndrome being a common disorder. Turner syndrome affects only girls, while XYY affects only boys.
A chromosomal 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 sperm each contribute 23 chromosomes when they combine 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 chromosomal disorder.
Sometimes an embryo will have the correct number of chromosomes, but one of the chromosomes will contain an abnormality. A piece can be missing, reversed, duplicated, or swapped for another chromosome. When this happens, the person will have a structural chromosomal disorder.
A fetus with a chromosomal disorder usually does not survive and the woman miscarries. It is estimated that more than half of miscarriages that occur during the first trimester are due to a chromosomal disorder in the embryo. When a birth takes place, 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 have a higher risk of giving birth to a baby with Down syndrome or another chromosomal 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 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 infertile and unable to go through puberty without medical intervention.
Triple X is another chromosomal disorder that only affects girls. Females with this disorder have an extra X chromosome. They are often tall and have no obvious physical or mental defects. The parents of a girl born with this chromosomal disorder may not even know that their daughter has it unless prenatal testing is done.
Some chromosomal disorders affect only boys. One of them is called XYY. A normal male has two Y chromosomes, while a boy with XYY has extra Y chromosomes. These individuals show normal development and their parents may not know their child has a chromosomal disorder unless it shows up during prenatal testing.
