Chromosomes are tiny structures inside the nucleus of almost every cell in the body. These threadlike structures carry genetic information that determines many of your physical characteristics. Each cell usually has 23 pairs of chromosomes. Extra or missing chromosomes result in chromosomal disorders.
Scientists have numbered chromosomes from 1 to 23. When a baby’s cells have three number 21 chromosomes instead of two, the baby has trisomy 21, which causes most cases of Down syndrome.
Down syndrome is one of the most common genetic disorders. It can result in a combination of birth defects, including some degree of mental impairment, certain facial features and, often, heart defects, vision and hearing impairment, and other health problems. Severity varies greatly from person to person. Life expectancy among adults with Down syndrome is about 55 years, but life spans vary widely with the severity of the condition.
Trisomy 13 and trisomy 18
Babies born with trisomy 13 have three number 13 chromosomes, and those with trisomy 18 have three number 18 chromosomes. These conditions are severe, and cause a combination of birth defects, including severe mental impairment and health problems affecting most organ systems. Unfortunately, 90 percent of babies born with trisomy 18 or 13 die by age 1, although a few have survived into their teens. Trisomy 18 is also called Edwards syndrome, and trisomy 13 is called Patau syndrome.
Turner syndrome, also called monosomy X, only affects girls. Girls with Turner syndrome have cells with only one X chromosome instead of the usual pair (either another X or a Y chromosome). They tend to be shorter than other girls their age and do not fully mature sexually as they grow up. Severity varies, and the condition can cause heart or kidney problems as well. Many problems affecting girls with Turner syndrome can be managed or corrected with the right medical treatment.