Some recessive genetic defects are carried only on the X chromosome, which means that mostly guys get them because they have only one X chromosome. Girls have two X chromosomes, so they would need to inherit two copies of the recessive gene in order to get the disease. Sometimes when the egg and sperm unite, the new cell gets too many or too few chromosomes. Children born with Down syndrome, which causes mental retardation, have an extra chromosome number 21. People who are concerned that they may carry defective genes can go through genetic testing so they can learn their children's risk of having a disease.
Pregnant women can also have tests done to see if the fetus they are carrying has genetic illnesses.
Genetic testing usually involves taking a sample of someone's blood or other bodily fluid, and checking it for flags, or indicators, of diseases or disorders. For specific medical advice, diagnoses, and treatment, consult your doctor.