I am going to deal mainly with genetic testing as it relates to pregnancy. Every human being has genes , these tiny little switches that determine hair color, eye color, and other traits are packed into 46 chromosomes inside our cells. The human sperm and egg cells are different from other cells in that each only has 23 unmatched chromosomes inside. When pregnancy begins and the egg and sperm are joined you start with a brand new cell, with 46 chromosomes. Each gene's instructions are either dominant or recessive.
Problems caused by dominant disorders can be either nonexistent or quite severe.
If only one parent has a recessive disorder gene, then the dominant gene from the other parent will prevent the disorder. If both parents were carriers of a recessive gene then there would be a one in four chance that each child would inherit the disorder. Recessively inherited disorders are often more serious. There are other types of disorders, such as X linked disorders, and carriers. This is a blood test done between 15 - 17 weeks of pregnancy.