Basic Genetics

Human cells contain 46 chromosomes each (diploid number); each sperm and each egg contain 23 chromosomes (haploid number). Chromosomes are composed of DNA and proteins that contain genes. Each cell contains 23 pairs of chromosomes-one chromosome of each pair is inherited from each parent.

Of the 23 pairs, 22 are autosomes (nons ex chromosomes) and one pair is the sex chromosomes, X and Y. In a normal woman, the sex chromosomes are represented by two X (XX) chromosomes, and in the normal man by X and Y (XY) chromosomes.
Chromosome analysis usually involves blood cells. Chromosomes are viewed through a microscope, then photographed and enlarged. They are then arranged in pairs, called a karyotype; a karyotype is used to identify abnormalities in the number of chromosomes.

With recent advances, doctors can examine individual chromosomes for abnormalities. We have learned that too much or too little chromosome material can cause an abnormality.

Incidence Of Genetic Disease
The hoped-for and anticipated outcome for every pregnancy is a normal, healthy baby. It is expected and taken for granted. However, that is not always the case. Almost half of all l st-trimester miscarriages are caused by some type of chromosomal abnormality. About 1 % of all babies born have a chromosomal abnormality; 4 to 7% of perinatal deaths (death after birth) are attributed to chromosomal problems.

Among newborns, 3 to 5% are born with a major congenital malformation, such as a heart defect; many of these birth defects have a genetic component. In the United States, this represents more than 250,000 children each year.

Mental retardation is another area of concern; about 3% of the general population is mentally retarded. It is estimated that 70% of mental retardation is caused by genetic problems. Genetic counseling may offer an indication this might occur.