The possibility for a chromosomal error increases with advanced maternal age. Chromosome disorders are common, affecting about 7 of 1,000 liveborn infants. An exception to this rule is the category of sex chrom-osome disorders which tends to be associated with children with developmental problems and either tall or short stature. These disorders result from the interaction of small variations in multiple genes and environmental factors. Multifactorial conditions recur in families but do not show the characteristic inheritance patterns of single gene disorders.
Multifactorial disorders are very common and account for the majority of birth defects .
They also include many common chronic conditions such as asthma, diabetes, coronary artery disease, hypertension, schizophrenia, and certain forms of cancer. The smallest living unit and the basic building block of all living things. The human body is composed of millions of cells. The nucleus of every cell contains a complete genetic blueprint for that person. DNA is the biochemical structures of the genetic information that codes or instructs the genes.