204
CHAPTER FOURTEEN
BIRTH DEFECTS
Centuries ago in the land of Judea the disciples asked a question, “Who did sin, this [blind] man, or his parents, that he was born blind?” A prevailing idea at that time was that some transgression lay at the foundation of all congenital disease. It is true that conditions classed as birth defects may be related to parental transgression. Evidence implicating drugs (cocaine, crack, LSD, etc.) as the cause of many chromosome defects and genetically transmitted disease accumulates every year. Some pharmaceutical drugs affect the growing embryo during key developmental phases. They may result in deformities of the cranium, limbs, heart, kidneys, or sense organs. Infectious diseases, such as syphilis, toxoplasmosis, and the cytomegalovirus can all produce serious damage to the unborn child. In order to understand genetic diseases let us now consider some methods by which information is passed to our offspring. In the nucleus of every cell lie specialized strands of nucleic acids called chromosomes. In human cells there are twenty-three pairs of these. Men and women differ only in the presence or absence of a Y chromosome (male — XY) or a pair of X chromosomes (female — XX). During the division of nonreproductive cells (mitosis) the chromosomes divide and duplicate themselves, forming identical nuclei in the “daughter” cells. When a reproductive cell divides, however, its chromosomes split and each resulting spermatozoa or ovum receives only one-half of each of the original twentythree pairs, or half the complement of the fertilized ovum. This process (meiosis), then, results in cell division without duplication of original chromosome pairs and prepares the mature sex cell for fertilization. In the chromosome lie a vast number of possible combinations (genes), each of which has the capability of governing growth, determining protein