Genetic Effects

Genetic information necessary for the production and functioning of a new organism is contained in the chromosomes of the germ cells - the sperm and the ovum. The normal human somatic cell contains 46 of these chromosomes; mature sperm and ovum each carry 23 chromosomes. When an ovum is fertilized by a sperm, the resulting cell, called a zygote, contains a full complement of 46 chromosomes. During the 9-month gestation period, the fertilized egg, by successive cellular division and differentiation, develops into a new individual. In the course of the cellular divisions, the chromosomes are exactly duplicated, so that cells in the body contain the same genetic information. The units of information in the chromosomes are called genes. Each gene is an enormously complex macromolecule called deoxyribonucleic acid (DNA), in which the genetic information is coded according to the sequence of certain molecular and sub-assemblies called bases. The DNA molecule consists of two long chains in a spiral double helix. The two long intertwined strands are held together by the bases, which form cross-links between the long strands in the same manner as the treads in a step-ladder.

The genetic information can be altered by many different chemical and physical agents called mutagens, which disrupt the sequence of bases in a DNA molecule. If this information content of a somatic cell is scrambled, then its descendants may show some sort of an abnormality. If the information that is jumbled is in a germ cell that subsequently is fertilized, then the new individual may carry a genetic defect, or a mutation. Such a mutation is often called a point mutation, since it results from damage to one point on a gene. Most geneticists believe that the majority of such mutations in man are undesirable or harmful.

In addition to point mutations, genetic damage can arise through chromosomal aberrations. Certain chemical and physical agents can cause chromosomes to break. In most of these breaks, the fragments reunite, and the only result may be a point mutation at the site of the original break. In a small fraction of breaks, however, the broken pieces do not reunite. When this happens, one of the broken fragments may be lost when the cell divides, and the daughter cell does not receive the genetic information contained in the lost fragment. The other possibility following chromosomal breakage, especially if two or more chromosomes are broken, is the interchange of the fragments among the broken chromosomes, and the production of aberrant chromosomes. Cells with such aberrant chromosomes usually have impaired reproductive capacity as well as other abnormalities.

Studies suggest that the existence of a threshold dose for the genetic effects of radiation is unlikely. However, they also show that the genetic effects of radiation are inversely dependent on dose rate over the range of 800 mrad/min (8 mGy/min) to 90 rads/min (0.9 Gy/min). The dose rate dependence clearly implies a repair mechanism that is overwhelmed at the high dose rate. Geneticists estimate that there are 320 chances per million of a "spontaneous" mutation in a dominant gene trait of a person. The radiation dose that would eventually lead to a doubling of the mutation rate is estimated to be in the range of 50-250 rads (0.5-2.5 Gy).