Genetic defects vary from trivial to life threatening. Although we tend to think of inherited conditions such as diabetes and muscular dystrophy as diseases, we often refer to cleft palates or color blindness as inherited defects. However, they are all the result of mutations in DNA, the genetic material. Not only are some diseases directly caused by mutations, but susceptibility to infectious disease and other damaging environmental factors, such as radiation, is also influenced by a variety of genes.
Precise rates of mutation are difficult to estimate, but for humans and apes, the mutation rate is around 5.0 × 10-8 per kilobase of DNA per generation. For rodents, the rate is some 10-fold less because fewer cell divisions are needed to form gametes from ancestral germ cells. Thus a considerable number of mutations are constantly accumulating in the germline of humans and other animals. Most of these have little or no effect, but a small percentage give rise to serious hereditary defects. A full listing of human hereditary defects, known as OMIM (Online Mendelian Inheritance in Man), is available on the Internet at http://www.ncbi.nlm.nih.gov/ entrez/query.fcgi?db=OMIM.
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