Lethal Genes in Man

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Lethal Genes in Man

Lethal Genes in Man
In man several hereditary diseases have lethal effects. Few important lethal genes of man are following:
1. Congenital ichthyosis- One of the most typical cases of a recessive lethal gene in man is expressed in congenital ichthyosis. At birth children afflicted with this disease have a crusted leathery skin with deep fissures down to the subcutaneous tissue; the fissures lead to bleeding, infection and death. Congenital ichthyosis occurs only when there occur homozygous condition for its recessive lethal genes
2. Amaurotic idiocy-A recessive allele in homozygous condition causes a fatal disease called Amaurotic idiocy in juvenile stage. Bearers of this genotype begin to lose their eye sight between the age of four to seven years. The complete blindness is followed by mental degeneration and finally death before adolescence.

3. Cooley's anemia-Among certain African tribes, a codominant gene Hb1s in homozygous condition produces a syndrome (disease) called sickle cell anemia which leads to death, generally At least by late adolescence. In the blood of suck persons the erythrocytes become distorted, many being essentially sickle-shaped. Such cells not only impede circulation by blocking capillaries, but also cannot properly perform their function of carrying oxygen and carbon dioxide to and from the tissues.

Under normal conditions, heterozygous (Hb₁^A Hb₁^S) manifest none of these symptoms, being outwardly indistinguishable from the normal homozygotes (Hb₁^A Hb₁^A).
Besides these cases of complete lethality in man, there are cases of sublethal genes. Sublethal genes produce less than 50% mortality. The examples of sublethal genes of man arc following:

1. Retinoblastoma-It is a human disease which is caused by a dominant mutant gene and is characterized by the growth of tumours in eyes. The gene causes mortality in 50% children only.
2. Epilopia- This disease is caused by a dominant lethal gene in heterozygous condition. It is characterized by mental deficiency, tumours in organs and abnormal growth of the skin. Persons sufferring from epilopia disease die in childhood but some of them survive and produce children.
3. Huntingdon's chorea-Huntingdon's chorea is lethal disease of man which is characterized by involuntary jerking of the body and a progressive degeneration of the nervous system, accompanied by gradual mental and physical deterioration. The patients of this disease die in the age of forty or forty five. This disease is caused by a dominant lethal gene.

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