[priyasaravanan_1406]

The presence of a mutant gene, chromosomal aberration and the complexity of the relationship of group of genes with the environmental factors (mutagens) are all well exhibited by various common and rare kind of diseases. The diseases developed as a result of mutation are classified under genetic diseases. A defective single gene (mutant) is powerful enough to develop a disease which may be even fatal to the affected person. The genetic disease is either inherent or found to develop in the offspring alone as a result of mutant gene present in the sex chromosomes.

Inheritance is the passing of disease from one generation to the next which usually occurs in three ways. They are Autosomal dominant inheritance, Autosomal recessive inheritance and X-linked inheritance.

Autosomal dominant inheritance: The presence of mutated allele in either one of the parents causes disease in the offspring. For example, combination of a male parent with one mutant allele (dD) with a normal female parent (DD) produces two mutated offspring (dD) and two normal offspring (DD). The individual with one normal allele (D) and one mutated allele (d) is called as a heterozygous individual (dD).

Autosomal recessive inheritance: The disease is developed in the offspring by the inheritance of mutant alleles one each from both the parents. For example, combination of a carrier male parent (Dd) with a carrier female parent (Dd) results in two carriers (Dd), a normal offspring (DD) and an affected offspring (dd) also called as homozygous individual.

X-linked inheritance: The presence of mutant allele in X – chromosome causes X-linked inheritance. The male population is the affected group because of the presence of only one X chromosome (XY) whereas the female with two X chromosomes (XX) may usually be carriers.

The point mutation and the gross mutation are to be blamed for the defective single gene disorders. Missense/silent mutations, nonsense mutation, frameshift mutation, splice site mutation and promoter mutation falls under one roof called point mutation. The insertion and deletion mutation, gene rearrangement and trinucleotide repeat mutation are classified as gross mutation. Some of the single gene mutation diseases are Hemophilia A and B, Thalassemia, sickle cell anemia, Duchenne muscular dystrophy, Becker muscular dystrophy, Fragile X syndrome, Huntington’s disease, Neurofibromatosis, Phenylketonuria and cystic fibrosis.

Hemophilia A: A condition of excess bleeding developed due to the mutation of Factor VIII gene and the mutation type is either frame shift or insertion or deletion mutation and it is a X-linked inherited disease.

Hemophilia B: Excess unusual bleeding due to the mutation of the promoter gene responsible for protein Factor IX which stimulates clotting of blood. Mutation of the gene arrests the blood clotting property of Factor IX causing unusual bleeding in the affected person and it is an X-linked inherited disorder.

Thalassemia: The anemic condition due to either splice site mutation or nonsense mutation of the gene responsible for β-globin resulting in the termination of β globin synthesis causes β Thalassemia. This is Autosomal recessive inherited disorder.

Sickle cell Anemia: The Autosomal recessive inherited disease occurs as a result of misense mutation to the gene coding β globin. Sickle cell anemia is represented by the presence of short lived sickle shaped red blood cell causing anemia and ischemia.

Huntington’s disease: The repetition of trinucleotide sequence causing mutation in the gene sequence coding for huntingtin causes Huntington disease, a Autosomal dominant inherited disorder. The disease is characterized by dementia.

Cystic Fibrosis: The Autosomal recessive disorder associated with lung damage symptoms due to the deletion mutation of the CFTR gene.

Neurofibromatosis: Autosomal dominant disorder due to the mutation of the NF-1(Type 1 disease) and NF-2 (type 2 disease) gene causing tumor of the nerve tissues.

If we see the Autosomal recessive type disorder, both the parents acts as carriers and the product receiving both the mutant allele (one from the father and the other from the mother) becomes subject for the genetic disease. The parents may be informed about the possibility of genetic disease in their child in advance if both the parents are identified as carriers. Gene Tracking is the technique employed to detect the carriers in a family. This involves the detection of restriction fragment length polymorphisms in a genomic DNA sample by southern blotting technique and also by using DNA repeat sequences like minisatellite DNA and microsatellite DNA.