Hereditary disorders, by definition, are derived from one's parents and are transmitted in the germ line through the generations. The term «congenital» simply implies «born with». Some congenital diseases are not genetic, as, for example, congenital syphilis. Not all genetic diseases are congenital; patients with Huntington's disease, for example, begin to manifest their condition only after the twenties or thirties.
Hereditary and some congenital diseases result from mutations.
MUTATIONS
Mutation is defined as a stable, heritable change in DNA. Mutations in somatic cells may be relevant to cancer or aging; mutations in germ cells have their impact on offspring of an individual. Types of mutations are the following (see fig. 3).
The concept that mutations are stable changes remains generally true, but the discovery of expanding triplet repeat mutations emphasizes that some mutations can be unstable either in somatic or in germ cells.
Mutations occur spontaneously during the process of DNA replication. Certain environmental influences, such as radiation, chemicals, and viruses, increase the rate of the so-called spontaneous mutations.
Classification of mutations
Based on the extent of genetic change, mutations may be classified into 3 categories.
I. Single-base or point mutations.
II. Chromosome mutations.
III. Genome mutations.
I. Single-base or point mutations
These involve only one or several base pairs in a gene.
Types of single-base mutations:
a) substitutions;
b) insertions;
c) deletions;
d) amplification of trinucleotide repeats.
II. Chromosome mutations
Chromosome mutations result from rearrangement of genetic material and give rise to structural changes in the chromosome. These involve millions of base pairs in the structure of a chromosome. Chromosome mutations include duplications, inversions, deletions, and translocations of a portion of one chromosome to another.