Get complete information on Molecular Basis of Gene Mutation

( i) Inversion:

Base sequence of a segment is reversed.

(ii) Transition Substitution:

One nitrogen base is replaced by another of similar type. It is induced by tautomerisation (alternate state due to rearrangement of hydrogen atoms, – NH₂ into -NH or – CO, into – COH), deamination, base analogue and ionisation.

(iii) Transversion Substitution:

One nitrogen base is replaced by another of different type.

(iv) Insertion Frameshift:

Addition of a nucleotide changes the reading of codons.

(v) Deletion Frameshift:

Deletion of a nucleotide changes the reading of codons. Gene mutations often change them into inactive, recessive or lethal state.

Same Sense, Mis-Sense and Non-Sense Mutations:

Due to wobble position, a change in one neucleotide of a codon does not change amino-acid specificity. This is called same sense (or silent) mutation, e.g. AGA = AGG = AGC. When a nucleotide change in one codon causes the change of one amino acid of a polypeptide, it is called mis-sense mutation.

If a mutation causes early termination of polypeptide synthesis, it is termed as non-sense mutation.

Hardy Weinberg Genetic Equilibrium:

A Mendelian population is an interbreeding group of organisms sharing a common gene pool. A gene pool is the total genetic information possessed by reproductive members in a population of sexually reproducing organisms.

Alleles in the pool have dynamic relations with other alleles and with the environment in which the organism reside. Environmental factors that collectively provide selection tend to alter allele frequencies and thus to cause evolutionary changes in local populations.

In 1908, an English mathematician, G.H. Hardy and a German physician, W.Weinberg, independently discovered, the principle concerned with the frequency of alleles in a population, called the Hardy- Weinberg equilibrium principles.

It states that at equilibrium, both allele and genotype frequencies remain constant from generation to generation. This occurs among diploid, sexually reproducing organisms with nonoverlapping generations in which mating is random and no selection or other factors are present for changing the allele frequencies or as long as the following assumptions are met:

1. The population size is very large

2. Random mating is occurring

3. No mutation takes place

4. No genes are input from other sources

5. (No migration takes place)

6. No selections occur.

Dominant alleles thus do not, in fact, replace recessive ones, Because their proportions do not change, the genotypes are said to be in Hardy- Weinberg Equilibrium.

In algebric terms, the H.W. equilibrium principle is written as an equation. Its form is what is known as bionomial expansion. For a gene with two alternative alleles, which one will call B and b, the equation looks like,

(p + q)² = p² + 2pq + q²

In statistics, frequency is defined as the proportion of individuals falling within a certain category in relation to the total number of individuals under consideration. Thus, in a population of 100 cats, with 84 black and 16 white cats, the respective frequencies would be 0.84 (or 84%) and 0.16 (or 16%).

In the algebraic terms of this equation, the letter p designates the frequency of one allele, the letter q designates frequency of alternative allele. By convention, the more common of the two alleles is designated p and rarer q. Because there are only two alleles, p plus q must always equal 1.

If we assume that the white cats are homozygous recessive for a gene designated b, and the black cats are, therefore, either homozygous dominant BB or heterozygous Bb, we can calculate the allele frequencies of the two alleles in the population from the proportion of black and white individuals. If q² – 0.16 (the frequency of white cats), then q = 0.4.

Therefore, p, the frequency allele B, would by 0.6 (1.0 – 0.4 = 0.6). We can now easily calculate the genotype frequencies, there are p² = (0.6)² x 100 (the number of individuals in the total population), or 36 homozygous dominant BB individuals.

In the absence of factors that alter them, the frequency of gametes, genotypes and phenotypes remain constant genertion after generation.