3. Genetic Variability

Genetic variability is a measure of the tendency of individual genotypes in a population to vary from one another. The variability of a trait describes how much that trait tends to vary in response to environmental and genetic influences. It is important to note that the sequence of nuclear DNA between any two humans is nearly 99.9% identical, and yet it is that 0.01% of DNA sequence differences that cause genetically determined variability among humans. On the other hand some DNA sequence differences have little or no effect on phenotype whereas others are directly responsible for causing disease. Between these two extremes the difference in DNA sequence is responsible for variation in phenotype, character, talents, susceptibility to specific diseases etc.

Basic forms of variation

1.Continuous variation: This is the case where the individuals in a population show a graduation from one extreme to another. For example, height of individuals in the human population follows a normal distribution curve (bell-shaped curve). Characteristics which show continuous variation are controlled not by one but by the combined effect of a number of genes and is called a polygene. Thus any characteristic which results from the interaction of many genes is called polygenic inheritance. The variable assortment of the genes during prophase 1 of meiosis ensures that individuals posses a range of genes from any polygenic complex.

2.Discontinuous variation: This is the case where there is a limited number of distinct forms within the population in other words there are no intermediate phenotypes. For example humans may be separated into groups according to their blood type i.e. 4 groups.

Recombination

Genetic recombination is the process by which the combinations of alleles observed at different loci in two parental individuals become shuffled in offspring individuals. Such shuffling can be the result of recombination via intra-chromosomal recombination (crossing over) and via inter-chromosomal recombination (also called independent assortment). In other words, it is a process by which a breaking of a strand occurs and then rejoined to a different DNA molecule therefore the offspring now having a different combination of alleles from their parents.

The crucial events of meiosis are those which are responsible for recombination, which means that the combinations of alleles passed by individuals to their offspring differ from those that were passed to the individuals by their parents. This helps to a level of genetic variation.

Independent assortment

Each pair of homologous chromosomes consists of one chromosome inherited from the father and one inherited from the mother. When a pair of homologous chromosomes separate/segregate at anaphase I, one member of each pair moves to opposite poles of the cell. It is important to note that the process is not selective to which chromosome of the homologous pair, paternal or maternal, is going to move to a specific pole of the cell. Therefore the two daughter cells contain new combinations of maternally and paternally inherited chromosomes. Hence we say that we have recombination due to independent assortment (on the equatorial plate) in metaphase I

Mutation

It is defined as a change in the DNA sequence of a cell's genome. Mutations can be divided into 3 classes or categories:

• Genome mutations: Mutations that affect the number of chromosomes.

• Chromosome mutations: Mutations that alter the structure of individual chromosomes. Also known as Gross mutations.

• Gene mutations: Mutations that alter individual genes.

All 3 types of mutations occur quite often in many different cells. However, if a mutation occurs in a germline cell, it may be passed on to future generations. On the other hand somatic mutations occur by chance in a subset of cells in certain tissues and result in somatic mosaicism that cannot be transmitted.

It is important to note the fact that many types of mutations are represented among the millions of DNA variants found throughout the genome in the normal population as well as among the vast numbers of alleles at individual loci in thousands of genetic disorders. Also another important pointer is that, mutations are the drive force of evolution but they can also be pathogenic.