Genotype Variation, Mutations and Recombination

Introduction
The genotype is the specific genetic makeup of an individual, in the form of DNA. and it codes for the phenotype of that individual. Any given gene will usually cause an observable change in an organism, known as the phenotype. However, genotype and phenotype are not always directly correlated. Some genes only express a given phenotype in certain environmental conditions. Conversely, some phenotypes could be the result of multiple genotypes.

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

Recombination due to crossing over at chiasmata This only affects recombination in genes located on the same chromosome. It occurs in Prophase I when homologous chromosomes pair up (one paternal and one maternal) a process called “synapsis”. The paternal and maternal chromosomes cross over forming chiasmata and exchange genetic information. Thus the recombinant sister cromatid formed contains genetic information from both father and mother.

Other mechanisms
 * 1) Conservative site specific recombination: This occurs when a mobile element of DNA is inserted into another strand of DNA. This is possible when the mobile element posses a segment of DNA that matches exactly the other strand, therefore allowing enzymes called integrases to insert the rest of the mobile element into the target strand. Site specific recombination systems are employed in many cellular processes, including bacterial genome replication, pathogenesis and differentiation. These systems present a potential basis for the development of genetic engineering tools.
 * 2) Transpositional recombination: Does not require an identical strand of DNA in the mobile element to match with the target DNA. The integrases involved introduce nicks in both the mobile element and the target DNA, allowing the mobile DNA to enter the sequence. The nicks are then removed by ligases.