Genetic Variation

Define: The variety of alleles in a population (for a specific characteristic)

Genetic variation is crucial to the survival of any species. It is what natural selection acts on and can ultimately lead to genetic change and evolution.

Each organism inherits two versions of ever gene called alleles, one from each parent. These two alleles are found on seperate chromosomes and separate from each other when gametes are formed during meiosis with one allele ending up in each gamete.

SEGREGATION OF ALLELES

Therefore during segregation, each gamete receives a single allele for a gene which results in a unique combination of alleles in the gamete and therefore increases genetic variation possible within a population.

This increases genetic

This increases genetic variation as half the gametes get one version of the gene and half get the other version. The offspring produced from each type of gamete will be genetically different (This happens with every pair of chromosomes in the cell).

INDEPENDENT ASSORTMENT

During meiosis the pairs of homologous chromosomes line up on the equator (bivalents). The chromosomes are pulled to opposite sides of the nucleus by spindle fibres and more end up in different gametes.During independent assortment, a pair of homologous chromosomes seperate randomly which results in each daughter cell having a unique set of chromosomes. This process increases genetic variation by creating a huge variety of gametes.The gametes will end up with the haploid number of chromosomes, but the origin of any particular one will be randomly selected from paternal or maternal chromosomes. As a result independent assortment is the major source of the genetic variability of offspring.

CROSSING OVER

When the homologous chromosomes pair up on the equator during part 1 of meiosis, sections of genetic material can be exchanged between the inner chromatids of the chromosomes. This results in a reshuffling of alleles, called recombination. This increases genetic variation by creating new combinations of alleles not found in the parents.Therefore crossing over/recombination is the process whereby there is an exchange of sections of homologous chromosomes resulting in different allele combinations than were present in either parent. This makes chromatids unique and therefore increase genetic variation within the gametes.Because recombination is random, its effect on the level of variation is different in each case, but has the potential to add significant changes to the already high degree of variability caused by independent assortment.

Sexual Reproduction (excluding Meiosis)

Random Fusion of Gametes

Once meiosis has occurred in the reproductive organs and the gametes have been produced, fertilisation of gametes occurs. Which gametes are fertilised in an organism is random, further increasing genetic variation within the species.

Random Selection of Mates

Mate selection also plays a part in which individuals contribute gametes, further increasing genetic variation within the species.

Mutations

The rate of mutations is normally very low, but certain agents called mutagens, can increase the rate at which they occur.These include Radiation e.g. UV, X rays Chemicals Temperature e.g. increasing temp of reproductive cells can increase mutation rates

Mutations occur spontaneously in two types of cells: Somatic cells (normal body cells) - Not inherited by offspring as they do not occur in gametes therefore only affect the organism they occur in Gametic mutations (gametes/sex cells)- Will be passed onto the next generation as long as the gamete is fertilised

Mutations that occur in gamete-producing cells cause a permanent, inheritable change in the DNA that can be passed onto the gene pool a population. As DNA codes for proteins, this can result in a non-functioning protein, which will often have a detrimental affect on the organism. Most mutations are recessive, which means that they are hidden and can remain in the population for a long time.Mutations create NEW ALLELES for NATURAL SELECTION processes and obviously contribute to increasing the genetic variation available within a gene pool.

Mate SelectionMany species select the mate they wish to have offspring with based on desirable phenotypes. This means certain combinations of alleles will come together. (Random = variation). This is not the case with all species as in some cases no mate selection is involved e.g. plants.

Introduction

Meiosis

Other Sources of Genetic Variation in Individuals