QVprep Biology Lessons

Genetic variation, Polymorphic genes, Causes of genetic variation, Factors that cause genetic variation

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Genetic variation: Genetic variation refers to the variation in alleles of genes. It occurs both within and among populations. Genetic variation is important because it provides the genetic material for natural selection. Genetic variation is brought about by mutation, which is a permanent change in the chemical structure of a gene. Polyploidy is an example of chromosomal mutation. Polyploidy is a condition wherein organisms have three or more sets of genetic variation. Genetic variation can also be identified by identifying phenotypic variation, genotypic variation or examining variation at the level of enzymes using the process of protein electrophoresis.


Polymorphic genes: Polymorphic genes have more than one allele at each locus. Half of the genes that code for enzymes in insects and plants may be polymorphic, whereas polymorphisms are less common in vertebrates.


Causes of genetic variation: Ultimately, genetic variation is caused by variation in the order of bases in the nucleotides in genes. Examination of DNA has shown that genetic variation can occur in both coding exon regions and in the non-coding intron region of genes.


Results of Genetic Variation: Genetic variation will result in phenotypic variation if variation in the order of nucleotides in the DNA sequence results in a difference in the order of amino acids in proteins coded by that DNA sequence, and if the resultant differences in amino acid sequence influence the shape, and thus the function of the enzyme.


Geographic variation in genes: Geographic variation in genes often occurs among populations living in different locations. Geographic variation may be due to differences in selective pressures or to genetic drift.


Factors that cause genetic variation: Mutations are the ultimate source of genetic variation. Mutations are likely to be rare and most mutations are neutral or deleterious, but in some instances the new alleles can be favored by natural selection. Genetic variation can also be produced by the recombination of chromosomes that occurs during sexual reproduction, called independent assortment Crossing over and random segregation during meiosis can result in the production of new alleles or new combinations of alleles. Furthermore, random fertilization also contributes to variation. Variation and recombination can be facilitated by transposable and transposed genetic elements, commonly known as endogenous retroviruses, LINEs, SINEs, etc. A variety of factors maintain genetic variation in populations. Potentially harmful recessive alleles can be hidden from selection in the heterozygous individuals in populations of diploid organisms. Natural selection can also maintain genetic variation in balanced polymorphisms that occur when heterozygotes are favored or when selection is frequency dependent.