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Other Sources of Genetic Variation
Mendelian Genetics can only account for a majority of the genetic variations in species seen on earth. Other sources of Genetic Variation include Chromosomes Crossing over, Mutations, and Genetic Drift.
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How do you get a chromosome that is totally different from the original chromosomes of both parents? Crossing-Over takes place during Meiosis. During the process, DNA lengths are shared between chromosomes. In the illustration at left, two chromosomes intertwine, and exchange one end of the chromosome with the other. The end Chromosome has a completely different chemical composition from the starting two chromosomes. |
Mutation takes place when an organism undergoes a spontaneous genetic change during replication. During the process of replication, the nucleotides of a chromosome are altered, so rather than creating an identical copy of DNA strands, there are chemical variations in the replicated strands. The alteration on the chemical composition of DNA triggers a chain reaction in the genetic information of an individual. For example, the mutation toward sickle cell anemia is based upon the production of one different amino acid, which in turn affects the polypeptide strands produced, etc. Mutations are usually non-beneficial to an organism, however, they are almost always recessive and unless two mutations are coupled together the mutation will not be expressed.
Gene flow refers to the passage of traits or genes between populations. The passage of genes from one population to another prevents high occurrences of mutation, and genetic drift. In genetic drift, random variation occurs because the genetic population is small, leading to the proliferation of specific traits within a population. For example, the population in the colonial history of Martha's Vineyard, settled in Massachusetts in 1642, had an unusually high occurrence of deafness among it's inhabitants. The high occurrence of deafness was a result of genetic drift, in that the population was so small that differing traits from outside populations could not enter in. To prevent genetic drift, genetic material must be shared between differing populations, even so, variations can occur. For example, the trait for sickle cell anemia is beneficial in some climates where there is a high rate of malaria. An individual with a heterozygous genotype for sickle cell anemia is more resistant to malaria. The concept of gene flow consequently ties directly into the concept of adaptation and natural selection.
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