Where Does Genetic Variation Come From?
Genetic variation is a very important aspect of survival for any given species. But where does this genetic variation come from? There are five factors that influence the genetic variation of organisms: mutations, crossing-over, segregation, independent assortment and fertilization.
Many events can cause damage to the DNA molecule, such as x-rays, chemicals like nicotine. When this damage occurs in a sexual reproductive cell, that influences the next generations and may cause mutations. Sometimes an entire strain of DNA is damaged, resulting in a decrease or lack of the cell’s ability to produce proteins. This is called a chromosome mutation and is a serious matter. If the damage is large enough, the cell can die. When too many cells die, so does the organism. But worse is when the cells start to divide and spread uncontrollably, as is the case with cancer.
At the very beginning of the meiotic process (the division that produces gametes), when the homologous chromosomes join, crossing-over occurs. This leads to a new combination of genes and a new chance of variation. While the chromosomes lie close to each other, they exchange equal portions of chromatids, and thus ‘trade’ genes. This can occur at a number of places and paves the way for a whole collection of possible genetic variations. Crossing-over is one of the processes that make it possible that you have the hair color of your father and the bone structure of your mother.
Segregation is the process that directly follows crossing-over, when the chromosomes part ways and move to the poles of the cell. At this point, alleles split up in such a way that one goes to the first daughter-cell and the other one to the second daughter-cell. Both alleles now have an equal chance to be passed to the next generation.
If, during meiosis, only one pair of chromosomes would split and move to the poles of the cell, two genetically different gametes would be produced. The cell that splits up to produce gametes, however, contains 23 pairs of chromosomes and all of these splice and move independently to the poles of the cell. This independent assortment is purely coincidental, only determined by the position of the chromatids in the equatorial plane before the chromosome splices. So how many genetic variations are possible through independent assortment? The answer is 2 to the power of 23, or 8.388.608.
And that is without crossing-over or mutations.
The female individual of a species thus possesses potentially millions of genetically different egg cells, while the male individual has millions of genetically different sperm cells. So, when the male and female gametes join, the possibilities for variation are virtually unlimited. That is why every human that has ever been born is genetically unique, except, of course, identical twins, which develop from the same fertilized egg cell.
These five factors, mutations, crossing-over, segregation, independent assortment, and fertilization, account for an incredible amount of genetic variation among sexually reproducing species.