Genetic Variation
In sexually reproducing species, genetic variation arises as a result of the behavior of chromosomes during meiosis and fertilization. Unlike mitosis, meiosis involves the independent assortment of chromosomes as well as crossing over. The process of random fertilization increases further genetic variation, but for now we will just focus on the variation that results from meiosis.
Before moving on, it is important to understand some basic terms associated with chromosomes: loci and alleles. In sexually reproducing organisms, genes exist as homologous pairs (with the exception of the X and Y chromosomes) that are at fixed locations called loci. The locus is simply the location of a gene on a chromosome. Genes that occupy the same locus are called alleles, and can be identical (homozygote) or vary (heterozygote). Alleles are responsible for a particular trait. Below is a diagram showing the alleles and locus on a pair of homologous chromosomes.
In sexually reproducing species, genetic variation arises as a result of the behavior of chromosomes during meiosis and fertilization. Unlike mitosis, meiosis involves the independent assortment of chromosomes as well as crossing over. The process of random fertilization increases further genetic variation, but for now we will just focus on the variation that results from meiosis.
Before moving on, it is important to understand some basic terms associated with chromosomes: loci and alleles. In sexually reproducing organisms, genes exist as homologous pairs (with the exception of the X and Y chromosomes) that are at fixed locations called loci. The locus is simply the location of a gene on a chromosome. Genes that occupy the same locus are called alleles, and can be identical (homozygote) or vary (heterozygote). Alleles are responsible for a particular trait. Below is a diagram showing the alleles and locus on a pair of homologous chromosomes.
Independent Assortment of Chromosomes
The random orientation of homologous chromosomes during metaphase of meiosis I contributes to the genetic variation found in sexually reproducing organisms. During metaphase I, the homologous pairs consisting of one maternal and one paternal chromosome orient themselves along the metaphase plate. The orientation along the metaphase plate is random. This random orientation results in either the paternal or maternal homolog being closer to a given pole. Furthermore, each pair of homologous chromosomes position themselves independently of other pairs during metaphase I. Now, the first meiotic division occurs with each pair sorting its maternal and paternal homologs into daughter cells independently of the other pairs. This independent sorting of the pairs is called independent assortment. Each daughter cell is the result of one unique possible combination of maternal and paternal chromosomes. The number of possible combinations due to independent assortment is equal to 2^n with n being the haploid number of the organism. In humans n=23, which results in about 8.4 million combinations of paternal and maternal chromosomes. Below is a short video illustrating this concept of independent assortment.
The random orientation of homologous chromosomes during metaphase of meiosis I contributes to the genetic variation found in sexually reproducing organisms. During metaphase I, the homologous pairs consisting of one maternal and one paternal chromosome orient themselves along the metaphase plate. The orientation along the metaphase plate is random. This random orientation results in either the paternal or maternal homolog being closer to a given pole. Furthermore, each pair of homologous chromosomes position themselves independently of other pairs during metaphase I. Now, the first meiotic division occurs with each pair sorting its maternal and paternal homologs into daughter cells independently of the other pairs. This independent sorting of the pairs is called independent assortment. Each daughter cell is the result of one unique possible combination of maternal and paternal chromosomes. The number of possible combinations due to independent assortment is equal to 2^n with n being the haploid number of the organism. In humans n=23, which results in about 8.4 million combinations of paternal and maternal chromosomes. Below is a short video illustrating this concept of independent assortment.
Crossing Over & Genetic Recombination
Crossing over occurs during prophase I. In crossing over, the homologous chromosomes wrap together or “cross over” each other. While intertwined, genetic material is exchanged resulting in genetic recombination. More specifically, crossing over occurs at the chiasma.
The figure to the left shows how the overlapping of chromosomes can result in genetic variation.
Below is an illustration that shows the results of crossing over and its effect on meiosis.
Crossing over occurs during prophase I. In crossing over, the homologous chromosomes wrap together or “cross over” each other. While intertwined, genetic material is exchanged resulting in genetic recombination. More specifically, crossing over occurs at the chiasma.
The figure to the left shows how the overlapping of chromosomes can result in genetic variation.
Below is an illustration that shows the results of crossing over and its effect on meiosis.
The genetic variation as a result of meiosis can be summed up in this short video.