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Main article: Chromosomal crossover
The most important and well-studied of these processes is the crossing over of two chromosomes during meiosis. After chromosomal replication, the four available chromatids are in tight formation with one another. During this time, homologous sites on two chromatids can mesh with one another, and may exchange genetic information. Immediately after replication, the tetrad formed by replication contains two pairs of two identical chromatids; after crossing over, each of the four chromatids carries a unique set of genetic information.
Enzymes known as recombinases catalyze the reactions that allow for crossover to occur. A recombinase will create a nick in one strand of a DNA double helix, allowing that strand to pull apart from its complementary strand and anneal to the double helix on the opposite chromatid. A second nick allows the second double helix to pull apart and anneal to the first, forming a structure known as a cross-strand exchange or a Holliday junction . The Holliday junction is a tetrahedral structure which can be 'pulled' by other recombinases, moving it along the four-stranded structure it creates. When the process is reversed, the two double helices have swapped DNA.
In most eukaryoteEukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. They include the animals, plants, and fungi, which are mostly multicellular, as well as various other groups called protists, many of whichs, a cellstained for keratin The cell is the structural and functional unit of all living organisms. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, (humans have an estimated 100,0 carries two copies of each geneDNA and to a chromosome (right). Introns are regions often found in eukaryote genes which are removed in the splicing process: only the exons encode the protein. This diagram labels a region of only 40 or so bases as a gene. In reality many genes are much, each referred to as an alleleAn allele is any one of a number of alternative forms of the same gene occupying a given locus (position) on a chromosome. An example is the gene for blossom color in many species of flower a single gene controls the color of the petals, but there may be. Each parent passes on one allele to each offspring. Even without recombination, each gameteGametes also known as sex cells, germ cells, or spores—are the specialized cells that come together during fertilization (conception) in organisms that reproduce sexually. In those species that produce two morphologically distinct types of gametes, and in contains a random assortment of chromatids, choosing randomly from each pair of chromatids available. With recombination, however, the gamete can receive a (mostly) random assortment of individual genes, as each chromosome may contain genetic information from two different chromatids.
Recombination results in a new arrangement of maternal and paternal alleles on the same chromosome. Although the same genes appear in the same order, the alleles are different. This process explains why offspring from the same parents can look so different. In this way, it is theoretically possible to have any combination of parental alleles in an offspring, and the fact that two alleles appear together in one offspring does not have any influence on the statistical probability that another offspring will have the same combination. This theory of " independent assortmentMendelian inheritance (or Mendelian genetics or Mendelism is a set of primary tenets that underlie much of genetics developed by Gregor Mendel in the latter part of the 19th century. Mendel (1822-1884), an Austrian monk, was interested in understanding va" of alleles is fundamental to genetic inheritance. However, there is an exception that requires further discussion.
The frequency of recombination is actually not the same for all gene combinations. This is because recombination is greatly influenced by the proximity of one gene to another. If two genes are located close together on a chromosome, the likelihood that a recombination event will separate these two genes is less than if they were farther apart. Genetic linkageGenetic linkage occurs when particular alleles are inherited together. Typically, an organism can pass on a allele without regard to which allele was passed on for a different gene. This is because chromosomes are sorted randomly during meiosis. However, describes the tendency of genes to be inherited together as a result of their location on the same chromosome. Linkage disequilibriumLinkage disequilibrium LD is the non-random association of alleles at two or more loci on a chromosome. It describes a situation in which some combinations of alleles or genetic markers occur more or less frequently in a population than would be expected describes a situation in which some combinations of genes or genetic markers occur more or less frequently in a population than would be expected from their distances apart. This concept is applied when searching for a gene that may cause a particular disease. This is done by comparing the occurrence of a specific DNA sequence with the appearance of a disease. When a high correlation between the two is found, it is likely that the appropriate gene sequence is closer.