Background
At the beginning of normal meiosis, a homologous chromosome pair (called a bivalent, made up of a chromosome from the mother and a chromosome from the father) intertwine and exchange sections or fragments of chromosome. The pair then breaks apart to form two chromosomes with a new combination of genes that differs from the combination supplied by the parents. Through this process of recombining genes, organisms can produce offspring with new combinations of maternal and paternal traits that may contribute to or enhance survival.
This recombination of genes, called the crossing over of DNA, can cause alleles previously on the same chromosome to be separated and end up in different daughter cells. The farther the two alleles are apart, the greater the chance that a cross-over event may occur between them, and the greater the chance that the alleles are separated.
The relative distance between two genes can be calculated by taking the offspring of an organism showing two linked genetic traits, and finding the percentage of the offspring where the two traits do not run together. The higher the percentage of descendants that do not show both traits, the farther apart on the chromosome the two genes are. Genes for which this percentage is lower than 50% are typically thought to be linked.
Genetic linkage can also be understood by looking at the relationships among phenotypes. Among individuals of an experimental population or species, some phenotypes or traits can occur randomly with respect to one another, or with some correlation with respect to one another.
The former is known as independent assortment. Today, scientists understand that independent assortment occurs when the genes affecting the phenotypes are found on different chromosomes or separated by a great enough distance on the same chromosome that recombination occurs at least half of the time.
The latter is known as genetic linkage. This occurs as an exception to independent assortment, and develops when genes appear near one another on the same chromosome. This phenomenon causes the genes to usually be inherited as a single unit. Genes inherited in this way are said to be linked, and are referred to as "linkage groups". For example, in fruit flies, the genes affecting eye color and wing length are inherited together because they appear on the same chromosome.
Read more about this topic: Genetic Linkage
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