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Linkage vs. Independent Assortment

Chromosomes contain hundreds to multiple thousands of genes. Genes that are on separate chromosomes are inherited independently of one another and are said to follow the principle of independent assortment (discussed in another section and illustrated by the second part of this animation). Remember that this principle states that an RrYy individual is expected to make equal numbers of RY, Ry, rY and ry gametes.

Genes that are located on the same chromosome are not free to participate in independent assortment and are referred to as linked genes. These genes tend to be inherited together as a unit, as you can see in the first part of this animation. In this case, the RrYy individual makes only RY and ry gametes because these alleles are present together on the parental homologs (shown in blue and pink) and are not separated during gamete formation.

When two genes are always inherited together in this manner, it is referred to as complete linkage. Complete linkage creates only parental gametes, meaning that the gametes that are produced are chromosome copies of the parent they came from. If all genes demonstrated complete linkage during gamete formation, then the result would be that each individual chromosome in a gamete would be either a maternal or paternal replica and the offspring would show little genetic variation from their parents. For this reason, complete linkage rarely occurs in nature and other modes of inheritance, such as crossing over, have evolved.





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