Linkage is the major exception or modification to the law of independent assortment. Genes located reasonably closely together on the same chromosome tend to be transmitted together, from generation to generation, more frequently than chance alone would allow for; therefore, they are said to be linked. The closer together the 2 loci are, the less likely they are to be affected by crossovers. Linear physical proximity along a chromosome cannot be considered an automatic guarantor of linkage, however. In fact, certain sites on each chromosome may be more vulnerable to homologous crossing over than others.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.