Online Open Genetics

Lesson 3 - Linked Genes

Mendel was lucky.  He studied a variety of traits in pea plants and his data were consistent with them being encoded by pairs of individual heritable units.  He didn’t call these “genes”; he had no idea about their chromosomal origins or chemical makeup.  It turns out that the genes he studied were either on different chromosomes and so assorted independently, or so far apart on the same chromosome that linkage could not be detected.

Symbols for a gene can be drawn on a page to communicate their position on a chromosome.  To do this, we use a forward slash (/) to demonstrate what is on each chromosome.  The figure below shows a cell that is heterozygous for A.  Click on it to see how the chromosomes can be simplified to be represented using a "slash".  Note that for simplicity, chromosomes are shown as unreplicated.

If we add another gene to the mix, it may be on the same chromosome as A, or on a different chromosome.  If it is on a different chromosome, it will be represented with a second slash and separated with a semicolon.  However, if it is on the same chromosome, both genes will occur on the left and the right of the slash.  No semicolon is used.  Finally, if there are three genes with only two linked, a combination of these rules will follow.  Click on each diagram below to see the shorthand method of demonstrating linkage.  Note that C is underlined when capitalized to make it clear.

unlinked genes linked genes in coupling linked genes in repulsion three genes, with two linked

Note that the middle two diagrams both represent a cell that is heterozygous for both A and C. However, their gametes will have different proportions of the combinations AC, Ac, aC, and ac.  The combination with both dominant traits together is called coupling while the second is repulsion.  Recombination between chromosomes that have alleles arranged in coupling results in ones that are in repulsion, and vice-versa.  You will use this information extensively when you do gene mapping!

Background image credit greyloch via flickr/