Incomplete linkage, crossing over
As you recall from the Basic Concepts portion of this web site, crossing over is the process by which homologous chromosomes exchange portions of their chromatids during prophase I of meiosis. Thomas H. Morgan is credited with first describing this phenomenon. He proposed that linked genes exist in a linear order along the chromosome and that a variable amount of genetic exchange could happen between any two linked genes.
The result of this exchange of genetic information is shown in the animation to the right. As you can see, portions of the black chromosome (shown in blue) are transferred to the red chromosome, and portions of the red chromosome (shown in yellow) are transferred to the black chromosome. Thus, the resulting gametes, called recombinants, will contain chromosomes that have unique gene combinations. Crossing over is an extremely important mechanism for increasing genetic diversity in animals.
The frequency with which crossing over occurs between any two linked genes is proportional to the distance between them. in other words, as the distance between two genes increases, the probability that a crossover will occur between them also increases. Genes that are completely linked do not show crossing over because they are too close to each other along the chromosome. Therefore, if two genes are completely linked there will be no recombinant gametes. Conversely, as the distance between two genes increases, the proportion of the recombinant gametes increases as well.
The percent recombination between two linked genes can be directly converted into units of genetic distance called centi- Morgans or map units. While these genetic distances are only estimates of the true physical distance between two genes, they are still extremely valuable, as we will discuss in the Applications portion of the web site.