Linkage and Mapping

Overview

This module addresses another situation that causes departure from Mendel: when two different traits are encoded by genes found on the same chromosome. Such genes do not assort independently, and do not follow Mendelian predictions. Crossing over during meiosis introduces another variable to the situation, but this has proven to be a useful tool for mapping the locations of genes on chromosomes. We will look at the concept of linkage, and learn how to use crossover rates to map genes.

Objectives

Understand the concept of linkage. Learn to recognize when two genes are linked.
Realize how the linkage of two genes results in a different set of gametes than if the two genes were unlinked.
Understand the nature of crossing over, and how this affects linked alleles.
Be able to map gene positions on a chromosome, using rates of crossover.

Linkage

Mendel's principle of independent assortment states that two traits assort independently during meiosis, but after the rediscovery of Mendel's work, it didn't take long before exceptions to this rule were observed. Examples were documented where one allele always assorted with another.

The explanation for this phenomenon is that each chromosome contains many more than one gene. For example, in humans, there are around 30,000-40,00 different genes in the haploid genome, and 23 chromosomes, for an average of about 1300 to 1700 genes per chromosome. Therefore, it shouldn't be too hard to find two genes on the same chromosome. Two such genes will segregate together during meiosis. Consider the following series of diagrams, and compare it to that of independent assortment.

Here is how two linked gene pairs might look in a double heterozygote, prior to meiosis. Note that only one homologous pair of chromosomes is shown.

Here are the homologous chromosomes after replication.

After the first meiotic division, the segregated chromosomes would look like this. In contrast to two unlinked genes, in this case there is only one segregation pattern, because there is only one homologous pair of chromosomes.

Here is how the cells would look after the second meiotic division. Note that there are only two types of gametes produced (in contrast with the four types in the case of unlinked genes). In this particular case, the dominant alleles always segregate together, and the recessive alleles always segregate together.

Next

1 | 2 | 3 | 4 | 5 | 6

Here is how two linked gene pairs might look in a double heterozygote, prior to meiosis. Note that only one homologous pair of chromosomes is shown.
Here are the homologous chromosomes after replication.
After the first meiotic division, the segregated chromosomes would look like this. In contrast to two unlinked genes, in this case there is only one segregation pattern, because there is only one homologous pair of chromosomes.
Here is how the cells would look after the second meiotic division. Note that there are only two types of gametes produced (in contrast with the four types in the case of unlinked genes). In this particular case, the dominant alleles always segregate together, and the recessive alleles always segregate together.