By studying how traits were inherited in pea plants, Gregor Mendel was able to formulate two basic laws of inheritance. The first, the law of segregation, states that each gamete (egg or sperm) cell contains only one version of the parent gene. The second law of inheritance, or the law of independent assortment, states that each version of a gene is inherited independently from other genes. Today, we know that some genes are in fact "linked" and are inherited together, but for the most part Mendel's laws have proved surprisingly robust.
Mendel, known today as the "Father of Genetics," was an Austrian monk who put his extensive knowledge of gardening to use unraveling some of the basic mysteries of inherited traits. Although Mendel's work was published in 1866, it was widely ignored until three European scientists, Hugo de Vries, Carl Correns and Erich von Tschermak, "rediscovered" his ideas in 1900.
Most organisms are diploid, which means that we carry two alleles (or versions) of each gene encoded in our DNA. We inherit one allele for each gene from our mothers, and one from our fathers. These alleles interact in various ways to determine what the final product for the gene will be.
All genes encoded in our DNA are arranged in pairs of chromosomes, which are threadlike structures found in the nuclei of our cells. Humans have 23 pairs of chromosomes.
Mendel's second law of inheritance is applied during meiosis, or the type of cell division responsible for the production of egg and sperm (gamete) cells. During meiosis, each chromosome pair splits and contributes one allele for every gene to the gamete cell.
If alleles assort independently of each other as predicted by Mendel's second law, then there would be no correlation between different traits seen in organisms. For example, there would be no relationship between flower color and stem length in the pea plants Mendel studied. If traits did not assort independently, we might see that purple flowers always occurred along with tall pea plants. But as Mendel showed when formulating his second law, there was no such relationship between traits.
For most traits Mendel's second law holds true. But we now know that alleles for some genes are, in fact, inherited together and do not assort independently. These genes are called "linked genes" and are located close together on the same chromosome. By chance, all the traits Mendel studied were located on different chromosomes. Genes on different chromosomes must follow Mendel's second law and assort independently.