What Is Mendel's Law of Segregation?

The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860s. One of these principles, now called Mendel's Law of Segregation, states that allele pairs (alternative forms of genes) separate or segregate during gamete formation and randomly unite at fertilization, giving offspring one allele from each parent.

The Four Concepts

There are four main concepts related to this law of segregation:

1. A gene can exist in more than one form or allele.
2. Organisms inherit two alleles for each trait.
3. When sex cells are produced (by meiosis), allele pairs separate leaving each cell with a single allele for each trait.
4. When the two alleles of a pair are different, one is dominant and the other is recessive.

For example, the gene for seed color in pea plants exists in two forms. There is one form or allele for yellow seed color (Y) and another for green seed color (y). In this example, the allele for yellow seed color is dominant, and the allele for green seed color is recessive. When the alleles of a pair are different (heterozygous), the dominant allele trait is expressed, and the recessive allele trait is masked. Seeds with the genotype of (YY) or (Yy) are yellow, while seeds that are (yy) are green.

Law of Segregation Examples

Mendel formulated the law of segregation as a result of performing monohybrid cross experiments on plants. Specific traits he studied exhibited complete dominance. In complete dominance, one phenotype is dominant and the other is recessive. Not all types of genetic inheritance, however, show total dominance.

In incomplete dominance, neither allele is completely dominant over the other. In this type of intermediate inheritance, the resulting offspring exhibit a phenotype that is a mixture of both parent phenotypes. Incomplete dominance is seen in snapdragon plants. Pollination between a plant with red flowers and one with white flowers produces a plant with pink flowers.

In codominance relationships, both alleles for a trait are fully expressed. Codominance is exhibited in tulips. Pollination that occurs between red and white tulip plants can result in a plant with flowers that are both red and white.