What is Mendel's Law of Segregation?

© Steve Berg

The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860's. One of these principles, now called Mendel's law of segregation, states that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization.

There are four main concepts involved in this principle:

• A gene can exist in more than one form.
  
• For each characteristic or trait, organisms inherit two alternative forms (alleles) of that gene.
  
• When gametes (sex cells) are produced, allele pairs separate leaving them with a single allele for each trait.
  
• When the two alleles of a pair are different, one is dominant and the other is recessive.
  

These four concepts are demonstrated in the image. The F1 plants (Gg) are both green because the allele for green pod color (G) is dominant over the allele for yellow pod color (g). When the F1 plants are allowed to self-pollinate, 1/4 of the F2 generation plant pods are yellow (gg). This trait (yellow pod color) is masked in the F1 plants because it is recessive.

The alleles for a trait separate when gametes are formed through a type of cell division called meiosis. These allele pairs are then randomly united at fertilization.