Mendel's Law of Segregation

Mendel's Law of Segregation. Illustration by Hugo Lin. © 2018 ThoughtCo.

Definition: 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 related to this principle. They are as follows:

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

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.

See: Genes, Traits and Mendel's Law of Segregation

Genetic Dominance

Mendel formulated the law of segregation as a result of performing monohybrid cross experiments on plants.

The specific traits that were being studied exhibited complete dominance. In complete dominance, one phenotype is dominant and the other is recessive. Not all types of genetic inheritance however exhibit complete 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 a plant with white flowers produces a plant with pink flowers.

In co-dominance relationships, both alleles for a trait are fully expressed. Co-dominance 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. Some people get confused about the differences between incomplete dominance and co-dominance. For information about the differences between the two, see: Incomplete Dominance vs Co-dominance.

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Bailey, Regina. "Mendel's Law of Segregation." ThoughtCo, Mar. 15, 2018, Bailey, Regina. (2018, March 15). Mendel's Law of Segregation. Retrieved from Bailey, Regina. "Mendel's Law of Segregation." ThoughtCo. (accessed April 25, 2018).