Law of Segregation (Mendel): Definition, Explanation & Examples

Gregor Mendel was an Augustinian monk studying inherited characteristics in Austria in the 19th century. He was interested in how an individual's characteristics or traits were passed on through generations. Between 1856 and 1863, he grew and studied thousands of pea plants to find out how heredity worked.

The theory of inheritance, at the time, proposed that the characteristics of an offspring were a mixture of the characteristics of the parents. Inconsistencies such as a blue-eyed child being born to brown-eyed parents raised doubts as to the accuracy of these ideas.

Mendel's work established that traits were the result of the presence or absence of the dominant allele of a gene. Mendel's law of segregation states that the two alleles of a gene that are found on a chromosome pair separate, with the offspring receiving one from the mother and one from the father. According to Mendel's law, the two alleles act in a segregated fashion and do not mix or change each other.

Gregor Mendel's Law of Segregation Explanation

Mendel studied the traits of pea plants and how observable characteristics were passed on from parents to offspring. He raised plants whose parents had the same traits and contrasted that with offspring whose parents had different traits.

The characteristics he studied included the following:

  • Flower color
  • Flower position on stem
  • Stem length
  • Pod shape
  • Pod color
  • Seed shape
  • Seed color

From his studies, he concluded that each parent had two versions of a gene. Advanced organisms have two sets of chromosomes, one from the mother and one from the father. A chromosome pair would have the two versions of the gene, called alleles. Various combinations of the alleles resulted in the different traits of the pea plants.

Law of Segregation Examples: Pea Plant Pollination

Pea plants can self-pollinate, or they can be pollinated by placing pollen from the stamens of a parent plant on the pistil of another plant.

Since Mendel was interested in the offspring of two plants with different traits, he removed the pollen-bearing tops of the stamens from some plants and pollinated their pistils with pollen from specific plants. This process allowed him to control plant breeding.

Mendel started by focusing on flower color. He worked with pea plants that had the same characteristics except for one trait and pollinated them in a monohybrid cross. His experiments included the following steps:

  1. Cross-pollinated true-breeding plants, some with purple and some with white flowers.
  2. Observed that the first generation or the F1 generation was all purple.
  3. Cross-pollinated members of the F1 generation.
  4. Observed that three quarters of the second generation or F2 generation was purple and one quarter was white.

From these experiments he was able to deduce that each one of the pair of alleles for a specific gene was either dominant or recessive. Plants with one or two dominant alleles exhibited the dominant trait. Plants with two recessive alleles exhibited the recessive trait. Plants could have the following combination of alleles:

  • Purple/purple for purple flowers.
  • Purple/white for purple flowers.
  • White purple for purple flowers.
  • White/white for white flowers.

Purple was the dominant allele and the possible combinations formed the basis for the 3:1 ratio of purple to white flowers.

Law of Segregation Definition: Supported by Model of Heritability

In Mendelian inheritance, the interaction between dominant and recessive alleles produce the organism phenotype, or the collection of observable characteristics. An organism that has two identical alleles is called homozygous.

Two different alleles, meaning a dominant and a recessive one, produce a heterozygous organism with respect to that gene. The genotype, or the collection of genes and alleles of the organism, is the basis for the organism phenotype.

The Mendelian law of segregation states that organisms randomly contribute an independent assortment of one of their two alleles to the offspring.

Each allele stays segregated from the other, but dominant alleles, when present, act to produce the dominant trait in the organism. When no dominant allele is present, the two recessive alleles produce the recessive trait.

Related topics:

References

About the Author

Bert Markgraf is a freelance writer with a strong science and engineering background. He has written for scientific publications such as the HVDC Newsletter and the Energy and Automation Journal. Online he has written extensively on science-related topics in math, physics, chemistry and biology and has been published on sites such as Digital Landing and Reference.com He holds a Bachelor of Science degree from McGill University.

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