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Mendelian Inheritance

The simplest form of inheritance was uncovered by an Austrian Monk named Gregor Mendel in 1865. From years of experiments using the common pea plant, Gregor Mendel was able to describe the way in which genetic characteristics are passed down from generation to generation.

Mendel’s Peas

Gregor used peas in his experiments, primarily because of the ease of controlling the fertilization – by transferring pollen from plant to plant using a tiny hairbrush.

Sometimes he transferred pollen from the same plant (self-fertilization) or from other plants flowers (cross-fertilization).

In one experiment , he cross fertilized smooth yellow pea plants with wrinkly green peas.

  • Every single pea from the first pollination, the first generation (F1) was smooth and yellow
  • However, when two smooth yellow peas from the first generation were crossed to produce a second generation (F2), the result was ~75% smooth yellow, 25% green wrinkly (3:1 ratio)

This outcome shows that genes for smooth yellow peas are dominant while the genes for wrinkly green peas are recessive.

The results from these, and further experiments led Gregor Mendel to come up with 3 principles of inheritance.

Gregor’s 3 Principles of Inheritance

1. The inheritance of each trait is determined by ‘factors’ (now known as genes) that are passed onto descendants

2. Individuals inherit one ‘factor’ from each parent for each trait

3. A trait may not show up in an individual but can still be passed onto the next generation

Genetic traits that follow these principles of inheritance are called Mendelian.

The Hardy-Weinberg Equation

The Hardy-Weinberg Equation: p2 + 2pq + q2 = 1

To estimate the frequency of alleles in populations, we can use the Hardy-Weinberg Equation.

p = frequency of the dominant allele (represented below by ‘A’)

q = frequency of the recessive allele (represented below by ‘a’)

For a population in genetic equilibrium, p + q = 1 (sum of both = 100%).


(p + q)2 = 1


p2 + 2pq + q2 = 1

  • p2 = frequency of AA (homozygous dominant)
  • 2pq = frequency of Aa (heterozygous)
  • q2 = frequency of aa (homozygous recessive)


  • 4 Black pigs (recessive)
  • 12 pink pigs (dominant)

q2 = 4/16 => 25%, or 0.25

Therefore, q = 0.5; derived from √0.25

The sum of both, p + q = 1

Therefore, p = 1 - 0.5 => 0.5

2pq = 2(0.5)(0.5) => 0.5 (frequency of heterozygous)

The Hardy-Weinberg Equation: p2 + 2pq + q2 = 1

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