Exceptions to Mendel's Principles

Exceptions to Mendel's Principles

1. Incomplete Dominance

  • Definition: Incomplete dominance occurs when two alleles are present, but neither allele is fully dominant over the other. This results in blending of both traits, producing a third phenotype that is distinct from both parents.
  • Example: Snapdragon Plants
      - When a red flowering (homozygous) snapdragon is crossed with a white flowering (homozygous) snapdragon, all F₁ offspring are pink (heterozygous).
      - When two pink flowering plants are crossed, the original phenotypes of red and white reappear in the ratio of 1 red : 2 pink : 1 white in the F₂ generation.

2. Codominance

  • Definition: Codominance is a form of inheritance where both alleles are fully expressed in the phenotype without creating a new phenotype; instead, both phenotypes are visible simultaneously.

  • Example: Human Blood Types
      - There are three alleles for blood type genes: A, B, and O. Each individual possesses two of these alleles.
      - Genotypes include: AA, AB, BB, AO, BO, and OO.
      - Phenotypes (blood types) are: A, B, AB, and O.

  • Other Examples:
      - Calico cats display both orange and black markings at once, resulting from codominance.
      - In Andalusian chickens, the offspring of black and white chickens exhibit speckled or blotchy feathers, showing black and white markings simultaneously.

3. Polygenic Traits

  • Definition: Polygenic inheritance refers to a single trait being controlled by multiple genes, leading to a wide variety of phenotypes. The prefix "poly" means many.
  • Examples:
      - Skin color in humans is controlled by around 7 different genes, leading to a continuum of shades.
      - Eye color is believed to be influenced by approximately 5 genes.
      - Height in humans is another example of a polygenic trait.

4. Epistasis

  • Definition: Epistasis occurs when one gene affects or is controlled by another gene.
  • Example: Mice Coat Color
      - The black coat color (B) is dominant over brown (b).
      - However, there is another gene (C) that dictates whether pigmentation will occur. A genotype with two recessive alleles (cc) results in no pigment, leading to a white coat color regardless of the genotype for the black or brown coat color.
  • Analogy: This can be likened to a circuit breaker: power (color) can only flow if the breaker (gene) allows it.

5. Pleiotropy

  • Definition: Pleiotropy occurs when a single allele has multiple distinct phenotypic effects.
  • Examples:
      - In Siamese cats, the same allele responsible for their unique coloration also causes them to have crossed eyes.
      - Marfan's syndrome is another example where one allele can lead to multiple effects, such as abnormally long fingers and legs as well as heart problems.
      - In rats, an allele that causes a lack of pigmentation can also make their eyes sensitive, leading to blindness.

6. Norm of Reaction

  • Definition: The norm of reaction refers to the range of phenotypes that can result from a single genotype depending on environmental factors.
  • Examples:
      - The color of hydrangea flowers can change to red or blue-violet, depending on the acidity or basicity of the soil.
      - Human height can be influenced by environmental factors, such as nutrition.
      - In some animals, like the Himalayan rabbit, snow owl, and arctic fox, fur or feathers change color in response to temperature (e.g., white in winter and brown in summer).
  • Implication: This highlights the interplay between genetics and the environment in determining phenotype.