Non-Mendelian, Codominance, Incomplete Dominance, etc. (cont.) - Jan. 23

Non-Mendelian Inheritance

• Altered Ratios: Phenotypes can be complicated due to various factors:

  1. Late onset conditions (e.g., Huntington's disease).

  2. Influencing factors such as other genes, environment, and chance; includes incomplete penetrance and variable expressivity.

  3. Interactions between alleles of one gene:

     • Incomplete dominance: Phenotypic ratio 1:2:1.

     • Codominance: Phenotypic ratio 1:2:1.

     • Recessive lethal alleles: Phenotypic ratio 2:1.

     • Multiple alleles (allelic series).

  4. Interactions between two genes:

     • Complementation: Phenotypic ratio 9:7.

     • Epistasis: Phenotypic ratio 9:3:4.

  5. Extra-nuclear inheritance (e.g., mitochondrial); this aspect is not covered in detail in the document.

Incomplete Dominance

• Flower Color Example:

  • Parental Generation: Purple x White.

  • F1 Generation: Lavender (self-fertilized).

  • F2 Generation reveals a blend of colors.

Key Concepts of Incomplete Dominance

• Phenotypic Ratio: 1:2:1 observed due to the intermediate phenotype.

• Genotype Contributions:

  • RR (homozygous dominant).

  • Rr (heterozygous).

  • rr (homozygous recessive).

Codominance

• Example: Human Blood Types (Glycoproteins).

  • Alleles IA and IB produce different proteins that add distinct sugars.

  • When crossed (IB x IA), the expected phenotypic ratio is 1 “A”: 2 “AB”: 1 “B” because both alleles are expressed in heterozygotes.

Environmental Influences on Phenotypes

• Case Study: Phenylketonuria (PKU):

  • PKU leads to mental retardation if untreated but is prevented with a low-phenylalanine diet.

  • All newborns tested for PKU since the 1960s.

Influence of Environment on Coat Color

• Siamese Cats: Temperatures affect pigmentation.

  • Color appears in extremities where the temperature is cooler, allowing the enzyme for pigment formation to function.

Hydrangea Color Variation

• The pH of the soil influences the color of hydrangeas:

  • Acidic soil (pH 4.5) leads to blue flowers due to aluminum availability.

  • Neutral soil (pH around 7.0) results in pink flowers.

Quantitative Traits

• Example: Wheat Kernel Color

  • Resulting phenotypes reflect a bell-shaped curve indicating multiple gene influences.

Epistasis: Gene Interactions

• Definition: One gene can affect or mask the expression of another gene.

• Example: Labrador Retriever Colors:

  • Black fur: B- E- (both genes functional).

  • Brown fur: bb E- (mutation at B).

  • Yellow fur: B- ee (mutation at E).

  • Yellow with brown skin: bb ee.

Molecular Mechanism in Labs

• Genes B and E control pigment production and incorporation into hair.

• Mutations in gene B lead to brown pigmentation while mutations in gene E lead to lack of pigmentation.