🧬 Non-Mendelian Inheritance

Incomplete Dominance

Neither allele completely dominant

• Heterozygote shows intermediate phenotype
• Blend of two alleles

Example: Snapdragon flowers

• RR = Red
• WW = White
• RW = Pink (intermediate)
• F₂ ratio: 1 Red : 2 Pink : 1 White

Note: Genotypic ratio = Phenotypic ratio (1:2:1)

Codominance

Both alleles fully expressed

• Heterozygote shows both phenotypes simultaneously
• No blending

Example: ABO blood type

• I^A I^A or I^A i = Type A
• I^B I^B or I^B i = Type B
• I^A I^B = Type AB (both A and B antigens)
• ii = Type O

Example: Roan cattle

• RR = Red coat
• WW = White coat
• RW = Roan (both red and white hairs)

Multiple Alleles

More than two alleles exist for a gene in population

• Individual still has only two alleles

ABO Blood Type:

• Three alleles: I^A, I^B, i
• I^A and I^B are codominant
• Both dominant to i
• 6 possible genotypes:
  • I^A I^A, I^A i → Type A
  • I^B I^B, I^B i → Type B
  • I^A I^B → Type AB
  • ii → Type O

Polygenic Inheritance

Multiple genes control one trait

• Produces continuous variation
• Range of phenotypes

Examples:

• Height: controlled by many genes
• Skin color: controlled by 3-4 genes
• Eye color: multiple genes
• Intelligence: highly polygenic

Characteristics:

• Bell curve distribution
• Environmental influence common
• Quantitative trait

Pleiotropy

One gene affects multiple traits

Example: Sickle cell disease

• Single gene mutation (hemoglobin)
• Multiple effects:
  • Sickle-shaped red blood cells
  • Anemia
  • Pain crises
  • Organ damage
  • Malaria resistance (heterozygotes)

Example: PKU (phenylketonuria)

• One enzyme deficiency
• Affects: brain development, skin pigmentation, hair color

Epistasis

One gene masks expression of another gene

• Gene interaction

Example: Labrador coat color

• Gene E: deposits pigment
  • EE or Ee = pigment deposited
  • ee = no pigment (yellow)
• Gene B: determines color
  • BB or Bb = black
  • bb = brown

Phenotypes:

• B_E_ = Black (9/16)
• bbE_ = Brown (3/16)
• _ _ee = Yellow (4/16)

Sex-Linked Traits

Genes on sex chromosomes (usually X)

X-linked recessive:

• More common in males (XY - only one X)
• Females need two copies (XX)

Examples:

• Hemophilia: blood clotting disorder
• Color blindness: red-green
• Duchenne muscular dystrophy

Notation:

• X^H = normal, X^h = hemophilia
• Males: X^H Y (normal) or X^h Y (affected)
• Females: X^H X^H (normal), X^H X^h (carrier), X^h X^h (affected)

Environmental Effects

Environment influences phenotype

Examples:

• Temperature: Himalayan rabbit coat color
• Nutrition: height in humans
• Light: chlorophyll in plants
• pH: hydrangea flower color

Key Concepts

1. Incomplete dominance: heterozygote is intermediate (blend)
2. Codominance: both alleles fully expressed
3. Multiple alleles: >2 alleles in population (ABO blood)
4. Polygenic: multiple genes control one trait (continuous variation)
5. Pleiotropy: one gene affects multiple traits
6. Epistasis: one gene masks another
7. Sex-linked: genes on sex chromosomes, often X-linked recessive
8. Environment can influence gene expression