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

Genotypes and Phenotypes:

• C^R C^R : Red flowers
• C^R C^W : Pink flowers (intermediate!)
• C^W C^W : White flowers

Cross: Pink × Pink (C^R C^W × C^R C^W)

         C^R     C^W
    |--------|--------|
C^R | C^R C^R| C^R C^W|
    |  Red   |  Pink  |
    |--------|--------|
C^W | C^R C^W| C^W C^W|
    |  Pink  | White  |

Phenotypic ratio: 1 red : 2 pink : 1 white

Key difference from complete dominance:

• Phenotypic ratio = Genotypic ratio (1:2:1)
• In complete dominance: 3:1 phenotypic, but 1:2:1 genotypic

Other examples:

• Wavy hair in humans (straight × curly → wavy)
• Hypercholesterolemia (heterozygotes have intermediate cholesterol)

(b) Codominance:

Definition: BOTH alleles fully expressed in heterozygote (no blending)

Example: ABO Blood Types

Alleles:

• I^A = produces A antigens (dominant)
• I^B = produces B antigens (dominant)
• i = produces no antigens (recessive)
• I^A and I^B are codominant to each other

Genotypes and Phenotypes:

Cross: Type A (I^A i) × Type B (I^B i)

         I^A      i
    |--------|--------|
I^B | I^A I^B|  I^B i |
    | Type AB| Type B |
    |--------|--------|
 i  |  I^A i |   ii   |
    | Type A | Type O |

Phenotypic ratio: 1 AB : 1 A : 1 B : 1 O

$\boxed{\text{All 4 blood types possible from this cross!}}$

Codominance vs Incomplete Dominance:

• Codominance: Both traits visible (AB has A AND B antigens)
• Incomplete: Blended trait (pink is blend of red and white)

Other codominant examples:

• MN blood groups (M, N, MN)
• Roan coat color in cattle (red + white hairs = roan)

(c) Polygenic Inheritance:

Definition: Multiple genes control ONE trait; produces continuous variation

Example: Human Skin Color

Model (simplified):

• At least 3 genes involved (A, B, C), each with 2 alleles
• Capital letters (A, B, C) = adds melanin (dark)
• Lowercase (a, b, c) = less melanin (light)
• Each capital letter has additive effect

Possible genotypes:

• Darkest: AABBCC (6 capital letters)
• Lightest: aabbcc (0 capital letters)
• Intermediate: Various combinations (1-5 capital letters)

Number of capital letters → skin tone:

• 0: Very light
• 1: Light
• 2: Light-medium
• 3: Medium
• 4: Medium-dark
• 5: Dark
• 6: Very dark

Cross: AaBbCc × AaBbCc

Offspring distribution (simplified):

Using probability:

• Each gene acts independently
• Aa × Aa: ¼ AA, ½ Aa, ¼ aa

Distribution of phenotypes:

• 0 capitals: 1/64 (very light)
• 1 capital: 6/64
• 2 capitals: 15/64
• 3 capitals: 20/64 (most common - bell curve peak)
• 4 capitals: 15/64
• 5 capitals: 6/64
• 6 capitals: 1/64 (very dark)

Bell curve distribution:

Frequency
    ^        ***
    |       *   *
    |      *     *
    |     *       *
    |    *         *
    |___*___________*___> Skin color
    Light  Medium  Dark

Characteristics:

• Continuous variation (not discrete categories)
• Bell-shaped curve (normal distribution)
• Environmental factors also contribute (sun exposure)
• Most offspring near middle (average of parents)

Other polygenic traits:

• Height (100+ genes!)
• Eye color (multiple genes, not just one)
• Intelligence (many genes + environment)
• Fingerprint patterns

Summary Comparison:

$\boxed{\text{Non-Mendelian: incomplete dominance, codominance, polygenic inheritance}}$