Non-Mendelian Inheritance Patterns:

(a) Incomplete Dominance:

Definition: Heterozygote shows INTERMEDIATE phenotype between two homozygotes

Example: Snapdragon flower color

Alleles:

• C^R = red allele
• C^W = white allele
• Neither is fully dominant

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:

| Genotype | Phenotype | Antigens | Can receive from | |----------|-----------|----------|------------------| | I^A I^A or I^A i | Type A | A | A, O | | I^B I^B or I^B i | Type B | B | B, O | | I^A I^B | Type AB | A and B | A, B, AB, O | | ii | Type O | None | O only |

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:

| Pattern | Heterozygote | Example | Ratio | |---------|-------------|---------|-------| | Complete dominance | Like dominant | Pea plants | 3:1 | | Incomplete dominance | Intermediate | Snapdragons | 1:2:1 | | Codominance | Both expressed | ABO blood | Varies | | Polygenic | Continuous variation | Height, skin color | Bell curve |

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