Non-Mendelian Inheritance

By the end of this lesson, you will be able to explain and give examples of incomplete dominance, codominance, multiple alleles, sex-linked traits, and polygenic inheritance.

Why Not All Traits Follow Mendel's Patterns

Mendel's pea plant traits happened to all have simple dominant-recessive relationships. Many traits don't work this way.

Incomplete Dominance

Neither allele is completely dominant. Heterozygotes show an intermediate phenotype.

Example: In snapdragons, red (R¹R¹) × white (R²R²) → pink (R¹R²).

The phenotype ratios change: R¹R¹ (red) : R¹R² (pink) : R²R² (white) = 1:2:1 (same as genotype ratio).

Codominance

Both alleles are fully expressed simultaneously in heterozygotes.

Example: ABO blood type (partially). Type AB individuals express both A and B antigens on their red blood cells.

Example: Roan cattle — some hairs are red, some white (both phenotypes visible in the same individual).

Multiple Alleles

More than two alleles exist in a population for one gene (though each individual still has only two).

Example: ABO blood type — three alleles: $I^A$, $I^B$, $i$.

Genotype	Blood Type	Antigens
$I^A I^A$ or $I^A i$	A	A
$I^B I^B$ or $I^B i$	B	B
$I^A I^B$	AB	A and B
$ii$	O	None

$I^A$ and $I^B$ are codominant; both are dominant over $i$.

Sex-Linked Traits

Sex chromosomes: females have XX, males have XY.

The X chromosome carries many genes unrelated to sex determination. Males have only one X — so X-linked recessive traits appear more frequently in males (no second X to mask the recessive allele).

Example: Color blindness — recessive, X-linked.

Carrier female: $X^C X^c$ (normal vision but can pass the allele)

Color-blind male: $X^c Y$

Color-blind female: $X^c X^c$ (both X chromosomes must carry the recessive allele — rare)

Polygenic Inheritance

Multiple genes (each with two or more alleles) contribute to a single trait, producing a continuous range of phenotypes (a bell curve distribution).