Non-Mendelian Inheritance

You’ll be able to

Distinguish incomplete dominance from codominance in heterozygotes
Predict blood-type offspring from the multiple-allele, codominant ABO system
Explain how polygenic traits and sex-linked genes deviate from simple dominance

When the heterozygote looks different

Not every gene has a clean dominant-recessive relationship. In incomplete dominance, neither allele fully masks the other and the heterozygote shows a blended, intermediate phenotype — cross a red snapdragon (CᴿCᴿ) with a white one (CᵂCᵂ) and every offspring is pink (CᴿCᵂ). In codominance, both alleles are fully and separately expressed in the heterozygote — not blended. Roan cattle (with distinct red and white hairs side by side) and human AB blood are the classic cases. The tell: a mix means incomplete dominance; both at once means codominance.

Multiple alleles and ABO blood type

A gene can have more than two alleles in a population — though any one diploid person still carries only two. The ABO gene has three alleles: Iᴬ, Iᴮ, and i. Iᴬ and Iᴮ are codominant to each other, and both are dominant over the recessive i. This yields four blood types from six genotypes: type A (IᴬIᴬ or Iᴬi), type B (IᴮIᴮ or Iᴮi), type AB (IᴬIᴮ, both antigens shown — codominance), and type O (ii, no antigen).

Polygenic and sex-linked patterns

Polygenic inheritance means a single trait is controlled by many genes adding together — human skin color and height are set by several genes, producing smooth, continuous variation (a bell curve) rather than a few discrete categories. Sex-linked genes sit on the X chromosome. Because males are XY, they have only one copy of every X-linked gene, so a single recessive allele (like the one for red-green colorblindness) is expressed — there is no second X to mask it. This is why X-linked recessive conditions appear far more often in males than in females.

Incomplete dominance vs. codominance

Incomplete: Aᴿ + Aᵂ → blended intermediate · Codominant: Iᴬ + Iᴮ → both shown at once

The heterozygote is the giveaway: an in-between phenotype (pink) is incomplete dominance; two full phenotypes displayed together (type AB) is codominance.

Worked example

A woman with type A blood whose genotype is Iᴬi has children with a man of type B blood whose genotype is Iᴮi. What blood types can their children have, and in what ratio?

1.List each parent’s gametes. The mother (Iᴬi) makes ½ Iᴬ and ½ i. The father (Iᴮi) makes ½ Iᴮ and ½ i.
2.Combine every gamete pair in a Punnett square: IᴬIᴮ, Iᴬi, Iᴮi, and ii — each 1 of 4 boxes.
3.Translate genotypes to phenotypes: IᴬIᴮ = type AB (codominance), Iᴬi = type A, Iᴮi = type B, ii = type O.

Answer: The children can be any of the four blood types in a 1 AB : 1 A : 1 B : 1 O ratio (¼ each) — including type O, even though neither parent is type O.

Checkpoint

In snapdragons, red (CᴿCᴿ) crossed with white (CᵂCᵂ) gives all pink (CᴿCᵂ) flowers. If two pink snapdragons are crossed (CᴿCᵂ × CᴿCᵂ), what phenotype ratio is expected in the offspring?

Watch out

Do not merge incomplete dominance and codominance. Incomplete dominance blends (red + white → pink, a new intermediate). Codominance shows both fully (Iᴬ + Iᴮ → type AB, with both A and B antigens present, not a blend). The heterozygote phenotype is how you tell them apart.

Checkpoint

A father with type O blood (ii) and a mother with type AB blood (IᴬIᴮ) have children. Which blood type is impossible among their children?

Checkpoint

Red-green colorblindness is X-linked recessive. A carrier mother (XᴴXʰ) and a father with normal vision (XᴴY) have children. What fraction of their SONS are expected to be colorblind?

On the exam

For sex-linked pedigree questions, remember the two shortcuts: a son inherits his single X-linked allele from his mother, and X-linked recessive traits are more common in males because their one X has no partner to mask a recessive allele.

Answer the 3 checkpoints as you read.