Distinguish patterns of inheritance beyond simple dominance, including incomplete dominance, codominance, multiple alleles, and polygenic traits (Ohio's Learning Standards for Science, Biology, B.H.2 and B.H.3).

What this topic is asking

Ohio standards B.H.2 and B.H.3 cover how offspring inherit traits and how those traits appear. Simple dominance (from Mendelian genetics and Punnett squares) is only the starting point. Ohio's Biology EOC also tests patterns beyond simple dominance: incomplete dominance, codominance, multiple alleles, and polygenic traits. The crosscutting idea is patterns: each non-Mendelian pattern has a tell-tale signature in the offspring that the EOC asks you to recognize.

Incomplete dominance: a blend

In incomplete dominance, neither allele is completely dominant, so the heterozygote shows an intermediate phenotype that looks like a blend of the two.

• A red snapdragon ($C^R C^R$) crossed with a white one ($C^W C^W$) gives all pink ($C^R C^W$) offspring.
• Crossing two pink plants ($C^R C^W \times C^R C^W$) gives a $1 : 2 : 1$ ratio of red : pink : white.

The signature is a third, intermediate phenotype in the heterozygote. Notice that here both alleles are written as capital letters with a superscript, because neither is recessive.

Codominance: both at once

In codominance, both alleles are fully expressed in the heterozygote, with no blending. You see both phenotypes side by side rather than a mix.

• A roan cow ($C^R C^W$) has a coat with both red and white hairs (not pink).
• Human AB blood type shows both the A antigen and the B antigen on the red blood cells.

The key contrast with incomplete dominance: incomplete dominance blends (pink); codominance shows both (red and white hairs, A and B antigens).

Multiple alleles: more than two versions

A gene with multiple alleles has more than two possible versions in the population, even though each individual still carries only two of them. The classic example is human ABO blood type, controlled by three alleles:

• $I^A$ (makes the A antigen),
• $I^B$ (makes the B antigen),
• $i$ (makes no antigen; recessive).

$I^A$ and $I^B$ are codominant with each other but both dominant over $i$. The possible blood types come from the combinations: type A ($I^A I^A$ or $I^A i$), type B ($I^B I^B$ or $I^B i$), type AB ($I^A I^B$, codominant), and type O ($ii$).

Polygenic traits: many genes, a continuous range

A polygenic trait is controlled by many genes acting together, so instead of a few clear categories it shows a continuous range of phenotypes.

• Human height, skin color, and eye color are polygenic.
• Because many genes (and often the environment, B.H.3) contribute, the trait varies smoothly across a population rather than falling into two or three groups.

Try this

Q1. State the difference between incomplete dominance and codominance. [2]

• Cue. In incomplete dominance the heterozygote shows a blended intermediate phenotype; in codominance both alleles are fully expressed at once with no blending.

Q2. Explain what is meant by a polygenic trait and give one example. [2]

• Cue. A polygenic trait is controlled by many genes, so it shows a continuous range of phenotypes; an example is human height (or skin color).