Explain patterns of inheritance beyond simple dominance, including incomplete dominance, codominance, multiple alleles, and polygenic traits (North Carolina Standard Course of Study, Biology, LS.Bio.7).

What this topic is asking

North Carolina LS.Bio.7 asks you to explain inheritance patterns beyond simple dominance. For the Biology EOC you need to recognize and tell apart incomplete dominance, codominance, multiple alleles (ABO blood type), and polygenic traits. The most-tested distinction is incomplete dominance (a blend) versus codominance (both shown at once), so be precise about it. Items usually give an example and ask you to name the pattern.

Incomplete dominance: a blend

The classic example is snapdragons: a red flower ($RR$) crossed with a white flower ($rr$) gives all pink ($Rr$) offspring. The pink is a mix, intermediate between red and white, because neither allele completely masks the other. Notice the heterozygote is a new, in-between appearance, not both colors at once. A cross of two pink ($Rr \times Rr$) gives a $1:2:1$ ratio that is also a $1$ red $: 2$ pink $: 1$ white phenotype ratio, because each genotype looks different.

Codominance: both shown at once

This blend-versus-both distinction is the single most common EOC item in this topic, so anchor it on the two examples: pink snapdragon (blend, incomplete) and AB blood or roan coat (both, codominance).

Multiple alleles and polygenic traits

Two further patterns broaden the picture.

• Multiple alleles. Some genes have more than two versions in the population. ABO blood type is controlled by three alleles: $I^A$, $I^B$, and $i$ (where $i$ is recessive). Any one person still carries only two of these, but the population has three, which is what "multiple alleles" means. (Type AB also illustrates codominance, since $I^A$ and $I^B$ are codominant.)
• Polygenic traits. Some traits are controlled by many genes acting together, producing a continuous range of phenotypes rather than a few categories. Human height and skin color are polygenic, which is why they vary smoothly across a population instead of falling into a few distinct types.

Try this

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

• Cue. Incomplete dominance gives a blended, intermediate phenotype; codominance shows both phenotypes fully and at the same time.

Q2. Explain why human height is described as a polygenic trait. [2]

• Cue. It is controlled by many genes acting together, producing a continuous range of heights rather than a few distinct categories.