Discuss observed inheritance patterns caused by various modes of inheritance, including dominant, recessive, codominant, incomplete dominance, sex-linked, polygenic, and multiple alleles (NGSSS SC.912.L.16.2; Reporting Category 2, Classification, Heredity, and Evolution).

What this topic is asking

The NGSSS benchmark SC.912.L.16.2 asks you to discuss inheritance patterns beyond simple dominant and recessive: incomplete dominance, codominance, multiple alleles, sex-linked traits, and polygenic inheritance. For the Florida Biology 1 EOC you need to recognize each pattern from a description or a cross and explain why it produces the phenotypes it does. The two most-tested are the incomplete-dominance-versus-codominance distinction and why sex-linked recessive traits are more common in males.

Incomplete dominance and codominance

These two are easy to confuse, and the EOC tests the difference directly.

The one-line distinction: incomplete dominance is a mix (pink), codominance shows both at once (spotted, or both markers present).

Multiple alleles: ABO blood type

In the ABO system, $I^A$ and $I^B$ are codominant to each other and both are dominant over $i$ (recessive). This gives four blood types: type A ($I^A I^A$ or $I^A i$), type B ($I^B I^B$ or $I^B i$), type AB ($I^A I^B$, showing both, codominance), and type O ($ii$). Blood type is the EOC's favorite example because it combines multiple alleles and codominance in one trait.

Sex-linked inheritance

A female with one recessive allele is a carrier: she does not show the trait but can pass it to her sons. This is why a sex-linked recessive trait can appear to skip from grandfather to grandson through a carrier mother.

Polygenic inheritance

Polygenic traits are controlled by several genes acting together, rather than one. Human height, skin color, and eye color are polygenic. Because many genes each add a small effect, the result is a range of phenotypes (a smooth gradient rather than a few distinct categories), often forming a bell-shaped distribution in a population. An EOC clue for a polygenic trait is a continuous range of values rather than two or three clear-cut types.

Try this

Q1. State the difference between incomplete dominance and codominance, with an example of each. [2]

• Cue. Incomplete dominance blends the traits (red and white give pink); codominance shows both fully at once (AB blood type, or red-and-white roan cattle).

Q2. Explain why red-green color blindness, a recessive X-linked trait, is more common in males than females. [2]

• Cue. Males have only one X chromosome, so a single recessive allele is expressed; females have two X chromosomes and usually need the allele on both to show the trait.