Apply concepts of statistics and probability to explain patterns of inheritance beyond simple dominance, including incomplete dominance, codominance, multiple alleles, polygenic, and sex-linked traits (Louisiana Student Standards for Science, High School Biology, HS-LS3-3).

What this topic is asking

Louisiana's LS3 standards (HS-LS3-3) ask you to explain the variation and distribution of traits, which means going beyond simple dominant-and-recessive inheritance. For LEAP 2025 Biology you should be able to recognize and explain incomplete dominance, codominance, multiple alleles, polygenic traits, and sex-linked inheritance. The test often gives a cross result (such as a blended or a both-shown phenotype) and asks you to name and explain the pattern.

Incomplete dominance: a blend

The key word is blend: the heterozygote looks like a mix of the two parents, not like either one. This is different from a dominant allele simply masking a recessive one.

Codominance: both shown

Multiple alleles and the ABO example

Multiple alleles means a gene has more than two versions in the population, even though any individual still inherits only two. The classic example is human ABO blood type, controlled by three alleles: $I^A$, $I^B$, and $i$. $I^A$ and $I^B$ are codominant with each other and both dominant over $i$. So $I^A I^B$ is type AB (codominance), $I^A I^A$ or $I^A i$ is type A, $I^B I^B$ or $I^B i$ is type B, and $ii$ is type O.

Polygenic traits: a continuous range

Polygenic traits are controlled by many genes acting together, so instead of a few distinct categories they show a continuous range of values. Human height and skin color are polygenic: people fall along a smooth spectrum rather than into two or three groups. The combined small effects of many genes (plus the environment) produce the bell-shaped distribution seen in these traits.

Sex-linked traits

Sex-linked traits are coded by genes on the sex chromosomes, usually the X. Because males are XY (only one X) and females are XX (two X chromosomes), a recessive allele on the X is expressed in a male whenever he inherits it (he has no second X to mask it). This is why X-linked recessive conditions such as red-green color blindness appear more often in males, while females are more often unaffected carriers.

Try this

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

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

Q2. Explain why X-linked recessive conditions are more common in males. [2]

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