Explain non-Mendelian patterns of inheritance, including incomplete dominance, codominance, multiple alleles, and sex-linked traits (Tennessee Academic Standards for Science, Biology I, BIO1.LS3).

What this topic is asking

The Tennessee LS3 standards extend inheritance beyond the simple dominant-and-recessive case. For the Biology I EOC that means recognizing incomplete dominance, codominance, multiple alleles (the classic example is ABO blood type), polygenic traits, and sex-linked inheritance, and being able to tell them apart. Items usually describe an offspring pattern that does not fit a clean 3:1 ratio and ask which pattern it is.

Incomplete dominance: a blend

The textbook example is flower color in snapdragons: red ($RR$) crossed with white ($rr$) gives all pink ($Rr$) offspring, because one red allele alone cannot make the flower fully red. A cross of two pinks ($Rr \times Rr$) then gives a $1$ red $: 2$ pink $: 1$ white ratio, where the genotype ratio and phenotype ratio are the same because every genotype looks different. The exam clue is a blended middle phenotype.

Codominance: both at once

The key difference from incomplete dominance: codominance shows both distinct phenotypes (red and white hairs side by side; A and B markers together), while incomplete dominance shows one blended phenotype (pink). Confusing these two is the most common error in this topic.

Multiple alleles: ABO blood type

A single gene can have more than two alleles in the population, called multiple alleles (an individual still carries only two). The classic example is ABO blood type, controlled by three alleles: $I^A$, $I^B$, and $i$. $I^A$ and $I^B$ are codominant to each other, and both are dominant over the recessive $i$. So $I^A I^A$ or $I^A i$ is type A, $I^B I^B$ or $I^B i$ is type B, $I^A I^B$ is type AB (codominance), and $ii$ is type O. Blood type combines two ideas the EOC likes: multiple alleles and codominance.

Polygenic and sex-linked traits

Polygenic traits are controlled by many genes acting together, producing a continuous range of phenotypes rather than a few categories. Human height, skin color, and eye color are polygenic, which is why they vary smoothly across a population rather than falling into two or three types.

Sex-linked traits are carried on the sex chromosomes, most often the X. Because males are XY (one X) and females are XX (two X's), a male needs only one copy of a recessive X-linked allele to show the trait, while a female needs two. This is why X-linked recessive conditions (such as red-green color blindness and hemophilia) appear more often in males.

Try this

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

• Cue. Incomplete dominance gives one blended, intermediate phenotype (pink); codominance shows both phenotypes fully and separately at the same time (red and white hairs, or A and B markers).

Q2. Explain why X-linked recessive conditions appear more often in males than in females. [2]

• Cue. Males are XY with only one X, so a single recessive allele on that X is expressed; females are XX and would need the recessive allele on both X chromosomes to show the trait.