Use mathematical models to predict and explain patterns of inheritance beyond simple dominance, including incomplete dominance, codominance, and multiple alleles (such as ABO blood type) (GSE SB3.b).

What this topic is asking

Standard SB3.b extends Punnett-square reasoning beyond simple dominance. For the Georgia Milestones Biology EOC you must recognize and work with three non-Mendelian patterns: incomplete dominance (a blended phenotype), codominance (both alleles shown fully), and multiple alleles (a gene with more than two versions, the classic example being ABO blood type). Items ask you to identify the pattern from a result or to predict a cross.

Incomplete dominance

The classic example is flower color in snapdragons: a red parent ($RR$) crossed with a white parent ($WW$) gives all pink offspring ($RW$), because the single red allele cannot fully mask the white. Cross two pink ($RW \times RW$) and you get a 1 red : 2 pink : 1 white ratio, where the genotype ratio equals the phenotype ratio (because each genotype looks different). The signature is a blend.

Codominance

In codominance, both alleles are expressed fully and separately in the heterozygote, so you see both phenotypes at once rather than a blend. An example is roan coat in cattle: a cross of red and white gives an animal with both red and white hairs mixed together, not pink. The signature is both shown, not blended.

Multiple alleles and ABO blood type

In multiple alleles, a gene has more than two possible alleles in the population, although each individual still carries only two. The textbook example is human ABO blood type, controlled by three alleles:

• $I^A$ (makes the A marker),
• $I^B$ (makes the B marker), and
• $i$ (makes neither; recessive).

The rules combine codominance and dominance: $I^A$ and $I^B$ are codominant (together they make type AB), and both are dominant over $i$. So the genotypes give: $I^A I^A$ or $I^A i$ = type A; $I^B I^B$ or $I^B i$ = type B; $I^A I^B$ = type AB; $ii$ = type O.

Try this

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

• Cue. Incomplete dominance gives a blended intermediate phenotype (pink); codominance shows both alleles fully and separately (red and white together).

Q2. A person has blood type O. State their genotype and explain why it is recessive. [2 points]

• Cue. Genotype $ii$; type O appears only when both alleles are the recessive $i$, since $I^A$ and $I^B$ are both dominant over $i$.