Non-Mendelian Genetics

Mendel worked with traits where one allele was cleanly dominant over the other, so a heterozygote looked exactly like the dominant homozygote. That is real, but it is not the whole story. In non-Mendelian patterns, the heterozygote can look like neither parent, or like both at once. The single habit to break is the reflex that says “heterozygote → show only the dominant trait.”

Two patterns do most of the work, and they are the ones students constantly swap. Incomplete dominance: the two alleles blend, so the heterozygote is an intermediate — a red snapdragon crossed with a white one gives a pink flower, a shade that is neither parent. Codominance: both alleles are expressed fully and separately, so the heterozygote shows both phenotypes at the same time — type AB blood carries both A and B antigens; a roan cow has patches of distinct red and white hairs, not a single blended pink. Blend versus both-at-once is the line between them; do not conflate the two.

Two more wrinkles round out the topic. Multiple alleles (more than two versions of a gene exist in the population, as in the ABO blood system) expand the possible genotypes and phenotypes. And sex-linked (X-linked) genes are inherited differently by males and females, so the same allele shows up at different rates in the two sexes. Underneath all of it, the Mendelian bookkeeping still holds: genotype is not phenotype, and each fertilization is independent.

When you are handed a cross and asked what the heterozygote looks like, do not jump straight to “dominant.” Walk through the type of inheritance first — the type is what tells you how to read the genotype.

1. Write the genotype, not the phenotype. List the two alleles the individual actually carries (for example R and r, or I^A and I^B). The alleles are the genotype; the trait you can observe is the phenotype. Keeping these separate is the habit that prevents most errors — the same phenotype can come from more than one genotype.
2. Ask what kind of inheritance it is. Simple dominance? Incomplete dominance? Codominance? Multiple alleles? Sex-linked? The type decides how the heterozygote is expressed. “Dominant” is only one option, and it means one allele masks the other — it does not mean that allele is stronger or more common.
3. Incomplete dominance → a blended intermediate. If neither allele fully masks the other, the heterozygote is a blend: red R and white r give a pink Rr snapdragon. The heterozygote is a new intermediate phenotype, distinct from both homozygotes.
4. Codominance → both, fully and separately. If both alleles are expressed at once without blending, the heterozygote shows both phenotypes side by side: I^AI^B is type AB blood with both A and B antigens; a roan coat shows separate red and white hairs. Both traits appear in full — not averaged into one.
5. Sex-linked → check the sex. For an X-linked gene, a male (XY) has a single X, so one recessive allele shows in his phenotype; a female (XX) needs two copies to show it and is otherwise an unaffected carrier. Read the same allele differently depending on the sex.

Through-line: identify the type of inheritance before you predict the phenotype. Only under simple dominance does a heterozygote show one trait; under incomplete dominance it blends, under codominance it shows both, and under sex linkage the answer depends on whether the individual is male or female.

Quick reference card. For each term, read what it is and how it changes the way a heterozygote is expressed — blend, both, or masked is the whole game.

The one distinction graders test again and again is incomplete dominance versus codominance. They sound similar — in both, the heterozygote differs from the dominant homozygote — but the mechanism, and the look, are different. Get this line clear and most of the topic falls into place.

Incomplete dominance = a blend. Neither allele fully masks the other, so the heterozygote is a single intermediate phenotype. Cross a red snapdragon (RR) with a white one (rr) and every Rr offspring is pink — a new shade that is neither red nor white. There is one blended trait, sitting between the two homozygotes.

Codominance = both, fully and separately. Here both alleles are expressed at once, without averaging. The heterozygote shows both phenotypes at full strength, side by side. Type AB blood (I^AI^B) makes both A and B antigens; a roan animal has a coat of distinct red hairs and white hairs, not a uniform pink. If you can see both parental traits separately, it is codominance; if you see one blended in-between trait, it is incomplete dominance.

The ABO system ties it together. ABO is a multiple-allele gene: three alleles exist in the population — I^A, I^B, and i. I^A and I^B are codominant with each other (genotype I^AI^B → type AB), while both are dominant over recessive i (type O is ii). Notice a genotype-vs-phenotype consequence: a person with type A blood could be I^AI^A or I^Ai. The same phenotype, two possible genotypes.

“Dominant” is about masking, not muscle. Calling an allele dominant says only that it masks the other in a heterozygote. It does not mean the allele is stronger, better, or more common. A dominant allele can be rare in a population, and a recessive allele can be the most common one there is — frequency and dominance are unrelated.