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Beyond Simple Dominance: Blends, Blood, and Multiple Alleles

Not every allele is cleanly dominant or recessive. Meet incomplete dominance (the blend), codominance (both at once), and multiple alleles — the system behind ABO blood groups, where three alleles share one gene.

Incomplete dominance: a true blend

Mendel's clean dominant/recessive pattern is the headline, not the whole book. In incomplete dominance, a heterozygous individual shows a phenotype that lies *between* the two homozygous parents. The classic example is the snapdragon: cross a true-breeding red flower with a true-breeding white one and the F1 are all pink — neither allele fully masks the other, so the single dose of red pigment produces an intermediate shade.

Incomplete dominance: red (RR) × white (R'R')   [R' = white allele]

F1: all R R'  ->  PINK (intermediate)

Now cross two pink F1 (R R' × R R'):

        R          R'
   -----------------------
 R |  R R   |   R R'  |
   -----------------------
 R'|  R R'  |  R' R'  |
   -----------------------

F2 phenotypes: 1 red (RR) : 2 pink (R R') : 1 white (R'R')

Note: phenotype ratio 1:2:1 EQUALS the genotype ratio,
because each genotype looks different.
With incomplete dominance the heterozygote is visibly intermediate, so the F2 phenotype ratio is 1:2:1, not 3:1.

Codominance: both, fully, at once

Codominance looks similar but is different in a crucial way: instead of blending, *both* alleles are expressed fully and separately in the heterozygote. Think of roan cattle, whose coat shows distinct red hairs and white hairs side by side rather than a uniform pink. The two contributions stay visible as themselves, not averaged.

Multiple alleles and the ABO blood group

So far we have spoken of two alleles per gene, but a *population* can hold many versions of the same gene — multiple alleles. Any one person still carries only two, but the gene as a whole has more variants to draw from. The textbook case is the ABO blood group, governed by three alleles: I^A, I^B, and i. I^A and I^B are codominant with each other, and both are dominant over i.

ABO: three alleles -> I^A, I^B, i
  I^A and I^B are codominant; i is recessive to both.

  Genotype        Blood type (phenotype)
  ----------------------------------------
  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   (codominance: BOTH shown)
  i i .................... Type O    (neither)

Cross: type A carrier (I^A i) × type B carrier (I^B i)

        I^B        i
   -------------------------
 I^A| I^A I^B | I^A i |
   -------------------------
  i | I^B i   |  i i  |
   -------------------------

Children: 1 AB : 1 A : 1 B : 1 O  -> all four types possible!
Three alleles plus codominance give four blood types — and two heterozygous parents can have children of any of them.