Explain co-dominance and multiple alleles using the human ABO blood groups. Give the genotypes for each group.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define both terms. Co-dominance: both alleles fully expressed in a heterozygote. Multiple alleles: more than two alleles of a gene in a population.
2 · Name the gene and its product. Gene I has alleles Iᴬ, Iᴮ and i; it decides the sugar on red blood cells. i makes no sugar.
3 · List the genotypes.
A: Iᴬ Iᴬ or Iᴬ i
B: Iᴮ Iᴮ or Iᴮ i
AB: Iᴬ Iᴮ (co-dominance)
O: ii
4 · Explain the population point. One person has only two alleles, so all three are seen only when a population is studied.
5 · Link to real life. Blood groups are matched before every transfusion.
6 · Close in one line. So gene I shows both co-dominance (Iᴬ and Iᴮ) and multiple allelism (three alleles).
Scoring tip: Write all six genotypes as a neat list or table. Say clearly that Iᴬ and Iᴮ are both dominant over i.
