The genotypes of husband and wife are IAIB and IAIO. Among the blood groups of their children, how many different genotypes and phenotypes are possible?
4 genotypes and 3 phenotypes
The ABO blood groups are controlled by the gene I, which has three alleles: IA, IB and i (written IO here). IA and IB are codominant, and both are completely dominant over IO; this is an example of multiple allelism.
Step 1 — Gametes: the husband (IAIB) forms IA and IB gametes; the wife (IAIO) forms IA and IO gametes.
Step 2 — Punnett square:
| Gametes | IA (wife) | IO (wife) |
|---|---|---|
| IA (husband) | IAIA | IAIO |
| IB (husband) | IAIB | IBIO |
Step 3 — Genotypes of the children: IAIA, IAIO, IAIB and IBIO, which are 4 different genotypes.
Step 4 — Phenotypes: IAIA and IAIO both give blood group A, IAIB gives blood group AB and IBIO gives blood group B, which are 3 different phenotypes. Blood group O (IOIO) cannot appear because the husband carries no IO allele.
A count of 4 phenotypes would require blood group O as well, and counts of 3 or 2 genotypes miss some of the four distinct gamete combinations.
4 genotypes and 3 phenotypes option is correct.
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