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ABO blood groups in humans are controlled by the gene I. It has three alleles – $I^A$, $I^B$ and i. Since there are three different alleles, six different genotypes are possible. How many phenotypes can occur?
A
Two
B
Three
C
One
D
Four
Detailed Solution
$I^A$ and $I^B$ are completely dominant over i, and they are co-dominant with each other.
The six genotypes are $I^AI^A$, $I^Ai$, $I^BI^B$, $I^Bi$, $I^AI^B$ and ii.
$I^AI^A$ and $I^Ai$ both give blood group A.
$I^BI^B$ and $I^Bi$ both give blood group B.
$I^AI^B$ gives blood group AB (both antigens expressed) and ii gives blood group O.
So four phenotypes are possible: A, B, AB and O.
The six genotypes are $I^AI^A$, $I^Ai$, $I^BI^B$, $I^Bi$, $I^AI^B$ and ii.
$I^AI^A$ and $I^Ai$ both give blood group A.
$I^BI^B$ and $I^Bi$ both give blood group B.
$I^AI^B$ gives blood group AB (both antigens expressed) and ii gives blood group O.
So four phenotypes are possible: A, B, AB and O.