Question

Which among the following regarding blood type of humans is incorrect?

• A. Gene I controls the ABO blood type in humans
• B. Gene I has three alleles $I^A,I^B$ and i, and all these alleles are found in humans, which imparts different blood types in an individual
• C. Because of codominance, sugars of both the alleles $I^A$ and $I^B$ get expressed together
• D. Gene I has three alleles $I^A,I^B$ and i, and all these alleles are found in every individual, which gives blood type to the individual

Answer 1

The correct option is D Gene I has three alleles $I^A,I^B$ and i, and all these alleles are found in every individual, which gives blood type to the individual

Mendel suggested factors (genes) have two forms or variants but there are many genes in a population that contain more than two different alternative forms(alleles). Alleles are responsible for differences in the phenotypic expressions for a character.
Two important examples of multiple alleles in human beings are ABO blood group system and human leukocyte-associated antigen (HLA). The ABO blood group system in humans is due to the three alleles of the gene I. I stands for “isohemagglutinin”, The gene (I) has three alleles- $I^A,I^B$, and i (or $I^O$). Multiple allelism is the property of a population, not an individual organism, for example, human beings are diploid in nature, so among the three alleles for the blood type, a particular individual can have only two of the alleles in the particular locus of the chromosome.
When both $I^A$ and $I^B$ come together in the chromosome, they express their phenotype codominantly.
$\begin{array}{cc}\text{Genotype}& \text{Blood Group}\\ I^A{I}^A& \text{A}\\ I^A{I}^O& \text{A}\\ I^B{I}^B& \text{B}\\ I^B{I}^O& \text{B}\\ I^A{I}^B& \text{AB}\\ I^O{I}^O& \text{O}\end{array}$
The alleles $I^A,I^B$ and i are the multiple alleles of the same gene coding for the blood type, and it is a phenomenon of a population, not an individual, one individual shows any two of the alleles and not all the three.