Question

The genotypes of a husband and wife are ${ I }^{ A }{ I }^{ B }$ and ${ I }^{ A }$i. Among the blood types of their children, how many different genotypes and phenotypes are possible?
(a) 4 genotypes; 4 phenotypes
(b) 3 genotypes; 3 phenotypes
(c) 3 genotypes; 4 phenotypes
(d) 4 genotypes; 3 phenotypes

Answer 1

Hint: ABO blood group system in humans is controlled by gene I, which has three alleles ${ I }^{ A }$, ${ I }^{ B }$and i. Also, ${ I }^{ A }$, ${ I }^{ B }$ are co-dominant and i is recessive. In co-dominant genes, neither allele is recessive.

Complete answer:
During gamete formation, the genotype of husband ${ I }^{ A }{ I }^{ B }$ will have two possible alleles ${ I }^{ A }$ and ${ I }^{ B }$ and the genotype of wife ${ I }^{ A }$i will have two possible alleles ${ I }^{ A }$ and i. Crossing over would produce 4 genotypes( ${ I }^{ A }{ I }^{ A }$, ${ I }^{ A }$i, ${ I }^{ B }{ I }^{ A }$, ${ I }^{ B }$i ) and 3 phenotypes (Blood group A, B, and AB)
The genotype is the genetic constitution of an individual cell or organism, e.g. genotype of hybrid tall - -Tt, pure tall- TT and dwarf- tt. The phenotype represents the observable characters of an organism, which develops as a result of interaction between the genotype and environment, e.g. phenotypic tall pea plant may be genetically TT and tt. In this case, possible genotypes and phenotypes of the children will be as follows:

MotherFather	${ I }^{ A }$	i
${ I }^{ A }$	${ I }^{ A }{ I }^{ A }$ (Blood group ‘A’)	${ I }^{ A }$i ( Blood group ‘A’)
${ I }^{ B}$	${ I }^{ B }{ I }^{ A }$ (Blood group ‘AB’)	${ I }^{ B }$i( Blood group ‘B’)

So, the correct answer is ‘4 genotypes; 3 phenotypes’.

Note:
ABO blood group system in humans exhibits multiple allelism as well as codominance.
Multiple allelism is a phenomenon wherein a trait or a character or gene is controlled by three sets of alleles, instead of two. Although at a time, a gene will be represented by only two alleles in any individual.
Co-dominance is a phenomenon when two different alleles in a heterozygote are expressed at the same time.
ABO blood group system is known to be discovered by Karl Landsteiner.