What are alleles?

$ABO$ blood grouping in humans is controlled by the gene $I$. It has three alleles: $I^A, I^B,$ and $i$. Due to these three different alleles,six different genotypes are possible. How many phenotypes can be formed from these?

With respect to $\text{ABO}$ blood groups,how many genotypes and phenotypes,respectively,are possible in the human population?

$A$ male with heterozygous blood group $B$ and a female with homozygous blood group $A$ have offspring. What are the possible genotypes and phenotypes in their progeny?

Genes located at the same locus of chromosomes are called

Mother is homozygous for blood group $B$,and father has blood group $A$ (genotype unknown). Therefore,the possible blood groups in the progeny are: