Answer:
A (Genotype)
Explanation:
All organisms possess genetic material which contains information required for certain functions/traits. This genetic material is stored as DNA and contains certain segment that encodes functional products (proteins) called GENE. The make-up of the genes responsible for a particular characteristics in an organism is called the GENOTYPE. Genes occur in variant forms called ALLELES, which are inherited from each parent.
These alleles are responsible for the varying expression of a particular trait e.g. allele T can be responsible for Tallness while the variant form, allele t can be responsible for shortness. In this example, we can see that height is the trait here but the different alleles encode varying expression of it. The combination of the alleles received by both parents, which collectively determines the trait to be expressed is the GENOTYPE.
N.B: The genotype refers to the physically invisible genes not the expressed trait itself, which is the phenotype.
In this case, an allele (gene variant) is responsible for the resistance of Red Blood cells and regulates oxygen carried by them. The phenotype is the resistance that the RBC's possess and its reduced ability to carry oxygen but the combination of alleles responsible for that trait is the GENOTYPE.
