Question

(30) The textbook discusses human blood typing (ABO) from both phenotypic and genotypic perspectives. If a particular red blood cell exhibits both the cell surface antigens designated glycoproteins A and B, we designated the blood type AB. This type of genetics is an example of multiple alleles and which of the following types of dominance?

(A) complete dominance (B) co-dominance

(C) partial dominance (D) non-dominance

Answer 1

Here, the correct answer is B) codominance.

Explanation -

If the red blood cell exhibits both the cell surface antigens designated glycoproteins A and B, we designated the blood type as AB. If the red blood cell exhibits in the cell surface antigen designated glycoproteins A, we designated the blood type as A. If the red blood cell exhibits in the cell surface antigen designated glycoproteins B, we designated the blood type as B. If the red blood cell exhibits none of cell surface antigens (designated glycoproteins A and B), we designated the blood type as O.

Blood group in humans is determined by three different alleles of the gene. Thus it is a case of multiple alleles.

Also, it shows the phenomenon of co-dominance. Co - dominance is the phenomenon in which the two alleles of the genes are equally dominant to each other and when the two alleles are present in the heterozygous condition both the alleles show their effects equally.

IA allele is responsible for the cell surface antigen glycoproteins A and thus gives blood type A. IB allele is responsible for the cell surface antigen glycoproteins B and thus gives blood type B. Both IA  and IB alleles are co-dominant to each other. While i is the recessive allele to both IA  and IB alleles. 'i' allele is responsible for none of the cell surface antigen glycoproteins and gives blood type O.

Thus, the following are the possible genotype and corresponding phenotype:

IAIA  / IAi = blood type A

IBIB​​​​​​​/ IB​​​​​​​i = blood type B

IAIB​​​​​​​ = blood type AB

ii = blood type O

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