Human ABO blood types are an example of which type of inheritance?

Question

BlackTom AI · Accepted Answer

Codominance occurs when both alleles are fully expressed in the heterozygote, so this concept explains the ABO system.

1) Relevant knowledge points and concepts
- Multiple alleles: the ABO blood group locus has three common alleles: IA, IB, and i.
- Codominance: two different alleles in a heterozygote are both expressed, producing a phenotype that displays characteristics of both alleles simultaneously.
- Recessive allele: an allele whose phenotype is not expressed in the presence of a dominant or codominant allele (here, i is recessive).

2) Step-by-step reasoning
- List genotype → phenotype relationships:
  - IAIA or IAi → blood type A (A antigen expressed)
  - IBIB or IBi → blood type B (B antigen expressed)
  - ii → blood type O (no A or B antigens)
  - IAIB → blood type AB
- Focus on IAIB: an individual with IA and IB produces both A and B antigens on red blood cells. The phenotype is not intermediate (like a blend) but shows both antigens fully.

3) Intermediate steps (mapping shown above)
- Demonstrates that heterozygote IAIB expresses both alleles’ products simultaneously (A and B antigens).

4) Why the correct answer is correct
- IA and IB are expressed at the same time in IAIB individuals, so neither allele masks the other; both phenotypes appear. That is the defining hallmark of codominance.

5) Why the other options are incorrect
- a. Incomplete dominance: would give an intermediate/blended phenotype (e.g., red + white → pink), which does not describe AB (AB displays both A and B, not a blend).
- b. Semidominance: often used synonymously with incomplete dominance or to indicate partial dominance; it does not fit the full expression of both alleles seen in ABO.
- c. Autosomal dominance: describes dominance of an allele on an autosome but does not capture the specific pattern of two alleles being equally expressed; also does not explain the IA/IB codominant relationship.

6) Conclusion
- Therefore, human ABO blood types exemplify codominance. This matches the provided answer: d. Codominance.

Human ABO blood types are an example of which type of inheritance?单项选择题

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