Which one of the following pairs of genes is linked on a single chromosome?

Correct Answer: D

Rationale: The correct answer is D because the genes for the gamma chain and alpha chain are located on the same chromosome, making them linked genes. This is known as genetic linkage. The other choices (A, B, C) involve genes from different chromosomes or gene segments, so they are not linked genes. Genes that are physically close together on the same chromosome tend to be inherited together, which is the basis of genetic linkage analysis.

Correct Answer: B

Rationale: The correct answer is B because the antibody-binding site is primarily formed by the hypervariable regions of both the heavy (H) and light (L) chains. These regions contain the most sequence diversity and directly interact with antigens. Choice A is incorrect because the constant regions do not directly participate in antigen binding. Choice C is incorrect as it only mentions H chains, disregarding the contribution of L chains. Choice D is incorrect as it focuses solely on the variable regions of H chains, neglecting the importance of the L chains in forming the binding site.

Correct Answer: D

Rationale: The correct answer is D. The affinity for antigen is determined by the variable region of the antibody, specifically the antigen-binding site. The heavy-chain constant region influences the other properties listed. A: The ability to cross the placenta is determined by the structure of the heavy-chain constant region, particularly the Fc region. B: Isotype (class) is determined by the type of heavy-chain constant region present in the antibody. C: The ability to fix complement is also influenced by the heavy-chain constant region, particularly the Fc region. Therefore, D is the only property that is not dependent on the structure of the heavy-chain constant region.

Correct Answer: D

Rationale: The correct answer is D because macrophages are responsible for processing antigens and presenting them to T cells, initiating an immune response. They are antigen-presenting cells that help activate T cells. Choice A is incorrect because macrophages do not make antibodies. Choice B is incorrect as macrophages do not lyse virus-infected cells; this is typically done by cytotoxic T cells. Choice C is incorrect as macrophages do not directly activate cytotoxic T cells but rather present antigens for T cell activation.

Correct Answer: D

Rationale: The correct answer is D because allotypes are indeed due to genetic polymorphism within a species. Allotypes are genetic variants of immunoglobulins, resulting from differences in amino acid sequences. This genetic diversity within a species allows for variability in the immune response. Choices A, B, and C are incorrect. Allotypes can be found on both heavy and light chains, they are not determined by class I MHC genes, and they are not confined to the variable regions.