Q.39
Dizygotic twins are connected to a single placenta during their embryonic development.
These twins
Options:
(A) have identical MHC haplotypes
(B) have identical TH cells
(C) have identical T cells
(D) can accept grafts from each other (both (A) and (B))
Understanding Dizygotic Twins and Transplant Compatibility: Debunking Myths
Dizygotic twins, often called fraternal twins, spark curiosity in genetics and immunology, especially around organ transplants and immune responses. This article dives into a common question about their embryonic development and what it means for MHC haplotypes, T cells, and graft acceptance. If you’re prepping for exams in molecular biology or biotechnology, read on for the correct answer and detailed breakdowns.
Dizygotic twins develop from two separate fertilized eggs (zygotes) and are genetically as similar as any siblings—about 50% shared DNA. Even if they share a single placenta (monochorionic-diamniotic), their immune systems remain distinct. They do not have identical MHC haplotypes, T cells, or automatic graft acceptance. The statement “connected to a single placenta” is often a distractor; sharing doesn’t equate to genetic or immunological identity like in monozygotic (identical) twins.
Why Dizygotic Twins Aren’t Immunologically Identical
Dizygotic twins arise from two sperm fertilizing two eggs, making them no more alike than non-twin siblings. A shared placenta (possible in ~20-30% of cases) allows nutrient exchange but keeps fetal blood separate via dividing membranes. This setup doesn’t mix their genomes or immune profiles.
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MHC (Major Histocompatibility Complex): Encoded by genes on chromosome 6, MHC molecules present antigens to T cells. Dizygotic twins inherit different parental alleles, so their MHC haplotypes differ—just like siblings.
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T Cells (Including TH Cells): These mature in the thymus, expressing unique T-cell receptors (TCRs) shaped by genetics and environment. Helper T (TH) cells and total T cells vary between dizygotic twins due to non-identical genomes.
In transplants, graft rejection hinges on MHC mismatch. Identical twins (monozygotic) succeed without immunosuppression; dizygotic twins face rejection risks like any siblings.
Detailed Explanation of All Options
Let’s break down each option with immunology basics, tailored for biotech and molecular biology students.
Option (A) have identical MHC haplotypes
Incorrect. MHC haplotypes are inherited haplotypes (sets of alleles). Dizygotic twins get random combinations from parents (e.g., one gets HLA-A1/B8/DR3, the other HLA-A2/B7/DR4). No placental sharing alters DNA.
Real-world example: Sibling bone marrow transplants check HLA typing; matches are ~25% likely, not guaranteed.
Option (B) have identical TH cells
Incorrect. TH (CD4+) cells are T helper cells with diverse TCRs for immune coordination. Genetic differences plus thymic selection mean dizygotic twins’ TH cells aren’t identical—unlike clones in identical twins.
Key fact: TCR diversity exceeds 1015 possibilities, amplified by non-identical MHC.
Option (C) have identical T cells
Incorrect. Encompasses cytotoxic (CD8+), TH, and regulatory T cells. All differ due to genetics. Shared placenta doesn’t induce tolerance via cell mixing; barriers (e.g., amnion) prevent it.
Metaphor: Think of T cells as custom keys—dizygotic twins have different locks (MHC) and keys (TCRs).
Option (D) can accept grafts from each other (both (A) and (B))
Incorrect. Graft acceptance requires near-identical MHC (A+B). Since neither holds, rejection occurs via host-vs-graft response. Studies (e.g., twin transplant registries) confirm dizygotic pairs need immunosuppression.
Evidence: Unlike 100% success in monozygotic twins, dizygotic rates mirror sibling stats (~40-60% 1-year survival with matching).
Key Takeaways for Exams and Research
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Dizygotic vs. Monozygotic: Fraternal (di-zygotic) = siblings; identical (mono-zygotic) = clones.
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Placental Sharing: Dichorionic (separate) is default; monochorionic risks (e.g., TTTS) don’t affect immunity.
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Bioinformatics Angle: Use tools like BLAST for MHC sequence alignment in twin studies.
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Clinical Relevance: Understanding this aids transplant ethics, immunology research, and genetic counseling.
This clarifies why no option fits—perfect for NEET, GATE, or biotech interviews. Sources: Immunology texts like Janeway’s Immunobiology (10th ed.) and twin studies in Nature Genetics.
