π Paralogs vs orthologs homologous genes (10 MCQs)
π From Principles of Biochemistry β’ 1. The Foundations of Biochemistry β’ 10 questions available
What is Paralogs vs orthologs homologous genes?
Definition:
Paralogs and orthologs are types of homologous genes that share a common evolutionary ancestry, but orthologs are genes in different species that diverged after a speciation event and often retain the same function, while paralogs are genes that arose by gene duplication within a genome and may acquire new functions over time, and distinguishing between them is crucial for understanding functional genomics and evolutionary relationships.
Working:
Orthologs work by performing similar functions in different species, and they are identified by phylogenetic analysis where the gene tree matches the species tree; paralogs arise from duplication, and one copy retains the original function, while the other may diverge, and these genes are often involved in functional diversification; the functional relationships can be studied by comparing sequences, and the equation helps infer homology; understanding these distinctions aids in predicting gene function and evolutionary history.
Example:
A simple example is the globin gene family: human hemoglobin alpha and beta chains are paralogs (duplicated from a common ancestor), while human hemoglobin alpha and mouse hemoglobin alpha are orthologs, and this distinction helps researchers understand the evolution of oxygen transport and use model organisms to study human genes.
Reason:
Understanding paralogs and orthologs is essential for genomics, as it aids in functional annotation, evolutionary studies, and medical research, allowing predictions of gene function across species and insights into the evolution of gene families, and it is central to comparative genomics.
π All Paralogs vs orthologs homologous genes MCQs
Q1. A gene in species X and a gene in species Y originated from the same ancestral gene after the species diverged. What relationship is most strongly supported if no duplication occurred before the divergence?
π Explanation: These genes are orthologs because they trace back to a common ancestral gene whose lineage separated when the two species diverged. Orthology generally reflects speciation, whereas paralogy results from gene duplication within an evolutionary lineage.
Q2. A genome contains genes A and B that arose when an ancestral gene duplicated before the lineage split into species 1 and species 2. Gene A from species 1 is compared with gene B from species 2. Which classification is most appropriate?
π Explanation: The duplication occurred before speciation, so the A and B lineages already represented different duplicated copies when the species diverged. Therefore, A and B are paralogs even when they are located in different species.
Q3. Researchers identify one enzyme gene in bacterium P and two related enzyme genes in bacterium Q. Phylogenetic analysis indicates that the duplication producing the two Q genes happened after P and Q diverged. Which inference is best?
π Explanation: Because the duplication happened after the two bacterial lineages separated, the two Q genes descended from a duplication event and are therefore paralogs. Each Q copy can still have an orthologous relationship with the ancestral counterpart in P.
Q4. A scientist assumes that two genes with 85% sequence similarity must be orthologs. Another researcher notes that a duplication occurred before the species separated. Why is the first conclusion potentially incorrect?
π Explanation: Sequence similarity supports homology but does not by itself distinguish orthologs from paralogs. Determining whether the relevant evolutionary split resulted from speciation or duplication requires evolutionary context, such as gene histories or phylogenetic relationships.
Q5. A laboratory wants to infer whether a newly discovered human gene performs the same ancestral function as a mouse gene. The genes are highly similar, but the human lineage contains evidence of an ancient duplication. What additional analysis would most directly improve the inference?
π Explanation: A phylogenetic analysis can place the genes relative to duplication and speciation events. If the human and mouse copies descend from the same post-speciation branch, orthology is more likely; if duplication separates them, paralogy must be considered.
Q6. A gene-family study reports the following number of homologous copies per species: Species A = 1, Species B = 1, Species C = 3, Species D = 3. The data alone suggest an expansion in the lineage leading to C and D. Which interpretation is most reasonable?
π Explanation: If C and D each possess three related copies while A and B possess one, a duplication before the C-D split could explain the shared expansion. The copies within the expanded family would commonly be paralogous because they arose through duplication.
Q7. Consider a graph where the horizontal axis represents evolutionary time and the vertical axis shows the number of copies of a gene family. The copy number remains 1 until time T, rises abruptly to 2, and later the two descendant lineages separate into different species. What event most likely occurred at T?
π Explanation: An abrupt increase from one copy to two indicates the formation of an additional gene copy, which is characteristic of duplication. If the two copies later persist through speciation, comparisons between them across species can reveal paralogous relationships.
Q8. A student reasons: 'If two genes are found in different species, they are orthologs; if they are found in the same species, they are paralogs.' Which correction is most accurate?
π Explanation: The student's rule uses location as a shortcut, but evolutionary relationships are determined by ancestry. Genes in different species can be paralogs if duplication predates speciation, and genes within one species can have complex histories involving duplication and lineage-specific events.
Q9. Two methods compare a human gene with genes from three other species. Method 1 selects the gene with the highest sequence identity. Method 2 builds a phylogenetic tree and identifies the candidate whose lineage separates at the expected speciation event without an intervening duplication. Which method provides stronger evidence for orthology?
π Explanation: Sequence identity can be misleading when paralogs are highly conserved. A phylogenetic approach that explicitly considers duplication and speciation events provides stronger evidence because orthology is defined by evolutionary history rather than similarity alone.
Q10. An ancestral genome contains one gene, which duplicates into copies X and Y. Later, the species carrying both copies splits into species M and N. A researcher compares X from M with Y from N and calls them orthologs because they occur in different species. What is the key flaw?
π Explanation: The critical event is the ancestral duplication that produced X and Y before species M and N diverged. Because the compared genes descend from different duplicated copies, their relationship is paralogous despite occurring in different species. Evolutionary timing overrides simple species-based classification.