What is TRUE about complex traits?

Question

BlackTom AI · Accepted Answer

When evaluating statements about complex traits, it helps to test each claim against what is known about how these traits arise and vary among individuals.
Option 1: 'They are caused by a single high-penetrance mutation and follow Mendelian inheritance.' This describes a simple Mendelian trait, not a complex trait. Complex traits do not hinge on a single mutation with strong effect; they arise from many genetic variants with small effects that interact with the environment, so this statement is incorrect.
Option 2: 'They are determined solely by genes and are not influenced by environment.' This overstates genetic determinism. In reality, environmental factors (nutrition, exposure, stress, lifestyle) interact with genetic predispositions to shape complex traits, making this claim false.
Option 3: 'A single genetic variant typically explains most of the trait’s variance.' Complex traits are polygenic; no single variant accounts for most variance. Instead, many variants contribute small effects, collectively influencing the trait, so this option is inaccurate.
Option 4: 'They must always be associated with diseases.' Complex traits include many normal, non-disease phenotypes (like height or intelligence) that vary continuously and are not inherently pathological. Therefore this statement is incorrect.
Option 5: 'They are polygenic and shaped by both genetic and environmental factors.' This aligns with current understanding: multiple genes contribute additively or interactively, and environmental context modulates their expression, producing the observed variation. This makes the statement correct and consistent with the science of complex traits.

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