diff --git a/gerbil-manager-web/src/genetics/lethality.ts b/gerbil-manager-web/src/genetics/lethality.ts
new file mode 100644
index 0000000..993698d
--- /dev/null
+++ b/gerbil-manager-web/src/genetics/lethality.ts
@@ -0,0 +1,99 @@
+/**
+ * Lethality rules applied to an offspring genotype distribution.
+ *
+ * Data-driven so the biology is easy to adjust:
+ *  - SpSp (Schecke homozygous): PRENATAL-LETHAL. Embryos die and are resorbed,
+ *    so they never appear as live young -> removed from the distribution, which
+ *    is then renormalised. Triggers a ScheckeLethal warning carrying the
+ *    fraction of young lost (~1/4 for Schecke × Schecke).
+ *  - ReRe (Rex homozygous): SEMI-LETHAL. Reduced viability but not modelled as a
+ *    hard removal (some survive) -> kept in the distribution, warning only.
+ */
+import { add, divide, toString, ZERO, type Fraction } from './fraction'
+import type { LocusKey } from './loci'
+import type { Genotype } from './genotype'
+import type { DistEntry } from './punnett'
+import { GeneticsWarningCode, type GeneticsWarning } from './warnings'
+
+interface LethalRule {
+  readonly locus: LocusKey
+  readonly allele: string
+  /** 'lethal' = remove from live-birth distribution; 'semi' = warn only. */
+  readonly kind: 'lethal' | 'semi'
+  readonly warning: GeneticsWarningCode
+}
+
+const LETHAL_RULES: readonly LethalRule[] = [
+  { locus: 'Sp', allele: 'Sp', kind: 'lethal', warning: GeneticsWarningCode.ScheckeLethal },
+  { locus: 'Re', allele: 'Re', kind: 'semi', warning: GeneticsWarningCode.RexSemiLethal },
+]
+
+function isHomozygous(g: Genotype, locus: LocusKey, allele: string): boolean {
+  return g[locus][0] === allele && g[locus][1] === allele
+}
+
+export interface LethalityResult {
+  /** Live-birth distribution (lethal genotypes removed, renormalised). */
+  readonly distribution: DistEntry<Genotype>[]
+  readonly warnings: GeneticsWarning[]
+}
+
+/**
+ * Apply lethality to a (merged) genotype distribution.
+ * The input probabilities are assumed to sum to 1.
+ */
+export function applyLethality(dist: DistEntry<Genotype>[]): LethalityResult {
+  const warnings: GeneticsWarning[] = []
+
+  // Total probability mass of fully-lethal genotypes (for the "young lost" stat).
+  let lethalMass: Fraction = ZERO
+  const survivors: DistEntry<Genotype>[] = []
+
+  for (const entry of dist) {
+    const hardLethal = LETHAL_RULES.some(
+      (r) => r.kind === 'lethal' && isHomozygous(entry.value, r.locus, r.allele),
+    )
+    if (hardLethal) {
+      lethalMass = add(lethalMass, entry.probability)
+    } else {
+      survivors.push(entry)
+    }
+  }
+
+  // Renormalise survivors over the surviving mass.
+  const survivingMass = survivors.reduce<Fraction>((acc, e) => add(acc, e.probability), ZERO)
+  const distribution =
+    survivingMass.num === 0
+      ? survivors
+      : survivors.map((e) => ({ value: e.value, probability: divide(e.probability, survivingMass) }))
+
+  if (lethalMass.num > 0) {
+    warnings.push({
+      code: GeneticsWarningCode.ScheckeLethal,
+      detail: {
+        youngLostFraction: toString(lethalMass),
+        youngLostPercent: Number(((lethalMass.num / lethalMass.den) * 100).toFixed(2)),
+      },
+    })
+  }
+
+  // Semi-lethal: warn if any surviving genotype is homozygous for a semi-lethal allele.
+  for (const rule of LETHAL_RULES) {
+    if (rule.kind !== 'semi') continue
+    const affected = distribution.reduce<Fraction>(
+      (acc, e) => (isHomozygous(e.value, rule.locus, rule.allele) ? add(acc, e.probability) : acc),
+      ZERO,
+    )
+    if (affected.num > 0) {
+      warnings.push({
+        code: rule.warning,
+        detail: {
+          affectedFraction: toString(affected),
+          affectedPercent: Number(((affected.num / affected.den) * 100).toFixed(2)),
+        },
+      })
+    }
+  }
+
+  return { distribution, warnings }
+}
diff --git a/gerbil-manager-web/src/genetics/warnings.ts b/gerbil-manager-web/src/genetics/warnings.ts
new file mode 100644
index 0000000..4c685c9
--- /dev/null
+++ b/gerbil-manager-web/src/genetics/warnings.ts
@@ -0,0 +1,16 @@
+/**
+ * Warning codes emitted by the genetics engine. The engine is language-neutral:
+ * it emits CODES only. German texts are mapped in de.ts (UI layer).
+ */
+export enum GeneticsWarningCode {
+  /** Schecke × Schecke: SpSp embryos die in utero; ~25% fewer live young. */
+  ScheckeLethal = 'SCHECKE_LETHAL',
+  /** Rex × Rex: ReRe is semi-lethal; reduced viability of homozygous young. */
+  RexSemiLethal = 'REX_SEMI_LETHAL',
+}
+
+export interface GeneticsWarning {
+  readonly code: GeneticsWarningCode
+  /** Optional structured detail for the UI (e.g. fraction of young lost). */
+  readonly detail?: Record<string, string | number>
+}