A geneticist is studying the inheritance of a trait in humans. She observes that two parents who are both carriers of a specific disease have offspring with a 75% chance of developing the disease. Which of the following is the most likely genotype of the affected offspring?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

Autosomal recessive disorders arise from loss-of-function mutations in genes encoding proteins essential for specific biochemical pathways. Consider an enzyme like hexosaminidase A (HEX A), whose absence causes Tay-Sachs disease. The HEXA gene on chromosome 15 encodes a lysosomal hydrolase that cleaves GM2 ganglioside. A single nucleotide substitution in exon 11 (1278insTATC) produces a frameshift, generating a premature stop codon and a truncated, nonfunctional polypeptide. In heterozygous carriers (Aa), one functional allele produces roughly 50% of normal enzyme activity—sufficient to prevent GM2 ganglioside accumulation in neurons. The wild-type allele's promoter remains accessible to transcription factors like SP1, enabling adequate mRNA transcription and translation at ribosomes bound to the rough endoplasmic reticulum. Properly folded HEX A traffics via mannose-6-phosphate tagging to lysosomes, where its active site hydrolyzes the terminal N-acetylgalactosamine from GM2 ganglioside.

Why Other Options Are Wrong

When an individual inherits two mutant alleles (aa), virtually zero functional enzyme reaches lysosomes. GM2 ganglioside accumulates in neuronal cell membranes, disrupting membrane fluidity and impairing synaptic transmission. The hydrophobic ganglioside aggregates interfere with calcium channel gating, ultimately triggering apoptotic cascades via cytochrome c release from mitochondria. This molecular phenotype manifests as progressive neurodegeneration. The recessive nature reflects a simple dose-response: one functional gene copy produces enough enzyme protein to maintain homeostasis, while zero copies cannot sustain the required reaction rate for substrate clearance. Homozygous dominant individuals (AA) produce 100% functional enzyme; heterozygous carriers (Aa) produce approximately 50%, remaining asymptomatic; homozygous recessive individuals (aa) produce negligible functional protein and develop disease pathology.

PILLAR 2 — STEP-BY-STEP LOGIC

The geneticist's observation that carrier parents (both Aa) produce 75% affected offspring signals a non-Mendelian inheritance pattern—likely incomplete penetrance combined with environmental triggers or haploinsufficiency at this locus. Standard Mendelian probability for Aa × Aa yields 25% aa, 50% Aa, and 25% AA. However, if heterozygous offspring (Aa) exhibit disease due to reduced enzyme activity falling below a critical threshold under certain physiological conditions, then both Aa and aa genotypes could display the phenotype. The 75% figure represents the combined proportion of Aa (50%) plus aa (25%) individuals who express clinical symptoms.

Despite this unusual phenotypic ratio, the question asks for the genotype of affected offspring specifically. Among those expressing the disease, both Aa and aa are represented. However, the aa genotype represents individuals who invariably develop the disease regardless of environmental modifiers, because they completely lack functional protein. Their cells cannot produce any wild-type enzyme—no correctly folded catalytic domain, no proper lysosomal targeting, no residual hydrolytic activity. The aa genotype thus constitutes the most reliable predictor of disease status. Furthermore, when the question specifies the genotype of affected offspring, it targets the homozygous recessive constitution that guarantees phenotypic expression through complete loss of gene function. The 75% observation confirms that both genotypes contribute to the affected pool, but aa remains the definitive affected genotype tied directly to the molecular absence of functional protein product.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A (AA) traps students who incorrectly assume the disease allele is dominant. If AA were the affected genotype, the cross Aa × Aa would produce only 25% AA offspring—contradicting the 75% observation entirely. This distractor reflects flawed logic inverting dominant and recessive allele relationships, failing to recognize that homozygous dominant individuals possess two functional alleles producing normal protein levels.

Option B (Aa) attracts students who correctly identify that 75% = 50% Aa + 25% aa but then select the most frequent genotype among affected individuals rather than the genotype most definitively linked to disease. While Aa individuals may show symptoms under this non-Mendelian scenario, the heterozygous state retains one functional allele capable of producing some normal protein. This distractor exploits confusion between statistical frequency and the mechanistic certainty of disease expression.

Option D (It cannot be determined) ensnares students who recognize the unusual 75% ratio and conclude insufficient information exists. This reflects overthinking—while the ratio deviates from classical 25% expectation for recessive disorders, the question provides enough context to identify aa as the affected genotype. The molecular reality that aa eliminates all functional protein production provides definitive genotype-phenotype correspondence.