A geneticist is studying the inheritance of a trait in plants. He observes that the offspring of two parents who are heterozygous for a specific gene (Tt) have a 1:1 ratio of T:tt. Which of the following is the most likely explanation for this ratio?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

Genomic (gametic) imprinting is an epigenetic regulatory mechanism in which specific alleles are transcriptionally silenced based exclusively on their parent of origin. The molecular basis of imprinting involves differential DNA methylation at cytosine residues within CpG islands clustered in differentially methylated regions (DMRs) adjacent to imprinted loci. During gametogenesis, DNMT3A and its cofactor DNMT3L catalyze de novo methylation of one parental allele's promoter or regulatory region—establishing the imprint before fertilization. Histone modifications, particularly trimethylation of histone H3 at lysine 9 (H3K9me3) and lysine 27 (H3K27me3), reinforce this transcriptionally repressed chromatin state. Crucially, the silenced allele retains its full DNA sequence; transcriptional machinery such as RNA polymerase II cannot bind the methylated promoter, so no mRNA is produced from that allele regardless of its dominant or recessive identity.

Why Other Options Are Wrong

In a Tt × Tt cross, standard Mendelian segregation during meiosis I—where homologous chromosomes separate to opposite spindle poles—produces gametes carrying T or t with equal frequency. A Punnett square predicts offspring genotypes TT, Tt, Tt, and tt in a 1:2:1 genotypic ratio, yielding a 3:1 phenotypic ratio of T_ to tt. However, if the T allele is imprinted in one parent's germline—silenced through methylation of its regulatory DMR—then that parent's gametes effectively contribute only the t allele functionally. Suppose the maternal T allele is epigenetically silenced: the mother's oocytes carry either a methylated, non-expressed T or an expressed t. Paternal gametes contribute T or t normally. Offspring inheriting the maternal imprinted T allele cannot transcribe it; they depend entirely on the paternal allele. Thus Tt offspring with the imprinted maternal T express only t phenotypically, collapsing the expected 3:1 T_ : tt ratio into an observed 1:1.

PILLAR 2 — STEP-BY-STEP LOGIC

The question stem establishes that both parents are heterozygous (Tt) and that offspring display a 1:1 phenotypic ratio of T_ : tt rather than the expected 3:1. Since parental genotypes are given as confirmed, the deviation must arise from differential allele expression—not from an error in genotype assignment. Gametic imprinting directly accounts for this: one parental copy of T is epigenetically silenced via methylation at its promoter's CpG dinucleotides, preventing transcription factor binding and RNA polymerase II initiation. Offspring who inherit a silenced T allele from the imprinting parent cannot produce functional T protein, regardless of their nominal Tt genotype. These individuals phenotypically mirror tt homozygotes. The net result is that only offspring receiving a paternally expressed T allele display the dominant phenotype, producing the observed 1:1 ratio. Named examples of this mechanism include the IGF2/H19 locus on human chromosome 11p15.5, where paternal IGF2 is expressed and maternal IGF2 is silenced by CTCF-binding insulator activity governed by DMR methylation status. Such parent-of-origin effects do not alter DNA sequence or Mendelian segregation mechanics—they modify post-zygotic gene expression through heritable epigenetic marks erased and re-established each generation in the germline.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A ("The parents are not truly heterozygous") is seductive because a Tt × tt test cross classically yields a 1:1 ratio—exactly matching the data. Students predisposed to question experimental assumptions may gravitate here. However, the stem explicitly states both parents are heterozygous, and trained geneticists confirm parental genotypes through controlled crosses or molecular genotyping before drawing inheritance conclusions. Selecting A requires rejecting stated premises, a logical error on the AP exam.

Option B ("There is a mutation in the gene") exploits students' tendency to default to mutation as an explanatory mechanism. While a spontaneous point mutation converting T to t in one parent's germline is theoretically possible, mutation rates are exceedingly low (approximately 10⁻⁵ to 10⁻⁸ per nucleotide per generation in most plant nuclear genes). Shifting an entire cohort's ratio from 3:1 to 1:1 would demand mutations in a substantial fraction of gametes—orders of magnitude above any realistic spontaneous mutation frequency. This option reflects a quantitative reasoning flaw: failing to evaluate whether the proposed mechanism can produce the observed effect at scale.

Option C ("The trait is influenced by multiple genes") invokes polygenic inheritance, a legitimate non-Mendelian concept covered in Unit 5. However, polygenic inheritance across multiple unlinked loci produces continuous phenotypic distributions (e.g., quantitative traits like seed weight or plant height governed by additive allele effects across many QTLs), not a clean 1:1 ratio at a single defined gene. Students selecting C conflate two distinct non-Mendelian mechanisms—polygenic inheritance and epigenetic imprinting—failing to recognize that only imprinting converts a monohybrid 3:1 into 1:1 without invoking additional loci. This distractor tests whether students can distinguish among non-Mendelian phenomena by their specific mechanistic signatures and predicted outcomes.