epistasis

**Definition:** Epistasis occurs when the allele(s) at one gene locus alter, suppress, or mask the phenotypic expression of alleles at a second, separate locus. The masking gene is "epistatic"; the masked gene is "hypostatic." Don't confuse it with dominance (interaction between alleles at the SAME locus).

**Mechanism:** Genes often act in the same biochemical pathway or regulatory cascade. If an upstream gene's product is nonfunctional, downstream genes cannot manifest their effects, whatever their genotype.

**Classic examples:** - **Bombay phenotype (hh):** The H gene (FUT1, fucosyltransferase) makes the H antigen, which is the precursor for A and B antigens. hh individuals lack H antigen, so A and B antigens cannot be made even if they carry IA or IB alleles. They type as O (no A, B, or H on RBCs) but have anti-A, anti-B, AND anti-H antibodies in serum. They can only receive blood from other Bombay donors. Because they carry IA or IB, they can pass these alleles to offspring, so O-type parents can have an AB or A/B child. - **Coat color in Labrador retrievers:** The E locus (pigment deposition) is epistatic to the B locus (black vs brown). ee dogs are yellow regardless of B genotype. - **Albinism:** Tyrosinase deficiency masks genes for other pigments. - **Cancer genetics:** Mutations in the same pathway (e.g., RAS, then downstream effectors) show epistatic relationships. Synthetic lethality (e.g., PARP inhibitors in BRCA-mutated tumors) is a related type of genetic interaction.

**Genetic ratios:** Epistasis modifies the classic Mendelian 9:3:3:1 dihybrid ratio. Examples: 9:3:4 (recessive epistasis), 12:3:1 (dominant epistasis), 9:7 (complementary gene action), 15:1 (duplicate genes).

**High-yield comparisons:** - **Epistasis:** interaction between different loci (one masks another). - **Dominance:** interaction between alleles at the same locus. - **Pleiotropy:** one gene, multiple phenotypic effects (e.g., Marfan syndrome). - **Locus heterogeneity:** mutations at different loci produce the same phenotype (e.g., tuberous sclerosis, retinitis pigmentosa). - **Polygenic inheritance:** many genes contribute additively to one trait (e.g., height). - **Variable expressivity / incomplete penetrance:** may result partly from modifier genes (epistatic effects) and environment.

**Clinical significance:** Explains unexpected blood type inheritance (Bombay), differences in disease severity among people with the same primary mutation (modifier genes, e.g., cystic fibrosis lung disease severity, or β-thalassemia severity modified by α-globin or BCL11A/HbF variants), and gene–gene interactions in complex diseases.

**Exam tip:** If a question describes parents with apparent type O blood having a child with type A, B, or AB, or a patient with type O blood who has anti-H antibodies and reacts with all ABO types, think Bombay phenotype and epistasis.