Non-Mendelian Genetics

Core Review

Beyond Simple Dominance

Incomplete Dominance: In a heterozygote, the phenotype is intermediate between the two homozygous phenotypes — a "blending" pattern. Classic example: snapdragon flower color. A cross between a red-flowered plant (RR) and a white-flowered plant (WW) produces pink-flowered offspring (RW). The F₂ cross of RW × RW produces 1 red (RR) : 2 pink (RW) : 1 white (WW). Note that the genotypic ratio (1:2:1) equals the phenotypic ratio — a hallmark of incomplete dominance.

Codominance: Both alleles are fully and simultaneously expressed in the heterozygote. Neither allele is recessive. The classic example is the ABO blood group system: the Iᴬ and Iᴮ alleles are codominant. An IᴬIᴮ individual expresses both A and B antigens on red blood cells — blood type AB. This is qualitatively different from incomplete dominance because both traits appear in full, not blended.

Multiple Alleles: Many genes exist in more than two allelic forms in the population (though any individual still carries only two). The ABO blood group is the classic example, with three alleles: Iᴬ (A antigen), Iᴮ (B antigen), and i (no antigen, recessive to both). The six possible genotypes and four phenotypes:

• IᴬIᴬ or Iᴬi → Type A
• IᴮIᴮ or Iᴮi → Type B
• IᴬIᴮ → Type AB (universal recipient; both antigens, no antibodies)
• ii → Type O (universal donor; no antigens, anti-A and anti-B antibodies)

Pleiotropy

Pleiotropy occurs when a single gene influences multiple, seemingly unrelated phenotypic traits. The classic example is Marfan syndrome, caused by a mutation in the FBN1 gene encoding fibrillin-1, a connective tissue protein. The single gene mutation produces tall stature, long limbs (arachnodactyly), lens dislocation, and aortic aneurysm. Another example: sickle cell disease — a single mutation in the β-globin gene causes anemia, pain crises, splenic damage, and resistance to malaria. On the DAT, pleiotropy = "one gene, many effects."

Epistasis

Epistasis occurs when the expression of one gene is masked or modified by another gene at a different locus. This is NOT the same as dominance (which involves alleles of the same gene). The gene that does the masking is "epistatic"; the gene whose expression is masked is "hypostatic."

Classic example: Labrador retriever coat color involves two genes. Gene B controls pigment type: B_ = black, bb = brown (chocolate). Gene E controls pigment deposition: E_ = pigment deposited, ee = no pigment deposited (yellow lab). If a dog is ee, it doesn't matter whether it's B_ or bb — it will be yellow. Gene E is epistatic to Gene B. The F₂ dihybrid ratio with recessive epistasis is 9:3:4 (instead of the Mendelian 9:3:3:1), which signals epistasis on the DAT.

Other epistatic ratios: dominant epistasis = 12:3:1; duplicate recessive epistasis (complementary gene action) = 9:7.

Polygenic Inheritance

Many traits (height, skin color, eye color) are controlled by multiple genes, each contributing a small, additive effect. This produces continuous (quantitative) variation — a bell curve in the population — rather than discrete categories. The number of contributing genes determines the range of phenotypes. Skin color in humans involves at least 3-4 genes with additive effects on melanin production.

Pedigree Analysis

Autosomal Dominant:

• Appears in every generation (no skipping).
• Males and females equally affected.
• Affected individuals have at least one affected parent.
• Example: Huntington's disease, achondroplasia.

Autosomal Recessive:

• Often skips generations.
• Males and females equally affected.
• Two unaffected parents can have an affected child (both are carriers).
• Consanguinity (related parents) increases risk.
• Example: cystic fibrosis, sickle cell disease, Tay-Sachs.

X-Linked Recessive:

• More males than females are affected.
• Affected males cannot pass the trait to sons (no male-to-male transmission).
• All daughters of an affected male are carriers.
• Carrier females pass the trait to ~50% of sons.
• Example: hemophilia A, red-green color blindness, Duchenne muscular dystrophy.

X-Linked Dominant (rare on DAT but worth knowing):

• Affected males pass the trait to ALL daughters, NO sons.
• Affected females pass to ~50% of offspring (both sexes).
• Example: vitamin D-resistant rickets.

Common Traps

• "Codominance and incomplete dominance are the same": No. Codominance shows both traits fully (e.g., AB blood type — both A and B antigens). Incomplete dominance blends (red + white = pink). If a heterozygote shows an intermediate phenotype, it's incomplete dominance.
• "Epistasis = dominance": Dominance involves alleles at the SAME locus. Epistasis involves genes at DIFFERENT loci. The DAT loves to test this distinction.
• Pedigree trap: An X-linked trait CAN appear in a female if the father is affected and the mother is a carrier (or affected). Don't rule out X-linked recessive just because one female is affected — check the broader pattern.