Non-Mendelian Inheritance: Incomplete/Codominance, Multiple Alleles, and Pleiotropy

Overview of Non-Mendelian Dominance and Allelic Diversity

• In genetics, not all traits follow Mendel's simple dominant/recessive patterns. Some traits show dominance relationships that are not complete or are codominant, and some genes have more than two alleles in populations. We also study how one gene can affect multiple, seemingly unrelated, traits (pleiotropy).
• This section covers two broad categories of non-Mendelian dominance: incomplete dominance and codominance, then expands to multiple alleles in populations and practical implications (blood types, transplantation). Finally, pleiotropy is discussed with classic examples.

Allele notation and why it matters for non-Mendelian patterns

• When dominance is not clear, alleles are often written with superscripts rather than simple uppercase/lowercase letters. Example: alleles written as $a1$ and $a2$ (often shown as $a^1$ and $a^2$ in figures).
• This helps distinguish alleles when neither is strictly dominant over the other.
• For complete dominance, we typically label one allele as dominant and the other as recessive; for incomplete/codominance, heterozygotes can show intermediate or dual expression.

Incomplete dominance

• Definition: Incomplete dominance occurs when the heterozygote has a phenotype that is intermediate between the two parental phenotypes, not a blend that matches either parent exactly but a new intermediate trait.
• Classic example: snapdragon flower color
  • Pure-breeding red allele: $r1$; pure-breeding white allele: $r2$.
  • Cross: $r1r1 imes r2r2$ yields all F1 as $r1r2$ with an intermediate pink phenotype.
• Genotype-to-phenotype expectations for the F1 and F2 crosses in incomplete dominance (using the red/white snapdragon example):
  • F1 cross results: all offspring are heterozygotes $r1r2$ with pink phenotype (intermediate).
  • When F1 intercrosses with itself: cross $r1r2 imes r1r2$ yields the phenotypic ratio
  • one quarter red: $\frac{1}{4}$ (genotype $r1r1$),
  • one quarter white: $\frac{1}{4}$ (genotype $r2r2$),
  • two quarters pink: $\frac{2}{4}$ (genotype $r1r2$).
• Significance: Shows that dominance can be neither complete nor codominant; the heterozygote is not simply the same as one parent but has a distinct intermediate phenotype.

Codominance

• Definition: In codominance, the heterozygote expresses both parental phenotypes simultaneously; neither allele is recessive.
• Examples:
  • Seed spotting in lentils: two alleles, $c^s$ (spotted) and $c^d$ (dotted), are codominant.
  • Cross spotted ($c^s c^s$) with dotted ($c^d c^d$) yields F1 heterozygotes $c^s c^d$ with a phenotype that expresses both traits.
  • When crossing F1 individuals, the expected phenotypic ratio among offspring resembles a 1:2:1 pattern for the three phenotypic classes: 1/4 spotted, 1/4 dotted, 2/4 expressing both traits.
  • Blood type (ABO system) as a clinically important codominant example:
  • Three alleles in the population: $I^A$, $I^B$, and $i$ (often written as $I^A$, $I^B$, and $i$).
  • Genotypes and phenotypes:
    • $I^A I^A$ → phenotype A
    • $I^A i$ → phenotype A
    • $I^B I^B$ → phenotype B
    • $I^B i$ → phenotype B
    • $I^A I^B$ → phenotype AB (codominant expression of both A and B sugars on red blood cells)
    • $ii$ → phenotype O
• Biochemical basis for ABO codominance (brief):
  • $I^A$ allele codes for an enzyme that adds N-acetylgalactosamine to the surface sugars on red blood cells.
  • $I^B$ allele codes for an enzyme that adds galactose to surface sugars.
  • $i$ allele produces no sugar on the red blood cell surface.
• Hematology/clinical relevance:
  • Blood compatibility depends on surface sugars; mismatched ABO blood types can trigger immune responses (antibody production) against transfused cells.
  • Transfusion compatibility examples:
  • AB individuals are universal recipients (they can receive from all ABO types) because they do not produce antibodies against A or B antigens.
  • O individuals are universal donors (they have no A or B antigens on their cells) but can only receive from type O due to antibodies against A and B antigens.
• Extension to multiple alleles in a population:
  • Blood type involves multiple alleles at the population level (three alleles: $I^A$, $I^B$, and $i$), but each individual carries only two alleles.
  • Other human multiple-allele systems exist (e.g., histocompatibility antigens).

Multiple alleles in a population (beyond two alleles per individual)

• Concept: A gene can have more than two alleles circulating in a population, even though any single individual carries only two copies.
• Blood type as a primary example (three alleles in population): $I^A$, $I^B$, and $i$.
  • In population genetics, dominance relationships can vary depending on which two alleles are being compared; relationships are context-dependent.
  • Among the pair $I^A$ vs $I^B$, the alleles are codominant. In comparisons with $i$, both $I^A$ and $I^B$ are dominant over $i$.
• Another example discussed: human histocompatibility antigens (HLA genes):
  • Genes discussed: HLA-A, HLA-B, and HLA-C (three MHC class I genes used in immune recognition).
  • Population-wide allele diversity is enormous: about 400 to 1,200 distinct alleles for each gene in the human population.
  • Alleles at these loci are codominant with each other (i.e., heterozygotes express both alleles' products on the cell surface).
  • Clinical relevance: matching HLA alleles between donor and recipient is crucial for tissue transplantation success (better matches reduce rejection risk).
• Practical note: Although individuals have only two alleles per gene, populations can harbor thousands of alleles across individuals, increasing diversity of immune recognition and transplant compatibility issues.

Practical implications in medicine: transplantation and tissue compatibility

• MHC/HLA diversity underlies why matching donor and recipient is challenging and important.
• The codominant expression of multiple alleles in individuals helps explain why even heterozygotes can present a broad range of antigens but also why mismatches can trigger strong immune responses.

Pleiotropy: one gene, multiple phenotypes

• Definition: Pleiotropy occurs when a single gene influences multiple, seemingly unrelated traits.
• Why it happens: Genes encode proteins that function in many tissues; a gene’s product may have different roles depending on the tissue or developmental stage.
• It is more common than one might think once we understand gene function and molecular pathways.
• Classic and instructive examples:
  • Maori men in New Zealand: a single gene contributes to both sterility and respiratory problems. These seemingly unrelated traits are linked by a single genetic cause (pleiotropy).
  • The yellow mouse (Lucien Guineau’s observations): a famous historical pleiotropy example with the Ay allele.

Pleiotropy example: the Ay allele in mice

• Observations:
  • There exists a dominant allele for coat color that yields yellow fur (Ay) and a standard allele for brown/agouti fur (A).
  • The Ay allele is dominant to A with respect to hair color, so Ay-bearing genotypes tend to be yellow.
  • However, Ay is recessive for viability (lethal when homozygousAyAy).
• Crosses and interpretation:
  • Cross 1: yellow mouse (AyA) with pure-breeding agouti (AA).
  • Offspring show a 1:1 ratio of agouti to yellow phenotypes, because the heterozygote AyA (yellow) appears among the offspring.
  • Cross 2: yellow mice crossed with yellow mice (AyA × AyA).
  • Among viable offspring (excluding those that do not survive due to AyAy lethality), the expected ratio is 2 yellow : 1 agouti, reflecting the lethal AyAy genotype being missing from the viable progeny.
• Conclusion:
  • Ay is dominant for coat color (yellow) but recessive for viability; AyAy is lethal.
  • This is a clear demonstration of pleiotropy: one gene (Ay) affects both color and viability.

Connections to Mendelian genetics and real-world relevance

• Underlying Mendelian principles still apply (segregation, independent assortment, Punnett square logic), but dominance relationships and allele counts can complicate phenotypic predictions.
• Incomplete dominance and codominance broaden the spectrum of possible phenotypes beyond simple dominant/recessive patterns.
• Multiple alleles in populations (e.g., ABO blood types, HLA genes) increase genetic diversity and have direct clinical implications (transfusions, organ/tissue transplantation, immune compatibility).
• Pleiotropy shows how single genes can influence multiple traits, which is important for understanding disease syndromes, drug effects, and evolutionary biology.

Key formulas and numerical references to remember

• Incomplete dominance phenotype ratios (example: red × white snapdragon):
  • F1 phenotype: intermediate (pink).
  • F2 phenotype ratio: $\frac{1}{4} ext{ red} : \frac{2}{4} ext{ pink} : \frac{1}{4} ext{ white}.$
• Codominance phenotype in F2 example (lentils):
  • Phenotype ratio: $\frac{1}{4} ext{ spotted} : \frac{2}{4} ext{ both spotted and dotted} : \frac{1}{4} ext{ dotted}.$
• ABO blood group genetics (alleles in population): three alleles $I^A,\, I^B,\, i$ with phenotypes A, B, AB, O depending on genotype. Common genotype-phenotype mappings:
  • $I^A I^A o ext{A},\, I^A i o ext{A}$
  • $I^B I^B o ext{B},\, I^B i o ext{B}$
  • $I^A I^B o ext{AB}$
  • $ii o ext{O}$
• ABO codominance between $I^A$ and $I^B$ in heterozygotes (e.g., $I^A I^B$ yields AB).
• HLA genes: number of alleles per gene in the human population typically ranges from $400 ext{ to } 1200$ per gene, and alleles are codominant with each other.
• Transplantation relevance: better donor-recipient HLA matches reduce rejection risk; mismatches increase risk.
• Ay/A allele pleiotropy example (summary ratios):
  • Phenotypic color: Ay dominant over A for fur color.
  • Viability: AyAy lethality (recessive lethality).
  • If crosses yield viable offspring only, you may observe a 2:1 ratio of yellow to agouti among viable pups when Ay is crossed with itself, due to the lethal AyAy genotype being nonviable.

Quick study questions (to test understanding)

• What is the difference between incomplete dominance and codominance? Give one example of each from the notes.
• In the snapdragon example of incomplete dominance, what is the F2 phenotypic ratio? Show the Punnett square results.
• Explain how three alleles at the population level for ABO blood groups lead to four phenotypes, and why AB is considered codominant.
• Why are HLA genes so variable in human populations, and why is this important for organ transplantation?
• Define pleiotropy and describe the Ay allele example, including how it can be dominant for color but lethal when homozygous.

End of notes