Genetic Extensions: Mendel’s Principles Expanded (Dominance, Penetrance, Lethal Alleles, and TS Alleles)

Incomplete Dominance and Codominance (Review and Key Concepts)

  • Quick review from last class:

    • Incomplete dominance
    • Codominance

  • Incomplete dominance

    • Definition: Heterozygote displays a blended phenotype rather than matching one parent.
    • Key idea: Neither allele is completely dominant; the heterozygote phenotype is intermediate.
    • Example: Snapdragon flower color
    • True-breeding red (RR) × true-breeding white (WW) → heterozygote (RW) with pink phenotype.
    • Conclusion: RW shows a blended result, not simply red or white.

  • Codominance

    • Definition: Heterozygote expresses both parental phenotypes simultaneously; both alleles are fully expressed.
    • Key idea: No blending; both phenotypic traits are observed.
    • Examples:
    • Blood type AB (IAIB): crossing IAIA or IAi with IBIB or IBi yields AB phenotype in offspring when genotypes combine IA and IB.
    • Lentil seed color (spotted/dotted): combinations can yield both phenotypic components present in the offspring.

  • Roan cattle problem (codominance in action)

    • Setup: Coat color can be red, white, or roan (mixture of red and white hairs).
    • Symbols used in class: superscripts for alleles (red = R, white = W; roan = RW) [notation used in problem].
    • Data from crosses:
    • Red × Red → all red
    • White × White → all white
    • Red × White → all roan
    • Roan × Roan → 1/4 red, 1/2 roan, 1/4 white
    • Genotypes inferred:
    • Red parent: RR
    • White parent: WW
    • Roan parent: RW
    • Punnett square result for RW × RW:
    • Genotype frequencies: P(RR)=14,  P(RW)=12,  P(WW)=14P(RR)=\tfrac{1}{4},\; P(RW)=\tfrac{1}{2},\; P(WW)=\tfrac{1}{4}
    • Phenotype frequencies: Red = 1/4, Roan = 1/2, White = 1/4
    • Interpretation: This is a classic codominance example; roan phenotype expresses both red and white hairs.
    • Conclusion: Coat color inheritance in this case is codominant.

  • Solved example problem: compact recap and steps

    • Given: Red x Red → all red; White x White → all white; Red x White → roan; Roan x Roan → 1/4 red, 1/2 roan, 1/4 white.
    • Purpose: identify inheritance mode and deduce parental/offspring genotypes.

Theotropy (Pleiotropy) and Lethal Alleles

  • Pleiotropy (theotropy): one gene influencing multiple, seemingly unrelated phenotypic traits.

    • Note: Two examples discussed in class illustrate how a single gene can affect several traits in different tissues.
    • Terminology: pleiotropy (also referred to as theotropy in the lecture notes).
    • Implication: Extends Mendel’s one-gene-one-trait idea; a single mutation can ripple into multiple phenotypic effects.
    • Examples referenced (not exhaustively detailed in transcript): Maori men study and mice examples showing pleiotropy, where a single allele correlates with multiple phenotypes.

  • Lethal alleles overview

    • Definition: Alleles that, when expressed, cause death of the organism (not necessarily immediately).
    • Mechanism: Often arise because the allele disrupts an essential, survival-required function (e.g., essential enzyme).
    • Two broad classes:
    • Recessive lethal alleles: require two copies to cause death.
    • Dominant lethal alleles: can cause death with only one copy (often lethal before reproduction; examples discussed include Huntington disease and Manx cat allele behavior).
    • Huntington disease (dominant lethal example):
    • Heterozygotes (Hh) show disease progression (neurodegeneration), usually later in life.
    • Homozygous dominant (HH) is typically lethal early (embryonic loss) and thus rarely observed.
    • Practical point: Many affected individuals are heterozygous, allowing transmission to offspring, which sustains the allele in the population.
    • Manx cat allele (two-phenotype locus behavior):
    • Allele ML is dominant for tailless phenotype but recessive for lethality when homozygous (ML ML).
    • Genotypes:
      • ML ML → lethal (cats do not survive)
      • MLM → tailless phenotype (viable)
      • mm → normal-tailed, viable
    • Conclusion: The same allele can have a dominant phenotype (tailless tail) and a recessive lethal effect when homozygous.
    • Summary implications:
    • Lethality (dominant or recessive) can alter Mendelian ratios in offspring.
    • Survival bias can influence observed phenotypic ratios in a population.

  • Practical recap and contrast with Mendel

    • Extensions of Mendel’s rules include: incomplete dominance, codominance, multiple alleles, and lethal alleles.
    • Some alleles can be lethal in certain genotypes, shifting expected phenotypic ratios.
    • Some relationships involve pleiotropy, where a single gene impacts multiple traits.
    • Two additional concepts to keep in mind: (i) dominance vs lethality (dominant vs recessive lethal alleles) and (ii) how survival affects observed outcomes.

Recap: Extensions of Mendel’s Work

  • Key takeaways from the lecture
    • Incomplete dominance and codominance extend Mendel’s simple complete dominance model.
    • Mendel’s two-allele-per-gene concept remains valid, but some genes have multiple alleles (e.g., ABO blood types).
    • Not all alleles are equally viable; some alleles are lethal in homozygous or other combinations, altering phenotype proportions.
    • A single gene can influence multiple traits (pleiotropy).
    • Some genes interact in more complex ways (e.g., complementary gene interaction, recessive epistasis) to produce phenotypes—these topics are set for future discussion.

Practice Problem: Platinum Foxes (Recessive Lethal Alleles)

  • Problem setup and reasoning
    • Cross: Platinum fox allele is lethal in homozygous form.
    • Symbols used: big P for platinum allele (dominant for the platinum phenotype), little p for silver allele.
    • Genotypes:
    • Platinum phenotype when genotype is PP (lethal) or Pp (platinum), or the plain silver phenotype when genotype is pp.
    • Cross considered: two platinum foxes (Pp × Pp), with viability considered only for offspring that are not lethal (i.e., exclude PP).
    • Genotypic outcomes from a PP × Pp cross (example explained for clarity):
    • Without loss of generality, using a standard cross Pp × Pp yields:
      • P(PP)=14,  P(Pp)=12,  P(pp)=14.P(PP)=\tfrac{1}{4},\; P(Pp)=\tfrac{1}{2},\; P(pp)=\tfrac{1}{4}.
    • Lethality: PP offspring are not viable.
    • Viable offspring composition and phenotypic ratio
    • Viable genotypes: Pp (platinum) and pp (silver).
    • Relative frequencies among viable offspring:
      • Pp: 0.5, pp: 0.25 → after excluding PP, the total viable fraction is 0.75.
      • Renormalized phenotypic ratio among viable offspring:
      • Platinum (Pp) = \frac{0.5}{0.75} = \frac{2}{3}
      • Silver (pp) = \frac{0.25}{0.75} = \frac{1}{3}
      • Hence, the phenotypic ratio among viable offspring is 2:1 (platinum:silver).
    • Additional interpretation
    • If we consider the phenotype of a heterozygote (Pp) as platinum, the cross demonstrates a recessive lethality because death occurs only in the homozygous lethal state (PP).
    • If we consider a dominant phenotype for the non-lethal trait, the heterozygote still shows the platinum phenotype because the P allele selects the dominant platinum phenotype in Pp individuals.
    • Takeaway: Recessive lethal alleles commonly yield a 2:1 phenotypic ratio among viable offspring in a monohybrid cross that yields a homozygous lethal class.

Incomplete Penetrance and Variable Expressivity

  • Core definitions

    • Penetrance: the proportion of individuals with a particular genotype that actually express the associated phenotype.
    • Expressivity: the degree or intensity with which a phenotype is expressed; can be uniform (unvarying) or variable.
    • Penetrance can be complete (100%) or incomplete (<100%).
    • Expressivity can be unvarying (same phenotype across individuals) or variable (different degrees of the same phenotype).

  • Descriptive scenarios (from the figures described in the lecture)

    • Complete penetrance with unvarying expressivity
    • All individuals with the genotype express the same phenotype with identical intensity.
    • Incomplete penetrance with unvarying expressivity
    • Some individuals with the genotype do not express the phenotype, but those who do express it show the same phenotype (no variation in expressivity among those expressing).
    • Complete penetrance with variable expressivity
    • All individuals with the genotype express the phenotype, but the intensity or extent of expression varies between individuals.
    • Typical real-world example: pew-behavior in beagles (piebald spotting)
    • Piebald spotting shows complete penetrance (spotting occurs in all individuals carrying the allele) but variable expressivity (spot size and pattern differ between individuals).
    • Common trait illustrating incomplete penetrance and variable expressivity: Polydactyly in humans
    • Dominant trait with incomplete penetrance: some individuals with the allele show no extra digits.
    • Variable expressivity: among those who express the trait, the number of digits varies (e.g., six, seven digits on hands or feet; differences between individuals).

  • Pedigree and family example: polydactyly

    • If the trait is dominant, an unaffected parent would imply incomplete penetrance in the affected offspring.
    • Observed variability in digits among affected individuals demonstrates variable expressivity.

  • Takeaway

    • Penetrance and expressivity add nuance to Mendelian ratios and explain why genotype-phenotype correspondence is not always 1-to-1.

Conditional Alleles and Temperature Sensitivity (TS Alleles)

  • Definitions

    • Conditional allele: a mutant allele that expresses different phenotypes under different environmental conditions.
    • Often temperature-sensitive (TS) alleles.
    • Key terms: permissive conditions (where wild-type or normal phenotype is observed) and restrictive conditions (where mutant phenotype emerges).

  • Mechanistic rationale

    • TS alleles frequently encode proteins that are destabilized or misfolded at higher temperatures.
    • At permissive (colder) temperatures, the protein folds and functions adequately, yielding the wild-type phenotype.
    • At restrictive (warmer) temperatures, the mutated protein misfolds or loses function, leading to a mutant phenotype.

  • Examples discussed in class

    • Siamese cats
    • Extremities (tail, ears, nose, paws) are darker due to temperature-sensitive melanin production.
    • Mechanism: an enzyme involved in melanin synthesis is temperature-sensitive; cooler areas permit enzyme activity, producing more pigment; warmer areas have reduced pigment production.
    • Himalayan rabbits
    • Similar temperature-sensitive pigment patterning as Siamese cats.
    • Demonstrated with a cool-off/ice-pack experiment: shaving fur and applying an ice pack reveals darker coloration as fur regrows in cooler conditions, demonstrating reversible TS phenotype expression.

  • Conceptual takeaway

    • TS alleles illustrate how environment interacts with genetics to shape phenotype.
    • Permissive vs restrictive conditions help explain why some populations exhibit a range of phenotypes for the same genotype.

  • Practical note

    • TS alleles are a useful teaching example to illustrate conditional expression and the interplay between genotype and environment.

Summary of Takeaways (Connection to Foundational Principles)

  • Mendel’s work extended by:

    • Incomplete dominance: heterozygotes produce intermediate phenotypes.
    • Codominance: heterozygotes express both parental phenotypes.
    • Multiple alleles: more than two alleles exist for a gene (e.g., ABO blood types).
    • Not all alleles are viable; lethal alleles alter expected Mendelian ratios.
    • Pleiotropy: a single gene can influence multiple traits.
    • Some genes interact in more complex ways (e.g., epistasis, complementary gene action) to shape phenotypes; these will be covered in future classes.

  • Practical problem-solving patterns to remember

    • When a phenotype suggests codominance, check for heterozygotes showing a mixed phenotype (e.g., roan).
    • For lethal alleles, consider viability of homozygotes and adjust phenotypic ratios accordingly (often a 2:1 ratio among viable offspring).
    • Penetrance and expressivity add complexity to genotype-phenotype predictions; use penetrance values and expressivity patterns to interpret real-world data.
    • Temperature-sensitive alleles illustrate environment-genotype interactions and can be used to explain conditional phenotypes.

Quick Reference Formulas and Ratios

  • Codominance (roan example) cross RW × RW:

    • Genotype frequencies: P(RR)=14,  P(RW)=12,  P(WW)=14P(RR)=\tfrac{1}{4},\; P(RW)=\tfrac{1}{2},\; P(WW)=\tfrac{1}{4}
    • Phenotype frequencies: Red = 1/4, Roan = 1/2, White = 1/4

  • Recessive lethal allele (platinum fox example) cross Pp × Pp:

    • Genotype frequencies: P(PP)=14,  P(Pp)=12,  P(pp)=14P(PP)=\tfrac{1}{4},\; P(Pp)=\tfrac{1}{2},\; P(pp)=\tfrac{1}{4}
    • Viable offspring fraction: 1 - P(PP) = 3/4
    • Phenotype frequencies among viable offspring:
    • Platinum (Pp) = \dfrac{\tfrac{1}{2}}{\tfrac{3}{4}} = \tfrac{2}{3}
    • Silver (pp) = \dfrac{\tfrac{1}{4}}{\tfrac{3}{4}} = \tfrac{1}{3}
    • Final viable phenotypic ratio: 2:1 (platinum:silver)

  • Penetrance and expressivity (conceptual)

    • Penetrance: percentage of individuals with the genotype who express the phenotype (0–100%).
    • Expressivity: degree of phenotype expression, which can be constant or variable across individuals.

  • TS alleles (permissive vs restrictive)

    • Permissive conditions: wild-type phenotype observed; stricter conditions can reveal mutant phenotype.
    • Restrictive conditions: mutant phenotype expressed; decoding often involves protein stability/folding at higher temperatures.

If you’d like, I can tailor these notes further to match your course format (e.g., more worked practice problems or a quick reference cheat sheet).