lecture 13: Gene Interactions

Mendelian Genetics Review

• Simple Dominance: One gene with two alleles affecting one trait.

  • Two alleles are inherited, one from each parent.

  • Example: Two alleles can create different traits, e.g., if there are four alleles, it would lead to two traits.

Allelic Interactions

• Allelic Interactions: How alleles interact with each other.

  • Includes terms such as incomplete dominance (blending of traits) and codominance (both traits expressed).

  • Can include lethal alleles, which are variants that can lead to organism death if arranged in a certain way.

Multiple Alleles

• Multiple Alleles: More than two variants of a gene in a population.

  • An individual can still only possess two alleles per gene.

  • Example: Blood types (A, B, O) produced by interactions of alleles.

Pleiotropy

• Pleiotropy: One gene affects multiple traits.

  • Example: A single gene affecting flower color and scent.

Epistasis

• Epistasis: Multiple genes affecting a single trait.

  • More complex interactions than earlier examples due to the involvement of additional gene products.

  • Can lead to variations in phenotype when combinations of genes are involved.

• Definition: One trait determined by multiple genes; each gene has two alleles.

• Gene Interaction Analogy: Painting a barn with multiple painters, each responsible for specific tasks contributing to the final color.

  • Example: Gene X has to function before Gene Y can contribute to the phenotype (the color).

Molecular Level Example of Epistasis

• If Gene X is non-functional, it inhibits Gene Y from displaying its trait.

  • Like missing a crucial step in a multi-step process (e.g., painting).

  • Downstream vs. Upstream Genes: Some genes' products are necessary before others can function.

Historical Context and Examples

• RC Punnett and William Bateson experimented with chickens and uncovered epistasis through comb shape variations.

  • Found that traits do not follow straightforward Mendelian genetics when they received unexpected phenotypical outcomes from certain gene combinations.

Ratios and Predictions

• Dihybrid Crosses: A classic two-gene crossover leads to specific phenotypic ratios (9:3:3:1).

  • Epistasis disrupts this ratio based on gene interactions.

• Recessive Epistasis: The incapability to express a dominant gene due to the recessive interaction of another gene can lead to a different phenotypic outcome.

Specific Case: Labrador Retrievers

• Black, Chocolate (Brown), and Yellow coat colors in Labrador Retrievers serve as a classic case of recessive epistasis:{

  • Black: Big B (for black pigment) and Big E (which expresses color).

  • Brown: Little b (which produces a lighter pigmented protein).

  • Yellow: Little e (which inhibits any pigmentation).

• Crosses: Self-crossing black labs yields a 9:3:4 ratio of color outcomes instead of the expected Mendelian ratios due to the interplay of multiple genes.

Key Takeaway for Epistasis

• Need to understand that epistasis involves how gene products interact to produce given traits or phenotypes.

• Must always check that you are provided with the necessary problem details to identify epistatic relationships.

• Remember that genotypic and phenotypic ratios will follow the rules of segregation and assortment from Mendelian genetics, but epistasis will alter phenotypic expression.