L17 - Extensions 3

Learning Objectives: At the end of this section, students should be able to:
- Explain how to determine if two recessive mutants with similar phenotypes have mutations in the same gene or different genes.
- Explain how gene interactions, including epistatic interactions, affect offspring phenotypic ratios.
- Identify types of genetic interactions from crosses and use the chi-square test to statistically test if observed phenotypes match expectations for a given type of inheritance.

The expression of one gene can mask the expression of a second gene, termed Epistasis (derived from the Greek term meaning "stand upon").
In double mutants, the phenotype expressed is that of one mutation, while the other mutation remains unnoticed.
Epistatic genes: A gene that masks the effect of another gene can be:
- Dominant: Requires only one allele to mask the other.
- Recessive: Requires two alleles to mask the effect.

Two genes influencing a single characteristic can exhibit independent assortment but not act independently in expression, resulting in a phenotypic ratio of $9:3:3:1$ for F2 generation for a single trait.

Dominance: Refers to the relationship between alleles of a gene, where one allele masks the expression of another.
- Complete dominance occurs when one allele completely masks the other at the locus.
Epistasis occurs when one gene masks the effect of another gene located at a different locus.

Recessive Epistasis: Illustrated by coat color in Labrador retrievers.
- B gene: Encodes black pigment (B = dominant; b = recessive for brown color).
- E gene: Encodes an enzyme essential for pigment deposition.
  - E = enables pigment deposition; e = inhibits pigment deposition.
- Genotype ee (homozygous recessive) masks the effect of the B gene, resulting in a yellow coat regardless of the B gene alleles.

In a cross between genotypes Bb Ee x Bb Ee, expected phenotypic ratios are derived:
- The diagnostic ratio for recessive epistasis is $9:3:4$ which indicates the presence of individuals with yellow fur (homozygous recessive 'ee').

Illustrated by fruit color in squash.
- W gene: Dominant W masks the expression of the Y gene.
  - W = dominant (no enzyme I); w = recessive (produces enzyme).
- Y gene encodes enzyme II involved in further coloration of the fruit.
- If genotype has W (W_), it will mask any effect from the Y gene, resulting in a specific fruit color.

Crossing Ww Yy by Ww Yy, phenotypic ratio expected is $12:3:1$ , indicating both dominant and recessive contributions.
Characteristics are classifiable to analyze expected phenotypic outcomes relative to interactions.

A Chi-square test is used to compare observed phenotypic ratios versus expected ratios. If p-value < 0.05 indicates significant deviation, suggesting the effect of non-random factors or epistasis.
The null hypothesis states that differences between observed and expected ratios are due to random chance.

Many traits are influenced by multiple genes (polygenic characteristics).
Traits can present continuous characteristics due to the additive effects of multiple genes during development.
Environmental factors can significantly influence phenotype expression, leading to variability.

External conditions (temperature, nutrition) can alter gene expression and impact observable traits.
- For example, the Himalayan allele exhibits temperature dependence affecting pigmentation.
Continuous (quantitative) traits such as height, weight, and blood pressure often reflect polygenic influences and are subject to environmental modulation.

Utilized to determine whether two mutations affecting the same phenotype are in the same gene (locus) or different loci.
- If offspring from a cross of homozygous mutants express a wild-type phenotype, mutations reside on different loci and exhibit complementarity.
- If mutants expressed the mutant phenotype, they are on the same locus.

9:7 Phenotypic Ratio: Indicative of complementary gene interactions where any homozygous mutation can block the phenotype produced by the other allele.
Connections made to various genetic phenomena such as penetrance and expressivity in pathological cases like cystic fibrosis or phenylketonuria (PKU).
- Variable expressivity (different manifestations of the same genotype) leads to varied presentations across instances of the same genetic condition.

Students are encouraged to practice with textbook examples to enhance proficiency in identifying genotypes and phenotypes resulting from various genetic interactions, particularly before the upcoming midterm assessments.
Key practice areas include:
- Understanding phenotypic ratios (9:3:3:1; 12:3:1) and applying Chi-square tests.
- Resolving complex interactions with gene queries for continuous traits to assess environmental impact.