Lecture 14: Gene Interaction pt2

Gene Interactions Overview

• Focus on epistasis, where one gene affects the expression of another.

Recessive Epistasis

• Defined as the homozygous recessive of one gene masking the effects of another gene.

• Typical ratio in a dihybrid heterozygous cross: 9:3:4.

• Requires both genes (A and B) to be involved in controlling phenotypes.

• Example: In a plant with two genes affecting color, if gene A is homozygous recessive, it will mask gene B.

• In genetic ratios, 16 combinations arise in dihybrid crosses, confirming that this involves two genes.

Double Recessive Epistasis (Duplicate Recessive)

• Occurs when homozygous recessive patterns in either gene produce the same phenotype.

• Results in a 9:7 phenotypic ratio when conducting a dihybrid cross.

• Example: In flower color genetics, both genes might yield white flowers when homozygous recessive.

• Dominant allele presence leads to purple flowers due to the functional proteins.

• Pathway: Precursor → Intermediate (C activated) → Purple (via P).

• Absence of either dominant allele (C or P) results in white flowers.

Dominant Epistasis

• Defined when a single dominant allele of one gene masks the effects of other alleles.

• In a dihybrid cross, the ratio observed is 12:3:1.

• Example: In melons, gene W is dominant and leads to a white phenotype, overshadowing the impact of gene Y.

• Presence of the W allele dictates phenotype regardless of Y's presence.

• Requires consideration of inheritance ratios from true breeding individuals and self crosses.

Double Dominant Epistasis

• Both genes express dominant characteristics that confer the same outcome.

• Results in a 15:1 ratio upon self-crossing.

• Example in wheat where either gene A or B can lead to red kernels.

• If either A or B is present, red color appears; only the double recessive results in a non-red phenotype.

Dominant Recessive Epistasis

• Distinguishes that dominant allele of one gene yields the same phenotype as the homozygous recessive of another gene.

• Example: Flower color in African violets where gene K (purple pigment) interacts with gene D (which inhibits K).

• Results in a 13:3 ratio from a dihybrid cross.

• Dominant allele K leads to pigment production only if D is not there.

General Observations of Epistasis

• Each epistatic interaction highlights how two or more genes collaborate to affect a single phenotype.

• Ratios produced from crosses can indicate types of epistasis:

  • 9:7 for double recessive,

  • 12:3:1 for dominant,

  • 15:1 for double dominant,

  • 13:3 for dominant recessive.

Practical Application in Genetic Problems

• In genetic problems, analyze given genotypes and phenotypes to infer possible outcomes.

• Remember the significance of dominance and recessive patterns in gene interactions.

• Drawn from specific examples, problems can analyze various outcomes, leading to determination of genotypic ratios.

Conclusion

• Understanding epistasis involves recognizing how gene interactions modify phenotypic expressions.

• Familiarity with ratios and pathways is essential.

• Direct observation of outcomes from experiments reinforces the principles of genetic inheritance.