Genes and Heredity: Gene Interactions

Independent Assortment and Dihybrid Crosses

• Dihybrid x Dihybrid Cross: Results in a classic phenotypic ratio of $9:3:3:1$.

  • $9$ $A- B-$

  • $3$ $A- bb$

  • $3$ $aa B-$

  • $1$ $aa bb$

• Dihybrid x Tester Cross: Results in a phenotypic ratio of $1:1:1:1$.

  • $1$ $Aa Bb$ : $1$ $Aa bb$ : $1$ $aa Bb$ : $1$ $aa bb$

Additive Gene Interactions: Chicken Combs

• Concept: Additive traits occur when independent contributions from different genes combine to produce a unique phenotype.

• Phenotypes and Genotypes:

  • Walnut comb: $R- P-$ (requires at least one dominant allele from both genes).

  • Rose comb: $R- pp$ (dominant $R$ only).

  • Pea comb: $rr P-$ (dominant $P$ only).

  • Single comb: $rr pp$ (double recessive).

• Interaction: Rose and Pea act as dominant "exclusive" traits that, when combined ($R-P-$), produce the Walnut phenotype.

Epistasis and Gene Interaction in Drosophila Wings

• Genes involved:

  • $wa^{+}$: Wild-type (wings present); $wa$: Wings absent (Wingless).

  • $vg^{+}$: Wild-type (normal wings); $vg$: Vestigial wing allele.

• F2 Phenotypic Ratio: $9$ Wild-type : $4$ Wingless : $3$ Vestigial.

• Genetic Mechanism:

  • $wa$ (Wingless) is epistatic to $vg$ (Vestigial). If the organism is wingless ($wa/wa$), the vestigial trait cannot be expressed.

  • Molecular Basis: The $wg^{+}$ gene produces a signaling protein (glycoprotein), while the $vg^{+}$ gene acts as a master regulator (transcription coactivator) for wing tissue identity.

Epistasis in Flower Development

• Genes examined:

  • $LFY$: Determines flower development. Mutants ($lfy$) lack flowers.

  • $AG$: Determines flower identity and part differentiation (stamens and carpels). Mutants ($ag$) have petals instead of reproductive organs.

• Epistatic Relationship: The $LFY$ gene is necessary for flower identity. If the organism is $lfy/lfy$, the differentiation of parts (controlled by $AG$) cannot occur.

Inheritance of Coat Color and Modifiers

• Locus interaction:

  • Black/brown locus ($B/b$): Controls the type of pigment produced. Black is dominant to brown.

  • Dense/dilute locus ($D/d$): Acts as a modifier gene by controlling the distribution of pigment granules. Dense ($D-$) is dominant to dilute ($dd$).

• Example Phenotypes:

  • Black + Dense = Black

  • Black + Dilute = Blue

  • Brown + Dense = Brown

  • Brown + Dilute = Silver

• Presence of Expression Locus ($E/e$): Determines if the color is expressed at all. The $ee$ genotype results in a cream coat regardless of the $B/b$ genotype, demonstrating epistasis.

Summary of Gene Interactions

• Gene Interaction: Alters the expected $9:3:3:1$ dihybrid ratio.

• Epistasis: Occurs when the phenotype produced by one gene locus depends on the genotype of a different locus.

• Epistatic vs. Hypostatic: A gene locus that masks or overrides the contribution of another locus is called epistatic.