Notes on Gene Interactions and Their Types

Gene Interactions Types

• Allelic interaction: Genetic interactions of the alleles of a single gene.

• Gene interaction: The expression of one gene depends on the presence or absence of another gene.

Allelic Interaction

• Types:

  • Dominant: An allele that expresses its effect even when present in just one copy.

  • Recessive: An allele that expresses its effect only when two copies are present.

Examples of Allelic Interaction:

• Haplosufficiency: A single normal allele is sufficient to provide adequate function.

• Haploinsufficiency: A single functional copy of a gene is not enough to maintain normal function.

Multiple Alleles

• Genes can have variations in their nucleotide sequences at various positions, each representing a different allele.

• Example: The ABO blood group system in humans is determined by three alleles at the I locus, referred to as "Isoagglutinogen."

Blood Types and Agglutination:

• Type O: Universal donor.

• Type AB: Universal recipient.

• The alleles add different sugar groups to the lipids in the blood cell membranes (the "H" antigen).

• A and B alleles are co-dominant, while O is recessive to either A or B.

Lethal Alleles

• Gene mutations that can lead to the death of an organism, often during early development.

Examples:

• Agouti Genetic Example:

  • A allele produces Raly protein necessary for embryonic development.

  • AY allele does not produce Raly protein and leads to a very high level of yellow pigment due to a 120,000 bp deletion.

• Manx Cat (Tailless):

  • ML (tailless) is lethal in homozygous condition.

  • M indicates the presence of a tail.

• Huntington Disease (HD):

  • Caused by mutant Huntingtin protein, which leads to neuronal death with delayed onset.

Pleiotropy

• Definition: A single gene affects multiple phenotypic traits.

• Example:

  • Sickle Cell Anemia: Beta-globin gene mutation can cause multiple effects including blindness and liver failure, resulting in various phenotypic traits (e.g., heart attack).

Epistasis

• Expression of one gene is influenced by the presence or expression of another gene.

Example:

• Labrador Retriever Coat Color:

  • Determined by two genes:

  • Gene B/b:

    • B: eumelanin (black pigment).

    • b: pheomelanin (brown pigment).

  • Gene E/e:

    • E: functional pigment deposition transporter.

    • e: mutated transporter that prevents deposition.

Suppression

• A second mutation can counteract or "suppress" the effect of an original mutation, restoring a normal or near-normal phenotype.

Molecular Mechanism of Suppression:

• Example involving Wg mutant and hairless mutant.

Synthetic Lethality

• This phenomenon occurs when mutations in two genes jointly lead to cell death, but mutations in either gene alone do not result in lethality.

Summary of Key Points

• Allelic Interactions:

  • Haplosufficiency, haploinsufficiency.

  • Lethal alleles, multiple alleles, pleiotropy.

• Gene Interactions:

  • Epistasis, suppression, synthetic lethality.