4. Gene Interaction and Dominance Relationships

4. Gene Interaction and Dominance Relationships

Types of Interactions

Gene interaction refers to the various ways in which genes and their alleles influence the expression of traits in organisms. These interactions can affect the number and type of phenotypes observed in a population.

• Multiple Alleles: There may be more than two alleles for a given gene within a population.

• Incomplete Dominance: Dominance of one allele over another may not be complete.

• Polygenic Traits: Two or more genes may affect a single trait.

• Gene-Environment Interaction: The expression of a trait may depend on the interaction of more than one gene and/or the interaction of genes with nongenetic factors.

Interactions between Alleles Produce Dominance Relationships

Mendelian Dominance and Its Molecular Basis

Mendel's experiments focused on traits with two alternative forms, where one form was completely dominant over the other. The molecular basis of dominance is determined by the protein product of each allele.

• Dominant and Recessive: These terms are based on phenotype, but the underlying cause is the activity of the protein products encoded by the alleles.

• Phenotype Determination: The overall phenotype is the result of the combined activities of the protein products from both alleles.

Functional Effects of Mutation

Loss-of-Function and Gain-of-Function Mutations

Mutations can alter the function of gene products, leading to changes in phenotype.

• Loss-of-Function: Significant decrease or complete loss of functional gene product.

• Gain-of-Function: Gene product acquires a new function or expresses increased wild-type activity.

Loss-of-Function Mutations

• Null (Amorphic) Mutations: Produce no functional gene product; often lethal when homozygous.

• Leaky (Hypomorphic) Mutations: Result in partial loss of function; severity depends on the level of residual activity.

Dominant Negative Mutations

• Multimeric Proteins: Composed of multiple polypeptides; susceptible to dominant negative mutations.

• Spoiler Effect: Mutant polypeptides interfere with normal protein function, leading to dominant negative phenotypes.

Gain-of-Function Mutations

• Hypermorphic: Produce more gene activity than normal.

• Neomorphic: Acquire novel gene activities not found in the wild type.

• These mutations are usually dominant.

Dominance Relationships and Examples

Haplosufficiency and Haploinsufficiency

• Haplosufficient: One copy of the wild-type allele is enough for normal function (e.g., R+ allele producing 50 units of enzyme).

• Haploinsufficient: One copy of the wild-type allele is not enough for normal function (e.g., T1 allele producing 10 units, T2 allele producing 5 units; 18 units required for wild-type phenotype).

Incomplete Dominance and Codominance

Incomplete Dominance

Occurs when heterozygous individuals display intermediate phenotypes between either homozygous type.

• Allele designations such as A1, A2 or B1, B2 are used.

• Heterozygote is typically more similar to one homozygous type than the other.

• Example: Flowering time in pea plants: T1T1 (early), T1T2 (intermediate), T2T2 (late).

Codominance

Heterozygotes express both alleles, resulting in a phenotype distinct from either homozygote.

• Detectable expression of both alleles in heterozygotes.

• Multiple patterns of dominance may exist (e.g., ABO blood type).

ABO Blood Type: Dominance and Codominance

Dominance Relationships of ABO Alleles

• Four blood types: A, B, AB, O.

• Three alleles: IA, IB, i.

• IA and IB are completely dominant over i, but codominant to each other.

• Type A: IAIA or IAi; Type B: IBIB or IBi; Type AB: IAIB; Type O: ii.

Blood Type	Response to Anti-A	Response to Anti-B	Possible Genotypes
A	Clumping	No clumping	IAIA or IAi
B	No clumping	Clumping	IBIB or IBi
AB	Clumping	Clumping	IAIB
O	No clumping	No clumping	ii

Molecular Basis of ABO Dominance and Codominance

• Blood group antigens are glycolipids anchored in the red blood cell membrane.

• Based on the H antigen, modified by the addition of an extra sugar (A or B) or no extra sugar (O).

Allelic Series

Multiple Alleles and Sequential Dominance

Some genes have more than two alleles, forming an allelic series with a specific order of dominance.

• Example: C-gene system for mammalian coat color (cats, rabbits, mice).

• C gene produces tyrosinase enzyme for melanin production.

• Four main alleles: C (full color), cch (chinchilla), ch (Himalayan), c (albino).

Allele	Phenotype	Type of Mutation
C	Full color	Wild-type
cch	Chinchilla (dilute)	Hypomorphic (leaky)
ch	Himalayan	Hypomorphic (temperature-sensitive)
c	Albino	Null (amorphic)

• C is dominant over all others; cch is partially dominant over ch; all are dominant over c.

• ch enzyme is temperature sensitive: functional at low temperatures (extremities), non-functional at high temperatures (body).

Lethal Alleles

Definition and Effects

Lethal alleles are single-gene mutations that cause death in the organism, typically when homozygous.

• Recessively inherited; only homozygotes die.

• Low frequency in populations.

*Additional info: These notes cover the first half of Chapter 4, focusing on gene interaction, dominance relationships, mutation effects, and allelic series, with examples and tables for clarity.*