Gene Interaction and Extensions to Mendelian Inheritance

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Gene Interaction and Extensions to Mendelian Inheritance

Types of Gene Interactions

Gene interaction refers to the phenomenon where the expression of a trait is influenced by more than one gene or by the interaction of genes with environmental factors. This chapter explores how alleles interact to produce dominance relationships and how multiple genes can affect a single trait.

Multiple alleles: More than two alleles may exist for a gene within a population.
Incomplete dominance: Dominance of one allele over another may not be complete.
Gene collaboration: Two or more genes may affect a single trait.
Gene-environment interaction: Expression of a trait may depend on both genetic and nongenetic factors.

Interactions Between Alleles: Dominance Relationships

Mendel studied traits with two alternative forms, where one form was completely dominant. Later research revealed the molecular basis of dominance, which is determined by the protein product of each allele.

Dominant and recessive alleles: These terms are based on phenotype, but the underlying cause is the activity of the protein products.
Haplosufficiency: One copy of a dominant allele is sufficient for normal function.
Haploinsufficiency: One copy of a wild-type allele is not enough for normal function.

The Functional Consequences of Mutation

Mutations can alter gene function in several ways, affecting the phenotype. The main categories are loss-of-function and gain-of-function mutations.

Loss-of-function: Significant decrease or complete loss of functional gene product.
Gain-of-function: Gene product acquires a new function or increased activity.

Wild type allele produces normal phenotype Null mutation produces no functional product Leaky mutation produces partial function Dominant negative mutation affects multimeric proteins Hypermorphic mutation produces excessive gene activity Neomorphic mutation produces novel function

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 activity level.
Dominant negative mutations: Affect multimeric proteins, causing abnormal interactions and loss of function.

Gain-of-Function Mutations

Hypermorphic mutations: Produce more gene activity than normal.
Neomorphic mutations: Acquire novel gene activities not found in wild type.

Incomplete Dominance

Incomplete dominance occurs when heterozygotes display intermediate phenotypes between either homozygous type. The heterozygote is often more similar to one homozygous type.

Example: Flowering time in pea plants shows intermediate days of first flowering in heterozygotes.

Incomplete dominance in flowering time of pea plants

Codominance

Codominance results in heterozygotes expressing both alleles distinctly, producing a phenotype different from either homozygote. The classic example is the ABO blood group system.

ABO blood type: Three alleles (IA, IB, i) produce four blood types. IA and IB are codominant, both dominant over i.
Antigen-antibody reaction: Blood type is determined by the presence of antigens and their reaction with antibodies.

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

Blood types and genotypes table

Molecular Basis of ABO Dominance and Codominance

The ABO antigens are glycolipids based on the H antigen, which is modified by the addition of specific sugars. The IA allele adds N-acetylgalactosamine, IB adds galactose, and i does not modify the H antigen.

Production of ABO blood group antigens

Allelic Series and Multiple Alleles

Some genes have more than two alleles, forming an allelic series with a specific order of dominance. The C gene system for mammalian coat color is a classic example.

C allele: Wild-type, produces full coat color.
cch allele: Chinchilla, produces diluted color.
ch allele: Himalayan, color on extremities only.
c allele: Null, produces albino phenotype.

Allelic series for coat-color determination in mammals

Variable Phenotypes: Penetrance and Expressivity

Phenotypic ratios can be complicated by variable expression of genotypes. Complete penetrance means a genotype always produces the same phenotype, while incomplete penetrance and variable expressivity introduce variability.

Sex-limited traits: Expressed in only one sex, though both carry the gene.
Sex-influenced traits: Expression differs between sexes for the same genotype.
Incomplete penetrance: Not all individuals with a genotype express the expected phenotype (e.g., polydactyly).
Variable expressivity: Individuals with the same genotype show different degrees or forms of the phenotype (e.g., Waardenburg syndrome).

Variable expressivity of Waardenburg syndrome

Gene–Environment Interactions

Environmental factors can influence gene expression and phenotype. For example, dietary modification can prevent the expression of genetic diseases like phenylketonuria (PKU).

Example: Restricting phenylalanine in the diet prevents PKU symptoms.

Pleiotropy

Pleiotropy occurs when a single gene affects multiple distinct traits. This is often seen in developmental genes and diseases such as sickle cell disease.

Sickle cell disease: Mutation in the β-globin gene causes multiple symptoms due to abnormal hemoglobin.

Pleiotropy in sickle cell disease

Gene Interaction Modifies Mendelian Ratios

Gene interaction describes the collaboration of multiple genes in producing a single phenotypic characteristic. This can alter the expected Mendelian ratios.

Anabolic and catabolic pathways: Genes work together in multi-step biochemical processes.
Epistasis: Interaction between genes can mask or modify the expression of other genes, leading to altered phenotypic ratios.