Extensions of Mendelian Inheritance

Introduction

Mendelian inheritance describes patterns of genetic transmission that follow the law of segregation and the law of independent assortment. However, many traits deviate from the simple dominant/recessive relationships described by Mendel. This chapter explores these extensions, providing insight into the molecular basis and phenotypic outcomes of various inheritance patterns.

Patterns of Mendelian Inheritance Involving Single Genes

Simple Mendelian Inheritance

• Definition: Inheritance pattern where a single gene with two alleles displays a strict dominant/recessive relationship, following Mendel's laws.

• Molecular Basis: 50% of the protein produced by a single copy of the dominant allele in a heterozygote is sufficient for the dominant phenotype.

• Example: Flower color in peas (purple vs. white).

Incomplete Penetrance

• Definition: Occurs when a dominant phenotype is not expressed even though the individual carries the dominant allele.

• Penetrance: The percentage of individuals with a particular genotype who exhibit the expected phenotype. For example, 60% penetrance means 60% of heterozygotes show the trait.

• Example: Polydactyly (extra fingers/toes) in humans.

• Molecular Basis: Environmental factors or other genes may suppress the effect of the dominant allele.

Expressivity

• Definition: The degree to which a trait is expressed in an individual.

• Example: Polydactyly can range from a single extra digit (low expressivity) to several extra digits (high expressivity).

• Influences: Modifier genes and environmental factors.

Incomplete Dominance

• Definition: The heterozygote exhibits a phenotype intermediate between the two homozygotes.

• Example: Four o'clock plant (Mirabilis jalapa): Red (CRCR) x White (CWCW) produces Pink (CRCW).

• Phenotypic Ratio: F2 generation shows a 1:2:1 ratio (red:pink:white).

• Molecular Basis: 50% of the functional protein is not enough to produce the full phenotype.

Codominance

• Definition: Both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits simultaneously.

• Example: ABO blood group system in humans (IA and IB alleles).

Heterozygote Advantage (Overdominance)

• Definition: The heterozygote has greater reproductive success than either homozygote.

• Example: Sickle-cell anemia: HbAHbS individuals are resistant to malaria and do not suffer from sickle-cell disease.

• Molecular Explanations:

  • Disease resistance (e.g., malaria resistance in sickle-cell carriers)

  • Homodimer formation (heterozygotes can form more protein variants)

  • Variation in functional activity (enzymes function over a broader range)

X-Linked Inheritance

• Definition: Inheritance of genes located on the X chromosome.

• Sex Differences: Males (XY) are hemizygous for X-linked genes; females (XX) have two copies.

• Example: Duchenne muscular dystrophy (DMD) is X-linked recessive; mostly affects males.

Sex-Influenced and Sex-Limited Inheritance

• Sex-Influenced: An allele is dominant in one sex but recessive in the other. Example: Scurs in cattle.

• Sex-Limited: A trait occurs in only one sex, often due to sex hormones. Example: Plumage in chickens, sperm production in males.

Lethal Alleles

• Definition: Alleles that can cause the death of an organism, often due to mutations in essential genes.

• Example: Manx cats (homozygous for the Manx allele die in embryonic development).

• Conditional Lethals: Only lethal under certain environmental conditions (e.g., temperature-sensitive alleles in Drosophila).

• Semi-lethal: Kill some, but not all, individuals with the genotype.

Wild-Type and Mutant Alleles

• Wild-Type Alleles: Prevalent in the population, code for functional proteins, and are usually dominant.

• Mutant Alleles: Result from mutations, often produce nonfunctional proteins, and are usually recessive.

• Genetic Polymorphism: The presence of two or more wild-type alleles in a population.

Protein Levels and Phenotype

Note: In simple dominance, 50% protein is sufficient for the dominant phenotype.

Human Genetic Diseases Caused by Mutant Alleles

Dominant Mutations

• Gain-of-Function: Mutant protein gains a new or abnormal function.

• Dominant-Negative: Mutant protein antagonizes the normal protein.

• Haploinsufficiency: One functional copy is not enough for the wild-type phenotype.

Environmental Effects on Gene Expression

• Definition: Environmental conditions can significantly impact phenotype.

• Example: Arctic foxes change coat color with seasons; PKU symptoms can be prevented by dietary management.

• Norm of Reaction: The range of phenotypes produced by a genotype under different environmental conditions.

• Example: Number of eye facets in Drosophila decreases as temperature increases.

Multiple Alleles and Blood Types

• Multiple Alleles: Many genes have more than two alleles in the population.

• ABO Blood Group: Three alleles (IA, IB, i) determine blood type.

Blood Transfusion: Matching blood types is critical to avoid agglutination and life-threatening reactions.

Sex Chromosomes and Inheritance

• Sex-Linked Genes: Found on sex chromosomes (X or Y).

• X-Linked: Males are hemizygous; more likely to express recessive X-linked traits.

• Y-Linked (Holandric): Genes transmitted only from father to son; rare in humans.

• Pseudoautosomal Inheritance: Genes found in homologous regions of X and Y chromosomes; inherited like autosomal genes.

Lethal Alleles and Their Effects

• Essential Genes: Required for survival; mutations often lethal.

• Nonessential Genes: Not required for survival; mutations may have no effect.

• Timing: Lethal effects can occur at different life stages (e.g., embryonic death vs. late-onset diseases like Huntington's).

Pleiotropy

• Definition: A single gene affects multiple phenotypic traits.

• Example: Cystic fibrosis affects lungs, skin, and digestive system due to a single defective protein (CFTR).

• Mechanisms: Gene product may function in multiple cell types or at different developmental stages.

Gene Interactions

• Definition: Two or more genes influence a single trait.

• Types:

  • Epistasis: Alleles of one gene mask the effects of another gene.

  • Complementation: Two parents with similar recessive phenotypes produce wild-type offspring.

  • Gene Modification: An allele of one gene modifies the phenotypic outcome of another gene.

  • Gene Redundancy: Loss of function in one gene has no effect unless both redundant genes are lost.

Epistasis and Complementation Example: Sweet Pea Flower Color

• Two genes (C and P) are required for purple color; homozygosity for either recessive allele (cc or pp) results in white flowers.

• F2 generation from a cross between two white varieties (CCpp x ccPP) yields a 9:7 ratio (9 purple:7 white).

Gene Modifier Effect: Rodent Coat Color

• Two genes (A and C) control coat color: A for agouti pattern, C for pigment production.

• Genotype aa produces black, cc produces albino, and at least one dominant A and C produces agouti.

• F2 ratio: 9 agouti : 3 black : 4 albino.

Gene Redundancy Example: Capsule Shape

• Genes T and V are redundant for triangular capsule shape; only ttvv produces ovate shape.

• F2 ratio: 15 triangular : 1 ovate.

Summary Table: Single Gene Inheritance Patterns

Key Equations and Concepts

• Penetrance:

• Phenotypic Ratios: In dihybrid crosses with gene interaction, ratios may deviate from 9:3:3:1 (e.g., 9:7, 9:3:4, 15:1).

Conclusion

Extensions of Mendelian inheritance reveal the complexity of genetic traits, including the influence of multiple alleles, gene interactions, environmental factors, and molecular mechanisms. Understanding these patterns is essential for predicting inheritance, diagnosing genetic diseases, and appreciating the diversity of phenotypes in nature.