Chapter 4
Extensions of Mendelian Inheritance
Introduction
Mendelian inheritance refers to inheritance patterns governed by specific laws, notably:
Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.
Law of Independent Assortment: Alleles of different genes assort independently of one another during gamete formation.
Simple Mendelian inheritance is characterized by:
A single gene with two different alleles.
Alleles that exhibit a simple dominant/recessive relationship.
Complex Inheritance Patterns
The chapter covers traits that deviate from the simple dominant/recessive relationship:
Although these inheritance patterns are more complex, they still adhere to Mendelian laws.
4.1 Overview of Mendelian Inheritance Patterns
Several patterns of inheritance exist when two alleles of a single gene influence trait outcomes.
Goals of Understanding Patterns:
Predict outcomes of genetic crosses.
Understand gene expression relationships to observable traits.
Mendelian Inheritance Patterns Involving Single Genes
Types and Descriptions:
Simple Mendelian Inheritance:
Inheritance: Follows Mendel's laws strictly with a dominant/recessive relationship.
Molecular: 50% of protein produced by one copy of the dominant allele in heterozygotes suffices to express the dominant trait.
Incomplete Penetrance:
Inheritance: Dominant phenotype isn’t expressed even if dominant allele is present (e.g., polydactyly where allele is present but normal phenotype results).
Molecular: Influenced by environmental factors or other genes counteracting the dominant allele's effect.
Incomplete Dominance:
Inheritance: Heterozygote phenotype is intermediate (e.g., red and white flowered parents producing pink offspring).
Molecular: Only 50% of protein equivalent to homozygote does not yield full trait expression.
Heterozygote Advantage:
Inheritance: Heterozygotes exhibit greater reproductive success than either homozygote; may confer increased health benefits (e.g., sickle cell trait provides malaria resistance).
Molecular: Enhanced resistance due to functional diversity in proteins.
Codominance:
Inheritance: Both alleles are expressed simultaneously without blending (e.g., AB blood type from A and B alleles).
Molecular: Each allele produces a distinctly functioning protein affecting phenotype uniquely.
X-linked Inheritance:
Inheritance: Genes located on the X chromosome exhibit unique patterns in males (hemizygous) and females (dihaploid).
Molecular: Heterozygous females can express dominant traits based on one X allele's expression.
Sex-influenced Inheritance:
Inheritance: Some alleles are dominant in one sex and recessive in another (e.g., scurs in cattle vary by sex).
Molecular: Sex hormones may regulate gene expression leading to differential phenotypic outcomes.
Sex-limited Inheritance:
Inheritance: Traits only expressed in one sex (e.g., sperm production in males).
Molecular: Dependent on sex hormone regulation affecting gene expression.
Lethal Alleles:
Inheritance: Alleles that can result in death, often resulting from loss-of-function mutations.
Molecular: Essential genes—proteins vital for survival—when mutated may cause a lethal phenotype.
4.2 Dominant and Recessive Alleles
Wild-type Alleles: Alleles prevalent in populations coding for normal protein functions.
Mutant Alleles: Modified alleles often leading to defective proteins, typically inherited recessively.
Recessive Phenotypes: Manifest only when an individual has two recessive alleles (not affecting heterozygote phenotype due to reasons such as:
50% protein functional sufficiency.
Up-regulation of normal genes in the presence of a defective allele.
Genetic Diseases and Mutant Alleles
Many genetic diseases stem from recessive alleles causing loss-of-function mutations. Examples include:
Phenylketonuria: Inability to metabolize phenylalanine, manageable through dietary restrictions.
Albinism: Lack of pigmentation due to defective tyrosinase production.
Tay-Sachs disease: Metabolic disorder leading to severe neurological deterioration.
Cystic Fibrosis: Impaired ion balance resulting in chronic lung infections.
Lesch-Nyhan Syndrome: Affects purine metabolism, resulting in severe behavioral and developmental issues.
Dominant Mutants
Less common than recessive alleles, typically arising from:
Gain-of-function mutations: Resulting protein exhibits abnormal or new function.
Dominant-negative mutations: Mutant proteins antagonize normal proteins.
Haploinsufficiency: Single copy of mutant allele is insufficient to express wild-type phenotype.
4.3 Environmental Effects on Gene Expression
Environmental factors can significantly influence phenotypes:
Example: Arctic fox coat color changes with seasons; temperature sensitivity demonstrated.
Example: Individuals with PKU exhibit traits depending on diet early in life.
4.4 Incomplete Dominance, Heterozygote Advantage, and Codominance
Incomplete Dominance: Phenotype of heterozygotes lies between both homozygotes, not following typical phenotypic ratios.
Heterozygote Advantage: Seen in traits that result in increased fitness against diseases like malaria.
4.5 Genes on Sex Chromosomes
X-linked Traits: Affect typically males more than females due to hemizygosity (e.g., Duchenne Muscular Dystrophy pattern shown in pedigrees).
X-linked traits often seen in males and carried by females.
Y-linked Traits: Rare in humans; only one Y gene typically affects male inheritance patterns.
4.6 Sex-influenced and Sex-limited Inheritance
Sex-influenced Traits: Exhibit dominance variation based on sex.
Example: Scurs in cattle driven by autosomal genes.
Sex-limited Traits: Influenced or expressed purely based on sex.
4.7 Lethal Alleles
Lethal alleles cause death during organism development or later in life (e.g., Huntington's disease has a late onset).
Conditional Lethal Alleles: Kill organisms under certain environmental conditions, such as temperature sensitivity.
Semilethal Alleles: Affect only a portion of the organism population.
4.8 Understanding Complex Phenotypes Caused by Mutations in Single Genes
Pleiotropy: Single gene influences multiple traits; examples include cystic fibrosis affecting diverse physiological systems.
4.9 Gene Interactions
Interactions can occur between different genes leading to complex inheritance patterns:
Epistasis: One gene affects the phenotypic expression of another.
Example: Flower color in sweet pea, requiring both gene products for expression.
Complementation: Phenomenon where two parents with similar recessive phenotypes yield wild-type offspring.
Gene Modification: An allele can modify the phenotypic effects of another gene's alleles.
Gene Redundancy: Loss-of-function alleles may not affect phenotypes if another gene compensates for its function, often due to gene duplication throughout evolution.