Genetics Analysis & Principles

Extensions of Mendelian Inheritance

4.1 Introduction

Mendelian Inheritance

• Mendelian inheritance encapsulates inheritance patterns following two laws:

  • Law of Segregation: Each individual carries two alleles for a trait, and these alleles segregate from each other during the formation of gametes.

  • Law of Independent Assortment: Alleles of different genes assort independently of one another during gamete formation.

• Simple Mendelian inheritance typically involves:

  • A single gene with two different alleles.

  • Alleles demonstrating a straightforward dominant/recessive relationship.

Summary of Chapter Goals

• This chapter will explore traits that deviate from the simple dominant/recessive relationship.

• Inheritance patterns of these deviations still adhere to Mendelian laws but exhibit more complexity and richness than originally recognized by Mendel.

4.2 Simple Inheritance Patterns

• Several variations exist in how two alleles of a single gene may determine the outcome of a trait.

• Table 4.1 explores different Mendelian inheritance patterns, aiming to:

  • Predict outcomes of genetic crosses.

  • Comprehend the relationship between molecular expression of a gene and the observable trait.

Table 4.1: Mendelian Inheritance Patterns Involving Single Genes

• Type: Simple Mendelian Inheritance

  • Description: Involves alleles adhering to Mendel's laws, maintaining a strict dominant/recessive relationship.

  • Molecular Aspect: 50% of protein produced by one copy of the dominant (functional) allele in a heterozygote is adequate to express the dominant trait.

• Type: Incomplete Penetrance

  • Description: Dominant phenotype not expressed even though the individual possesses a dominant allele.

  • Example: An individual carrying the polydactyly allele exhibiting a normal number of fingers.

  • Molecular Aspect: Dominant allele present but protein encoded may not exert full effects due to environmental factors or interference from other genes.

• Type: Incomplete Dominance

  • Description: Heterozygote displays a phenotype that is an intermediate between the two homozygotes.

  • Example: A cross between homozygous red and white-flowered plants produces heterozygous pink-flowered offspring.

  • Molecular Aspect: 50% of the protein from a functional allele in the heterozygote is insufficient to achieve the same trait as produced by the homozygous counterpart.

• Type: Overdominance

  • Description: Heterozygote exhibiting traits more beneficial than either homozygote.

  • Molecular Aspect: Heterozygotes may possess enhanced resistance to diseases or produce proteins functioning under varied conditions.

• Type: Codominance

  • Description: Heterozygote expresses both alleles simultaneous without an intermediate phenotype.

  • Example: In blood types, an individual with A and B alleles will display AB phenotype.

  • Molecular Aspect: Codominant alleles code proteins functioning distinctly in the heterozygote, affecting the phenotype uniquely.

• Type: X-linked Inheritance

  • Description: Involves genes on the X chromosome; male (hemizygous) has one copy and female has two copies.

  • Molecular Aspect: Heterozygous females produce enough protein from one dominant allele to exhibit its trait; males express their single X-linked gene.

• Type: Sex-influenced Inheritance

  • Description: Effect of sex on the phenotype; alleles recessive in one sex may be dominant in another.

  • Molecular Aspect: Sex hormones may regulate gene expression influencing phenotypes.

• Type: Sex-limited Inheritance

  • Description: Traits occurring only in one sex (e.g., mammary gland development).

  • Molecular Aspect: Sex-specific hormones are essential for producing specific phenotypes.

• Type: Lethal Alleles

  • Description: Alleles potentially lethal to the organism.

  • Molecular Aspect: Often loss-of-function alleles encoding essential proteins; mutations may lead to severe dysfunction or complete loss of activity.

Dominant and Recessive Alleles

• Wild-type Alleles: Prevalent alleles in populations that produce functional proteins in proper amounts.

• Mutant Alleles: Modified alleles often defected in functional expression and inherited recessively.

Genetic Diseases and Mutant Alleles

• Genetic diseases often arise from mutations in recessive alleles leading to nonfunctional proteins. Table 4.2 presents examples involving specific conditions:

  • Phenylketonuria: Mutation prevents phenylalanine metabolism, potentially causing mental defects if untreated.

  • Albinism: Defect in tyrosinase resulting in lack of pigmentation.

  • Cystic Fibrosis: Mutated chloride transporter, leading to severe respiratory problems and organ dysfunction.

  • Lesch-Nyhan Syndrome: Mutation in a purine metabolism enzyme, resulting in severe neurological effects.

Comparison of Protein Levels Among Genotypes

• Genotypes and corresponding levels of functional protein:

  • PP (homozygous dominant): 100% functional protein results in purple phenotype.

  • Pp (heterozygote): 50% functional protein also gives purple phenotype.

  • pp (homozygous recessive): 0% functional protein results in white phenotype.

Incomplete Penetrance

• In certain cases, a dominant allele does not manifest in a phenotype even if the individual carries it (e.g., polydactyly).

• Penetrance expresses the extent to which a dominant allele manifests phenotypically within a population. E.g., 60% penetrance indicates that 60% of carriers exhibit the trait.

Expressivity

• Expressivity refers to the degree of trait expression.

• Variations in expressivity can be influenced by environmental conditions or interactions with other genes. For example, varying numbers of digits in polydactyly can show low to high expressivity based on individual cases.

Environmental Effects

• Environmental conditions can significantly impact phenotype.

• The arctic fox as an example changes coat color based on temperature, representing a temperature-sensitive allele.

• Phenylketonuria exemplifies how diet (environment) is pivotal in managing symptoms.

Incomplete Dominance

• In incomplete dominance, the heterozygote exhibits a phenotype intermediate between both homozygotes, demonstrated by flower color in four o'clock plants.

• In certain crosses, a 1:2:1 phenotypic ratio arises, contrasting with the expected 3:1 in simple dominance situations.

Overdominance

• Overdominance showcases a heterozygote's superiority.

• Sickle-cell anemia serves as a reference, where heterozygotes display enhanced malaria resistance without experiencing anemia themselves.

Multiple Alleles

• Many genes manifest multiple alleles, extending beyond simple dominant/recessive relationships. The ABO blood type is a classic example emphasizing three alleles (IA, IB, and i).

  • ABO Blood Types: IA and IB are codominant, while i is recessive.

• Blood transfusions require careful matching of donor and recipient types to avoid agglutination reactions due to antibody-antigen interactions.

Genes on Sex Chromosomes

• Sex-linked traits often show differential expression based on the sex chromosome composition.

• X-linked traits largely affect males who are hemizygous, while Y-linked traits are rarer and only transmitted from father to son.

Sex-influenced Traits

• These traits depend on sex, wherein an allele may be dominant in one sex and recessive in the other, yet are still autosomal.

  • Example: Scurs in cattle (dominant in males but recessive in females).

Lethal Alleles

• Lethal alleles can cause organism death typically inherited recessively and associated with mutations in essential genes.

• An example includes Manx cats, where homozygous lethal alleles lead to embryonic death.

Pleiotropy

• Pleiotropy denotes when a single gene influences multiple traits.

  • Cystic fibrosis is a noted instance, linking different physiological manifestations to one mutated gene.

Gene Interactions

• Gene interactions manifest when individual genes contribute collectively to traits.

  • Epistasis: One gene masks the phenotypic effects of another, seen in various biochemical pathways.

  • Complementation: Crosses between mutants illustrating that two mutations in different genes can restore a wild-type phenotype.

Summary of Concepts

• Understanding these patterns and mechanisms allows for more intricate predictions on inheritance and phenotype expression, emphasizing the complexity found beyond simple Mendelian models.