Study Notes for Non-Mendelian Genetics

Unit 5: Non-Mendelian Genetics

Introduction to Non-Mendelian Genetics

  • Previous discussion centered around Mendelian genetics focusing on predictable ratios (e.g., 3:1, 9:3:1).

  • Mendel's experiments with pea plants demonstrated the laws of segregation and independent assortment.

  • Current focus: Traits that do not follow Mendelian ratios.

Mechanisms Causing Deviations from Mendelian Ratios

  • Several mechanisms lead to altered inheritance patterns:

    • Varying dominance relationships between alleles.

    • Interaction of multiple genes to determine a trait.

    • Linkage of genes to sex chromosomes.

    • Traits passed through non-nuclear DNA (e.g., mitochondria and chloroplasts).

  • These mechanisms highlight the complexity of inheritance beyond Mendelian principles.

Varying Degrees of Dominance

Complete Dominance

  • Defined by Mendel: A scenario where a homozygote and a heterozygote present the same phenotype.

  • Example: Purple flowers in homozygous dominant and heterozygous plants.

Incomplete Dominance

  • Definition: When neither allele is fully dominant, leading to a blended phenotype in the heterozygote.

  • Example:

    • Snapdragons: Crossing a homozygous red flower with a homozygous white flower yields F1 offspring with pink flowers.

    • Emphasis on blending of the phenotype, with distinct alleles remaining unchanged.

Co-Dominance

  • Definition: Both alleles are expressed simultaneously in the phenotype.

  • Example:

    • Human blood groups: Type A blood displays both A and B antigens on red blood cells.

    • Rhododendrons: Crossing a red homozygous flower with a white homozygous flower results in F1 heterozygotes with patches of both red and white.

Multiple Alleles

  • Definition: Genes can have more than two alleles, leading to multiple phenotypes.

  • Example:

    • Human blood types have three alleles: A, B, and O, resulting in four phenotypes: A, B, AB, and O.

    • Dominance relationships:

    • Type A: Homozygous for A or heterozygous (A dominant over O).

    • Type B: Homozygous for B or heterozygous (B dominant over O).

    • Type AB: Co-dominance of A and B.

    • Type O: Homozygous recessive (OO).

Practice Problems

  1. Gray Mouse Phenomenon: A black homozygous dominant mouse crossed with a white homozygous recessive mouse produces gray heterozygous offspring.

    • Explanation: Incomplete dominance, where the heterozygous offspring shows a phenotypic blend.

  2. Cattle Color: Cattle display red (C^R), white (C^W), and roan (C^R/C^W).

    • Explanation: Co-dominance.

    • Phenotypic ratio from crossing a red cow with a roan cow yields a 1:1 ratio of red to roan.

  3. Blood Type Possibilities: A woman with type A blood has a child with a man with type B blood.

    • Possible genotypes:

      • Woman: Homozygous for A or heterozygous (A dominant over O).

      • Man: Homozygous for B or heterozygous (B dominant over O).

Multiple Genes Impacting Traits

  • Traits often depend on interactions between multiple genes:

Epistasis

  • Definition: The phenotypic expression of one gene affects the expression of another gene, common in coat color inheritance.

  • Example Informing Us: In Labrador retrievers and certain mice, one gene influences pigment production while another determines if pigment will be deposited.

    • Albino mice example: If a gene coded for pigment does not allow pigment to be deposited, the phenotype remains albino.

Polygenic Inheritance

  • Definition: When multiple genes influence a single phenotype leading to continuous variation.

  • Examples: Human height and skin color show a range rather than discrete categories.

Pleiotropy

  • Definition: A single gene can influence multiple traits across different systems.

  • Example: Marfan syndrome caused by mutations to the FBN1 gene, impacting connective tissues across multiple body systems, affecting:

    • Limb development

    • Cardiovascular system

    • Vision

Sex Chromosome Inheritance

  • Thomas Hunt Morgan's work with fruit flies demonstrated

Sex-Linked Genes

  • Definition: Genes located on sex chromosomes.

  • Types:

    • Y-linked genes: Rare, affect only males.

    • X-linked genes: More common, lead to distinctive inheritance patterns.

Patterns of X-Linked Inheritance

  • Fathers pass X-linked alleles to daughters; not sons (fathers transmit Y to sons).

  • Mothers can pass X-linked alleles to both daughters and sons.

Implications of X-Linked Inheritance

  • For recessive X-linked traits:

    • Females express the trait only if homozygous.

    • Males express the trait if they inherit it from their mother (hemizygous).

    • Higher prevalence of X-linked disorders in males due to having one X chromosome.

Examples of X-Linked Disorders

  • Duchenne muscular dystrophy: Progressive weakening of muscles.

  • Hemophilia: Inability to properly clot blood.

  • Color blindness: Inability to accurately perceive colors.

X Inactivation in Females

  • Females possess two X chromosomes, one of which becomes mostly inactive during development.

    • This inactive chromosome forms a Barr body.

    • X inactivation equalizes gene dosage and expression levels between males and females.

Linked Genes

  • Overview of how physical proximity of genes on a chromosome influences inheritance patterns, discussion to follow in subsequent lessons.