Extensions 2

POWER Innovation

  • World-class Extensions of Mendelian Genetics II

  • Chapters: 14 and 15 (Concepts 14.3 and 15.5)

  • Reference: Campbell Biology, 12th Edition

True or False?

  • Statement: Alleles of one gene can mask or modify the expression of another gene.

Learning Objectives

  • Describe epistasis.

  • Define polygenic inheritance.

  • Understand the basis of non-Mendelian inheritance.

  • Explore the concept of genomic imprinting.

  • Analyze the inheritance of organelle genes.

Extending Mendelian Genetics for Two Genes

Example: Coat Color Dilution

  • Concept: Autosomal recessive gene that creates a lighter coat color.

  • Requirement: Two copies of the dilute allele are necessary to produce the phenotype.

Epistasis

  • Definition: A type of gene interaction where the phenotypic expression of one gene alters the expression of another gene.

  • Example: Labrador Retrievers

    • Gene 1: Black coat color (B) is dominant to brown (b)

    • Gene 2: Pigment deposition (E) is dominant

    • The gene (E/e) is epistatic to the gene (B/b).

Dihybrid Cross and Epistasis

  • Cross Analysis:

    • Sperm: BbEe

    • Eggs: BbEe

  • To calculate offspring genotypes:

    • 1/4 BE

    • 1/4 bE

    • 1/4 Be

    • 1/4 be

  • Genotypes of Offspring:

    • BBEE, BbEE, BBEe, BbEe, bbEE, BbEe, bbee, etc.

  • Phenotypic Ratio:

    • 9:3:4 pattern observed in phenotypes when considering epistasis.

Detailed Mechanism of Epistasis

  • Interaction of proteins in coat coloration:

    • Colorless Protein E: Stimulates eumelanin synthesis when present.

    • Protein B: Responsible for eumelanin deposition.

    • Interaction Outcomes:

    • Eumelanin deposited densely with BB, Bb, or ee.

    • No eumelanin deposited with ee resulting in light coloring (e.g., Pheomelanin).

Extending Mendelian Genetics for Many Genes

Polygenic Inheritance

  • Definition: An additive effect of two or more genes on a single character.

  • Variations characterized as quantitative characters.

  • Environmental Influences: Traits that are multifactorial and influenced by the environment.

  • Examples: Skin color, height, and weight.

Polygenic Inheritance Example: Height

  • Controlled by multiple genes (A, B, C):

    • Each “tall” allele (A, B, or C) contributes to one unit of tallness.

  • Allelic Combinations:

    • AABBCC = very tall (6 units)

    • aabbcc = very short (0 units)

    • AaBbCc or AABbcc = intermediate height (3 units)

  • Implications of Incomplete Dominance: Presence of alleles creates cumulative effects on phenotype.

Genetic Impact on Birth Weight in Beef Cattle

  • Studies indicate 121 genetic markers linked to 103 genes influence birth weight.

  • Graph depicting the correlation of genetic markers across different chromosomes showing statistical significance.

The Environmental Impact on Phenotype

Influences Examples:

  • Wind and Sun: Affects tree leaves’ size, shape, and greenness.

  • Nutrition: Affects height and weight.

  • Experience: Correlates with intelligence.

  • Soil Quality: Influences flower coloration in plants (e.g., pink to blue in hydrangeas affected by soil pH).

Multifactorial Characters

  • Traits that depend on multiple genes combined with environmental influences.

  • Phenotypic range is dependent on the environment in which genotype is expressed.

Case Study: Hydrangea Flowers

  • Soil Acidity: Affects color; acidic soil leads to bluer flowers due to free aluminum availability.

Temperature Sensitivity in Siamese Cats

  • Temperature-sensitive enzyme: Active at lower temperatures, inactive in warmer temperatures; explains dark extremities on cooler areas of the body.

Genomic Imprinting

  • Concept illustrating that some phenotypes depend on parent-origin alleles.

  • Involves gene silencing during gamete formation.

  • Affects only a small fraction of mammalian genes, often that are critical for embryonic development.

Case Study: IGF2 Gene

  • Function: Required for normal prenatal growth.

  • Expression Pattern: Only paternal allele is expressed.

Organelle (Extranuclear) Genes

  • Found in organelles such as mitochondria and chloroplasts.

  • Inheritance: Maternally inherited due to zygote cytoplasm originating from the egg.

Human Examples of Mitochondrial Genes

  • Diseases: Mitochondrial myopathy, Leber’s hereditary optic neuropathy, affecting ATP production which leads to muscular and nervous system disorders.

Lesson Recap

  • Key topics reviewed:

    • Epistasis

    • Polygenic inheritance

    • Non-Mendelian inheritance

    • Genomic imprinting

    • Inheritance of organelle genes