Biology · MU Sofia

17. Genes and alleles. Interactions between allelic genes, patterns of inheritance. Interaction between non-allelic genes. Penetrance and expressivity. Heredity and environment.

Covers the basic terms of genetics, then Mendel's laws of segregation and independent assortment, followed by several forms of epistasis and duplicate gene effects. It moves on to genetic crosses (monohybrid, dihybrid, trihybrid and test crosses), inheritance patterns including X-linked, codominance and mitochondrial traits, linked genes, and complementary and modifier genes. It closes with penetrance, illustrated by a Huntington's disease example, expressivity, and the role of environment in shaping the phenotype.

Inside: 2 sections · 5 labelled pictures · 3 min

Topic 17 of 64 in the MU Sofia syllabus for Biology (Dental Medicine, Year 1). The free AceMed note covers it in 2 sections, about 440 words (3 min of reading).

In this note

  1. Introduction
  2. Inheritance Patterns:

Introduction

Definitions:

  • Gene: A segment of DNA or RNA responsible for a specific trait.
  • Allele: Variants of a gene; one from each parent.
  • Genotype: The gene set an organism carries; phenotype is the visible trait manifestation.
  • Diploid Eukaryotes: Organisms with two copies of each gene, either identical (homozygous) or varied (heterozygous).
  • Dominant and Recessive Traits: Dominant traits overshadow recessive ones; the latter are only visible when a dominant allele is absent.

Mendel's Laws Simplified:

  • Law of Segregation: Each parent passes only one allele for each trait to their offspring.
  • Law of Independent Assortment: Genes for different traits are inherited separately.

Epistasis Variants:

  • Dominant Epistasis: A dominant allele hides another gene's effect.
  • Recessive Epistasis: A recessive allele suppresses a different gene's effect.
  • Duplicate Recessive Epistasis: Two recessive alleles combine to influence a phenotype.
  • Duplicate Genes Additive Effect: Genes at different locations enhance each other's effects.
  • Duplicate Dominant Genes: Two genes yield the same phenotype, not additively.
  • Dominant and Recessive Interaction: Specific AA and bb combinations produce a unique phenotype.

Please watch the recommended video for further explanation

Types of Genetic Crosses:

  • Represented by capital letters for dominant traits and lowercase for recessive.
  • Generation tracking: P (parental), F1 (first generation offspring), F2 (second generation offspring).
  • Monohybrid Cross: Examines one trait inheritance.
  • Dihybrid Cross: Studies two trait inheritances.
  • Trihybrid Cross: Investigates three trait inheritances.
  • Test Cross: Determines an organism's genotype by breeding with a known recessive.

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