Mendelian Inheritance Summary

Mendelian Inheritance Notes

Overview

  • Focus on classical genetics, established by Gregor Mendel.
  • Studies inheritance of traits in whole organisms, primarily pea plants.

Gregor Mendel

  • Austrian monk (1822-1884) and gardener at Brno monastery.
  • Studied at Vienna University; applied mathematics to biological concepts.
  • Published findings on pea plant inheritance in 1866, rediscovered in 1900.
  • Recognized as the "Father of Genetics".

Key Methods of Mendel's Experiments

  • Investigated simple qualitative traits (e.g. flower color, seed shape), varying one trait at a time.
  • Controlled cross-pollination of pea plants.
  • Repeated experiments extensively and employed statistical methods.
  • Studied two generations simultaneously (P1 and F2).

Mendel's Conclusions

  1. No blended inheritance: Traits do not mix (e.g., no pink flowers).
  2. Recessive traits can reappear: Traits can be hidden in one generation and show in the next.
  3. Phenotype vs Genotype: Outward appearance doesn't always reflect genetic inheritance.
  4. Dominance: One allele can mask another (dominant vs recessive).
  5. Consistent ratios: F2 ratio approximates 3:1 for dominant to recessive phenotypes.

Genetic Terminology

  • Genes: Mendel's factors discovered to be DNA units located on chromosomes.
  • Alleles: Different forms of a gene (e.g., red vs. white flower color).
  • Homozygous vs Heterozygous: Homozygous has identical alleles, heterozygous has different alleles.

Monohybrid Cross

  • Involves one trait at a time (e.g., red vs. white flowers).
  • Punnett Square: Tool to predict genetic combinations.
  • F2 generation results in 3:1 ratio of dominant to recessive traits.
  • Mendel's First Law: Alleles segregate during gamete formation.

Test Cross

  • Used to determine genotype when phenotype is dominant.
  • Crossing with homozygous recessive reveals genetic composition of dominant phenotype.

Pedigrees

  • Charts showing inheritance of traits through generations, useful in tracking genetic diseases.

Gene and Phenotype Relationship

  • Genes code for proteins that determine phenotypes (e.g., flower color).
  • Dominant alleles typically produce functional proteins; recessive alleles may result in inactivity.

Sex Determination

  • In humans, males (XY) and females (XX); sex determined by Y chromosome.
  • X inactivation occurs in females, ensuring equal gene expression with males.

Sex-Linked Traits

  • Traits carried on sex chromosomes (most often X). Examples:
    • Eye Color in Fruit Flies: Red (R) vs White (r).
    • Hemophilia: X-linked condition due to recessive alleles affecting blood clotting.

Codominance

  • Situation where both alleles contribute to phenotype (e.g., snapdragon flower color).
  • Sickle Cell Anemia: Example of codominance.

Lethal Alleles

  • Mutations that can cause early death (e.g., some human genes cause genetic diseases).

Multiple Alleles

  • More than two alleles exist for a gene in a population (e.g., blood types).

Dihybrid Cross

  • Involves two traits (e.g., seed shape and color).
  • Shows independent assortment of genes (Mendel's Second Law), 9:3:3:1 ratio.

Gene Linkage

  • Genes on the same chromosome can be inherited together (linked) unless separated by crossing over.

Hardy-Weinberg Principle

  • Describes allele frequency stability under certain conditions in populations (no mutations, gene flow, etc.).
  • Any changes indicate evolution is occurring.

Evolutionary Factors

  • Mutations, gene flow, inbreeding, genetic drift, and selection influence allele frequencies in populations, producing evolutionary changes.