Biology: Chromosomes and Mechanism of Inheritance

Heredity and Inheritance

• Heredity is the transmission of genetic information from one generation to the next.
• Gregor Mendel is known as the father of genetics for his work on heredity using hybridization techniques.

Gregor Mendel's Contributions

• Mendel postulated principles of heredity and introduced the term "factors" which are now known as genes.
• He carefully planned experiments, used a large sample, made meticulous recordings, and chose well-defined contrasting characters.

Seven Pairs of Contrasting Characters

• Mendel studied seven pairs of contrasting characters in pea plants, including:
  • Seed shape: round vs. wrinkled
  • Seed color: yellow vs. green
  • Pod shape: inflated vs. constricted
  • Pod color: green vs. yellow
  • Flower color: purple vs. white
  • Flower location: axial vs. terminal
  • Plant height: tall vs. dwarf/short

Genetic Terminology

• Character: a specific feature that an organism possesses.
• Trait: an inherited character with its variant form that can easily be seen (e.g. tall or dwarf).
• Factor: a unit of heredity responsible for the inheritance and expression of a genetic character (now known as a gene).
• Gene: a specific segment of DNA responsible for the inheritance and expression of a character.
• Alleles or Allelomorphs: two or more alternative forms of a given gene (factor) located on identical loci on homologous chromosomes.
• Dominant: an allele that is expressed even in the presence of an alternative allele.
• Recessive: an allele that is not expressed in the presence of an alternative allele.

Genotype and Phenotype

• Phenotype: the external appearance of an organism that can be easily seen for any trait (e.g. tallness or dwarfness).
• Genotype: the genetic constitution of an organism that decides the trait (e.g. TT, Tt, or tt).
• Homozygous (pure): having identical alleles for a particular trait, producing only one type of gamete (e.g. TT or tt).
• Heterozygous (hybrid): having pairs of contrasting alleles for a particular trait, producing two types of gametes (e.g. Tt).
• Pure line: an individual or population that is homozygous for one or more traits.
• Monohybrid: a cross considering only one heritable trait (e.g. cross of pure tall and pure dwarf plants).
• Dihybrid: a cross considering two heritable traits.
• F1 generation: the first generation formed by mating of pure parents having contrasting characters (e.g. progeny of TT x tt).
• F2 generation: the second generation formed by self-pollination of F1 individuals.