Patterns of Inheritance

Patterns of Inheritance

Heritable Variation

• Organisms exhibit various traits:
  • Traits can be categorized as wild-type (commonly seen) or variants.
  • Variations arise from the principles of inheritance.
  • Patterns of inheritance follow specific rules.

Example of Inheritance Patterns: Parakeets

• Crossbreeding between different parental traits:
  • Wild-Type vs. Sky-Blue:
  • P Generation: Wild-type x Wild-type → F1: All Wild-type
  • P Generation: Wild-type x Sky-blue → F1: All Wild-type, F2: 3/4 Wild-type, 1/4 Sky-blue.

Gregor Mendel's Contributions

• First to systematically study inheritance using scientific methods.
• Conducted experiments to uncover genetic principles using pea plants.

Pea Plant Traits Studied by Mendel

• Mendel analyzed several traits:
  • Dominant traits:
  • Purple flower, Tall plant
  • Recessive traits:
  • White flower, Dwarf plant
  • Traits included seed shape, color, pod shape, and flower position.

Monohybrid Cross

• Definition: A cross between parents differing in one trait.
  • Allows exploration of inheritance patterns.
• Mendel's findings:
  1. Alleles exist in different forms (dominant/recessive).
  2. Each trait is controlled by two alleles: one from each parent.
  3. Dominance/Recessiveness: Only dominant alleles manifest in phenotype.
  4. Law of Segregation: Gametes carry one allele for each character due to separation during gamete formation.

Genetic Makeup: Phenotype & Genotype

• Phenotype: Observable characteristics of an organism.
• Genotype: The genetic makeup, encompassing all gene forms.

Mendel's Laws of Inheritance

Law of Segregation

• States allele pairs segregate during gamete formation, thus only one allele from each parent is passed on during fertilization.
• Each haploid gamete (n) combines to form a diploid organism (2n).

Alleles and Homologous Chromosomes

• Alleles for genes are located on homologous chromosomes.
• Conditions:
  • Homozygous: Identical alleles (true breeding).
  • Heterozygous: Different alleles.

Punnett Square Example

• Visual tool to predict genetic ratios:
  • Cross of PP (purple) and pp (white): Results in 1 PP: 2 Pp: 1 pp following a 3:1 phenotypic ratio.

Independent Assortment

• When studying two traits, findings indicate:
  • Dependent Assortment: If traits are inherited together, indicating genes located on the same chromosome.
  • Independent Assortment: Traits segregate independently if genes are on different chromosomes.

Principle of Independent Assortment

• States different allele pairs are inherited independently during gamete formation, leading to various trait combinations.

Mendel's Application to Human Traits

• Evidence shows Mendel’s principles apply to human genetics (e.g., dominant and recessive traits like freckles vs. no freckles).

Extensions Beyond Mendelian Genetics

• Some traits are explained by:
  • Incomplete Dominance: Traits blend (e.g., Red x White snapdragons yield Pink).
  • Multiple Alleles: Example: ABO blood types in humans (IA, IB, i).
  • Pleiotropy: Single gene affects multiple traits (e.g., sickle-cell anemia).
  • Polygenic Inheritance: Traits influenced by multiple genes (e.g., skin color).

Chromosome Theory of Inheritance

• Proposes that genes located on chromosomes dictate inheritance patterns due to the behavior of chromosomes during meiosis and fertilization.

Summary Diagram of Mendelian Principles

• Segregation and Independent Assortment in Gametes: Describes how alleles separate and assort into gametes impacting genetic ratios in offspring.
• Phenotypic ratios in F2 generation derived from F1 mating illustrates the application of Mendel's laws.