Lecture 18 - Heredity 2024

Heredity Overview

• Heredity: The transmission of genetic information from one generation to the next (from parent to offspring).

• Heritable Trait: A characteristic of an organism (morphological, physiological, or behavioral) determined by one or more genes.

• Phenotype: The observable traits in an individual resulting from their genotype.

• Genotype: The alleles an individual possesses for a specific heritable trait.

Genetic Definitions

• Allele: An alternative form of a gene which causes variation in phenotypes.

• Homozygous Genotype: An individual with two identical alleles for a specific gene (e.g., RR or rr).

• Heterozygous Genotype: An individual with two different alleles for a specific gene (e.g., Rr).

• Dominant Allele: An allele that expresses its phenotype when present in either homozygous (RR) or heterozygous (Rr) conditions.

• Recessive Allele: An allele that expresses its phenotype only in the homozygous condition (rr).

Mendel’s Experiments with Peas

• Monohybrid Cross: Examines the inheritance of a single trait.

  • Example: Cross between homozygous round seed (RR) and homozygous wrinkled seed (rr) leads to all offspring genotypes being Rr (heterozygous) and all phenotypes being round seeds.

  • When two heterozygotes (Rr) are crossed, the offspring phenotypic ratio is 3:1 (3 round to 1 wrinkled).

Principles of Segregation and Independent Assortment

• Principle of Segregation: During meiosis, alleles segregate into different gametes. This was demonstrated through Mendel's crosses.

• Principle of Independent Assortment: Alleles of different genes segregate independently during gamete formation.

Dihybrid Cross

• Involves two traits: For example, round (R) vs. wrinkled (r) seed shape and yellow (Y) vs. green (y) seed color.

• Cross between RRYY (round and yellow) and rryy (wrinkled and green) results in all RrYy offspring.

  • Following generation results in a phenotype ratio of 9:3:3:1.

Genetic Diversity

• Meiosis promotes genetic diversity through:

  • Segregation of Alleles: Different alleles are separated into gametes.

  • Independent Assortment: Alleles on non-homologous chromosomes assort independently.

  • Crossing Over: Occurs during meiosis I, leading to the exchange of DNA segments between homologous chromosomes.

• Importance of Genetic Diversity: It increases the likelihood that some individuals will survive environmental changes, thereby reducing extinction risk.