Classical Genetics: Mendel's Principles

Classical Genetics Introduction

  • Genetics is the study of how traits are passed from parents to children.

  • The basic framework of genetics was set by Mendel.

Gregor Mendel: "Father of Modern Genetics"

  • Mendel lived in the Czech Republic in the mid-1800s and became a monk.

  • He questioned the idea that traits simply blend together.

  • He started genetic experiments with mice but switched to pea plants because:

    • There was a conflict at the monastery about the mice.

    • Pea plants were better for experiments; they reproduce quickly, have many offspring, and follow simple genetic rules.

  • His work, which included a lot of math, was largely ignored when first published in an obscure journal.

  • It was rediscovered almost 100 years later.

  • He figured out how traits are passed through generations.

Key Genetic Terminology

  • Alleles: Different versions of a gene (e.g., a straight hairline or a widow's peak).

  • Loci: The specific location of a gene on a chromosome.

  • Dominant Alleles: These traits are always expressed if present. They are shown with a capital letter (e.g., ext{A}). If an individual has one dominant and one recessive allele, the dominant trait is what you see.

  • Recessive Alleles: These traits are only expressed if two copies are present. They are shown with a lowercase letter (e.g., ext{a}). Recessive traits can sometimes appear to skip generations.

  • Homozygous: Having two identical alleles for a gene.

    • Homozygous dominant: Having two dominant alleles ( ext{AA}).

    • Homozygous recessive: Having two recessive alleles ( ext{aa}).

  • Heterozygous: Having two different alleles for a gene ( ext{Aa}). The dominant trait will be expressed. (Do not say "heterozygous dominant").

  • True Breeders: Individuals who are homozygous ( ext{AA} or ext{aa}) and consistently produce offspring with the same trait.

  • Phenotype: The observable physical characteristics of an individual, resulting from their genes and environment (e.g., purple flowers, straight hairline).

  • Genotype: The specific set of alleles an individual has for a trait (e.g., ext{PP}, ext{Pp}, ext{pp}).

Generational Terms

  • P Generation: The first set of parents in a genetic cross.

  • ext{F1} Generation: The first group of offspring from the P generation cross.

  • ext{F2} Generation: The second group of offspring, resulting from breeding ext{F1} individuals together.

Mendel's Laws of Inheritance

  • Law of Dominance: In a heterozygote, one allele (dominant) will hide the expression of the other allele (recessive).

  • Law of Segregation: During the creation of sex cells, the two alleles for a trait separate, so each sex cell gets only one allele. (Mendel figured this out without knowing about chromosomes or meiosis).

  • Law of Independent Assortment: Alleles for different genes separate independently during sex cell formation. This means inheriting one trait does not affect the inheritance of another, especially if the genes are on different chromosomes or very far apart on the same chromosome.