Basic Principles of Heredity - Mendelian Genetics

Mendelian Genetics: The foundation of modern genetics, established by Gregor Mendel through experiments with pea plants.

Gene: An inherited factor that determines a characteristic.
Allele: Alternative forms of a gene (e.g., tall or short).
Locus: Specific location of a gene on a chromosome.
Genotype: The genetic makeup of an organism (set of alleles).
Phenotype: The observable characteristics or traits of an organism.
Homozygote: Individual with two identical alleles at a locus (e.g., TT or tt).
Heterozygote: Individual with two different alleles at a locus (e.g., Tt).

Pea Plant Studies: Mendel used Pisum sativum to study inheritance, focusing on seven traits.
Monohybrid Cross: An experiment that considers one trait at a time (e.g., tall vs. short).
- When breeding a tall plant (T) with a short plant (t):
- F1 Generation: All offspring are tall (Tt) indicating dominance of the tall allele.
- F2 Generation: When F1 plants are self-fertilized:
  - Ratios of tall to short plants were approximately 3:1.

Law of Segregation: Each individual has two alleles for each trait, which segregate during gamete formation.
- Only one allele from each parent is passed to the offspring.
Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

Traits expressed in F1 hybrids (tall plants) are called dominant, while those that disappear in the F1 generation (short plants) are called recessive.
Dominant vs. Recessive Frequencies: Example ratios observed in Mendel’s experiments include:
- 75% dominant traits (e.g., tall) vs. 25% recessive traits (e.g., short).

Purpose: A method to predict the genotypic and phenotypic ratios of offspring from genetic crosses.
Example: Cross of Tt x Tt yields:
- 1 TT : 2 Tt : 1 tt for genotype and 3 tall : 1 short for phenotype ratio.

Involves Two Traits: Example involves seed shape (Round vs. Wrinkled) and color (Yellow vs. Green).
- Results revealed a 9:3:3:1 phenotypic ratio.
- Example: Cross between RRYY and rryy yields all RrYy in the F1 generation.

Probability Concepts:
- Multiplication Rule: Calculating probability for independent events, e.g., two alleles.
- Addition Rule: Probability of mutually exclusive events, e.g., phenotype possibilities.

Albinism: An example of an autosomal recessive disorder where normal pigmentation is dominant (A) over albinism (a).
- Crosses can show potential offspring phenotypes based on parental genotype combinations.

Mendel’s research laid the groundwork leading to modern genetics theories and concepts, emphasizing that traits are passed as discrete units rather than blending.