The focus is on the study of heredity, genetic variation, and the transmission of traits across generations, particularly in sexually reproducing organisms.
Historical Background
Early genetic studies occurred before the discovery of DNA and chromosomes.
Selective breeding was an early observation that influenced both artificial selection (by humans) and natural selection (by Darwin).
Original beliefs about inheritance included the idea that traits were fixed and unchanging.
Theories of Heredity
Blending Theory: The idea that traits of parents blend in offspring (e.g., mixing paint colors).
Particulate Theory: Early proposal that traits are inherited as discrete units (like a deck of cards).
Gregor Mendel
Conducted experiments in the mid-1800s, focusing on the garden pea plant to study inheritance.
Introduced terminology:
Factors: Now referred to as genes, these are units that determine specific traits.
Traits: Various forms of factors, e.g., flower color (purple or white).
Emphasized the importance of controlled breeding to track inheritance patterns.
Mendel's Experiments
Mendel studied seven traits in pea plants, such as:
Flower color (purple or white)
Flower position (terminal or axial)
Pod shape (inflated or constricted)
Pod color (yellow or green)
Seed shape (round or wrinkled)
Seed color (yellow or green)
Stem length (tall or short)
Generations:
P1 (Parent Generation): Crossed true-breeding plants differing in one trait.
F1 Generation: All hybrids exhibiting the dominant trait.
F2 Generation: Allowed F1s to self-pollinate, revealing a 3:1 phenotypic ratio for dominant to recessive traits.
Mendel's Laws of Inheritance
Law of Segregation:
The two alleles for a trait segregate during the formation of gametes, ensuring offspring inherit one allele from each parent.
Example: In a cross of homozygous purple and homozygous white flowered plants, the F1 generation exhibits only purple flowers, while the recessive trait can reappear in the F2 generation.
Law of Independent Assortment:
Genes for different traits are inherited independently of each other, leading to new trait combinations.
Example: A dihybrid cross could produce a 9:3:3:1 phenotypic ratio.
Concepts of Genotype and Phenotype
Genotype: The genetic makeup (homozygous dominant, homozygous recessive, or heterozygous).
Phenotype: The observable characteristics influenced by the genotype.
Test crosses can help determine an organism's genotype by crossing it with a homozygous recessive individual.
Probability in Genetics
Genetic probabilities can be calculated using multiplication and addition rules.
Product Rule: Multiplying probabilities of independent events occurring in a specific combination.
Sum Rule: Adding probabilities of different combinations that can lead to the same outcome.
Example of using probabilities with dihybrid crosses illustrates the computational complexity in genetic predictions.
Conclusion
Mendel's research laid the groundwork for the field of genetics, explaining how traits are inherited and establishing foundational laws that continue to influence biological sciences today.