Basic Principles of Heredity

Gene: An inherited factor encoded in DNA that influences characteristics.
Allele: Variants of a gene that exist in different forms (e.g., dominant and recessive).
Locus: Specific location on a chromosome where an allele is found.
Genotype: The complete set of alleles possessed by an organism.
Heterozygote: An organism with two different alleles at a given locus.
Homozygote: An organism with identical alleles at a given locus.
Phenotype: Observable traits or characteristics of an organism.
Characteristic or Character: Features exhibited by an organism, such as color or size.

A genotype determines the potential for traits, but environmental factors also play a significant role in phenotypic expression.
Some traits are primarily influenced by genetics, while others are more affected by environmental conditions.
For example, the shape of Mendel's peas was mainly determined by genotype, whereas the height of an oak tree is significantly influenced by environmental factors.

Monohybrid Cross: A cross between individuals differing in one trait, often involving homozygous parents.
In Mendel's experiments, he crossed pure-breeding peas with round and wrinkled seeds, leading to predictable offspring traits in the first (F1) and second (F2) generations.
F1 generation exhibited only the dominant phenotype, while both traits appeared in the F2 generation in a 3:1 ratio of dominant to recessive traits.

Mendel hypothesized that organisms contain two alleles for each trait that segregate during gamete formation.
The concept of dominance was established; dominant alleles mask the expression of recessive ones.
The principle of segregation states that alleles separate with equal probability into gametes during reproduction.

A testcross determines the genotype of an unknown individual by crossing it with a homozygous recessive organism.
Phenotypic ratios from typical crosses include:
- 3:1 for heterozygous parents.
- 1:1 for a heterozygous and homozygous recessive cross.
- Uniform progeny in various homozygous crosses.

Dihybrid Cross: Crossing organisms differing in two traits, producing a 9:3:3:1 ratio among progeny.
Mendel demonstrated the principle of independent assortment, indicating that alleles for different traits segregate independently during gamete formation.

Principle of Segregation: Each organism has two alleles for each trait, which segregate during gamete formation.
Dominance Concept: In the presence of both alleles, only the dominant trait is expressed in the phenotype.
Independent Assortment: The segregation of alleles for one trait does not influence the segregation of alleles for another trait.