Genetics - A. V. Sivolob 2008

Formal Genetics: Patterns of Trait Inheritance
Mendel's Laws

One of the fundamental concepts in genetics is the trait—a distinct, specific property of an Organism or its part that differentiates one individual from another. In genetic terms, a trait can be any characteristic at the level of external features, physiological or biochemical specifics: weight, color, height, the presence of certain types of substances in Organs and Cells, a set of specific Proteins, and so on. The entire set of traits of a given organism is called its phenotype. Mendel realized that traits used in genetic analysis must manifest themselves consistently (regardless of age, environmental conditions, etc.) and exhibit two or more alternative forms of expression.

The most crucial experimental approach in genetics (virtually the only one at the dawn of its development) is crossing—the natural or artificial union of two Gametes during Fertilization. Crosses are denoted by the multiplication sign "×". In genetic diagrams, the female genotype, designated by the symbol "♀" (the mirror of Venus), is placed first, followed by the male genotype with the symbol "♂" (the shield and spear of Mars). In crossing schemes, the genotype refers not to the entire set of an organism's genes, but only to those associated with the traits being analyzed.

Dominant and recessive alleles (see Chapter 1) of the same Gene are typically denoted by the same uppercase and lowercase letters, respectively. The offspring resulting from a cross between two individuals with contrasting alternative traits are referred to as hybrid progeny, and an individual member of such progeny as a hybrid. Parental organisms used in crosses are designated by the letter P (from the Latin parentes, meaning parents), while their offspring are denoted by the letter F (from the Latin filii, meaning children). Usually, a numerical index corresponding to the filial generation number is subscripted to the letter F.