Human traits (such as blood type and skin color) are determined by their genes that encode specific proteins. The observable physical or biochemical characteristic of an organism determined by these proteins is called “phenotype.” The complete hereditary information of an organism is called “genotype.”

In organisms that undergo sexual reproduction, each parent contributes one half of the inherited genes; therefore, the genetic composition varies between generations. In organisms that undergo asexual reproduction such as fission, sprouting, vegetative reproduction, or sporulation, the genetic composition of offspring is completely identical to that of the parent.

In many cases, expression of traits is not determined by a single gene. For example, expression of skin color in humans is regulated by multiple genes. However, the expression of some traits, such as ABO blood types, may depend on a single specific gene. Traits of the latter type are known as Mendelian inheritance traits.

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