Reciprocal crosses showed each parent made an equal contribution to the offspring. Evidence indicated factors could be hidden or unexpressed, these are the recessive traits. The term phenotype refers to the outward appearance of a trait, while the term genotype is used for the genetic makeup of an organism. Upper case letters are traditionally used to denote dominant traits, lower case letters for recessives. The Principle of Segregation proposes the separation of paired factors during gamete formation, with each gamete receiving one or the other factor, usually not both.
Organisms carry two alleles for every trait.
These traits separate during the formation of gametes. The two traits, if considered to inherit independently, fit into the principle of segregation . Instead of 4 possible genotypes from a monohybrid cross, dihybrid crosses have as many as 16 possible genotypes. Mendel realized the need to conduct his experiments on more complex situations. A cross concerning two traits is known as a dihybrid cross.