true-breeding

Mendel used crosses between true-breeding (homozygous) pea plants for each of the seven traits.

First, two homozygous true-breeding 1 types are crossed and the phenotype ratio of the F1 offspring is observed.

Palomino is created by a genetic mechanism of incomplete dominance, hence it is not considered true-breeding.

This is done by crossing one of the F1 progeny back to one of the true-breeding P1 parents.

Mendez was a true-breeding (homozygotic) grey, meaning that all of his foal were also grey.

Zeddaan was a true-breeding (homozygous) grey, meaning that he produced only grey foals.

It should be noted that a hybrid plant is not usually hybrid for all characteristics nor does a true-breeding strain breed true for all characteristics.

Finally, he performed "test crosses" (back-crossing descendants of the initial hybridization to the initial true-breeding lines) to reveal the presence and proportion of recessive characters.

A cross such as Mendel's, where true-breeding (largely homozygous) opposite parents are crossed in a controlled way to produce an F1, is a special case of hybrid structure.

Fisher was sceptical of Mendel's 1:2 ratio of true-breeding (homozygotes) to mixed progeny (heterozygotes), and remarked Mendel's results as, "Too good to be true."