This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. A cross between an individual exhibiting the dominant phenotype of a trait and an individual that is homozygous recessive for that trait in order to determine the genotype of the dominant individual.

v. To subject to a testcross.

n. a cross between an organism whose genotype for a certain trait is unknown and an organism that is homozygous recessive for that trait so the unknown genotype can be determined from that of the offspring

"If the mystery mutant and the known mutant sit on different chromosomes or a long way from each other on the same chromosome the grandkids of your ‘testcross’ will have an even mix of either one of the mutations, both or neither."

crossing    crossbreeding    hybridisation    hybridizing    interbreeding