testcross vs crossbreeding

testcross and crossbreeding both are nouns.

Word	Noun	Adjective	Verb	Adverb
testcross	Yes	No	No	No
crossbreeding	Yes	No	No	No

As nouns, crossbreeding is a hypernym of testcross; that is, crossbreeding is a word with a broader meaning than testcross:

testcross: 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
crossbreeding: (genetics) the act of mixing different species or varieties of animals or plants and thus to produce hybrids

Other hypernyms of testcross include cross, crossing, hybridisation, hybridization, hybridizing, interbreeding.

testcross (noun)	crossbreeding (noun)
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	(genetics) the act of mixing different species or varieties of animals or plants and thus to produce hybrids
	reproduction by parents of different races (especially by white and non-white persons)