Q.What is test cross?

A back cross is any cross between an F1 hybrid and either one of its two original parents (dominant or recessive). A test cross is a specific type of back cross — the F1 hybrid is crossed only with the homozygous recessive parent — used to determine whether an individual showing the dominant phenotype is homozygous or heterozygous for that trait, since its own phenotype alone cannot distinguish the two.

Because the recessive tester parent contributes only recessive gametes, the offspring's phenotypes directly reveal the unknown parent's gametes: if the unknown individual is homozygous dominant ($RR$), all offspring show the dominant phenotype; if heterozygous ($Rr$), offspring split $1:1$ between dominant and recessive phenotypes. Test crosses are valued for being simple, repeatable and predictable, and are also used practically to introduce useful recessive traits into self-pollinated crop varieties. Every test cross is, by definition, a back cross (against the recessive parent), but a back cross against the dominant parent is not a test cross — which is why the two terms, though related, are not interchangeable. …