True or false? A mating between a tetraploid individual and a diploid individual produces biologically fit offspring.

A cross between a diploid individual and a tetraploid individual produces triploid offspring. There are many species of plants that are triploid, but those come from crosses between two triploid individuals. In those cases, the chromosomes from one parent match up with the chromosomes from the other parent just fine. The same is not true with a cross of a tetraploid and a diploid. In that case, one third of the chromosomes are unmatch, causing many offspring to die, to be sterile, or to be generally unfit. Long story short, the answer you seek is False.