Basic Crosses

If we are dealing with a single gene that has a dominant allele (B) and a recessive allele (b) then there are only a few possible crosses that could ever occur. If you know the progeny genotypes that result from these crosses, then working with crosses that involve more than one gene later in the course becomes easy.

So, memorize these crosses and make sure that you understand them!

BB x BB and bb x bb are trivial. Obviously BB x BB can only yield BB progeny and bb x bb can only yield bb progeny.

BB x Bb will yield ½ BB and ½ Bb (all dominant phenotype)

Bb x bb will yield ½ Bb (Dominant phenotype) and ½ bb (recessive phenotype)

Bb x Bb will yield ¼ BB, ½ Bb and ¼ bb. This, of course, is a 3:1 (Dominant:Recessive) phenotype ratio, the famous result that Mendel got from his F₁ crosses.

That is all of them. If you know these cold it makes later extensions quite easy.