If we are tracking two different genes at the same time we refer to the crosses as dihybrid. For example, we may have two genes A and B, which affect completely different characteristics. So A may be an eye colour gene, while B may be finger webbing gene.

Let A = Brown eyes and a= blue eyes

Let B= normal fingers and b = webbed fingers.

A brown eyed, normal fingered homozygous person has children by a blue-eyed, webbed fingered homozygous person.

If two persons of the heterozygous type were to meet and produce offspring the possible outcomes would be as follows:

Notice that we have 9 A*B* : 3 A*bb : 3 aaB* : 1 aabb , where the asterisk represents any partner allele.

Thus we expect 9 Brown-eyed, normal fingered : 3 Brown-eyed, webbed fingered : 3 Blue-eyed, normal fingered : 1 blue eyed, webbed fingered.

If we performed a backcross between the heterozygous type and a pure bred recessive we would normally get: a 1 : 1 : 1 : 1 ratio amongst the offspring.

There are some cases where this does not happen and these involve genes which are on the same chromosome, which prevents them from being randomly assorted. Such genes are said to be 'linked'. (See Gene systems for further details)