This is the method used to determine the heritability of a simple recessive trait, which is a question of probability.  Probability is the chance that a given event will occur.  With simple recessive inheritance there are three possible genes which can produce six possible combinations of breeding.

Each individual has 2 alleles which make one gene and will receive one allele (50%) from the father and one allele (50%) from the mother.  Alleles can either be dominant usually indicated by a capital letter (A) or recessive usually indicated by small letter (a).  For instance the color black is dominant (A) over the color red which is a recessive color (a), so whenever the black allele is present in the gene controlling color, then the animal will be black in color.  Since the recessive trait is unknown or hidden it often produces unhealthy or undesirable traits.  Sometimes it also provides desirable traits, like the lovely red color in Welsh Springer Spaniels.  

The 2 alleles can produce the following 3 genetic combinations.

The offspring will have one of the following combinations with one allele from the father and the other from the mother:

Each parent provides one of two possible alleles to the offspring and below is the probability that an offspring will inherit the two alleles depending on the status of the parents.