When the parents are heterozygous individuals (have both dominant and recessive alleles), they can have an offspring with a recessive trait (homozygous recessive). This can be explained with the following cross. For example, if both parents have black eyes (B)- black eye dominant and (b)- blue eye recessive, this means their phenotype is black but they have both alleles (Bb). Now, their children can have black or blue eyes, and that depends on the random passing down of the alleles. So their off-springs would have a 25% (1/4) probability of having blue eyes, if both parents would pass on the (b) blue allele to the offspring, hence the homozygous recessive condition would be seen.
