Non-random mating in which males with some characteristics tend to breed with females having that kind. If two parents tend to be more alike than is to be expected by chance, then "positive assortative mating" is occurring.
The tendency of like to mate with like. Mating can be assortative for a certain genotype (e.g., individuals with genotype AA tend to mate with other individuals of genotype AA) or phenotype (e.g., tall individuals mate with other tall individuals).
Assortative mating (also called assortative pairing) takes place when sexually reproducing organisms tend to mate with individuals that are like themselves in some respect (positive assortative mating) or dissimilar (negative assortative mating). In evolution, therefore these two types of assortative mating have the effect of reducing and expanding the range of variation, respectively, when the assorting is cued on heritable traits.