镶嵌现象是指从单一受精卵发育而成的同一个体的细胞有不同的遗传组成、染色体结构或染色体数目的现象。镶嵌现象可以和嵌合体相对比，在嵌合体的胚胎发育早期，多个受精卵相融合而导致同一个体中不同基因型的出现。

In genetics, a mosaic or mosaicism denotes the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg.

Mosaicism can result from various mechanisms including chromosome non-disjunction, anaphase lag and endoreplication. Anaphase lagging appears to be the main process by which mosaicism arises in the preimplantation embryo. Mosaicism may also result from a mutation during development which is propagated to only a subset of the adult cells.