When does a mutation in a mosaic person typically occur?

A mutation in a mosaic individual typically occurs after fertilization. Mosaicism arises when a mutation occurs in a single cell during early development. This means that not all cells have the mutation; rather, the mutation is present only in a subset of cells, leading to a mosaic pattern of normal and mutated cells.

In the context of embryonic development, this can happen at any time after the fertilization of the egg, as the zygote divides and forms more cells. If a mutation emerges after several cell divisions, those mutated cells will proliferate, leading to a mosaic individual with some cells exhibiting the mutation and others not.

This understanding of genetic mosaicism is crucial in genetics counseling, as it affects the phenotype of the individual and may have implications for inheritance patterns and the risk of passed down genetic abnormalities. The timing of the mutation's occurrence, thus, is pivotal for understanding the overall genetic makeup of the individual and how it relates to any clinical manifestations they may present.