Is it true that the gene-targeted mutation is present in all cells of a chimera?

A gene-targeted mutation is not necessarily present in all cells of a chimera. Chimeras are organisms that are composed of cells with different genotypes, often resulting from the fusion of different embryos or the introduction of genetically modified cells into an existing organism. In the case of gene-targeted mutations, these changes are typically introduced into specific cells, such as embryonic stem cells, which can then contribute to only a subset of the cells in the resulting chimera.

This selective incorporation means that only some cells of the chimera will carry the gene-targeted mutation, while others may retain the original unmodified genotype. Consequently, the chimeric organism will exhibit a mixture of cells with both the mutation and the normal gene. This principle is foundational in genetic engineering and developmental biology, where understanding the distribution of mutated versus non-mutated cells is crucial for studying gene function and the effects of genetic modifications.