If a genetic edit is performed on an embryo, every developing cell in that embryo will carry the edit, including the germline cells (sperm and egg).
"If you do it in an embryo, all of a sudden every cell in the developing embryo will have it, including sperm and egg." (said at 2:02:45)
The speaker's statement describes the fundamental distinction between somatic cell editing and germline (embryo) editing: editing at the one-cell zygote stage aims to introduce the genetic modification into all descendant somatic and germline lineages (sperm and egg), thereby making it heritable. However, in practice, editing an embryo does not guarantee that every single developing cell carries the edit due to the major biological and technical hurdle of genetic mosaicism (where Cas9 cleavage or repair occurs after cleavage divisions begin, leading to a mixture of edited and unedited cells or different edit types across blastomeres). Thus, while the theoretical/conceptual principle of germline editing is correctly stated, the categorical claim that 'every cell' will have it requires qualification.
- context: Mosaicism in CRISPR/Cas9-mediated genome editing. (Developmental biology 2019)
"However, using this technology for generating gene-edited animals involves a number of obstacles. One such obstacle is mosaicism, which is common in founder animals. This is especially the case when the CRISPR/Cas9 system is used in embryos." (abstract, passage verified)
pubmedfull study (doi) - context: CRISPR/Cas gene editing in the human germline. (Seminars in cell & developmental biology 2022)
"The ease and efficacy of CRISPR/Cas9 germline gene editing in animal models paved the way to human germline gene editing (HGGE), by which permanent changes can be introduced into the embryo... and that various hurdles (i.e. loss-of-heterozygosity and mosaicism) need to be overcome before clinical applications should be considered." (abstract)
pubmedfull study (doi)