Origin and transmission of de novo L1 insertions in mouse pedigrees. Above, two-generation pedigree originating from parental mice SRA/SRB. Below, three-generation pedigrees originating from parental mice SRC/SRD and SRE/SRF. F1 animals are designated by the parental pair from which they arose (AB, CD, EF); F2 animals are so indicated. De novo L1 T_F insertions #1–#11 are color coded. In subsequent figures, schematics of each insertion are likewise color coded. With respect to each insertion, filled shapes indicate heterozygous animals, vertical hatching indicates germline mosaicism, horizontal hatching indicates somatic mosaicism, and vertical and horizontal hatching together indicates both somatic and germline mosaicism. Circles represent female animals; squares represent males. Dashed lines indicate matings. Animals for which mRC-seq was performed on somatic tissue gDNA are indicated with an asterisk; animals for which whole-genome sequencing (WGS) was performed on somatic tissue gDNA are indicated with a plus sign. For mouse SRCD14, mRC-seq was performed on somatic tissues as well as the germ cell fraction of the left and right testicles. Polymorphic retrotransposon insertions are not depicted in this figure.