Fig. 1: Constraints on gene editing by inter-homologue homologous recombination (IH-HR) in the early human embryo.

a, Possible repair outcomes after a Cas9-induced DSB at the paternal MYBPC3^∆GAGT locus. Red and blue circles indicate unique maternal and paternal genetic variants, respectively. IH-HR results in gene conversion of the paternal allele by the wild-type (WT) maternal allele. The repair outcome can be a non-crossover or a crossover. Only one outcome of crossing-over is shown, in which the recombined chromosomes segregate to the same nucleus. The alternative is that the recombined chromosomes segregate to different daughter cells, such that loss of heterozygosity would occur on the chromosome from the point of the IH-HR event to the telomere in both daughter cells, one with homozygosity for the maternal chromosome and the other for the paternal chromosome. This outcome would be expected in half of the crossing-over events that underwent IH-HR in the G2 phase of the cell cycle. NHEJ can lead to small indels (not shown), but events are also possible that result in the deletion of a primer-binding site used for genotyping (as shown). b, Schematic of possible repair outcomes after Cas9 cleavage in the human zygote from panel a. m, maternal chromosome; p, paternal chromosome. c, Parthenogenesis after fertilization failure with (top) and without (bottom) second polar body (PB) extrusion. Outcomes of a–c are indistinguishable in genotyping assays using flanking PCR primers alone. d, Schematic of intracytoplasmic sperm injection (ICSI) followed by progression through the first cell cycle during day 1 of development. The number of maternal and paternal genomes is indicated at each phase of the cell cycle. e, Immunofluorescence of a mouse zygote at telophase of the second maternal meiotic division. Note that only the maternal genomes are attached to microtubules, while the paternal genome begins to form an interphase nuclear membrane to replace the sperm membrane. BF, bright field. f, Progression of human zygotes through the first cell cycle from the two-pronuclear stage to prometaphase, when the two genomes can be removed from the egg by a needle. Note the separation of the two genomes (arrows and dashed circles). NEBD, pronuclear envelope breakdown. g, Cell cycle progression during day 1 in fertilized mouse zygotes. Of 23 mouse eggs, none showed direct contact between the maternal and paternal genomes. Scale bars, 10 μm. Images one and four (from the left) in f are from Egli et al.¹⁴; images four to six (from the left, top) in g are from Egli et al.¹³.