Meiotic recombination results in the formation of DNA crossovers (CO) that are critical for ensuring the correct segregation of homologous (maternal and paternal) chromosomes at the first meiotic division. Chromosome segregation errors show striking sexual dimorphism: In humans, 20-80% of eggs versus 2.5-7% of sperm are aneuploid, likely due in large part to errors in CO formation. Meiotic recombination is initiated by the formation of DNA double strand breaks (DSB) that are then repaired via various pathways to achieve a tightly regulated frequency and distribution of COs across the genome. These DSB repair events occur in the context of the synaptonemal complex (SC), a proteinaceous structure that forms along the chromosome axes, tethering homolog pairs together. SC length correlates strongly with CO number, and most studies in human and mouse report females have higher CO rates due to their longer SC length. Paradoxically, meiotic recombination in females is highly error-prone, implying critical sex differences in CO formation cannot be explained by a correlation with SC length. I hypothesize that sexual dimorphism in CO rates is the product of key differences in molecular features of meiotic prophase I, namely the factors that orchestrate meiotic recombination and chromosome axis assembly. Unlike common laboratory mice (e.g., B6) and humans, wild-derived PWD male mice have higher CO number despite their shorter SCs, challenging the dogma that CO rates are inextricably linked to SC length. Thus, I propose to address my hypothesis using mice from diverse genetic backgrounds to dissect the molecular and genetic factors underlying sexually dimorphic CO rates. In Aim 1, I will examine dynamic localization of meiotic recombination proteins in male and female PWD and B6 mice to elucidate how sexually dimorphic CO rates progressively manifest through prophase I. Using high resolution imaging methods, I will characterize the accumulation of critical DSB repair factors (including RAD51, RPA2, MSH4, RNF212, and MLH1) to pinpoint sexually dimorphic differences in CO regulation. In Aim 2, I will evaluate cohesin-mediated chromatin organization in male and female B6 and PWD mice. Using CUT&Tag to profile REC8 and RAD21L cohesin distributions, I will identify sex differences in cohesin axis assembly and how they correlate with early DSB repair intermediates. In Aim 3, I will use the recombinant mouse lines of the Collaborative Cross to map genetic loci associated with sex differences in CO number and SC length. Collectively, these studies will be the first to examine the molecular and genetic factors that influence sexually dimorphic CO rate and SC length in diverse mouse genetic backgrounds. Insights gained from this project will provide critical understanding of why recombination errors are more common in females. Over the course of this project, I will receive invaluable training in the use of super-resolution microscopy, computational analysis of genomics data, and quantitative genetics of complex traits. Along with career development mentoring, these skills will be critical to the development of my independent research program.