The songbird germline-restricted chromosome (GRC) is a mysterious oddity that does not follow Mendel’s law of inheritance, in fact, it has long been thought to be strictly transmitted through the matriline. However, our recent study on the zebra finch, Taeniopygia guttata, has shown that the GRC can also be inherited from the father. Intriguingly, the ability of males to pass on their GRC appears to vary widely between families, raising the puzzle how this can be evolutionary stable, given that DNA is generally expected to evolve towards increased spreading ability. Following up on this unexpected discovery, I here propose to clarify the genetic costs (and benefits) of paternal GRC inheritance. Specifically, using four captive populations of zebra finch with pedigree and detailed long-term breeding data, I will examine the population-wide variation in the ability of paternal GRC inheritance. Then, I will conduct RNA sequencing and identify key genes that link to the elimination of GRC in male meiosis by comparing the RNA expression profiles between testes from three males with high and three with low fractions of GRC-carrying sperm from each of the four populations. Additionally, we are applying different RNA sequencing technologies for our samples, yielding 24 RNA-seq libraries, 24 small-RNA-seq libraries and 4 samples for Spatial Transcriptomics. This project will be the first one to clarify the enigmatic nature of paternal GRC inheritance, regarding its frequency of occurrence and proximate genetic causes.