As part of a longstanding collaboration, Sally Sheldon worked with Julie McCandless (Law, LSE) to investigate the legal determination of parenthood following collaborative reproduction in the UK. In a paper published in Relatedness in Assisted Reproduction: Families, Origins and Identities (a collection edited by a group of researchers working at Cambridge), they discuss the importance that law attaches to genetic links and also how such significance is influenced by gender. Sheldon notes that: ‘science will continue to push moral, cultural, legal and, indeed, biological boundaries, in the construction of parenthood. In this paper, we take the example of the donation of mitochondrial DNA (mtDNA), offering an analysis that we hope might cause people to pause before they resort to the idea that children born of this technology are ‘three parent babies’ or even, indeed, that they have three ‘genetic parents’.