A Genealogical Look at Shared Ancestry on the X Chromosome

Abstract

2015; Rosenberg 2016; Shringarpure et al. 2016). In this article, we look at the inheritance of chromosomal segments on the X chromosome among closely related individuals. Our genetic ancestry models are structured around biparental genealogies back in time, an approach used by many previous authors (e.g., Donnelly 1983; Chang 1999; Rohde et al. 2004; Barton and Etheridge 2011). If we ignore pedigree collapse, the genealogy of a present-day individual encodes all biparental relationships back in time; e.g., the two parents, four grandparents, eight great-grandparents, 2k greatk22 grandparents, and in general the 2k ancestors k generations back; and we refer to these individuals as one’s genealogical ancestors. Note that throughout this article, kth generation ancestors refers to the ancestors within generation k, not the total number of ancestors from generations 1 to k. A genealogical ancestor of a present-day individual is said to also be a genetic ancestor if the present-day individual shares genetic material by descent from this ancestor. We refer to these segments of shared genetic material as being identical by descent, and in doing so we ignore the possibility of mutation in the limited number of generations separating our individuals. Throughout this article, we ignore the pseudoautosomal region(s) (PAR) of the X chromosome, which undergoes crossing over with the Y chromosome in males (Koller and Darlington 1934) to ensure proper disjunction in meiosis I (Hassold et al. 1991). We also ignore gene conversion that is known to occur on the X (Rosser et al. 2009). Here, we are concerned with inheritance through the X genealogy embedded inside an individual’s genealogy, which includes only the subset of one’s genealogical ancestors who could have possibly contributed to one’s non-PAR X chromosome. We refer to the individuals in this X genealogy as X ancestors. Since males receive an X only from their mothers, a male’s father cannot be an X ancestor. Consequently, a male’s father and all of his ancestors are excluded from the X genealogy (Figure 1). Therefore, females are overrepresented in the X genealogy, and as we go back in one’s genealogy, the fraction of individuals who are possible X ancestors shrinks. This property means that genetic relationships differ on the X compared to the autosomes, a fact that changes the calculation of kinship coefficients on the X (Pinto et al. 2011, 2012) and also has interesting implications for kin-selection models involving the X chromosome (Rice et al. 2008; Fox et al. 2009).