A Populationwide Coalescent Analysis of Icelandic Matrilineal and
Patrilineal Genealogies: Evidence for a Faster Evolutionary Rate of mtDNA
Lineages than Y Chromosomes

Agnar Helgason,1,2 Birgir Hrafnkelsson,1 Jeffrey R. Gulcher,1 Ryk Ward,2 and Ka´ri Stefa´nsson1
1deCODE Genetics, Reykjavı´k; and 2Institute of Biological Anthropology, University of Oxford, Oxford

Historical inferences from genetic data increasingly depend on assumptions about the genealogical process that
shapes the frequencies of alleles over time. Yet little is known about the structure of human genealogies over long
periods of time and how they depart from expectations of standard demographic models, such as that attributed
to Wright and Fisher. To obtain such information and to examine the recent evolutionary history of mtDNA and
Y-chromosome haplotypes in the Icelandic gene pool, we traced the matrilineal and patrilineal ancestry of all
131,060 Icelanders born after 1972 back to two cohorts of ancestors, one born between 1848 and 1892 and the
other between 1798 and 1742. This populationwide coalescent analysis of Icelandic genealogies revealed highly
positively skewed distributions of descendants to ancestors, with the vast majority of potential ancestors contributing
one or no descendants and a minority of ancestors contributing large numbers of descendants. The expansion and
loss of matrilines and patrilines has caused considerable fluctuation in the frequencies of mtDNA and Y-chromosome
haplotypes, despite a rapid population expansion in Iceland during the past 300 years. Contrary to a widespread
assumption, the rate of evolution caused by this lineage-sorting process was markedly faster in matrilines (mtDNA)
than in patrilines (Y chromosomes). The primary cause is a 10% shorter matrilineal generation interval. Variance
in the number of offspring produced within each generation was not an important differentiating factor.We observed
an intergenerational correlation in offspring number and in the length of generation intervals in the matrilineal and
patrilineal genealogies, which was stronger in matrilines and thus contributes to their faster evolutionary rate.
These findings may have implications for coalescent date estimates based on mtDNA and Y chromosomes.