Montréal, Que, Canada. Did men and women contribute equally to the lineage of contemporary populations? Did our ancestors, particularly Homo sapiens (modern man), lean more toward polygamy or monogamy? These questions are important since they address the basis for transmitting ancestral genetic characteristics down though the generations.

Genetic diversity refers to the total number of characteristics in the genetic makeup of a species. It is a measure of relative biodiversity that differs from genetic variability, which describes the degree to which genetic characteristics can tend to vary.

To answer questions about our family and sexual arrangements, Dr. Damian Labuda headed a team that analyzed genomic data from three population samples of African, Asian and European origin. Labuda is an investigator at the Sainte-Justine University Hospital Research Center and a professor at the Department of Pediatrics of the Université de Montréal. The study team's findings appear in the American Journal of Human Genetics.

Genetic Population History

In a strictly monogamous population, one would expect to have an equal number of breeding females and males and, therefore, a breeding sex ratio of one female to one male. In a population where males tend to have more than one female mate, more females than males contribute to reproduction; for this reason the breeding ratio exceeds one. The authors of this study estimate that the breeding ratio varies between 1.1 and 1.4 according to population: 1.1 in Asia, 1.3 in Europe and 1.4 in Africa.

Modern man or Homo sapiens would, therefore, usually have been monogamous while exhibiting tendencies toward polygamy over the course of evolutionary history. These findings are consistent with studies in evolutionary psychology and anthropology that depict contemporary human populations.

Innovative Analytical Method

To estimate the breeding sex ratio based on genomic data, the authors developed a novel method to capitalize on how females carry two X chromosomes, whereas males carry only one. Consequently, during the recombination process, X chromosomes can only exchange their genetic information with females.

An excess of breeding women causes an excess of recombination signals in terms of quantifiable X chromosomes. This new method is more reliable than the previous approaches that quantified the breeding ratio using another method. It may be applied to any species for which data on genomic diversity are available.

"Our results allow better understanding of the genetic population structure and demonstrate once more the importance of population genomics in genetic epidemiology. Being able to analyze the female-male ratio in the history of humans provides new insights into the evolution of our species, which, in turn, leads to better understanding of ourselves through the knowledge of our past," says Dr. Labuda.

CitationFemale to male breeding ratio in modern humans -- an analysis based on historical recombinations. Damian LabudaJean-François Lefebvre, Philippe Nadeau and Marie-Hélène Roy-Gagnon. The American Journal of Human Genetics 2010; ePub ahead of print. doi:10.1016/j.ajhg.2010.01.029

Abstract

Was the past genetic contribution of women and men to the current human population equal? Was polygyny (excess of breeding women) present among hominid lineages? We addressed these questions by measuring the ratio of population recombination rates between the X chromosome and the autosomes, pX/pA. The X chromosome recombines only in female meiosis, whereas autosomes undergo crossovers in both sexes; thus, pX/pA reflects the female-to-male breeding ratio, ß. We estimated ß from pX/pA inferred from genomic diversity data and calibrated with recombination rates derived from pedigree data. For the HapMap populations, we obtained ß of 1.4 in the Yoruba from West Africa, 1.3 in Europeans, and 1.1 in East Asian samples. These values are consistent with a high prevalence of monogamy and limited polygyny in human populations. More mutations occur during male meiosis as compared to female meiosis at the rate ratio referred to as a. We show that at a ? 1, the divergence rates and genetic diversities of the X chromosome relative to the autosomes are complex functions of both a and ß, making their independent estimation difficult. Because our estimator of ß does not require any knowledge of the mutation rates, our approach should allow us to dissociate the effects of a and ß on the genetic diversity and divergence rate ratios of the sex chromosomes to the autosomes.

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TS-Si is dedicated to the acceptance, medical treatment, and legal protection of individuals correcting the misalignment of their brains and their anatomical sex, while supporting their transition into society as hormonally reconstituted and surgically corrected citizens.

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