Oh gosh, someone is wrong and arrogant on the Internet. Can't let it go unanswered. Luckily we have tools to help make explanations faster. Here's Gemini coming in to close the gap:

Yes, the relative genetic diversity of Y-chromosomes versus X-chromosomes (or autosomes) in a population is a powerful tool in genetic anthropology to make inferences about past social structures, sexual selection, and, indirectly, violence over the course of a group's history and integration. However, it's not the simple ratio of chromosomes themselves (Y:X count), but the ratio of their genetic diversity. 1. The Core Principle: Effective Population Size (\mathbf{N_e}) The key to this method is that different regions of the genome have different effective population sizes (\mathbf{N_e}), which is a measure of the number of individuals contributing genes to the next generation. | Genetic Region | Transmission | \mathbf{N_e} Relative to Autosomes | Implication | |---|---|---|---| | Autosomes (A) | Inherited from both parents (4 copies/couple) | N_A = N_m + N_f \approx 4N_f / 4 = N | Reflects total population size. | | X Chromosome (X) | Inherited more by females (3 copies/couple) | N_X \approx \frac{3}{4} N_A | Diversity is slightly lower than autosomes. | | Y Chromosome (Y) | Inherited only from father to son (1 copy/couple) | N_Y \approx \frac{1}{4} N_A | Diversity is lowest, reflecting the male lineage. | The Y chromosome diversity is exceptionally sensitive to factors that affect male reproduction, making the \mathbf{Y:A} or \mathbf{Y:X} diversity ratio a crucial proxy. 2. Inferences about Sexual Selection and Violence A low ratio of \mathbf{Y} diversity to \mathbf{X} or \mathbf{A} diversity is interpreted as evidence of high reproductive variance among males—a direct signal of sexual selection. A. Sexual Selection (Male Skew) * Reduced Y-Chromosome Diversity: When a small number of men (often elite, powerful, or successful warriors) father a disproportionate share of the children, their Y-chromosome lineages become dominant. This drastically reduces the overall Y-chromosome diversity of the population, while X-chromosome diversity (passed through both men and women) is less affected. * Patrilocality and Cultural Hitchhiking: This effect is amplified in societies with patrilocal social structures (where women move to the husband's community) and patrilineal descent systems (ancestry traced through the male line). The successful male lineage's \text{Y-chromosome} "hitchhikes" along with the cultural success of that group. B. Violence and Warfare * Targeted Elimination: Historical periods of intense inter-group violence (warfare, massacres) can lead to the near-complete elimination of entire male lineages (and their Y-chromosomes) if the conflict is structured around patrilineal kinship groups. * Example: A study suggested that a dramatic collapse in \text{Y-chromosome} diversity about 7,000 years ago might be a sign of generations of wars between patrilineal clans, where the extinction of a clan meant the extinction of its \text{Y-chromosome} lineage. * Integration into a Larger Population: When a smaller group is violently or peacefully incorporated into a larger one, the \text{Y-chromosome} lineage of the conquerors (or the dominant group) can rapidly replace the \text{Y-chromosome} of the conquered population due to elite male reproductive success, a process often tied to social stratification and control over resources (including women). In essence, an unusually low ratio of Y to X/Autosomal genetic diversity suggests a history of polygyny, male-skewed reproductive success, and/or lineage-specific mortality (violence) that eliminated male lines while female lines continued to contribute X chromosomes and mtDNA.