Male pattern baldness is inherited from your mother's side of the family.

The folk rule circulated for decades with the confidence of settled science: if you want to know whether a man will go bald, look at his mother's father. The reasoning had a grain of genuine genetics behind it. Baldness, the received wisdom held, was tied to the X chromosome, and because a man inherits his X chromosome exclusively from his mother, his maternal grandfather was the relevant predictor. This explanation appeared in popular science writing, in family advice columns, and in the relaxed authority of dinner-table conversation. It had enough biological plausibility to resist casual scrutiny.

The X chromosome connection is not fiction. A variant of the androgen receptor gene, which sits on the X chromosome, has been associated with male pattern baldness since at least 2001, when researchers published evidence linking its polymorphisms to androgenic alopecia. Men who carry certain versions of this gene are substantially more likely to lose hair. Because men have only one X chromosome and inherit it from their mothers, the maternal grandfather rule captured something real. The trouble was that it captured only part of the picture and then promoted that partial truth into a complete explanation.

The genetic architecture of male pattern baldness, as researchers began to understand more clearly in the 2000s, is not a single-gene story. The trait is polygenic: risk accumulates across many variants distributed across multiple chromosomes. A genome-wide association study published in Nature Genetics in November 2008, led by Axel Hillmer and colleagues, reported a newly significant locus on chromosome 20p11. This finding, confirmed across several independent cohorts, demonstrated that inheritance from the paternal line also contributes to baldness risk. A man who carries risk alleles at both the androgen receptor locus on the X chromosome and at the chromosome 20 locus faces odds roughly seven times higher than a man who carries neither. The chromosome 20 locus has nothing to do with the maternal grandfather.

Subsequent genome-wide association studies expanded the picture further. A 2017 study in Nature Communications identified 71 susceptibility loci associated with male pattern baldness, collectively accounting for approximately 38 percent of genetic risk. These loci are scattered across the genome, on chromosomes from both parents. The androgen receptor gene remains the single strongest individual contributor, but it is one voice in a larger genetic conversation that includes contributions from both sides of the family tree.

The maternal grandfather rule persists partly because it is memorable, partly because it contains a real signal, and partly because genetics education in schools rarely went beyond Mendelian inheritance and single-gene explanations. When teachers in the 1960s and 1970s illustrated sex-linked inheritance, baldness was a convenient example: it is visible, familiar, and the X-chromosome connection was plausible and easy to explain in forty minutes. The rule became a teaching shortcut, and like many teaching shortcuts, it outlived the science it was based on by several decades. Conversations about baldness genetics today require a different vocabulary: genome-wide association, polygenic risk scores, and the recognition that complex traits rarely honor the clean hierarchies that made them easy to teach.