The answer is both.
While baldness in men has been attributed to their mothers for quite some time now, baldness can, in fact, be inherited from both sides of the families.
For years, many people have believed the myth that the genes for male pattern baldness are passed down from a mother to her son on her x-chromosome. The conventional wisdom has been that men could simply take a look at the heads on their mum's side of the family and get a pretty good indication of how theirs might wind up looking.
But doctors say it's more accurate to blame both of your parents for thinning hair. Scientists still aren't sure exactly which genes determine whether we lose hair and where those genes come from, but they think both sides of the family contribute. Second, your mum has two x-chromosomes, which makes it difficult to look to any one family member for clues about your hair's future.
The Hair Growth Cycle
Hairs last about three to five years on our head, then fall out and get replaced with new hairs in a 3 to 4 month-long cycle. Each hair follicle is independent and goes through the growth cycle at different times, otherwise all your hair would fall out at once. Instead, you only shed a certain number of hairs a day – up to 80 hairs on a healthy head of hair.
When a hair falls out and isn't replaced — or gets replaced with a much thinner hair — then we start to go bald. The scientific term for common hair loss is androgenic alopecia. If you have it, you're not alone: roughly half of all adults will lose some hair by the time they're 40.