M.R.
No clue, but my dad had very little body hair (I grew more leg hair) and he had 6 daughters, 1 son. My brother could grow more facial hair than my dad, but not a ton.
My husband has less body hair than his dad, but still has dark, thick hair EVERYWHERE. My FIL has a back like his chest, my husband's back is less hairy. However, he grows the most amazing mountain man beard every winter. My 6 month old has obvious thick dustings of hair all over. I hope it isn't phasing out, but it might just be genetic/hereditary and depend on which side is more dominant.