Those who are MW will be muggles, but there is still a chance for their children.

FORBES: Harry Potter, Science Teacher

In Potter's world, most people are muggles, normal folks with no magical power.

FORBES: Magazine Article

Likewise, it could explain how Harry Potter's mother was born of muggles.

FORBES: Harry Potter, Science Teacher

Calling all Muggles: Princess Cruise Lines is launching a Harry Potter-themed cruise on the Golden Princess, which will depart from Los Angeles on 27 November.

BBC: In brief: Harry Potter cruise announced

If two muggles who are MW have a child, there is a one in four chance that the child will be WW--and able to cast spells.

FORBES: Harry Potter, Science Teacher

In the scenario, there are two versions of the gene for magical ability--the M version, which creates muggles, and the W version, which is needed for wizardry.

FORBES: Harry Potter, Science Teacher