Bare seconds after being attacked and bitten by a random Infected, the rapid-acting Infection turns Zoey’s mother into a violent, rabid monster herself. She savagely attacks Zoey’s father, and then launches herself at a shocked Zoey. Zoey’s father stops the attack with a gunshot to Infected mom’s head.
But Zoey’s dad has already been bitten by Infected mom…
The tragedy is compounded when weeks later, a doctor researching the outbreak reveals that resistance to the Infection is carried by the father, most likely meaning that immunity from becoming a crazed living zombie is recessive the way that colourblindness is.
Susceptibility to the Infection must be a dominant trait carried on the X chromosome.
Since Zoey is immune, that means both her X chromosomes must carry the recessive immunity. Therefore she inherited one X chromosome with the recessive immunity from her mother (whose other X chromosome has the dominant susceptibility, hence mom’s turning Infected)… And one from her father.
Which means her father wouldn’t have turned Infected.