KEY:
- Yellow = Carrier
- Grey = Not a carrier and not affected
- Blue = Affected
Reasoning:
If you take a close look you can see that the child affected by sickle cell has a sister who is not affected, and you may be wondering why. Well, this is because sickle cell is a s3x linked trait, X and Y chromosomes. And you know that males have one X and one Y, while females have two Xs.
Sickle cell is inherited by the X chromosome, and since males don't have two of those chromosomes then if their X chromosome is faulty, they don't have a back up chromosome, unlike women. For a woman to have sickle cell, she would have to have two faulty X chromosomes inherited by both of her parents.