Under what genetic conditions can a girl be colorblind?

Having a carrier mother and colorblind father.

The correct answer is Option (3) → Having a carrier mother and colorblind father.

Explanation:
Colorblindness is a sex-linked recessive trait found on the X chromosome. Since girls have two X chromosomes (XX), for a girl to be colorblind, she must inherit two copies of the defective gene—one from each parent.

For a girl (XX) to be colorblind:

She must inherit two affected X chromosomes:

• One from her colorblind father (X^cY → passes Xᶜ to daughters).
• One from her carrier mother (X^cX → 50% chance to pass Xᶜ).

Other Options:

• Carrier mother + normal father (X^cX × XY):
• Daughter has 50% chance to be a carrier (XᶜX) but cannot be colorblind (needs XᶜXᶜ).
• Normal mother + colorblind father (XX × X^cY):
• Daughter will be a carrier (X^cX) but not colorblind.
• Both 1 and 2: Incorrect (neither alone causes colorblindness in girls).

Key Genetic Cross:

Carrier Mother (X^cX) × Colorblind Father (X^cY):

• 25% chance of colorblind daughter (X^cX^c).

Answer: Having a carrier mother and colorblind father