A couple plans to have a child. The father is red-green colourblind (XbY) and the mother has normal vision but is a carrier (XBXb). What is the probability their child will be red-green colourblind?

Here's the completed genetic diagram and probability calculation:

• Parental genotypes:

  • Person 3 (red-green colourblind male): $X^b Y$
  • Person 4 (female with normal colour vision, heterozygous): $X^B X^b$

• Gametes:

  • Person 3: $X^b$, $Y$
  • Person 4: $X^B$, $X^b$

• Punnett Square:

  $$\begin{array}{|c|c|c|} \hline \text{Gametes} & X^B & X^b \\ \hline X^b & X^B X^b & X^b X^b \\ \hline Y & X^B Y & X^b Y \\ \hline \end{array}$$

• Offspring genotypes: $X^B X^b$, $X^b X^b$, $X^B Y$, $X^b Y$

• Offspring phenotypes:

  • $X^B X^b$: Normal vision female (carrier)
  • $X^b X^b$: Red-green colourblind female
  • $X^B Y$: Normal vision male
  • $X^b Y$: Red-green colourblind male

• Probability of offspring having red-green colour blindness: Out of 4 possible offspring, 2 will be red-green colourblind ($X^b X^b$ and $X^b Y$).

  $$\text{Probability} = \frac{2}{4} = \frac{1}{2}$$

  The probability of offspring having red-green colour blindness is $\boxed{\text{50\%}}$.