3. Lisa's father has red-green colour blindness, but[Lisa has normal colour vision Lisa's/husband Petek also has red-green colour blindness. If this condition is caused by a recessive sex-linked gene, what are the chances of that the boys will be colour blind? What are the chances of the girls being colour blind? What are the chances of the girls being a carrier but having normal vision?

Lisa's sons have a 50% chance of being colorblind, her daughters have a 0% chance of being colorblind, and a 50% chance of being carriers.<br /><br />Here's why:<br /><br />* **Red-green colorblindness is sex-linked recessive:** The gene is located on the X chromosome. Since it's recessive, a female needs two copies of the gene (one on each X chromosome) to express the trait. A male only needs one copy (on his single X chromosome) to express the trait.<br /><br />* **Lisa's genotype:** Since Lisa's father is colorblind (XcY), he passed his affected X chromosome (Xc) to Lisa. Since Lisa has normal vision, she must have also received a normal X chromosome (X) from her mother. Therefore, Lisa is a carrier (XXc).<br /><br />* **Peter's genotype:** Peter is colorblind, so his genotype must be XcY.<br /><br />* **Possible offspring:**<br /><br /> * **Sons:** Sons inherit a Y chromosome from their father (Peter) and an X chromosome from their mother (Lisa). Lisa can pass on either her normal X or her affected Xc. Therefore, there's a 50% chance a son will inherit XcY (colorblind) and a 50% chance he will inherit XY (normal vision).<br /><br /> * **Daughters:** Daughters inherit an X chromosome from each parent. They will *always* inherit an Xc from Peter. From Lisa, they have a 50% chance of inheriting the normal X and a 50% chance of inheriting the Xc. This means there's a 50% chance a daughter will be XXc (a carrier with normal vision) and a 50% chance she will be XcXc. However, the question states Lisa has *normal* color vision. This eliminates the possibility of her having the XcXc genotype and therefore eliminates the possibility of her daughters being colorblind.<br />