A colour-blind man marries a woman with normal sight who has no history of colour blindness in her family. What is the probability of their grandson being colour blind?
A colour-blind man marries a woman with normal sight who has no history of colour blindness in her family. What is the probability of their grandson being colour blind?
The correct option is C 0.25
The correct option is C.
Explanation of correct option:
- Colour blindness is a type of disorder in which the person faces trouble in seeing or identifying certain colors like red, blue, and green.
- It is caused by genes located on the X chromosome.
- When a color-blind man (XcY) marries a normal woman (XX), all the sons will be normal (XY) and the daughters will be normal/carrier (XcX) as shown in table 1.
- Assuming their son marries a woman with normal eyesight, the progeny of their son will be normal.
- However, their daughter is a carrier. If the daughter marries a color-blind man, the progeny of their daughter will comprise 25% color blind sons (Table 2).
- Therefore, the probability of the color blind grandson is 0.25.
Table 1: Normal woman (XX) x Colour blind man (XcY)
Gametes Xc
Y
X
XXc
(Carrier daughter)
XY
(Normal son)
X
XXc
(Carrier daughter)
XY
(Normal son)
Table 2: Carrier woman (XXc) x Normal man (XY)
Gametes Xc
Y
X
XXc
(Carrier daughter)
XY
(Normal son)
Xc
XcXc
(Color blind daughter)
XcY
(Color blind son)
Final answer: A color blind man marries a woman with normal sight who has no history of colour blindness in her family. The probability of their grandson being color blind is 0.25.
0.25
The correct option is C.
Explanation of correct option:
- Colour blindness is a type of disorder in which the person faces trouble in seeing or identifying certain colors like red, blue, and green.
- It is caused by genes located on the X chromosome.
- When a color-blind man (XcY) marries a normal woman (XX), all the sons will be normal (XY) and the daughters will be normal/carrier (XcX) as shown in table 1.
- Assuming their son marries a woman with normal eyesight, the progeny of their son will be normal.
- However, their daughter is a carrier. If the daughter marries a color-blind man, the progeny of their daughter will comprise 25% color blind sons (Table 2).
- Therefore, the probability of the color blind grandson is 0.25.
Table 1: Normal woman (XX) x Colour blind man (XcY)
| Gametes | Xc | Y |
X | XXc (Carrier daughter) | XY (Normal son) |
X | XXc (Carrier daughter) | XY (Normal son) |
Table 2: Carrier woman (XXc) x Normal man (XY)
| Gametes | Xc | Y |
X | XXc (Carrier daughter) | XY (Normal son) |
Xc | XcXc (Color blind daughter) | XcY (Color blind son) |
Final answer: A color blind man marries a woman with normal sight who has no history of colour blindness in her family. The probability of their grandson being color blind is 0.25.
