Possible answer:

Inheritance of colour blindness

Colour blindness is a condition that affects the perception of colours, making it difficult or impossible to distinguish certain hues or shades. It is usually inherited as a sex-linked trait, which means it is linked to genes on the X chromosome. Females have two X chromosomes, while males have one X and one Y chromosome. Therefore, a male who inherits a defective X chromosome from his mother will have a higher chance of being colour blind than a female who inherits the same gene, because a female needs both X chromosomes to be affected to express the trait.

Types of colour blindness

There are different types of colour blindness, depending on which cone cells in the retina of the eye are affected. The most common type is red-green colour blindness, which affects about 8% of males and 0.5% of females of European ancestry. This type can be further divided into protanopia (lack of red-sensitive cones) and deuteranopia (lack of green-sensitive cones). Another type is blue-yellow colour blindness, which is rarer and usually affects both sexes equally. A more severe type is achromatopsia, which is the total absence of colour vision and affects about 1 in 30,000 people.

Possible scenario for a colour blind girl

Based on the information given in the question, we can deduce that:

- The girl is colour blind, which means she has inherited a defective X chromosome from one of her parents.
- The girl is rare, which suggests that the trait is not common in her family or population.
- The girl's mother or father or both have the gene for colour blindness, but their vision is normal, which means they have one normal X chromosome and one defective X chromosome. They are carriers of the trait but do not express it.
- The girl's maternal grandfather is mentioned as a possible source of the colour blindness gene, but not the paternal grandfather.

Therefore, we can rule out options b) and d) as unlikely or incomplete explanations for the girl's condition. We can also infer that the girl's mother must have a defective X chromosome, since she is the only one in the family who could pass it on to her daughter. However, we cannot determine with certainty whether the father is a carrier or not, since he could have inherited a normal X chromosome from his mother and a defective Y chromosome from his father, who is not mentioned in the question.

Possible explanation

One possible explanation for the girl's condition is that:

- Her mother is a carrier of the gene for red-green colour blindness, which she inherited from her father, who was colour blind. Her mother's other X chromosome is normal.
- Her father has normal vision, but he is a carrier of the gene for blue-yellow colour blindness, which he inherited from his mother, who was a carrier. His other X chromosome is normal.
- The girl inherited her mother's defective X chromosome, which contains the gene for red-green colour blindness. Therefore, she is also red-green colour blind, but her father's normal X chromosome prevents her from being blue-yellow colour blind.

This scenario is plausible because:

- It explains why the girl is rare, since she has inherited two different types of colour blindness genes from her parents, who are carriers but not affected.
- It shows how a girl can be colour blind even if her mother has normal vision, if the mother is a carrier of the gene and the father is

The explanation is simple, since father is colorblind the genetic allele would be X'Y and the wife's grandfather was colorblind so she must've inherited his affected X' chromosome from him, so mother is heterozygous for colorblind. and after crossing the father and mother we can get a colorblind female