Question

A couple with normal vision has a child with colour blindness. Since we know that colour blindness is an X-linked disease, how would you indicate the mother in the pedigree chart?

• A. With a circle that is completely shaded
• B. With a circle that has 50% shaded area
• C. With a completely shaded circle and a diagonal line across it
• D. With a completely shaded circle and a horizontal line below it

Answer 1

The correct option is B

With a circle that has 50% shaded area

In the mentioned case, the mother would be a carrier of colour blindness. A person who is a carrier for a certain genetic disorder is not affected by it. To signify that the female parent is a carrier of the defect, a circle that has 50% shading is used in the pedigree chart. So the correct indication here will be a circle with 50% of its area shaded, indicating that one of her X chromosomes carries the defective allele, and the other X chromosome has a normal allele.