Question

Your maternal grandpa is colorblind but both your mom and dad are not affected. What are the chances of your sister being colorblind? What are the chances of your brother being colorblind? (Colorblindness is X-linked recessive)

Regarding the problem above, if your sister’s husband is colorblind, what are the chances that their first son will be colorblind?

Answer 1

Answer: So there is no chance of your sister being colorblind but there is 50 % of your sister will be the carrier of the color blind gene. There are 50 % chances of your brother being colorblind.

Color blindness is recessive sex-linked (X-linked) hereditary traits. Males are more likely to be color blind because they have only one X chromosome whereas females have two X chromosome, therefore, in females color blindness only occurs when both the X-chromosomes will have mutations for the gene. If females inherit one normal X- chromosome and one with mutation, she will be the only carrier and color blindness will not appear (since it is a recessive gene). A carrier is not affected because the trait is recessive.

In a given problem color blind Maternal grandpa is with genotype: X Y

Maternal Grandmother is normal with genotype: X X

In a given problem father is with normal genotype: X Y

Mother is normal (but carrier for the gene) with genotype: X X

1^st find out the probability of your mothers genotype for colorblindness and then the probability of your siblings through the Punnett square as shown in figure below.

Your grandpa is color blind because of that they will have all normal sons and will have daughters who all carry the color blind gene (see 1^st Punnett square). It means though your mother did not display the color blindness but she is the carrier for the gene.

If your father is normal for the gene and your mother is carrier for color blind gene but she is not a color blind, then there is a 50% chance that her sons will be color blind and a 50% chance that her daughters will be carrier for the color blind gene (see 2^nd Punnett square).

A Cross Between color blind grandpa and normal grand mother X: Normal X: Carrier Genotype of grandpa:XY Genotype of grandma: XX X Y 우 X XX X Y 100 % probability of daughters being gene carrier. X (xx XY A Cross Between carrier mother and normal father X: Normal X): Carrier Genotype of Father: XY Genotype of Mother:Xx X Y O+ X X X X Y 1. 50 % probability of sons being color blind and 50% probability of sons being normal. 2. 50 % probability of daughters being gene carrier and 50% probability of being normal. X XX XY

So there is no chance of your sister being colorblind but there is 50 % of your sister will be the carrier of the color blind gene. There are 50 % chances of your brother being colorblind.

So you can observe that females can only be colorblind if she has both the faulty chromosomes for the disease. All the sons get the X chromosomes from their mother and Y chromosome from father. If mother gives the X chromosome with the mutant gene to her son then he will be color blind, but if he receives the normal X chromosome he would not be color blind. Color blind father will not contribute disease to son because it passes only Y chromosome to the son. Hence, the mother is carrier and has passed the disease to his sons.