Expert: Florence M Rollwagen - 6/25/2008

Question
QUESTION: Can a normal vision father and a normal vision mother produce a daughter with any type of colorblindness?

ANSWER: Hi Belle, thanks for your question.

I'm assuming that you're referring to ordinary X-linked color blindness, not any of the other more rare conditions that are located on over 15 chromosomes.

X-linked color blindness is a gene carried on the X-chromosome.  Because males are XY, they donate an X chromosome to their daughters.  Females are XX, and donate one X chromosome to each child.  Since the trait is carried on the X chromosome, men cannot be carriers, since they have only one X chromosome.  So the normal vision father cannot produce a child with color blindness.

Women, who have two X chromosomes can be carriers, if one chromosome carries the trait and one chromosome carries the normal allele.  So carriers have the potential to donate the trait to a child.  But because women can donate only X chromosomes, the trait is seen only in boy children (who inherit the Y chromosome from their fathers).  Daughters may be carriers if they inherit the trait from one of their mother's X chromosomes.  Daughters may be normal if they inherit the normal X chromosome from their mother.

I hope this answer helps you.  Please write back if you have more questions.

FM Rollwagen, PhD

---------- FOLLOW-UP ----------

QUESTION: So it is possible that they could produce a colorblind daughter through any of the other more rare conditions that are located on over 15 chromosomes.  My biological father and mother are normal vision and my sister is colorblind, but she has not been tested for type.


Answer
Hi again, Belle:  If your sister has not been tested for color blindness type, how do you know she's color blind?  

There is a form of color blindness (yellow green) that is located on Chromosome 7 and is not sex linked.  This means that both parents could be carriers (they have the gene but it is not expressed) and contribute the defective gene to their daughter.  That is, if she has this form.

Without knowing what type of color blindness she has, it is impossible to come up with a genetic explanation.

Check out this article on Wikipedia for more information:
http://en.wikipedia.org/wiki/Color_blindness#Total_color_blindness

I hope this answer has helped you.  Please write back if you have more questions.

FM Rollwgen, PhD