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PCAT Biological Processes – Color Blindness

  • by Sam
  • Feb 19, 2018
  • PCAT Question of the Day

Color blindness (B) is a recessive (b) sex-linked disorder that results from a mutation on the X chromosome. What would be the most likely phenotypes of the male and female progeny when a colorblind man and a woman with no family history of color blindness mate?

  1. All of the male and none of the female offspring will be colorblind
  2. Half of the male and half of the female offspring will be colorblind
  3. None of the male and none of the female offspring will be colorblind
  4. None of the male and all of the female offspring will be colorblind
Click for Explanation

This question asks the examinee to determine the offspring phenotypes that result from cross between a woman with normal vision and a colorblind man. To answer this question, the easiest method is requires the use of a Punnett square.

Since the woman has no family history of colorblindness, it is safe to assume that she is homozygous dominant (XBXB). The colorblind male on the other hand has the mutated allele (XbY).

According to the Punnett square above, none of the males and none of the females will be affected by this recessive disorder. Therefore, none of the male and none of the female offspring will be colorblind making C the correct answer.

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