Expert: Walter Hintz - 11/13/2007

Question
Hello,again! I need help with two questions.

1.- If an individual exhibits the dominant trait, do you know the genotype? Why or why not?

2.- A son is color blind, but both parents are normal. How can I give the genotype of the mother- and the father-and how can I explain the pattern of inheritance.


Answer
1. The genotype cannot be determined by observing the phenotype unless the trait is recessive since the alleles may be different. (heterozygous)  For example there are two genotypes for type A blood  AO  and AA.

2. Colorblindness is sex linked.  If a son is colorblind and the father is not then the mother must be carrying the colorblind gene. Look here:
 Let X = normal gene
 Let Xc= colorblind gene

           XXC    x    XY

          X           Y

  X    XX           XY


  XC  XXC          XCY (colorblind son)