Punnett square to show geno and pheno types of offsprings

Question

for humans brown eyes are dominant (B) over blue eyes(b) what are the possible genotypes and phenotypes of their offspring showing Punnett square

Answer 1

The brown eyed parent could be BB or Bb.  (The B allele is dominant, and if present is expressed.  In other words, the brown-eyed parent could be homozygous, BB or heterozygous Bb.)

 

The blue-eyed parent has to be bb, since blue eyes are a recessive allele.  Now (Case 1) put the homozygous brown-eyed parent and the homozygous recessive blue-eyed parent into a Punnett square and see what you get.  Do the same for a heterozygous brown-eyed parent and a blue eyed parent (Case 2).  Are the results the same, or different? 

 

Ans: Different.  In case 1 all the children are heterozygous for brown eyes.  In case 2, you get two heterozygous brown-eyed children and 2 homozygous recessive blue-eyed children. 

 

Case 1. father:         B    B         

           mother:   b   Bb   Bb                     

                             b   Bb   Bb       

 

  Case 2.    father:            B       b

             mother:      b      Bb      bb

                                  b      Bb       bb