Problem 20

The pedigree below traces the inheritance of a vary rare biochemical disorder in humans. Affected individuals are indicated by filled-in circles and squares. Is the allele for this disorder dominant or recessive?

The allele is most likely dominant because the #2 individual (see below) with the trait marries a woman with the trait and 50% of their offspring are normal. If the trait were recessive one would expect the following:

100% of offspring would have the disease, which is not the case.


What genotypes are possible for the individuals marked 1, 2, and 3?



  1. Bb (heterozygous)
  2. Bb ( if 2's genotype were bb he would not have the disease and if BB all his children would have the condition.)
  3. bb (all normal individuals are homozygous recessive)