Answer
b.
Work Step by Step
Option a. is incorrect because cystic fibrosis is a recessive trait and is expressed if an individual has two copies of the defective gene.
Option c. is incorrect because with Down's Syndrome, an individual has three copies of chromosome 21 instead of two.
Option d. is incorrect because colorblindness is due to a defective gene carried only on the X chromosome.
Option b. is the correct answer because the ABO blood group system is governed by the actions of three different alleles.
