Emma and Anna are identical twins who were adopted by different families a few weeks after their birth. Although genetically identical, they grew up with different physical and psychological characteristics. For example, though both inherited a tendency to grow large, Anna was slim and athletic because of the active lifestyle practiced in her adoptive family. This variability can be explained by how:
a. each zygote is unique.b. longevity genes can make an individual less vulnerable to certain diseases.c. for each genotype, a range of phenotypes can be expressed.d. mutated genes can be a source of genetic variability.