Emma and anna are identical twins who were adopted by different families a few weeks after 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 due to the active lifestyle practiced in her adoptive family. this variability can be explained by how: a. longevity genes can make an individual less vulnerable to certain diseases. b. for each genotype, a range of phenotypes can be expressed . c. each zygote is unique. d. mutated genes can be a source of genetic variability.