Why is the concordance rate not 100% in monozygotic twins for a disorder?

• A. Environmental influences contribute to development
• B. Monozygotic twins are genetically different
• C. Genes completely determine behavior
• D. Twin studies are unreliable

Answer: (B)

The idea being tested is that concordance in monozygotic twins isn’t 100% because there can be genetic differences between twins who started from the same fertilized egg. Although monozygotic twins are formed from one zygote and are almost genetically identical, small genetic changes can accumulate after the split, such as somatic mutations, and differences in gene expression can arise through epigenetic changes. These subtle genetic and regulatory differences can influence whether a disorder develops, leading to cases where one twin has the disorder and the other does not. Environmental factors also play a role, but the statement that monozygotic twins can be genetically different directly explains why concordance isn’t perfect.