Which of these sibling pairs will share the greatest number of genes?
A) two brothers who were born of the same two parents 18 months apart
B) a brother and sister who are fraternal twins
C) two brothers who develop from the same fertilized egg
D) two sisters who are fraternal twins
Answer: C
Rationale: Monozygotic twins come from a single ovum (egg), which makes them genetically identical. In contrast, dizygotic twins (fraternal twins) come from two separate eggs fertilized by two different sperm cells, therefore, they share the same number of genes as do non-twin siblings.
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