The correct option is
A. An ovum is fertilized by a sperm and then divides into two, each forming a separate embryo.
Detailed Explanation:
- Understanding Identical Twins:
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Identical twins, also known as monozygotic twins, occur when a single fertilized egg (ovum) splits into two separate embryos. This means that both embryos originate from the same zygote, which is the result of the fertilization of an ovum by a sperm.
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Process of Formation:
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The process begins when a sperm successfully fertilizes an ovum, creating a zygote. After fertilization, this zygote undergoes a series of cell divisions (cleavage). In the case of identical twins, during the early stages of development, the zygote divides into two separate cell masses. Each of these masses continues to develop independently into a separate embryo, leading to the formation of two genetically identical individuals.
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Genetic Similarity:
- Since both embryos come from the same zygote, they share the same genetic material, which is why identical twins are genetically identical. They will have the same DNA, which is a key characteristic of monozygotic twins.
Why the Other Options are Incorrect:
- Option B: An ovum fertilized by two sperms and then divides into two, each forming an embryo:
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This scenario describes a situation that is biologically impossible. An ovum can only be fertilized by one sperm at a time. If two sperms were to fertilize the same ovum, it would lead to a condition called polyspermy, which typically results in an abnormal zygote that cannot develop properly. Therefore, this option is incorrect.
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Option C: Two ova are fertilized at the same time, each forming an embryo:
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This describes the formation of fraternal twins (dizygotic twins), not identical twins. In this case, two separate eggs are fertilized by two separate sperm cells, resulting in two genetically unique individuals. Each twin has a different genetic makeup, as they come from different zygotes. Thus, this option does not pertain to identical twins.
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Option D: Two ova are fertilized by a single sperm, each forming an embryo after fertilization:
- Similar to option C, this scenario is not possible. A single sperm cannot fertilize two separate ova. Each ovum requires its own sperm for fertilization. This option also describes a situation that does not lead to identical twins and is therefore incorrect.
Summary of Key Points:
- Identical twins (monozygotic) arise from a single fertilized ovum that splits into two embryos.
- The process involves one sperm fertilizing one ovum, leading to the formation of a zygote that divides.
- Fraternal twins (dizygotic) result from two separate ova being fertilized by two different sperm, leading to genetically unique individuals.
- Understanding the differences between monozygotic and dizygotic twins is crucial for grasping human reproductive biology.
This thorough understanding of the formation of identical twins will help you in your studies and any related exam questions.