Question Video: Explaining What Occurs if a Zygote Develops into Two Embryos | Nagwa Question Video: Explaining What Occurs if a Zygote Develops into Two Embryos | Nagwa

Question Video: Explaining What Occurs if a Zygote Develops into Two Embryos Biology • Third Year of Secondary School

What usually happens when a zygote develops into two embryos?

02:07

Video Transcript

What usually happens when a zygote develops into two embryos? (A) This question is misleading; a zygote cannot develop into two embryos. (B) They develop into fraternal twins, each with a separate placenta. (C) They develop into fraternal twins that share a placenta. (D) They develop into fraternal twins with a shared amnion. (E) They develop into identical twins that share a placenta.

An egg can be fertilized by a sperm cell in order to produce a fertilized egg, or a zygote. The individual cell of this zygote can then become an embryo by dividing into two cells, then four, and so on. This embryo can then implant into the endometrium of the mother to form a pregnancy. Eventually, the embryo develops into a fetus that exchanges nutrients and waste products with the mother through the placenta, as we can see here. This is what happens when a single fetus forms, but what about twins?

This can happen different ways, but one way is when a single zygote splits to form two separate embryos. This split usually occurs between eight and 12 days after fertilization. Because these embryos are both from the same zygote, they are identical. These can develop into monozygotic, or identical, twins. Monozygotic twins have two identical fetuses and often share the same placenta as we can see here. Less frequently, two placentas can form. Another type of twinning can occur when two separate zygotes are fertilized to form nonidentical, or fraternal, twins. But this isn’t what the question is referring to.

In the question, we are asked what usually happens when one zygote develops into two embryos. The answer to this is given by answer choice (E). They develop into identical twins that share a placenta.

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