Question

What are identical twins ?
A. Heterozygous
B. Homozygous
C. Monozygotic
D. Dizygotic

Answer 1

Hint: The two offsprings which are produced in the same pregnancy are called as twins. Depending upon the number of zygotes formed in pregnancy, there are types of twins as monozygotic and dizygotic twins.

Complete Answer:
- As the name says, monozygotic twins are the twins which are formed from a single zygote whereas dizygotic twins are the ones which are formed from two different zygotes formed in the same pregnancy.
- The twins which are formed from a single zygote or monozygotic twins are identical twins because they look similar and the twins which are formed from different zygotes are called fraternal twins.
- In case of monozygotic or identical twins, fertilization occurs only once and then the fertilized egg or the single celled zygote splits and develops into two identical offspring having the same genetic constitution.
- In case of dizygotic or fraternal twins, fertilization occurs twice. This happens when a single egg is released from both the ovaries at the same time, since in a single ejaculation millions of sperms reach the female reproductive tract, two of them fuse with the two released eggs to form two different zygotes.
- The genetic constitution of the two zygotes are different, therefore both of them are not similar but different. In case of identical twins, sex of the offsprings is always same, whereas in case of fraternal twins, sex of the offsprings may or may not be same.
- Since the monozygotic or the identical twins are derived from a single zygote, the entire chromosome content of them is the same resulting in having the same genetic characteristics such as hair colour, eye colour, complexion etc.
- There are more complications in twin pregnancy than the one in normal pregnancy. Many times in case of twin pregnancy, early birth or premature birth can take place when their organs or immune system are not completely formed.
- This pregnancy can be risky for mother too as she can develop anemia and gestational hypertension. The babies developing are also at a greater risk of getting congenital diseases of heart, spine or even gastrointestinal tract.
- Sometimes out of two twins during pregnancy, one may even die and this phenomenon is called vanishing twin syndrome.

The correct option is (C) .

Note: Sometimes in case of monozygotic twins, the zygote is not completely split apart and this results in conjoined twins. The twins are physically connected to each other and often die shortly after they are born. They may be connected to the chest, abdomen or head. They are physically, mentally and sexually normal.