Birth of twins in some cases are identical but in some
cases are not. Why?
In some woman, after the formation of zygote , the zygote
cell completely divides into two cells.
Each such cell acts as a separate zygote and later by repeated mitotic
divisions form two separate embryos. After these two embryos transform into two
individuals which resemble exactly each other in all features. This is due to
the same individuals are developed from the same zygote and so they have the
same genetic constitution. Due to this genetic similarity, the sexes of both
the individiuals also be the same. Such a twins born for a woman are said to be
identical twins.
But in some woman, two egg cells are released at a time in
the monthly menstrual cycle of a woman. These two eggs will be fertilized by two
different sperms during fertilization. The individuals formed from these
zygotes will have different genetic constitution and so the two individuals do
not resemble each other. In this type of twins, the two individuals may be of
different sexes or of the same sex. Such a twins born for a woman are said to
be fraternal twins.
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