How twins are born ?

Fraternal Twins (commonly known as "non-identical twins") usually occur when two fertilised eggs are implanted in the uterine wall at the same time that is when the mother releases two eggs and both become fertilized by two different sperms. The two eggs form two zygotes. and these twins are therefore also known as dizygotic.

Dizygotic twins. like any siblings. have a very small chance of having the exact same chromosone profile. but most likely have a number of different chromosones that distinguish them. Dizygotic twins may be a different sex or the same sex. just as with any other siblings. Like singleton siblings. they share 50% of their DNA.

Studies show that there is a genetic basis for fraternal twinning ;that is. non-identical twins do run in families. However. it is only the female that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum. Your likelihood of having fraternal twins is dependent upon the woman carrying a fraternal twin gene and can also be affected by heredity. race. marital age and number of children previously born. Two-thirds of all twin births result in same sex fraternal twins and one-third are different sex fraternal twins. About two-thirds of all twin births are fraternal.

Identical Twins occur when a single egg is fertilized by a single sperm to form one zygote (monozygotic) but the zygote then divides into two separate embryos.(a recent theory proposes that monozygotic twins are formed after a blastocyst essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half, leaving the same genetic material divided in two on opposite sides of the embryo) The biological mechanisms that prompt the single fertilized egg to split in two remain a mystery. The two embryos develop into foetuses sharing the same womb. Depending on the stage at which the zygote divides. identical twins may share the same amnion (in which case they are known asmonoamniotic) or not (diamniotic). Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic