Identical twins occur when a single fertilized
egg splits sometime within the first few days after fertilisation.
This accounts for around 30% of twins. When the young embryo
splits, each half takes the exact same DNA, so identical twins
have exactly the same genetic makeup. |
Non-identical or fraternal twins are created
by two separate eggs and two separate sperm that are fertilised
during the mother's same menstrual cycle. These types of twins
share half their genetic information and are no more or less
similar than normal siblings. |
Semi-identical twins occur when two sperm fertilize
a single egg. This egg then splits to form twins which each
have identical DNA from their mother, but different DNA from
their father. To test for semi-identical twins, DNA from both
parents is required. |
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