What about twins? Yesterday's post was about the most celebrated twins - conjoined. We know about identical and fraternal. What else is there?
Mirror image This is a subtype of identical twins that happens when the egg splits 7 to 12 days after fertilization, instead of during the first week. By this time, the growing embryo has already developed a left and right side. These twins are identical but mirror images of each other. For example, their hair may spiral in opposite directions, their teeth may start coming in on opposite sides of their mouth, and one may be right-handed while the other is left-handed.
Parasitic - fetus in fetu and cardiac twins Parasitic twins are a kind of conjoined twin where a smaller twin is dependent on a larger one. The smaller twin isn’t fully formed and may not have vital organs such as a fully formed brain or heart.
The small twin can form anywhere on the other twin’s body and appear as anything such as a small unnoticeable lump, a second nonfunctional head, or extra limbs attached to random body parts.
Semi-identical This type is the result of two separate sperms fertilizing a single egg. To survive, this egg must then split into two with each half having exactly the right number of chromosomes.There have only been two reported cases of semi-identical twins
Boy/girl monozygotic (identical) In very rare cases, identical twins can be different sexes. These twins begin as identical male twins. Like all males, they both have XY sex chromosomes, instead of XX like all females do.
Very soon after the egg splits in two, a genetic mutation causes one twin to lose its Y sex chromosome, changing it to X0. This mutation is called Turner syndrome.
Since there’s only one X chromosome, the twin looks female but has developmental difficulties starting at birth and fertility problems later in life. The other baby isn’t affected.
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