Aug 27, 2015 11:36 PM
Epigenetics And The Evil Twin
http://biologicalexceptions.blogspot.com...-twin.html
EXCERPT: Last week’s topic on parasitic twins was a bit depressing for me. This week, let’s focus on some amazing kinds of monozygotic (MZ) twins. Yes, there are many kinds of monozygotic twins, some we have talked about (conjoined, parasitic, absorbed, vanished) and some we’ll talk about today. Just because they’re monozygotic doesn’t mean that they only come in one type.
The popular idea is that MZ twins are “identical,” but nothing could be further from the truth. It may not even be the case that they share the same genes, but more about that later. When an embryo splits, each new embryo usually gets the same chromosomes. But a lot can happen after that....
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Twins That Share More Than Clothes
http://biologicalexceptions.blogspot.com...othes.html
EXCERPT: [...] Now consider what would occur if the early embryo undergoes a mosaic producing event, and then splits into two embryos – mosaic monozygotic (MZ) twins. A mosaic embryo, no matter what mechanism leads to the mosaic could become mosaic twins, as long as the chromosome change in some cell occurs before the split of the embryo into two embryos. Each twin will end up with some cells of one chromosomal profile, and some of the other. Given all that must happen to create them, is it any wonder that mosaic MZ twins are rare?
But there is another way for a mosaic individual, or MZ twins to form as well. Consider the case of two male gamete cells fertilizing the same egg (called polyspermy). When the zygote cell divides, one cell may get some of male gamete #1 chromosomes and some of male gamete #2 chromosomes. Male gametes carry either an X or a Y, so it is conceivable that some cells will be XX and some will be XY. If the embryo split, could you get MZ twins that are one boy and one girl?
Some people might want to call these twins chimera twins, but chimeras come from the fusion of two embryos into one (two complements of mom’s DNA). Mosaics form from one zygote only. For instance, another mosaic twin case formed after an XXY Klinefelter syndrome embryo lost the Y chromosome in some cell(s) (maybe from an anaphase lag). Then the embryo split and you had an XXY male and an XX female.
Can you think of another way to end up with mosaic twins?...
http://biologicalexceptions.blogspot.com...-twin.html
EXCERPT: Last week’s topic on parasitic twins was a bit depressing for me. This week, let’s focus on some amazing kinds of monozygotic (MZ) twins. Yes, there are many kinds of monozygotic twins, some we have talked about (conjoined, parasitic, absorbed, vanished) and some we’ll talk about today. Just because they’re monozygotic doesn’t mean that they only come in one type.
The popular idea is that MZ twins are “identical,” but nothing could be further from the truth. It may not even be the case that they share the same genes, but more about that later. When an embryo splits, each new embryo usually gets the same chromosomes. But a lot can happen after that....
- - - - - - - - -
Twins That Share More Than Clothes
http://biologicalexceptions.blogspot.com...othes.html
EXCERPT: [...] Now consider what would occur if the early embryo undergoes a mosaic producing event, and then splits into two embryos – mosaic monozygotic (MZ) twins. A mosaic embryo, no matter what mechanism leads to the mosaic could become mosaic twins, as long as the chromosome change in some cell occurs before the split of the embryo into two embryos. Each twin will end up with some cells of one chromosomal profile, and some of the other. Given all that must happen to create them, is it any wonder that mosaic MZ twins are rare?
But there is another way for a mosaic individual, or MZ twins to form as well. Consider the case of two male gamete cells fertilizing the same egg (called polyspermy). When the zygote cell divides, one cell may get some of male gamete #1 chromosomes and some of male gamete #2 chromosomes. Male gametes carry either an X or a Y, so it is conceivable that some cells will be XX and some will be XY. If the embryo split, could you get MZ twins that are one boy and one girl?
Some people might want to call these twins chimera twins, but chimeras come from the fusion of two embryos into one (two complements of mom’s DNA). Mosaics form from one zygote only. For instance, another mosaic twin case formed after an XXY Klinefelter syndrome embryo lost the Y chromosome in some cell(s) (maybe from an anaphase lag). Then the embryo split and you had an XXY male and an XX female.
Can you think of another way to end up with mosaic twins?...