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Why are toirtoiseshell cats always female? Could there ever be a male toirtoiseshell?
The answer to the first question lies in something called Mosaicism. This is when groups of cells in different parts of the body have different genetic properties. In mammalian females, this happens due to a feature of the X chromosone. Only one of the two X chromosones can be active in each cell. But the same X chromosone is not activated in all parts of the body; instead, you tend to get regions of cells with the first chromosone activated, and regions with the second activated.
Now, there is a gene on the X chromosone of cats which in one form codes for ginger hair, and in another codes for black hair. A male cat could only inherit either the X with the black hair gene or the X with the ginger hair gene. A female cat, by contrast, can inherit both X chromosones. The mosaic effect leads to some regions of skin cells expressing the ginger gene, and other regions expressing the black gene, resulting in a patchwork appearance.
But tortoiseshell cats are only the most visible example of the mosaic effect. In fact, any gene on the X chromosone may express itself 'spottily'. There was a New Scientist article not too long ago that speculated on the possible mental effects of this — could the differences between men and women be explained by the fact that women have, in effect, two different types of brain cell? Well, it's something to think about, even if it is rather "out there" as a theory.
So, on to male tortoiseshells. There is in fact a disorder called Klinefelter's syndrome, where a male has XXY chromosones rather than XY. This can cause male tortoiseshells, as again there are two X chromosones available to code for two different colourations. But Klinefelter's leads to health difficulties and sterility, so let us rephrase: could there ever be a fully healthy male tortoiseshell?
Well, there is in fact a way. It's to do with something called Chimerism. A chimera (named after the monster made up of parts of different animals) is an organism formed from the merging of two groups of genetically different cells. This can happen naturally, when an egg is fertilised by two sperm, when a single sperm fertilises two eggs, or when a pair of fertilised eggs merge together (this last case is sort of the opposite of identical twins).
A chimeric person can often look like anybody else, and often the only way to show that somebody is a chimera is through careful DNA testing. Sometimes, though, the effect is more obvious — such as when somebody has patches of differently-coloured skin, or when they are part male and part female. (Chimerism is a frequent cause of hermaphroditism.)
Some ineresting chimeras have been made in the lab, too. For example a "Geep" was made in 1987 by fusing goat and sheep embryos. (With 60 and 54 chromosones respectively, goats and sheep are almost impossible to mate naturally.) Chicken/quail and rat/mouse chimeras have also been made. A rabbit/human embryo was also created, by Chinese researchers, but quickly killed off.(Such a thing would be human transformation; an unforgivable sin!) 8^)
A chimeric woman recently became a minor celebrity; Lydia Fairchild was the subject of a channel 5 documentary called The Twin Inside Me. Lydia had three children with her partner Jamie, but split up with him while still pregnant with the third. She applied for welfare support, and DNA testing was done to show that Jamie was in fact the father. The test came back with surprising results: Jamie was the father, but Lydia couldn't have been the mother!
Immediately, Lydia was wading through court cases for fraud, and people were demanding for the children to be taken away from her. But the birth of her third child was coming up, and the judge ordered the presence of a witness at the birth. Blood was taken from the child and from Lydia, and once again they did not match. Eventually, somebody twigged that chimerism was responsible; a cervical smear was taken, which did match the children.
So back to male tortoiseshells. Chimerism provides the possibility of male tortoiseshells: two eggs are fertilised; one with an X chromosone with the black gene, and one with an X chromosone with the ginger gene. These eggs merge into a single organism. As in mosaicism, the different cells form regions with different genetic properties. The difference is that instead of the same genes being expressed differently, the regions are genetically completely different.
Miaow!
More info on these topics (and a cute baby geep) here.
The answer to the first question lies in something called Mosaicism. This is when groups of cells in different parts of the body have different genetic properties. In mammalian females, this happens due to a feature of the X chromosone. Only one of the two X chromosones can be active in each cell. But the same X chromosone is not activated in all parts of the body; instead, you tend to get regions of cells with the first chromosone activated, and regions with the second activated.
Now, there is a gene on the X chromosone of cats which in one form codes for ginger hair, and in another codes for black hair. A male cat could only inherit either the X with the black hair gene or the X with the ginger hair gene. A female cat, by contrast, can inherit both X chromosones. The mosaic effect leads to some regions of skin cells expressing the ginger gene, and other regions expressing the black gene, resulting in a patchwork appearance.
But tortoiseshell cats are only the most visible example of the mosaic effect. In fact, any gene on the X chromosone may express itself 'spottily'. There was a New Scientist article not too long ago that speculated on the possible mental effects of this — could the differences between men and women be explained by the fact that women have, in effect, two different types of brain cell? Well, it's something to think about, even if it is rather "out there" as a theory.
So, on to male tortoiseshells. There is in fact a disorder called Klinefelter's syndrome, where a male has XXY chromosones rather than XY. This can cause male tortoiseshells, as again there are two X chromosones available to code for two different colourations. But Klinefelter's leads to health difficulties and sterility, so let us rephrase: could there ever be a fully healthy male tortoiseshell?
Well, there is in fact a way. It's to do with something called Chimerism. A chimera (named after the monster made up of parts of different animals) is an organism formed from the merging of two groups of genetically different cells. This can happen naturally, when an egg is fertilised by two sperm, when a single sperm fertilises two eggs, or when a pair of fertilised eggs merge together (this last case is sort of the opposite of identical twins).
A chimeric person can often look like anybody else, and often the only way to show that somebody is a chimera is through careful DNA testing. Sometimes, though, the effect is more obvious — such as when somebody has patches of differently-coloured skin, or when they are part male and part female. (Chimerism is a frequent cause of hermaphroditism.)
Some ineresting chimeras have been made in the lab, too. For example a "Geep" was made in 1987 by fusing goat and sheep embryos. (With 60 and 54 chromosones respectively, goats and sheep are almost impossible to mate naturally.) Chicken/quail and rat/mouse chimeras have also been made. A rabbit/human embryo was also created, by Chinese researchers, but quickly killed off.
A chimeric woman recently became a minor celebrity; Lydia Fairchild was the subject of a channel 5 documentary called The Twin Inside Me. Lydia had three children with her partner Jamie, but split up with him while still pregnant with the third. She applied for welfare support, and DNA testing was done to show that Jamie was in fact the father. The test came back with surprising results: Jamie was the father, but Lydia couldn't have been the mother!
Immediately, Lydia was wading through court cases for fraud, and people were demanding for the children to be taken away from her. But the birth of her third child was coming up, and the judge ordered the presence of a witness at the birth. Blood was taken from the child and from Lydia, and once again they did not match. Eventually, somebody twigged that chimerism was responsible; a cervical smear was taken, which did match the children.
So back to male tortoiseshells. Chimerism provides the possibility of male tortoiseshells: two eggs are fertilised; one with an X chromosone with the black gene, and one with an X chromosone with the ginger gene. These eggs merge into a single organism. As in mosaicism, the different cells form regions with different genetic properties. The difference is that instead of the same genes being expressed differently, the regions are genetically completely different.
Miaow!
More info on these topics (and a cute baby geep) here.
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This also means, since mosaicism and chimerism are random as far as we can tell, that cloned tortoiseshells will look different. The same is true of any kind of brindled or striped marking - you have high-level control of what proportion of cells express which pigment, but not exactly which cells.
Also the fact that chimerism is particularly common in cattle was of clinical significance - cows can very frequently have two sets of immunological identities, blood groups, and so on, so surgical research for skin grafts and organ donation, using cows as model, failed to match humans. Pigs match much better. Picked that up from Peter Medawar's autobiography, Memoirs of a Thinking Radish, which I highly recommend.
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My blood group most definitely got passed to the bone marrow donation register when I registered (pause for brief moment of smugness due to moral high ground, to be paid for by several days of severe aching if and when).
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Essentially, it's not like a blood transplant where there is some of your own blood which would fight off non-matching blood - all the old bone marrow is destroyed and replaced with the donor's.
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