Playing God
I'll take Robin's suggestion first on human cloning.
I'm not sure if he wants to know whether I am for or against human cloning or what I think the future of cloning is, so I'll touch on both subjects.
Dolly, the first living cloned sheep, had quite a few health problems, among them severe arthritis and a lung disease. She lived only six years and in ( Read more... )
I'm not sure if he wants to know whether I am for or against human cloning or what I think the future of cloning is, so I'll touch on both subjects.
Dolly, the first living cloned sheep, had quite a few health problems, among them severe arthritis and a lung disease. She lived only six years and in ( Read more... )
It's not, and never has been, a means to copy an individual. That's literally impossible, because genes are only part of the mix. There's also all the epigenetic stuff, and there's the environment (which encompasses everything that happens from in-utero to eventual death).
We also are very familiar with clones, in the form of identical twins. As long as people remember that, I think most/all of the potential craziness will be avoided. Sure, you could kill your twin and rip out his heart and replace your own, but we don't DO that, for a whole bunch of reasons that I'm sure are obvious.
Therapeutic cloning (which you talk about) is only a stopgap measure. It may very well turn out to be a sideshow in the stage of medical progress, kept for human reproduction purposes and perhaps a replacement blood supply, and nothing more.
We're not that far away from regenerating lost limbs and organs. We're also not as far away from perpetual physical youth, which would mean that regeneration would happen quickly and effectively. Both techniques will be the means by which we keep ourselves in good maintenance.
Stem cells... well, useful to learn, perhaps even necessary, useful in the interim.. but probably not the way of the future.
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Identical twins occur when a single fertilized egg splits into two distinctly separate eggs and then grow to maturity. Most fertilized eggs split into to completely separate eggs, but not all result in twin births. Sometimes the egg does not separately cleanly and the result is what was once called Siamese twins, or twins that share organs or body parts, and often circulatory systems. Some are joined by a mere bridge of skin and others are more intertwined, as in the case of Lisa and Elisa Hanson who were born joined at the head. Dermoid cysts, which contain hair, teeth, etc., are thought to be the result of a 'lost' twin or twin that didn't fully differentiate and grow to maturity as an identical twin.
Multiple births are usually the result of more than one egg being fertilized and gestated at the same time: fraternal twins. They share the same womb and some DNA markers, but not identical DNA.
The theory that identical twins are clones is incorrect. Cloning, as previously stated, is birth resulting from an egg not fertilized with sperm. el
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Note that I said identical twins, not fraternal, siamese, or any of the other variations or malformations, which would not be clones.
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In the identical twin case, a sperm and an ovum merged to create a fertilized egg, with a full compliment of DNA, which divided into two separate individuals with identical DNA, making them clones of each other.
In the current manufactured clones, a sperm and an egg created a fertilized egg, which developed into an organism that was born, and then a portion of a cell (that portion with a full compliment of DNA) was taken and inserted into an ovum (not containing any DNA) which then was essentially a fertilized egg.
They're both clones, except the manufactured clone creation's involvement with sperm happened at a different point. Nontheless, you end up with a 2nd organism with the same DNA, just as with identical twins. Thus, a clone.
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The division of a single fertilized egg (or zygote) between 1 and 14 days postconception results in monozygotic twins. They share virtually all their genes and, with very rare exception due to unusual embryological events, are of the same sex. A common assumption is that because monozygotic co-twins have a shared heredity, their behavioral or physical differences are fully explained by environmental factors. However, monozygotic twins are never exactly alike in any measured trait, and may even differ for genetic reasons.
Sometimes chromosomes fail to separate after fertilization, causing some cells to contain the normal chromosome number (46) and others to contain an abnormal number. This process, mosaicism, results in monozygotic co-twins who differ in chromosomal constitution. There are several other intriguing variations of monozygotic twinning. Splitting of the zygote after day 7 or 8 may lead to mirror-image reversal in certain traits, such as handedness or direction of hair whorl. The timing of zygotic division has also been associated with placentation. Monozygotic twins resulting from earlier zygotic division have separate placentae and fetal membranes (chorion and amnion), while monozygotic twins resulting from later zygotic division share some or all of these structures. Should the zygote divide after 14 days, the twins may fail to separate completely. This process, known as conjoined twinning, occurs in approximately 1 monozygotic twin birth out of 200. The many varieties of conjoined twins differ as to the nature and extent of their shared anatomy. Approximately 70% of such twins are female. There do not appear to be any predisposing factors to conjoined twinning.
Clones share the exact DNA, when viable, of the source, not so with monozygotic twins, or identical twins. No matter how much they look alike, identical twins are not exactly alike. A clone is exactly like the source from which the DNA was taken.
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I'm talking about the general case, which is to say the case where nothing goes wrong in cell division, in other words the basis case. Thus, siamese twins and other examples you mention are a different discussion.
You say yourself that monozygotic co-twins have a shared heredity, and they do, because they have the same genes. Clones also have the same genes.
Now, I know you're going to point to the next bit in that paragraph, the part that says However, monozygotic twins are never exactly alike in any measured trait, and that's true. However, it is also true for clones
It's important to note that environmental factors can affect both cell division and the expression of genes, and the environment in any given point in space is never going to be exactly the same as any other point in space.
Or to put it another way, there's literally no such thing as clones.. unless you have a more relaxed colloquial definition of clone, which are two individuals that have the same DNA, in which case manufactured clones and identical (monozygotic) twins both qualify.
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