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The color of a person's eyes are genetically predetermined. If I knew how and in which gene(s) colors are encoded, I could change these genes in the ovary and sperm and thus change the eye color of the new human being. I only had to work on one or two cells.

What would I need to do to change the eye color of an adult human? Manipulate the genes in all body cells? Or would a limited number of cells suffice (e.g. those in the eye)?


Eye color is just an example. Your answer does not have to deal with this trait.

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You would only have to manipulate the relevant cells. Not all genes are expressed in all cells, many of them are regulated in a cell type specific (and also development specific) way. –  Chris Apr 7 at 8:23
    
Take a moment and think - why would you need to change genes in all your body cells to change the eye color ? –  biogirl Apr 7 at 8:33
    
@biogirl I'm a biological idiot. Last time I heard of genes was 30 years ago in school. I thought that mutations in cells happen often, and that this mutation is held in check by the surrounding cells that have the "correct" gene. So it seems to me that a larger number of cells need to be changed for a change not to appear as malignant mutation to the rest of the body (e.g. blood cells). Also, since cells constantly die and are recreated (at least in some parts of the body), a limited change might simply grow out and the original eye color return. –  what Apr 7 at 8:38
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It is very difficult indeed to change genes in an adult organism. Try reading about gene therapy. Its a practical application of what you are trying to imagine ! –  biogirl Apr 7 at 8:44
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Cells are not intelligent like that. They can not look at each other, read each other's DNA, compare it, and decide to kill an outlier. If a DNA mutation causes a cell to go haywire, the immune system might recognize that its behavior is not right, and kill it, or the cell's own suicide genes might kick in, and kill the cell. But those failsafes don't always work, and if the effect of the mutation was minor, nothing bad would happen to the mutated cell. –  swbarnes2 Apr 7 at 16:41
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With some exceptions (one being red blood cells), all cells in the body contain the complete genome of the individual. The expression of the genes however is, as mentioned in the comments, heavily regulated and different in various tissues. Early efforts at gene therapy have had mixed results, including some fatalities, and the field is still highly experimental.

Depending on the trait you want to change, you might have to manipulate the genes and/or gene expression in just a particular tissue, or in several. The rates of cell death and replenismhent varies between tissues, and will affect the stability of any introduced mutation. Some traits may depend on too many genes to effectively manipulate with current technology, and eye color does depend on several genes. From the Wikipedia page on Eye color:

Eye color is an inherited trait influenced by more than one gene. These genes are sought using associations to small changes in the genes themselves and in neighboring genes. These changes are known as single-nucleotide polymorphisms or SNPs. (...) There is evidence that as many as 16 different genes could be responsible for eye color in humans; however, the main two genes associated with eye color variation are OCA2 and HERC2, and both are localized in Chromosome 15.

So depending on the original and desired color, you may or may not get away with changing a single gene. However, the pigments that determine eye color are produced in two separate tissues in the eye, making matters a bit more complicated:

In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin in the iris pigment epithelium (located on the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma

Note that cellular density also plays a role, and this presents a much harder property to change with gene therapy, if it is at all possible.

Eye diseases have been a popular field for gene therapy research and trials lately, and there seems too have been quite some progress. However, the possiblity of using gene therapy for changing eye color has understandably not received much attention, and so there are few if any papers to consult. What can be said though, is that eye color is a complex trait, with a partially known genetic basis. It is plausible that gene therapy can be used to change pigment production and thus eye color, but even so there would likely to be many limitations. on the changes that can be made.

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Nice, thank you +1. I edited my question and added a note that eye color is just meant as an example. If you have another example that illustrates the invovled principles better, feel free to add it to your answer. –  what Apr 7 at 13:18
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