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Thread: Questions on genetics

  1. #1 Questions on genetics 
    Forum Freshman
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    Dec 2007
    I am presuming that all our cells in our body have more or less the same genetic information since we grew from a single zygote. The main genes concerning eye colour are on chromosome 15 and 19. I am presuming therefore, that a cell in the liver has a copy of chromosome 15 and 19 somewhere in its nucleus. What does a liver do with this information? Does it actually use this genetic information?

    What if, say, a new cell in the liver was created (as the result of mitosis etc.) that had a fault in chromosome 19 strictly concerning eye colour. Could this single cell ever have an effect on the colour of an already developed eye? I expect the cell would be destroyed rather instantly, but what if it wasn't?

    A further question then, can a person ever develop a genetic disorder such as down syndrome after birth? If so, what cells would have to be made with an extra chromosome 21 (the cause of down syndrome)?

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  3. #2  
    Forum Cosmic Wizard paralith's Avatar
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    Jun 2007
    Different types of cells express different genes and/or express genes to different degrees, depending on the function of the cell. Which genes they do express and how they express them is determined by what are called epigenetic mechanisms - mechanisms that control the expression activity of genes. Some epigenetic mechanisms are other genes that serve a controlling function, other mechanisms are environmental determinants, such as temperature, salinity, physical contact with other cells, movement, presence/absence of hormones or other molecules, etc.

    The hypothetical mutated liver cell in your question above won't effect eye color at all. That liver cell probably doesn't express any genes related to eye color, or maybe they do but at very low levels that cause no significant effect. In fact, I'm pretty sure that even in eye cells, the eye color genes are only activated in proto-iris cells during development, and are then turned off once the eyes are completely formed. There are many genes like this that are only active at certain times, such as development, and are otherwise not expressed or expressed at lower levels, etc.

    Similarly, in order for a genetic disorder to occur in a human after birth, the genes responsible for the disorder would have to be expressed (or not expressed, in many cases) in all the right areas and at all the right times. If just one cell has the mutation, very little will happen, and most likely the cell will be found and killed anyway. Take Duchenne's muscular dystrophy, for example. (I'm less familiar with the function of Down's syndrome.) This disease results from a mutation in the gene dystrophin. This mutation causes the cells of skeletal muscles to continually break down and die during an affected individual's lifetime. This progressive loss of muscle mass usually causes death before the age of 30. For an originally healthy person to have this same disease, the great majority of their skeletal muscle cells would have to have the mutated gene. Just one or a few isolated cells that contract that particular mutation will probably die before they can proliferate enough to cause any significant effect.

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