Nature vs Nurture

Discussion in 'Optometry Archives' started by Dr Judy, Mar 19, 2005.

  1. Dr Judy

    Dr Judy Guest

    Refractive error is due to a mismatch between the dozen or so refractive
    components of the eye and/or a failure of the emmetropization mechanism to
    correct the error present at birth or that develops later in life. Since
    all human growth is controlled by genes and the proteins they produce, no
    one can argue that corneal curvature, corneal thickness, lens curvature,
    anterior chamber depth, posterior chamber depth and retinal thickness (the
    major refractive components), the emmetropizaton mechanism and the rate of
    eye growth is not controlled by genes. We also know that environment can
    influence if, when and how strongly the genes and their proteins act.

    So, refractive error is a product of the individual's genetic makeup and
    it's interaction with the enviroment. Any attempt to describe refractive
    error as either genetic or environmental is doomed to fail.

    For a public friendly discussion of nature vs nuture see this summary of a
    PBS NOVA program on the issue:

    and I would highly recommend this book, Nature via Nuture by Matt Riley: I
    read it over Christmas this year.

    a small snip of the amazon review:
    "Editorial Reviews
    In the follow-up to his bestseller, Genome, Matt Ridley takes on a
    centuries-old question: is it nature or nurture that makes us who we are?
    Ridley asserts that the question itself is a "false dichotomy." Using
    copious examples from human and animal behavior, he presents the notion that
    our environment affects the way our genes express themselves.
    Ridley writes that the switches controlling our 30,000 or so genes not only
    form the structures of our brains but do so in such a way as to cue off the
    outside environment in a tidy feedback loop of body and behavior. In fact,
    it seems clear that we have genetic "thermostats" that are turned up and
    down by environmental factors. He challenges both scientific and folk
    concepts, from assumptions of what's malleable in a person to
    sociobiological theories based solely on the "selfish gene."

    Dr Judy
    Dr Judy, Mar 19, 2005
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  2. Dr Judy

    Can i present the argument put forward by 'unreasonable people' that
    although the component parts of the human body is described by genes
    myopia itself does not show a genetic relationship?

    First some statements that i imagine we can agree on? Please let me

    1. Each part of the eye is controlled by genes.

    2. Ridleys statement is reasonable and appears true.

    2. There is an emmetropisation process that produces more normal
    sighted eyes than can be produced by chance (this was true 20 years ago

    3. Environmental factors are known to have an effect on influencing
    myopia because statistically identical twins do not always have the
    same refractions

    Ok. Now to put this together.

    **Somehow** by some method with each eyes gene expressing itself
    differently the majority of eyes have no significant error or
    refraction (true as of 1965 approx) because an emmetropisation process
    seems to be working so that instead of a normally distributed bell
    curve, normal sight is a strong peak in the spectrum of refractive
    errors visible to left and right of that peak.

    Can you let me know if we are in aggreement so far?


    andrewedwardjudd, Mar 20, 2005
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  3. Dr Judy

    Dr Judy Guest

    You can present it, but finding evidence will be tougher.
    I don't agree with your description of the refractive error bell curve; it
    is a normal distribution curve with steep slope, slightly skewed to
    hyperopia with a very small secondary peak (with normal distribution around
    that peak) in the high myopia area

    It is not just the emmetropization process that causes the bell curve, the
    natural variation of the eye components before emmetropization takes place
    means that the refractive error of newborns and very young children follows
    a normal curve. Emmetropizaton steepens the slope of the curve so that
    higher percentages are at "normal" or near "normal".

    We will never separate genetic from environmental, that is Ridley's point.
    It is like the old chicken and egg question.

    Is refractive error genetic or environmental if:

    Baby inherits steep cornea gene and strong emmetropization gene from
    parents, so that the neonatal myopia present due to steep cornea is quickly
    compensated for by slowing growth of the eye in response to myopic defocus
    during babyhood?

    Baby has gene that causes rapid growth of eye if baby eats too much sugar, a
    sluggish emmetropization gene and indulgent grandparents who visit daily and
    give him sweets. After every visit with Grannie, the eye grows rapidly and
    the emmetropization process has a limited response to the resulting myopic

    Female baby has gene that causes growth of eye is response to high estogren
    levels and a emmetropizaton gene that stops working at age 6. At puberty,
    myopia develops.

    Male baby has gene that causes growth of eye is response to high estogren
    levels and a emmetropizaton gene that stops working at age 6. At puberty,
    no myopia develops.

    Female fetus has the sluggish emmetropization gene, the rapid growth
    stimulated at puberty gene and a gene that causes steep corneas if mother
    eats asparagus six times in one week during the second trimesterand .
    Second trimester starts in April when asparagus is at it's peak and Mom
    loves asparagus. Baby born with steep corneas and low hyperopia,
    emmetropization doesn't respond well.

    I'm sure we could think up lots of other possiblities, the point is made:
    how can we separate genes and environment.

    Dr Judy
    Dr Judy, Mar 20, 2005
  4. Dear Dr Judy,

    I suppose the uncertain area is:

    How capable is the emmetropisation process if all
    environmentally/behaviourally negative factors were altered to be
    posative factors?

    It appears that those who are aware of genetic and environmental
    influences can be divided into essentially two opposites camps

    1. Myopia cannot be improved - possibly it can be halted for some forms
    of myopia. It can vary up or down but generally it varies down until
    middle age, improves slightly for several decades and then varies down

    2. Myopia can be improved if environmentally/behaviourally negative
    effects are changed to environmentally/behaviourally posative effects.

    So then we come to Twin studies to decide this one way or another.

    In the context of these studies I am interested in the relationship of
    family habits, family anxiety, twin dominance/rivalry/birth order,
    loneliness etc etc etc in the development of myopia.

    If these influences are very important in myopia we would expect the
    most *similar* refractions in the following order when studying MZ and
    DZ twins.

    1. MZ Raised together in the same family who are treated *most*
    identically by care givers (inside the family and at school ect) and
    other family members **and** who have the most supportive relationship
    with each other **and** who are most alike physically. These twins
    may be so alike that outsiders particularly could not treat each twin
    as a different person because they could not tell the difference
    between them.

    2. Then some kind of confusion in ordering of differently treated (MZ
    twins who are *not* supportive of each other) and similarly treated (DZ
    twins who *are* supportive of each other)

    3. Finally the *most* differently treated DZ twins who are not
    supportive of each other and most unalike physically.

    To my knowledge this has never been tested in this manner.

    Genetic studies of twins assume environment/behaviour is not relevant,
    and yet a test of environmental/behavioural factors in a twin study
    would produce the same statistical results in favour of
    environmental/behavioural factors.

    Similarly "Familial high myopia" is said to be inherited in a
    "Mendelian" manner. This is assumed because it occurs in 3
    generations with some members having high myopia over -6D.

    Meanwhile environmentally/behaviourally we can conclude that "family
    habits, family anxiety,dominance/rivalry etc etc etc"
    are likely to be passed from generation to generation.

    In conclusion:

    1. If we are in the genetic camp the 'Classical twin study' method and
    "familial high myopia" are concrete 'very scientific evidence' that
    myopia has a very strong genetic component influencing it so that
    emmetropisation is unable to restore normal sight.

    2. If we are in the environmental/behavioural camp we notice that
    current gene studies are concluding that "different genes seem to be
    the cause of high myopia in different families" (but still no genes
    yet found for these families)

    I have written to the British Twin study group to ask them if they are
    prepared to examine the most refractively different set of MZ twins for
    behavioural differences. These twins seem to have the capability of
    providing some very useful data on the environmental/behavioural
    reasons why myopia developes.

    andrewedwardjudd, Mar 20, 2005
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