Tara Kuther wrote:
> Tipsters--
>
> Last night during a discussion of dominant-recessive inheritance in a
> Child Development course, I used eye color as an example (the classic
> brown = dominant, blue = recessive). The text mentions that gray, green,
> blue, and hazel eyes are recessive to brown, prompting a student to ask
> what happens when a child inherits two different recessive alleles (say,
> blue and green)?
>
> Looking through the archives I found a few posts on eye color changes,
> that I recalled from last year, but nothing on this. What do you think?
>
A couple years ago, I wrote the following post about eye color. Perhaps
this will help:
> From: IN%"[EMAIL PROTECTED]" 17-SEP-1997 04:07:48.86
> Subj: Update on Inheritance of Eye Color
> From: [EMAIL PROTECTED]
>
> TIPSters,
>
> I was unsatisfied with the answer that I gave as to the question of
> the
> inheritance of eye color; so I looked at a few general textbooks of human
> genetics to see what they had to say about this question. It seems that
> several
> people on TIPS expressed an interest in the genetics of eye color, so I
> thought
> it might be of general interest to post the results. Levitan (1988,
> _Textbook
> of Human Genetics_ [3rd edition], New York: Oxford University Press)
> stated
> that:
> the absence of pigment in the outer part of the iris causes the eye
> to
> appear blue for the same reason that an uncloudy sky appears blue:
> as
> light is scattered (...by the unpigmented iris diaphragm in the
> eye),
> the portion with shortest wave length, the blue, is scattered most
> and
> reflected back to the eye of the beholder. A pigmented iris absorbs
> light and reflects back a color, green, hazel, or brown, depending
> on
> the nature and intensity of the pigment present. (p. 342)
> He reported that there is a continuous variation of eye color between
> blue and
> dark brown, and that it has not been a simple task to classify these
> color
> variants:
> Attempts to erect meaningful genetic classes such as blue, yellow,
> green, hazel,light brown, and dark brown within this continuous
> spectrum
> generally have been unsuccessful. This is especially true in the
> lower
> part of the range, where persons with small amounts of pigment and
> completely pigmentless eyes are very difficult to distinguish, all
> referred to as "blue" by the layman. (p. 342)
> He suggests that there are three loci (genes), each with two variants
> (alleles), where the uppercase allele at each locus produces pigment and
> the
> lowercase allele produces no pigment: P1/p1, P2/p2, P3/p3. The greater is
> the number of pigment alleles inherited by a person, the darker is
> his/her
> eyes.
>
> In fact, a book by Mange & Mange (1990, _Genetics: Human Aspects_
> [2nd
> edition], Sinauer: Sunderland, Mass.) included a table that makes this
> hypothesis explicit. They modified an eye-color classification scheme
> developed
> by Francis Galton (1889, _Natural Inheritance_) and included the number
> of
> pigment alleles that might be associated with each eye color:
>
> Eye Color Hypothesized # of Pigment
> Alleles
>
> light blue 0 (p1/p1, p2/p2, p3/p3)
> blue 1 (e.g., P1/p1, p2/p2,
> p3/p3)
> blue green 2 (e.g., P1/p1, P2/p2,
> p3/p3)
> hazel 3 (e.g., P1/p1, P2/p2,
> P3/p3)
> light brown 4 (e.g., P1/P1, P2/p2,
> P3/p3)
> brown 5 (e.g., P1/P1, P2/P2,
> P3/p3)
> dark brown/black 6 (P1/P1,P2/P2,P3/P3)
>
> But they stated that (as of 1990, at least) the inheritance of eye color
> has
> not yet been worked out. They asserted that it is virtually certain,
> however,
> that the trait is polygenic in nature. This is an assertion that they
> repeated
> in 1994 in another book (_Basic Human Genetics_, Sinauer: Sunderland,
> Mass.):
> It turns out that eye color is a complex rather than a simply
> inherited
> trait, and--although alleles for darker colors tend to be dominant
> over
> alleles for lighter colors--several loci are probably involved, and
> it is
> possible for two blue-eyed parents to produce a brown-eyed child.
> (p. 80)
> They quote one famous geneticist, Victor McKusick, as saying that, "My
> monozygotic twin brother and I, brown-eyed, had blue-eyed parents and
> blue-eyed
> sibs" (p. 80). They don't mention whether or not it was this fact that
> caused
> him to take up the study of genetics in the first place.
>
> From what I can tell, the "myth" that eye color is a single-gene
> trait
> with a dominant brown-eyed allele and a recessive blue-eyed allele is due
> to
> the finding that, in some family pedigrees, it seemed to be inherited as
> such.
> I would bet that a further reason why this myth has been propagated is
> that it
> makes a nice, neat story that helps in the teaching of basic genetics to
> undergraduates.
> --
Jeffry P. Ricker, Ph.D. Office Phone: (480) 423-6213
9000 E. Chaparral Rd. FAX Number: (480) 423-6298
Psychology Department [EMAIL PROTECTED]
Scottsdale Community College
Scottsdale, AZ 85256-2626
"The truth is rare and never simple."
Oscar Wilde
"No one can accept the fundamental hypotheses of scientific psychology
and be in the least mystical."
Knight Dunlap
