chidsey@smoke.BRL.MIL (Irving Chidsey) (04/06/90)
In article <1138@wsu-cs> kkc@pandora.cs.wayne.edu (Kwok K. Chan) writes:
<
<This is a post for my friend who does not have access to USENET
<
<I am trying to write a paper related to human genetics and want to
<find out some more information of the following questions:
<
<1. I have heard when two close relatives get married, their children
< will have a great chance of having defective genes. In other word,
< it is very likely their children are mentally retarded, physically
< deformed, etc. Is there any medical journal, article, statistical
< data or evidence that documented this type of issue?
<
<2. What about two persons with same last name but are not relatives at
< all. Will they face the same risk?
<
<Thank you very much.
<
<Please reply to me via email : kkc@cs.wayne.edu
My understanding is that the risks are different. Related people
are more likely to share the same version (allele?) of a gene than unrelated
people. Most defective genes are recessive, not dominant, so you have to
inherit two bad copies to get in trouble. Unrelated people are less likely
to pass on the same defective genes so their offspring are less likely to
have problems.
It is the enhanced possibility of getting two copies of the same
baddie that does you in.
Irv
--
I do not have signature authority. I am not authorized to sign anything.
I am not authorized to commit the BRL, the DOA, the DOD, or the US Government
to anything, not even by implication.
Irving L. Chidsey <chidsey@brl.mil>lamoran@gpu.utcs.utoronto.ca (L.A. Moran) (04/11/90)
Dan Briggs (dbriggs@nrao.edu) writes:
"Here are some simples arguments why I think that *all*
kids should look like a pretty even mix of traits from
both parents...
[statistical stuff deleted...]
I'm not claiming that this model is correct, but just
wanted to make the point that statistical factors will
cause the distribution to peak sharply about the mean
as total number of traits rises.
Now observationally, this is bunk. I look like my mother,
(allowing for the obvoius gender based differences :-). My
sister looks like my father. I've no good way of quantifying
this, but wouldn't be surprised to find that I'm in the
70%-30% ballpark myself. That is I am much farther from the
mean value than my crude model can allow.
So crew, your job is to tell me (or speculate) which one of
the assumptions I made was the most stupid..."
This one is easy. It is not your assumptions or your model which are
incorrect but your observations! If you were to make a list of all
observable human phenotypes it would include things like;
height, weight, eye color, bone size, length of foot, blood type,
skin pigmentation, shape of ears, size of breasts, facial hair,
external genitalia, length of fingers, amount of body hair,
shape of cheekbones, position and number of teeth, knee shape,
flexibility of joints, hair texture, MHC antigens, enzyme isotypes,
shape of hips, muscle bulk, baldness, type of ear wax, etc. etc.
Imagine that your entire family was lined up, naked, on a stage. Any truely
objective observer would undoubtedly conclude that you more closely resemble
your father in most physical characteristics. Your sister presumably looks
more like your mother since they both have two X chromosomes. If you
concentrate on those physical characteristics that are not sex-linked it is
very likely that neither you nor your sister will show a marked preference
for either parent provide that all phenotypes are given equal weight.
Now, why is it that you think that you have inherited more traits from
your mother than from your father? The answer is that we tend to rely on a
small number of visible facial clues to distinguish individuals. For
Caucasians the most important are skin color, eye color, hair color, and the
overall shape of the face. Other ethnic groups rely on different criteria to
recognize individuals.
Thus, you still claim that you look more like your mother, in spite of
the fact that your physique is probably more like your father's, and your
voice is unlike your mother's, and you have hair on your face!
Consider eye color among European Caucasians as an example. If one
parent has blue eyes and the other parent has brown eyes (but is
heterozygous), then half of their children will have blue eyes and half will
have brown eyes. We are inclined to pretend that the blue-eyed child looks
much more like the blue-eyed parent in spite of the fact that they may not
resemble each other in any other physical characteristics. In this case all
of the children will "seem" to resemble one of the parents and not the other.
The same bias holds for other prominent facial characteristics.
When you claim that you "look more like your mother" you are probably
focusing on only a few features of your anatomy and ignoring many that seem
less important or are normally hidden by clothes. (By the way, do you know
your blood type and that of your parents?) You correctly suggested this
possibility when you wrote:
"Here are a few possible ideas..
(3) The number of potentially observable traits is
actually fairly small. Or more precisely, the
number of such traits that people notice when
comparing people is fairly small."
For the record, human genetics is no different than the genetics of
plants, fruit flies and nematodes. You really are a combination of the
genes of both your parents and this can be proven easily in the laboratory.
If there was an anomaly of the sort you suggest (ie. are 70% mother!) then
it would have been noticed long ago.
-Larry Moran