[PW] ? DNA question

[Kay Lancaster]

> >  When the
> >results were in, the Professor made a crack in front of the class that
> >her DNA had one female strand and two male strands (or lines?) and she
> >should "go home and talk to your mother."
> 
> What a nasty man.

I agree.  I'd add to that that I'd strongly consider a 
conversation with the Dean about that comment. And maybe a lawyer 
-- I do believe that's a slander on her mother.  And I say that 
as an old biology prof and someone who's not one to run to the 
legal system.

This just doesn't make any real sense to me no matter which way I 
turn the comment.  Nor can I imagine a college class doing human 
DNA analysis -- last time I heard, most of the basic tests for 
paternity analysis were about $100 a throw.  

The only things I can figure from this (other than the teacher 
needs to watch his mouth) is that a mistake was made in the 
analysis, the prof didn't take into account mutations, or she may 
be a chimera.

The chimera idea is the most interesting of the three -- she 
would have with cells with two genetic heritages, her own and 
cells from a non-identical twin, probably a twin sister, absorbed 
during early embryonic development.  The twin then was resorbed 
in utero. Here's a newspaper story that's reasonably accurate and 
comprehensible: 
http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2003/11/13/nivf113.xml

Mistakes in analysis are pretty common, particularly in college 
class situations -- I'm guessing there was a lab exercise to find 
your blood group (cheap and believable in a college lab 
situation) and then do a family tree with blood groups marked for 
any relative who might know theirs.  The student could have 
supplied some information like "I'm AB and my parents are A and 
O" -- theoretically impossible and most likely attributable to a 
poorly run test (blood that dries on the slide before you get the 
Anti-A and Anti-B serum on it, can look like AB if you're a 
novice at reading the results -- repeat testing might show O).
Poorly stored reagents are likewise a possibility for goofing up 
such a test -- no student required. <g>  

With such a test, there are also people who have rare blood types 
that do not classify correctly with the standard ABO blood 
grouping testing.  Some of the cases are talked about here:
http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=110300
And then there are the people who have demonsterably "impossible" 
blood types, including cis-AB -- where one parent has both A and 
B alleles (apparently sitting near each other on a DNA strand, 
instead of "proper alleles" across from each other) -- the child 
inherits BOTH A&B from one parent, i (the recessive allele for O 
phenotype) from the other.

Apparent mutations also frequently come to light when students 
are asked to do family trees with something like blood groups or 
eye color displayed on them... the immediate reaction from a 
teacher is likely to be "adopted!" or "not possible!", but it is 
indeed very possible.  Many genes are subject to modifiers -- the 
inheritance patterns are not the simple Mendelian patterns shown 
in first year biology textbooks.  We commonly talk about "blue 
eyes" and "brown eyes", but there is a wide range of eye color, 
from nearly colorless, icy blue through lavenders and greys to 
hazels and greens and browns.  I ran into this in my own family 
in high school biology... my mom's eyes were dark brown, my 
father's very blue.  I have grey eyes.  Going back a generation, 
my father's parents both had blue eyes.  So did my mom's.  Tilt! 
Tilt! Tilt!  And yet my mom was the spitting image of her father 
and other members of her father's family...  and my grandmother 
was a very strait-laced lady -- those two factors leave me no 
doubt that her legal parents were her biological parents.  But, 
my grandfather had been gassed several times in WWI with mustard 
gas, a known mutagen... that's one possible explanation that 
makes sense biologically.  Also there's the possibility of just 
plain ol' random mutation -- also not all that uncommon.  

I always made a point of telling that story to my classes, just 
because these sorts of genetic analyses are fraught with 
potential for emotional (and sometimes legal) landmines.  Even 
then, I'd probably see at least one anxious student out of a 
class of 150 or so during office hours with "impossible" family 
trees, and we'd have to spend a few hours with Mendelian 
Inheritance in Man (and occasionally with a real geneticist) 
figuring out the possibilities before the student was happier.

There are a bunch of other possibilities as to what triggered the 
ill-advised remark that I'm not really competent to explain -- 
(I'm really a botanist, not a human biologist or geneticist), but 
I'd suggest a two-pronged approach to the problem: the student 
should talk to another biologist about the possible biology 
issues, and to the dean about the apparent case of foot-in-mouth 
disease.

If your patron gets more information, or if her sister would like 
to "talk" to me, I'd be happy to do what I can to try to figure 
out the meaning of the remark -- noting, of course, that I'm more 
used to working with organisms that are happily polyploid than
with organisms that are almost all diploids.  

Oh, and the XYY idea doesn't hold water well... an individual 
with XYY karyotype has a normal male phenotype.  Incidence 
appears to be about 1/1000 males.

And then there's the First Law of Biology Labs: "Under the most 
carefully controlled conditions, biological material does what it 
damn well pleases".  <g>

Kay Lancaster  kay at fern.com