From: oberste@fcs260c.ncifcrf.gov (Oberste)
Subject: Re: Counting Chromosomes
Organization: Frederick Cancer Research and Development Center
Date: Tue, 12 Jan 1993 17:15:06 GMT

In article <1993Jan11.223233.4530@cbnews.cb.att.com> jmk@cbnews.cb.att.com (joseph.m.knapp) writes:
>pkteagarden@bobcat.lmc.edu (Phil Teagarden) writes:
>>Mathematically speaking it works like this:  You have approx. 1/2 of the 
>>GENES of your parents and 1/4 of your grandparents, 1/8 of your great 
>>grandparents.  A little calculation will soon reveal that after about 8 or 9 
>>generations, you aren't much of them anymore!  This is good because sexual 
>>reproduction makes sure that (evolutionarily speaking) this VARIATION 
>>insures our survival! 
>
>You got that right, Phil!
>
>But 9 generations back is comprised of 2^9 or 512 people. So assuming
>random mixing of genes, yes, only 1/512 of my genome on the average comes
>from any one of those people, but taking 100,000 as the number of genes
>then about 200 genes will come from each person. Not too shabby. You'd
>have to go back something like 15 generations before it became likely
>that a significant subset of that generation contributed no genes at all.
>Like hey, maybe Nicholas Knapp (b. ca. 1600) and I share the same gene
>that codes for, say, acetylgalactosaminyl transferase. :-) My respect 
>for him has gone up immensely!
>
However, many (likely most) genes are relatively conserved, ie. there are
only a few variants. Soooo, that AGAT gene may have come from Nicholas
Knapp or from any of the thousands of others who may share that particular
allele (version of the gene). As different as we all appear to be, we are
remarkably similar :-).

>But random mixing can't be assumed either, I'm sure, since Jim Freed's
>explanation (thanks Jim) stated that there are 1 to 6 fixed points on each
                                                       ^^^^^^^^^^^^
>chromosome where it may split up and cross over, so some groups of genes
>will move as a unit. And the sex chromosome case is interesting. Looks to me
>like this genetic code is a big mess and it's time to do a top down
>redesign.
>
Those points aren't fixed, rather they are more or less random. Ie., there
are crossovers at random points, resulting in an average of 1 to 6 crossover
events per chromosome. Whether groups of genes more as a unit depends largely
on the distance between them. If gene A and gene B are far apart, it is more
likely that one of those 1 to 6 crossovers will occur between them. If they
are very close together, a crossover between them is less likely. An analogy:

Imagine shooting arrows at a barn (the broad side, of course :-) ). You could
probably hit the barn somewhere between the left and right ends of the barn
pretty easily, but it would be very difficult to hit the crack between two
particular adjacent boards. On the other hand, if you shot enough arrows, you
would eventually (according to Statistics) hit that crack.

This is the basis of gene mapping.

-Steve (a former Genetics instructor and current Molecular Biologist)


-- 
Steve Oberste                            Internet: oberste@ncifcrf.gov
LCMS, PRI, NCI-FCRDC
PO Box B                        "Never put off until tomorrow that which
Frederick, MD 21702-1201            you can do the day after tomorrow"