X-VMS-To: IN%"ROOTS-L@vm1.NoDak.edu"
Newsgroups: soc.roots
Date:         Mon, 11 Jan 1993 13:19:00 EST
Reply-To:     JMFREED@CC.OWU.EDU
From:         JMFREED@CC.OWU.EDU
Subject:      Counting Chromosomes, etc., continued
Comments: To: ROOTS-L@vm1.NoDak.edu

In further response to the discussion on "counting chromosomes", Joe
Knapp asked if "genetic recombination" was the term to be applied to
the process in which chromosomes exchange genetic material.  Technically,
two different types of "recombination" exist:  (1), the "classic Mendelian"
type which involves two traits that may show up in different combinations
among the offspring due to segregation of the alleles at meiosis and
recombining at randon in fertilization.  For example, in Mendel's peas,
true-breeding tall, red-flowering plant crossed with a short white-flowering
plant may produce in its "grandchildren" plants which are tall and white-
flowering; i.e., a new combination.  We see this in humans as well, with
each individual showing some traits from the maternal line and some from
the paternal line.  (2) the SECOND type of recombination as Joe suggested
is in the actual exchange of genetic information between paired
chromosomes, in a process called "crossing over", occuring during
meiosis (production of gametes).
How's that for your biology 101 lesson today?
Tom Lincoln asked who frequent is this second type of recombination or
"crossing over" of genetic information between paired chromosomes.
That is more difficult to answer, so I'll provide netters with a quote
from Mange and Mange, "Genetics:  Human Aspects", Sinauer Associates, Inc,
Sunderland, MA, 1990, page 190:
     "Very little information is available on human females, but in
     human males the tetrads of autosomes show from one to four
     (sometimes five, rarely six), with an average of about two
     chiasmata per chromosome pair.  The XY pair in human males
     sees to have none."
What the above is saying is that crossing-over occurs with practically
every non sex chromosome (i.e., non X or Y chromosome) in the production
of every gamete, and in fact may more likely have two crossing over
events (two chiasmata), and may have as many as six.  Notice the hesitation
about stating the same about human females.  Also the hesitation about
the XY pair (the word "sees" in the quote should be "seems").  I
recall (with no documentation to provide at this time) that there is
some evidence for rare crossing over between the X and Y chromosome.

Then back to Joe Knapp's statement, with which I would heartily agree,
at least if females have any similar frequency of crossing over, "thinking
in terms of chromosomes is not the right way to go, but one should consider
..hundreds of thousands of genes...in which case it's likely one has some
genetic information in common with all ancestors in generations quite far
back."        Only one possibly change here, geneticists are not possible
yet to know exactly how many genes humans have on their chromosomes.  One
well-accepted estimate is 100,000 genes, and probably not in the ranges
of hundreds of thousands of genes....
Hope the above is of some "interest" to netters; just consider it as a
free lesson in basic genetics today, as we are all profiting fomr
(from) free lessons in the science (or art?) of genealogy every day
by reading and contributing to the information on the network.
Jim Freed, Delaware, Ohio..