Linkage Newsletter

Jurg Ott OTT at NYSPI.bitnet
Wed Feb 5 08:57:01 EST 1992

              Linkage Newsletter
        Vol. 6   No. 1   January 1992

Published by Jurg Ott, Columbia University, New York.
     e-mail: OTT at NYSPI.BITNET

Editorial Assistant:  Katherine Montague
     Fax: +1-212-568-2750    Tel. +1-212-960-2507

Postal address:  Columbia University, Box 58
     722 West 168th Street, New York, NY 10032


        The Linkage Newsletter is now going into its sixth year of
publication.  Last year only two issues were mailed out, but we are
again planning on three issues for this year.  As in the past, the
Newsletter will focus on all aspects of interest to researchers in
human linkage analysis with particular emphasis on statistical
problems and computer programs.  Comments from readers are always
        Several ad hoc versions of the LINKAGE programs in the C
language exist.  An "official" C version is also forthcoming - Mark
Lathrop and Peter Cartwright have a trial C version in operation.
The next LINKAGE version will be available both in C and Pascal.


        We currently have close to 900 individuals on our address list
and keep receiving new requests for the Newsletter.  To allow us to
consolidate our address list and remove any entries for people who
are no longer interested in receiving the  Newsletter, please fill
out the questionnaire attached and return it to us, preferably by
        All recipients are requested to fill out the questionnaire or
their names will be removed from the mailing list!


        This spring, two introductory linkage courses will be held,
one in New York and one in Zurich, Switzerland.  The dates are as

Zurich:   April 13-16, 1992, in the Computer Center at the
University of Zurich, Irchel campus.  This course is organized
jointly with Prof. Eric Kubli in Zurich.

New York: May 19-22, 1992, in the microcomputer classroom of the
Health Sciences library, Columbia University, New York.

Registration is open for both courses until one month prior to the
beginning of each course, but we always have more applicants than
space available, so we encourage a rapid response.  For information
and application forms, please write (preferably by fax) to
Katherine Montague, course coordinator, at the address given above.
        There will again be an advanced linkage course at Columbia
University this fall but a date has not yet been fixed.


        Bug in SLINK program

        Dr. Dan Weeks at the University of Pittsburgh sent in the
following text:

October 1, 1991: Clipping Bug in SLINK
        There is a bug in SLINK dealing with how SLINK clips up
through the pedigrees.  This bug mainly causes problems if the
pedigree is numbered so that a father is numbered before his child,
who is numbered before its mother.  Please correct your version of
SLINK as indicated below or order a new copy from us.

Find this line in SLINK.PAS in the procedure collapseup:

IF ((p^.foff<>NIL) AND (p^.geneloc=0)) OR ((p^.foff<>NIL) AND (NOT

and replace it by:

{10/1/91 Correction to clipping problem }
{ Step 1: Find the spouse }
    IF (p^.foff<>NIL) THEN
     IF p^.male THEN
{ Step 2: Collapseup if there are children AND either spouse }
{         has not yet been assigned a genotype }
    IF (p^.foff<>NIL) THEN
    IF ((p^.geneloc=0) OR (q^.geneloc = 0) OR (NOT noloop))
{Old code did not test whether spouse was done or not }
{IF ((p^.foff<>NIL) AND (p^.geneloc=0)) OR ((p^.foff<>NIL) AND (NOT

        Users of SLINK are encouraged to order the current program
version.  In the last few weeks several changes have been imple-
mented which should make the programs more useful.  Details are
given below.
        The default buffer size (128 bytes) for the input or output
files (whichever is usually larger) was increased to 4K bytes.
This resulted in the addition of an extra line of code in non-
standard Pascal.
        The critical levels for the maximum lod score (previously
fixed at 1, 2, and 3) are now user defined.  At start-up of the
analysis programs, the user is prompted to enter 3 critical values.
To run the analysis programs in batch mode, one may invoke them,
for example, as MSIM < MCRIT.DAT where MCRIT.DAT is a file with one
line containing 3 critical values.
        The quadratic interpolation routine, QUADMAX, in the MSIM
program has been rewritten, because it occasionally seemed to give
nonsensical results.  It is now based on formulas (8.8)-(8.14) in
Ott (1991) and interpolates the maximum lod score given three pairs
of values (Z, t).  It does not carry out any extrapolation beyond
the smallest or largest theta value.  If the two interval lengths
differ by more than a factor of 4, no quadratic interpolation is
carried out, because the quadratic approximation may not fit the
lod score curve very well in those circumstances.

        Prospero Pascal and OS/2

        All our Pascal programs have been updated to conform to
standard Pascal as much as possible.  Executable versions are
compiled with Prospero Pascal because this compiler can produce
programs running under OS/2.  Most programs (eg. SLINK, TLINKAGE)
are now available in compiled forms running under DOS and OS/2.
The major advantages of running under OS/2 are that 1) programs can
run in the background (in an OS/2 window) while one carries out
other work in the foreground, and 2) larger problems can be
analyzed under OS/2 than under DOS.  For example, I recently used
ILINK to analyze 1,000 pedigrees comprising a total of 22,000
individuals (2 loci, 2 and 4 alleles) with OS/2.
        For a while we had some problems with the OS/2 versions.
Every so often the system would halt a program and announce that it
had led to a protection violation.  After removing two compiler
switches previously used (checking for stack space, initializing
heap space to zero) these errors no longer occur.

        Program bugs and problems

        Users of the LINKAGE programs have reported the bugs and
problems outlined below.  I am grateful for any error reports -
other people working in this field will surely appreciate being
made aware of potential problems.
        In the PREPLINK program, for quantitative trait locus types,
a multiplier for the variance in heterozygotes versus that in
homozygotes can be chosen provided that only two alleles and a
single trait phenotype are defined.  With more than two alleles or
one trait, this multiplier has no effect in the program but still
occurs in the datafile, which may be misleading (Joseph Terwil-
liger).  There is presently no check in the LINKAGE programs
whether a loop in the data is made known to the programs or whether
the number of loops declared exceeds the constant MAXLOOP.  In
either case, the program may terminate normally but provide an
incorrect result.  The latter problem is relatively easy to fix,
but it will take some programming to create a loop detecting
procedure.  We are currently working on such a procedure.  Some
(but not all) undeclared loops lead to a program error in that the
stack space is exhausted (Marcy Speer).
        In the TLINKAGE programs, if two null loci (two-locus disease
models) are specified, they must be listed in the locus order in
such a way that the null locus with a higher locus number is listed
after the other null locus.  For example, if 1 and 2 are the null
loci, locus orders 3 1 4 2 and 1 2 4 3 are all right, but the locus
order 2 1 3 4 leads to a cryptic error message of Array bound
exceeded (Chantal Merette).
        In the CFACTOR program, which is invoked before the CILINK
program, when all family data are uninformative for linkage, the
TEMPPED.DAT output file is empty.  In that case, CILINK crashes
when it tries to read TEMPPED.DAT.  A check has been implemented in
the Prospero Pascal version of CILINK to test for an empty
TEMPPED.DAT file (Chantal Merette).
        In the LODSCORE program, in procedure getpen (file inl.pas),
line 26 should be commented out or deleted.  The line to be deleted
        FOR i:=1 TO nallele DO read(datafile,pen[0,i,2,l]); (Marie-
Claude Babron).

        Tom Burroughs, Department of Psychiatry, Washington
University, St. Louis, reports in their Linkage Bulletins two
problems with the LINKAGE programs on a Sun 4.1.1 computer:

        Problem 1:  PREPLINK won't read and write data for numbered
alleles properly.  Solution:  A line of code must be added after
the 101st line of procedure writedata.  Below, the line to be
inserted and the three lines preceding it are shown:

  numbers: begin end;  { <= insert this line}

        Problem 2:  LCP won't run properly in OpenWindows in a command
tool.  After typing lcp, about 12 lines are written to the screen
at which point the screen locks up.  Solution:  Invoke LCP in a
shell tool with the scroll mode off, rather than a command tool.
While the highlighting and boxes which surround the menu do not
appear as neatly as in the PC version, the program will run this


        As outlined in a previous issue of this Newsletter, the BIOSCI
electronic newsgroup network comprises various newsgroups, some of
which are of particular interest to people working in linkage and
genome analysis.  The network allows its users to contact people
around the world without having to learn a variety of computer
addressing tricks.  Any user can simply post a message to his/her
regional BIOSCI node and copies of the message will be distributed
automatically to all other subscribers on all of the participating
        The following newsgroups may be of particular interest to
readers of this newsletter:

--------------------  -----------------------------------------
CHROMOSOME-22         Mapping/Sequencing of Human Chromosome 22
GENETIC-LINKAGE       Newsgroup for genetic linkage analysis
HUMAN-GENOME-PROGRAM  Human genome issues, NIH sponsored
--------------------  -----------------------------------------

        Information about BIOSCI may be obtained from
biosci at;  to subscribe to one of the newsgroups,
send a message in plain English to one of the following addresses,
depending on your geographic location:

Your location         Address
-------------------   ----------------------
Continental Europe    biosci at
United Kingdom        biosci at
North/South America   biosci at
-------------------   ----------------------

        The GENETIC-LINKAGE newsgroup is an ideal forum for exchange
of ideas and discussion of current topics.


        In the May 1991 (Vol. 5 No. 2) issue of this Newsletter, run
times (in seconds) of our new benchmark problem (available on disk
5b, directory bench) were shown.  Dr. Dan Weeks provided the
following additional information, where for comparison the first
four lines are copied from the previously published table:

Machine                                    Run time
Toshiba 5200 (80386/7-20)                       449
Vaxstation 3100 model 38                        167
AT compatible 80486-33                           98
Sun Sparcstation 1+                              45

SUN SparcStation 2, 33MHz
  SUN Pascal 2.0                                 31
Macintosh IIfx (68030-40MHz
  MPW Pascal) LINKAGE 5.04                      257

        The SUN Pascal compiler seems very sensitive to the optimi-
zation level.  At the default optimization, the compile time is
much faster but the program then takes 82.3 seconds to run!  The
30.2 "user" seconds is with the -O3 optimization level.
        We often receive requests for information on Macintosh
versions for the LINKAGE programs.  Dr. Weeks started developing
such a version while he was in New York and now has a test version
working.  However, only the analysis programs have been converted
but not LCP or LSP.  There is thus no very useful Macintosh version
available.  Please address all requests for information to Dr.
Weeks at University of Pittsburgh, e-mail:
        dweeks at


        Q1: We are analyzing our family data with the help of LINKAGE,
HOMOG, and LINKAGE UTILITY programs.  For one highly polymorphic
DNA marker, no evidence for linkage was found when the whole sample
of 14 families was analyzed, although some individual families
showed complete linkage.  The HOMOG program provided considerable
support for heterogeneity: p=0.0058 (H2 vs. H1), p=0.0316 (H1 vs.
H0), and p=0.0037 (H2 vs. H0).  After the exclusion of 7 families
showing a conditional probability of linkage of less than 0.05, the
maximum lod score for the remaining families reaches 5.09 at zero
recombination fraction.  Does this provide significant evidence for
        A1: The classical criterion for linkage is a maximum lod score
of at least Z=3, which corresponds to a likelihood ratio for
linkage of at least 1000:1 or a p-value of at most 10-Z=0.001.  In
the presence of an admixture of linked and unlinked families, the
overall maximum lod score may be small or even zero but a
significant test for heterogeneity would clearly provide evidence
for linkage (in some of the families).  However, to retain the
stringency of the criterion for linkage, the test for heterogeneity
should only be declared significant when the associated likelihood
ratio is at least 1000:1, which is not the case for your data. (For
statistical reasons, the current HOMOG programs no longer provide
p-values.)   Since your results do point in the direction of
heterogeneity, adding more families may well lead to significance.

        Q2: What is the interpretation of the generalized lod score
calculated by ILINK?  It reaches 8.07 in our family sample for a
three-point analysis between the disease and two highly polymorphic
marker loci.
        A2: For three loci, the generalized lod score calculated by
ILINK is the logarithm to base 10 of the likelihood ratio, L(t1,t2)
/L(1/2,1/2).  It measures the overall evidence that the three loci
are linked as opposed to being all unlinked.  When one locus is a
disease locus and the other two are markers, one usually knows that
the markers are linked with each other.  The question of interest
is then generally whether the disease is linked with the two mark-
ers, given that the markers are linked.  The generalized lod score
overstates the evidence for linkage between disease and the
markers.  A better measure for linkage is obtained, for example,
from the LINKMAP program, which assumes a fixed map distance
between the markers.


        A revised version of my Analysis of Human Genetic Linkage
(1991) is now available for $47.50 from the publisher (Johns Hopkins
University Press, Baltimore) or through bookstores.  Readers
(notably Drs. Deborah Meyers and Marcy Speer) have alerted me to
the following missprints:
        Page 38, Problem 2.2:  Replace 200 cM by 100 cM.
        Page 117, lines 21-23:  The last sentence in this paragraph
should read:  The second child has genotype 121/222 or 122/221,
each of which requires at least one recombination in the father or
the mother.
        Page 137, first line, should read:
 ...between the loci C and D.
        Page 148, line 11:  Replace fdd by fDD.
        Page 149, table 7.1, line d1/d1:  replace 1/2 by (1/2)r for P(g;r)
(as on line above it).

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