README.TXT for DRIFT.EXE
A program that illustrates genetic drift through stochastic 
simulation

by 
Mark Young & Robert Zyskowski
Animal & Veterinary Sciences Group
PO Box 84
Lincoln University
Canterbury  8150
NEW ZEALAND

1.0	INTRODUCTION
This simple computer program graphically represents the effect of 
genetic drift on gene frequency of one or more populations for a 
single locus, two allele model in a closed population using 
stochastic simulation methods. The user can vary number of 
populations, size of these populations, starting gene frequency, 
number of generations, the genetic model (absence or presence of 
dominance) and the degree of selection acting against one allele.  
For each run of the simulation, a series of lines are successively 
plotted within a frame.  Each line represents the gene frequency of 
one population across time. The program is designed for interactive, 
computer-aided learning in conjunction with appropriately designed 
tutorial notes for courses in genetics, animal breeding and 
molecular biology.

2.0	COMPUTING REQUIREMENTS
The program file is DRIFT.EXE.  This file will run under DOS or 
WINDOWS 3.x on an IBM PC, with or without a mouse. Code was written 
to be computationally efficient and to run on low specification PC's 
provided they have a VGA or better display.  For large simulations 
(many populations of large size for a large number of generations) 
more powerful PC's (Pentium 100+Mhz) are recommended.  The latter 
simulations are not recommended for an interactive teaching 
situation as they can take a long time to run.

3.0	USING THE PROGRAM
DOS users - copy DRIFT.EXE to the working directory and type DRIFT 
at the DOS prompt e.g.
	C:\WORK\DRIFT  <return>

WINDOWS users - In file manager select the file DRIFT.EXE then 
select RUN from the FILE menu.

A title screen appears; type any key to continue.

3.1	THE MAIN SCREEN
This contains a frame for plotting gene (allele) frequency against 
time (generations).  One line appears when the program starts for 
one population, size 15, for 35 generations, initial gene frequency 
= 0.5.

Push RUN repeatedly for successive simulation runs for this set of 
parameters with output as one line per run.  Note that each run 
should produce a unique output on the basis of sampling theory (the 
chances of obtaining the same output are usually extremely low).  To 
see lines for multiple populations superimposed on each other, 
increase the number of populations per run (see below).

3.2	CHANGING SIMULATION PARAMETERS
You can use a mouse (slowest) or the arrows on the keyboard 
(fastest) to change simulation parameters.
  Select POPULATION SIZE and type return.  Now increase or decrease 
the population size then push return.
  NUMBER OF POPULATIONS can be changed under the OPTIONS menu.
  Escape (ESC) will move back a level, while repeated use of ESC 
will select EXIT.  You must type return to actually end the 
program.

3.3	ADDING THE EFFECTS OF SELECTION
Without selection this program simulates random genetic drift.  If 
the effects of selection are added then the gene frequency changes 
are the sum of random sampling and selection effects as described by 
Falconer (1989).  Selection is switched on by choosing SELECTIVE 
BREEDING under the OPTIONS menu. 

Selection effects are based on the formulae of Falconer and 
represent a force acting AGAINST the allele for which gene frequency 
is being studied.  

Three (3) models of gene interaction can be selected under the 
OPTIONS menu;
1. additive - where selection acts against the homozygote form of the 
allele and also the heterozygote but to a lesser extent.
2. dominant - where selection acts against the homozygote form of the 
allele and the heterozygote to the same extent.
3. recessive - where selection acts against the homozygote form of 
the allele only

Degree of SELECTION can be varied from 0 (zero) to 1.0 under the 
OPTIONS menu when selective breeding is switched on.

4.0	SAVING OUTPUT
Copies of the plot frame can be saved as bitmap files using SAVE 
GRAPH under the OPTIONS menu.

5.0	LICENSING STUFF
The program is freeware but acknowledgment of the authors is 
requested when using the package and when producing written material 
for use with it.

Mark Young
SAC, Edinburgh, SCOTLAND
email:	m.young@ed.sac.ac.uk
24 June, 1997