Is maybe this what are you looking for?
chromo=40;
gene=20;
for i=1:1:chromo
for j=1:1:gene
pop=randi([0,1]);
A(:,j)=[pop]
end
B(i,:)=A(:,j)
end
[m,n]=size(B) %just for test
B is matrix with m=40 and n=20, so you have 40 choromosomes with each having 20 genes.
Best of luck.
