I am a new R user.  i have a question about Mahalanobis distance.actually i have 300 rows and 7 columns. columns are different measurements, 300 rows are genes. since genes can 
classify into 4 categories. i used dist() with euclidean distance and cmdscale to do MDS plot. but find out Mahalanobis distance may be 
better. how do i use Mahalanobis() to generate similar dist object which i can use MDS plot? second question is if should i calculate mean for 
every categories for every measurement first and do 4*4 distance matrix, or i should calculate pairwise distance first and then find category 
means since i only care about relative position of 4 categories in MDS 
plot. Thank you very much.


      
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