Detecting trisomy 21 from exome data

[omerfaruk]

Hello,

I am trying to detect trisomy 21 from exome data.

I have tried:
200 exome bam files -> samtools idxstats and get a distribution for chr 21.

I have divided chr21 reads / (total reads - x - y)
I got the same for all chr 9 and tried to get a ratio between 21 and 9 and still couldnt get a thing.

I am now trying depth of coverage from gatk and i ll see if i can get sth from there.

What can i do more, better?

Best regards,