Why are you testing for allelic imbalance using exome (DNA-seq) data? You only use DNA-seq data for genotyping sites and get high-quality heterozygous sites and then use expression (RNA-seq) data from the same organism on those sites to get allelic imbalance results.

You can get a good understanding on the concept of allelic imbalance or allele-specific expression from this paper:

https://genomebiology.biomedcentral.com/articles/10.1186/s13059-015-0762-6