Question

In a VCF file, which specific field and flag combination indicates that a variant call is statistically supported by a high number of reads but likely represents a mapping error rather than a true biological variant?

Accepted Answer

In a VCF file, a variant call that is supported by many reads but likely represents a mapping error is identified by a high DP field value combined with the MQ0 flag. The DP field, which stands for Depth, represents the total number of reads covering a specific position in the genome. When this number is unusually high, it suggests that the software has forced many reads into a single location. The MQ0 flag indicates Mapping Quality Zero. Mapping quality is a statistical score that measures the confidence that a read originated from a specific genomic location. An MQ0 flag means the read could not be uniquely mapped to one location, suggesting it might actually belong elsewhere but was placed there due to repetitive sequences or structural errors. When these two conditions appear together, it signals a mapping artifact, where the sheer volume of reads creates a false appearance of a biological variant that is actually just an accumulation of improperly placed sequencing data.

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In a VCF file, which specific field and flag combination indicates that a variant call is statistically supported by a high number of reads but likely represents a mapping error rather than a true biological variant?