Introduction

CODIS and paternity testing, what do these topics have in common? Answer: testing for specific parts of a person’s genome!

A person’s genome contains regions of DNA that are unique from person to person and parts that are the same. For example, the gene (DNA segment) which makes the protein insulin is present in all humans. But there are regions between functional genes that are unique in individuals. These are the regions that are used for identification.

Learning

Humans have 46 chromosomes or 23 pairs. One in each pair was received from the mother (in egg) and one from the father (in sperm), this is referred to as a homologous pair.

On the chromosomes are functional units called genes. This is the DNA code or directions to tell the cell to make a specific substance, a protein. An allele is a term for alternate forms of a gene. For example, one allele in a gene pair could code to produce brown pigment in hair and the other allele in that gene pair could code for black hair. They both code for pigment, but different colors of pigment.

Short Tandem Repeats or STRs

Located between genes, or the coding regions of a chromosome, are short tandem repeats called STR’s. A short tandem repeat (STR) in DNA occurs when a pattern of two or more nucleotides are repeated and the repeated sequences are directly adjacent to each other.

…CCC TCAT TCAT TCAT TCAT TCA…

• Each person carries a unique combination of repeats. The above example has 4 repeats of TCAT.

These areas of the genome are unique from individual to individual (sort of like a fingerprint) and are “anonymous” (control no known trait or function). These regions are routinely used in forensic investigations for DNA-based individual identification.

CODIS

CODIS is the acronym for the “Combined DNA Index System” and is the generic term used to describe the FBI’s program of support for criminal justice DNA databases, as well as the software used to run these databases. It is a central location for law enforcement agencies to compare DNA from a suspect to that of known convicted felons. As the database gets larger the more likely a match can be found. Currently there 13 loci recorded in the database, but it’s soon to be increased to 20.

In addition to convicted felons, the CODIS database is also used to help identify missing persons, discovered human remains, and biological relatives of missing persons.

STR regions have been identified on 13 autosomes (non-sex chromosomes), listed below by name.

DNA is tested for at the 13 specific locations (loci). At a location, a person has two sets of repeats, one from each chromosome (remember – homologous pair – one from each parent). Each set is measured, and the number of repeat copies is recorded.

This can be used to match fetal material to specific parents. These 13 regions, loci, are tested for in the mother, father, and the fetal material. For example, in the table, the left column is the name of the loci, next is mother’s number of repeats at that loci, next is the fetal number of repeats at that loci, and the last column are the results of the father’s number of repeats at that loci.

As you can see the fetal material has one number of repeats that match the mother and the other number that matches the father’s number of repeats.

Summary

STR – Short tandem repeats in DNA occurs when a pattern of two or more nucleotides are repeated and the repeated sequences are directly adjacent to each other. They do not code for anything known.

CODIS – is the acronym for the “Combined DNA Index System”, the FBI’s program of DNA databases and software. Contains the results of the number of STR’s from 13 loci on human chromosomes from convicted felons, arrestees, missing persons etc.

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Sources:

Variable Number Tandem Repeats. (2021, March 23). Retrieved May 22, 2021, from https://bio.libretexts.org/@go/page/24779

“CODIS, Combined Database Index System.” By FBI, 2018. Retrieved from: https://www.fbi.gov/file-repository/combined-dna-index-system-codis-brochure.pdf/view, Licensed under: CC (applicable license here)