DNA & Genes - Forensic Science Notes (Year 10)

DNA contains the genetic code for producing proteins that make up the body.
A gene is a DNA segment with genetic information for certain proteins and physical features.
Each person has different sequences of nucleotides in genes, resulting in different versions of physical properties.
Half of a person’s DNA comes from their biological father, and half from their biological mother.

DNA analysis is considered the gold standard in forensic science.
When comparing two people’s DNA, more than 99% will be identical.
Despite the high similarity, differences exist that allow forensic scientists to distinguish between DNA samples.

Even with humans' DNA being 99% identical, enough differences exist to make each person’s genetic sequence unique.
Forensic approaches have evolved, leading to several methods for matching DNA evidence to individuals.
Most forensic DNA analysis focuses on regions of the genome that vary in length between individuals.

Regions in the genome that vary in length are called Short Tandem Repeats (STRs).
STRs are short sequences, typically 2 to 5 DNA bases, that repeat several times in a row.
The number of times the bases repeat varies between people.
Therefore, the length of DNA where the STRs are located also varies between people.
Comparing the lengths of DNA between individuals allows forensic scientists to match DNA samples.
There are 20 commonly used locations in the human genome where the number of copies of a specific STR varies between people.
STRs are given complicated names, but 'STR 1’ is used as an example.
At STR 1, the 'GATA' sequence is repeated between 5 and 16 times.

Humans have two copies of each chromosome, one from each biological parent, meaning they have two alleles for each STR region.
The term genotype refers to different combinations of alleles that a person could have.
Someone who is “9, 9” at STR 1 has inherited 9 GATA repeats from each parent.
Someone who is “5, 9” at STR 1 has inherited 5 GATA repeats from one parent and 9 from the other.
A person who is “16, 7” at STR 1 has inherited 16 GATA repeats from one parent and 7 from the other.

When we think of DNA, we often think of genes, which contain the code to make proteins.
However, the majority of the genome does not code for proteins; this is referred to as non-coding DNA.
Non-coding DNA can have roles like protecting the ends of chromosomes or regulating genes.
Scientists don’t always know the functions of some non-coding DNA.
Non-coding regions are more variable between individuals than coding regions.
For this reason, forensic scientists analyse non-coding regions of the genome when creating DNA fingerprints.

Understanding Short Tandem Repeats (STRs) is essential in genetics and forensic science.
Comparing STR genotypes allows investigators and researchers to match DNA samples accurately.
In forensics, this technique can link a suspect to a crime scene or confirm familial relationships.
By examining several STR regions and comparing the number of repeats at each site, scientists can determine whether two DNA samples came from the same person or identify a potential match from a database.