This page on the science behind DNA testing is the webmaster's
interpretation of a section of Professor David Sadava's
course, "Understanding genetics: DNA, genes and their real-world application," produced for
the Great Courses in 2008.
- Genetics is the science of heredity. It has made enormous advances during the
past 50 years.
- All life on earth is composed of cells. All cells from living beings contain special molecules
which are called chromosomes. Chromosomes are made up of pairs of genes in the shape of the double helix.
Genes, themselves, are made up of millions of base pairs. Thus, chromosomes contain the complete
genetic information (DNA) for every living being.
- The dominant gene in a pair of genes is expressed as protein or phenotype. A phenotype is what
gives a living being its structure and all its other characteristics.
- Mutations occasionally occur in genes
and may be passed down to the next generation. This is known as evolution.
- Humans (like all other mammals) have 23 chromosomes and about 24,000 genes. Unlike the rest of
the chromosomes, the pair of chromosomes known as the X chromosome and the Y chromosome do not
match. Women have two X chromosomes. Men have an X chromosome and a Y chromosome.
The Y chromosome determines male sex and has very few genes.
By 2003, scientists had been able to sequence the entire human genome ( 24,000 genes). Only
about 2% of the genes are actually expressed as a phenotype. Over half of the remaining genes
are noncoding repeated sequences.
About 99% of the human genome is the same for all people.
Even so, because there are billions of base pairs,
everyone, except identical twins, triplets, etc., is genetically unique.
The way genes appear in sequence in the chromosome can be studied. Short tandem repeats (STR)
are DNA sequences a few base pairs long which are repeated side by side in a pattern that is inherited.
There are about 10,000 STRs scattered throughout the human genome. Only a small number of known STRs
are used in DNA identification.
The Y chromosome is inherited through males and has over 30 regions that are nonselected
and mutate at a constant rate. If we compare males around the world with one another we can
determine when they had a common ancestor. Using gene sequencing, scientists have determined
that the original common ancestor of all human males lived in Africa about 70,000 years ago.
Using genetic sequencing, scientists can also identify other more recent common ancestors.
For example, there is a unique set of STRs which is only found in the chiefly line of the Grants. People
with the surname "Grant" who do not have this set of STRs are not related to the Chief. They
are probably clansmen who adopted the surname Grant in the 16th or 17th century out of loyalty
to their laird at that time.
Scientists working on the
Grant DNA Project use genetic markers (STRs) to determine if and how men with the surname
"Grant" are related to each other.
DNA in the mitochondrion, a part of the cell outside the nucleus, that is passed on
through females (NOT males), can also be used as a molecular clock. This method is not currently
used by the
Grant DNA Project.
Science is advancing so fast, that, according to Paul Jones of Canada's History
Magazine (February-March 2013), it is now possible to have a DNA testing service examine
Please post a comment
or send information to the editors.
Back to DNA Testing.