BACKGROUND SCIENCE

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.



    Double Helix


  • 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 700,000 markers.

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