The Blaker Society DNA Project

DNA 101.. Y-Chromosome Testing

DNA 101.. is the first of Two articles that attempt to take the extremely complex and confusing subject of Genetics and DNA and simplify it into layman terms. These articles addresses DNA only as it applies to Y-Chromosome testing and genealogy. Technical terms are defined in this same context.


Deoxyribonucleic acid (DNA) is the chemical inside the nucleus of all cells that carries the genetic instructions for making living organisms. A DNA molecule consists of two strands that wrap around each other to resemble a twisted ladder. The sides are made of sugar and phosphate molecules. The “rungs” are made of nitrogen-containing chemicals called bases. Each strand is composed of one sugar molecule, one phosphate molecule, and a base. Four different bases are present in DNA - adenine (A), thymine (T), cytosine (C), and guanine (G). The particular order of the bases arranged along the sugar - phosphate backbone is called the DNA sequence; the sequence specifies the exact genetic instructions required to create a particular organism with its own unique traits.

Each strand of the DNA molecule is held together at its base by a weak bond. The four bases pair in a set manner: Adenine (A) pairs with thymine (T), while cytosine (C) pairs with guanine (G). These pairs of bases are known as Base Pairs (bp).

These Base Pairs (bp) are the basis of Y-chromosome testing.


Chromosomes are paired threadlike "packages" of long segments of DNA contained within the nucleus of each cell. In humans there are 23 pairs of chromosomes. In 22 pairs, one chromosome is received from the individual's mother, the other from the father. The 23rd pair is different. In females this pair has two like chromosomes called "X". In males it comprises one "X" and one "Y," two very dissimilar chromosomes. It is these chromosome differences which determine sex.

The Y-Chromosome

Human sex is determined by the X and Y chromosomes. A female has 2 X-Chromosomes and a male has an X and a Y-Chromosome. When a child is conceived it gets one chromosome from its mother and one chromosome from its father. The chromosome from the mother will always be an X, but the chromosome from the father may be either X or Y. If the child gets the X she will be a girl, if the child gets the Y he will be a boy.

This Y-Chromosome has certain unique features:

The presence of a Y-Chromosome causes maleness. This little chromosome, about 2% of a father's genetic contribution to his sons, programs the early embryo to develop as a male.

It is transmitted from fathers only to their sons.

Most of the Y-Chromosome is inherited as an integral unit passed without alteration from father to sons, and to their sons, and so on, unaffected by exchange or any other influence of the X-Chromosome that came from the mother. It is the only nuclear chromosome that escapes the continual reshuffling of parental genes during the process of sex cell production.

It is these unique features that make the Y-Chromosome useful to genealogists.

Testing the Y-Chromosome

The Y-Chromosome has definable segments of DNA with known genetic characteristics. These segments are known as Markers. These markers occur at an identifiable physical location on a chromosome known as a Locus. Each marker is designated by a number (known as DYS#), according to international conventions. You will often find the terms Marker and Locus used interchangeably, but technically the Marker is what is tested and the Locus is where the marker is located on the chromosome.

Although there are several types of markers used in DNA studies, the Y-Chromosome test uses only one type. The marker used is called a Short Tandem Repeat (STR). STRs are short sequences of DNA, (usually 2, 3, 4, or 5 base pairs long), that are repeated numerous times in a head-tail manner. The 16 base pair sequence of "gatagatagatagata" would represent 4 repeats of the sequence "gata". Differing numbers of repeats at a given locus are referred to as alleles. The variation of the number of repeats of each marker enables discrimination between individuals.

Reading the Test Results

The table below is a shorten version of the actual table used to show our DNA test results. It shows some of the 25 markers that most of the participants had tested. Table2

The colomns across the top of the table are the Locus. The second set of numbers across the top of the matrix are DYS# (the actual marker names).

The numbers down the left side of the table identify the participant in the DNA project.

The numbers under each marker represent the number or repeats for each participant at that specific marker.