Chromosomes

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, both members are essentially identical, one deriving 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". These repeats are referred to as Allele. The variation of the number of repeats of each marker enables discrimination between individuals.

If you have...

=>DYS426=12 and have DYS392=11 then you are probably a member of haplogroup R1a1 (R-M17). About 5% of the men reading this will be members of this haplogroup.

=>If you have DYS426=12 and DON'T have DYS392=11 then you probably belong to haplogroup R1b (R-P25). About 2/3's of the men reading this will belong to R1b.

=>If you have DYS426=11 and DYS455=8 then you are probably a member of haplogroup I1a. About 20% of the men reading this will belong to I1a.

These three rules will give a quick estimate of Y haplogroup among men of European descent in about 9 out of 10 cases.

The importance of haplogroups for genealogists lies in the fact that you will share many marker values with anyone else who belongs to the same haplogroup. The tables below gives the modal haplotypes for the three most common haplogroups. Half of the men in R1b will match at 18 or more markers with the 25 marker modal R1b haplotype shown in the table below. Therefore the uniqueness of your paternal line's Y-chromosome signature is contained in the markers where you differ from your haplogroup's most common alleles. Having these differences scattered among 25 markers allows enough possible variations to make the Y-chromosome signature of each paternal line unique from most all other lines in a haplogroup.