Why only males?

Most DNA testing done now for genealogical purposes is done on structures in male cells called Y-chromosomes.  The reasons for this are explained below.  The explanation may seem complicated if you do not remember your high school or college course's coverage of genetics.  The explanation may seem too simplistic if you are familiar with genetics.  Forgive me for the former, I am not worried about the latter.

Humans normally inherit 23 segments of DNA in units called chromosomes from each of their parents.  These 23 pairs of chromosomes include all of the instructions for producing the proteins that direct our development as individuals.  One of the pairs of chromosomes has been dubbed the twenty-third pair and one type of chromosome in that pair determines ones gender by its presence or absence.  The gender-determining chromosome is called the Y-chromosome and its presence in your cells will cause you to develop as a male.  A normal male has one Y chromosome and one X chromosome in most of their cells.  Normal females have two X chromosomes in most of their cells and no Y-chromosomes at all.  Since only males possess the Y chromosome then only males can pass them along to their offspring.  

The Y chromosome is somewhat unique in a couple of important respects, when compared with all of the other chromosomes.  First, the Y-chromosome seems to be important in heredity only for one biological reason, it has one or more genes that code for maleness.  Secondly, the Y-chromosome is the only nuclear chromosome that is passed along from generation to generation with almost no change.  All of the other nuclear chromosomes are "reshuffled" every generation due to natural recombination processes.  The genetic reshuffling is usually good for the long-term success of a species, although it makes it more difficult for researchers to follow DNA patterns from one generation to the next.  Most of the Y-chromosome is not involved in one of the most important recombination events, one called "crossing over."  Since crossovers happen only on a very small part of the Y-chromosome, then little or no DNA exchange happens with other chromosomes.  This lack of crossing over makes the Y-chromosome an almost ideal tool to use for following unchanged DNA sequences from generation to generation in families, at least from father to son and so on.

The Y-chromosome may be passed along in a family for hundreds or thousands of years with very few changes.  The changes that happen in the DNA that the chromosomes are made from are called mutations.  These mutations may be harmful if they occur in an "important" part of the DNA or may seem harmless if they occur in other parts of a chromosome.  Only about one percent of our DNA seems to be useful for coding for our proteins, the rest seems to be unused sequences, often left over from our distant ancestors.   All mutations are potentially useful to someone who is attempting to follow family lineages by examining DNA.  Mutations serve as "markers" that may be traced through time (and therefore geographically too) in families.  A mutation or marker that happens in DNA that may be passed along to offspring might be useful as a marker that can be found in all following generations.  It is these markers (mutations) that show who is related to whom, thus revealing kinship.

This male-only testing situation applies only to Y-chromosome DNA,  There is another kind of DNA in our cells that is found in small bodies or organelles called mitochondria.  All organisms except bacteria have mitochondria.  Mitochondria have their own DNA and those structures are passed along from mothers to all their children.  The mitochondrial DNA is usually referred to as mtDNA to distinguish it from DNA in a cell's nucleus, like the Y-chromosome, the X chromosome and the other 22 types of chromosomes in our nuclei.

Females are not excluded from Y-chromosome tracing in families.  Most females have a father, paternal grandfather, brother or same-surname cousin who may be able to have the family's Y-DNA analyzed by having their Y chromosome DNA tested.