THE X-CHROMOSOME, Y-CHROMOSOME AND MITOCHONDRIAL DNA
Humans have 23 pairs of chromosomes. 22 of these are equally matched. The exception, pair number 23, the sex chromosomes, are matched in the female (XX) and mismatched in the male (XY). We say mismatched because the Y-chromosome is shorter than its matching X-chromosome. The Y-chromosome has been called the male chromosome and the X-chromosome has been called the female chromosome. These names are not completely accurate descriptions but they have found their way into common usage. Because the Y-chromosome is shorter than the X-chromosome and is found only in males, some genes present on the mismatched X-chromosome are present without any matching alleles. Thus, genetic defects on the Y-chromosome affect only males.
Despite the social importance (to the males in the population. of the Y-chromosome it seems to be a genetic wasteland. Other chromosomes typically have thousands of genes packed into their DNA. The Y-chromosome, to date, has been found to have only 20 genes. There may be a few more but the number is very small when compared to any other chromosome. It is an odd coincidence that the Y-chromosome which imparts "maleness" is barren of great amounts of genetic information. The genes that do exist on the Y-chromosome may be grouped into two categories. One group is called male specific genes or testis specific genes. These seem to switch a developing fetus from female to male. The others are called the housekeeping genes. This second group of genes seems to be involved in maintaining cellular function and these have counterparts on the X-chromosome.
One explanation of why the Y-chromosome is shorter than the X-chromosome involves the deletion of useless genes. Part of the normal genetic function of chromosomes is the exchange of genetic material. As the chromosomes align in pairs within cells, they often swap bits of DNA. This means that genes found on one chromosome find their way into others, usually their matching mate. Occasionally, bits of DNA and the associated genes travel from one chromosome pair to another. For example, a few genes from chromosome number 3 may work their way into chromosome number 20. The Y-chromosome, being the bearer of maleness does not completely combine with its paired X-chromosome. Perhaps many millions of years ago these two were very similar. But as time went by the Y-chromosome started not to recombine with the X-chromosome. Old habits are hard to break and we find that even today the ends of the Y-chromosome do indeed recombine with the ends of the X-chromosome in a region where each has the same few genes. By not recombining the Y-chromosome has isolated its genes. This means that genes which find their way onto the Y-chromosome never get much of a chance to pass their code on to the genome of any offspring. Over the millennia these extraneous genes have been cut from the Y-chromosome, leaving it much shorter than its X-chromosome counterpart.
Having a Y-chromosome that doesn't mingle its genes with an X-chromosome and having that same Y-chromosome determine maleness means that any genes on the Y-chromosome must be passed from father to son and only from father to son. Testing this mechanism is not as easy as it seems. Because most humans are very similar in their genetic makeup and most physical characteristics are determined by genes on chromosomes other that the Y-chromosome. However, a verification of this mechanism has been found and reported in the popular press (Finding Genetic Traces of Jewish Priesthood, THE NEW YORK TIMES, January 17, 1997.. This example forms an interesting coincidence of science and religion.
There is a Jewish tradition that holds that the descendants of Aaron, the older brother of Moses, are priests. Priests are different from rabbis and are the only ones permitted to perform certain blessings and rituals. The only way for a man to become a Jewish priest is to be born the son of one and then to take up the mantle of the title. It is not an office that can be granted by the temple or a congregation. Surnames of Cohen, Kohen, Kahane and Kahn, which mean 'the priest' are common among the priestly class. As you can imagine after many generations some men would still hold the title of priest and others would change their beliefs and religious practices. Many no longer practice the priesthood yet their family oral tradition tells them that they are descended from priests. Researchers tested the genetic markers of 188 Jewish men from several continents. They also asked these men if they were descendants of priests. They found a striking correlation of genetic markers that distinguished those descendants of priests from Jewish laymen. The simplest explanation for their finding is that the men descended from Jewish priests may be considered very distant cousins all descended from one man. That is they have the Y-chromosome of Aaron. This is an example of where scientific method and the cultural history give the same results and tell the same story.
To find genetic markers on the female chromosome that might have been passed from mother to daughter is not such an easy task. Because the X-chromosome does swap bits of DNA with its paired X-chromosome, we can never be sure that the markers we examine come from the mother's side or the father's side. However, there is a way to study the female to female lineage using biotechnology. During conception an ovum containing the X-chromosome and cytoplasm is joined by a tiny sperm containing almost no cytoplasm. This means that the cytoplasm for a fertilized egg comes exclusively from the mother. Cytoplasm of mammalian cells contains mitochondria. Mitochondria are the sites of cellular respiration. They provide the biochemical powerhouses that keep our cells functioning. The structure and function of mitochondria it spelled out by a single loop of DNA found within each mitochondrion. Appropriately this DNA is called mitochondrial DNA (mtDNA..
There is some debate as to whether we should consider this mitochondrial DNA (mtDNA. as part of the human genome. This mtDNA is vital to the function of all our cells. Without this information our cells could not sustain life. Even small mutations in the sequences of base pairs that make up the mtDNA can cause disorders of the metabolism.
The mtDNA is passed from a mother to her offspring, through the cytoplasm of the egg. mtDNA is passed to all of the offspring, male and female. The male offspring, however, can not pass this mtDNA on to their children because they simply do not transfer any cytoplasm to a fertilized egg. The female offspring will and do transfer the original mtDNA to each of their subsequent offspring. Thus, any mtDNA found in any person must have originated from that person's mother. More than originating from the mother, this mtDNA must be an exact copy of the mother's mtDNA.
By examining and studying mutations and common sequences found within mtDNA from different parts of the world, we can tell, with reasonable certainty, which population is descended from any other. Several such studies designed to correlate the origin of mtDNA were carried out and all came to the same conclusion. It is romantically called the Eve Hypothesis. It claims that all of us descend from some common ancestor female (Eve. from somewhere in central Africa. Needless to say this result, as the history, beliefs, and scientific data.