BIo Tech

 
Just before the cell divides, the DNA of each chromosomereplicates(makes a copy of itself.)  Following this, the chromosomes coil up tightly which allows them to be sorted and moved to the new cells without tangling and breaking apart.  This sorting and moving process occurs before the cell divides and is calledmitosis.  For a brief discussion of mitosis, click here.
 
During this period of cell division, chromosomes appear as dense, bulky objects when the cell is viewed through a microscope. At very high  magnifications, they have the shape of a fuzzy, bulky X.  Each half of the X comprises one chromatid--an exact copy of the original chromosome.  The two chromatids (often referred to as "sister chromatids") are joined  together at a specific small region called thecentromere.  
 
Using a microscope, it is possible to count and characterize the individual chromosomes during the time they are coiled and condensed.  A photograph of the entire set of chromosomes can  be made. Then the images of the individual chromosome can be cut out and arranged by shape and size in an orderly arrangement called akaryotype(see below).  (This is a nightmare project to the untrained eye, a simulation of which is often assigned to 10th grade students as a learning exercise!) 

 
In most higher plants and animals, including humans, chromosomes from the body cells can be matched up in pairs.  The two chromosomes of a pair are calledhomologous chromosomes.  The members of most homologous pairsof chromosomes look alike.  They are the same length, their centromeres are in the same position, they show the same pattern of light and dark bands when stained, and they carry genes for the same inherited characteristics, line up on the chromosome in the same order.   
 

 
The occurrence of pairs of chromosomes in our karyotype is a result of our sexual origins.   We inherit one member of each chromosome pair from each parent.  So the 46 chromosomes in our somatic cells are actuallytwo sets of 23 chromosomes—a maternal set (from our mother) and a paternal set(from our father.)A cell with two of each kind of chromosome is called adiploid celland is said to contain a diploid, or 2n, number of chromosomes.   
 
In humans, the homologous pairsare defined and numbered andcarry the genes for the same trait in each person. For example, human chromosome #1 contains, along with many others, the genes for the Rh blood protein and for a starch-digesting enzyme in the saliva.  However, the corresponding genes on the two homologous chromosomes are not necessarily identical. For instance, some chromosomes have a gene for the protein that makes a person Rh-positive, and some have a gene coding for a different version of this protein (Rh-negative) at the Rh location.  Different versions of the same gene are referred to as alleles.  An individual with two genes the same for a trait is said to behomozygousfor that trait.  A person with two different alleles for the same trait is heterozygousfor that trait.

In human males, the partners of 22 of the pairs of chromosomes look similar, but the twenty-third pair is mismatched with two unlike chromosomes, called X and Y (see the far right chromosome pair in the bottom row of the male karyotype above.)  In the cells of a female, both members of homologous pair #23 are X chromosomes (far right pair of chromosomes in the bottom row in the female karyotype.) The X and Y chromosomes are called thesex chromosomes,because they differ between the sexes and because they carry the genes that determine the sex of the individual.  The other 22 chromosomes are calledautosomal chromosmesor simplyautosomes.

It is the Y chromosome that is of major interest to the genealogist because, as it is handed from father to son, virtually unchanged, it becomes a signature or fingerprint for the surname which is passed down in the same way in many cultures. For a chart demonstrating how the Y chromosome is passed through a family,click here.