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Basic Genetics
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Basic Genetics For The Family Historian

Elementary genetics provides us with the fact that the 23rd chromosome, in the human genome, is the chromosome that determines sex.  Males have both an "X" and a "Y" 23rd chromosome, but females only carry an "X" for their 23rd chromosome.

The human egg will becomes a female embryo if the male sperm that initially reaches the egg carries an X-chromosome.  The egg will becomes a male embryo if the male sperm that initially reaches the egg carries a Y-chromosome. Thus you can see the Y-chromosome is passed down from generation to generation only through the male line.  In order to better understand how we arrived at this point, we need to reach for the next level.

The complete set of instructions for making an organism is called its genome. Found in every nucleus of a personís many cells, the human genome consists of tightly coiled threads of deoxyribonucleic acid (DNA) and associated protein molecules, organized into structures called chromosomes.

In humans, as in other higher organisms, a DNA molecule consists of two strands that wrap around each other to resemble a twisted ladder whose sides, made of sugar and phosphate molecules, are connected by rungs of nitrogen, containing chemicals called bases. Each strand is a linear arrangement of repeating similar units called nucleotides, which are each composed of one sugar, one phosphate, and a nitrogenous 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. These sequences specify the exact genetic instructions required to create a particular organism with its own unique traits.

The two DNA strands are held together by weak bonds between the bases on each strand, forming base pairs (bp). Genome size is usually stated as the total number of base pairs. The human genome contains roughly 3 billion base pairs (bp).

These three billion base pairs (bp) in the human genome are organized into 24 distinct, physically separate microscopic units called chromosomes. All genes are arranged linearly along the chromosomes.  The nucleus of most human cells contains 2 sets of chromosomes. One set is provided by each parent. Each set has 23 single chromosomes; 22 autosomes and an X or Y sex chromosome. A normal female will have a pair of X chromosomes in this 23rd chromosome; a normal male will have an X and Y pair.

 If you wish more knowledge on this subject, a great site to gain more depth is the Primer On Molecular Genetics located at http://www.ornl.gov/hgmis/publicat/primer/intro.html

 This Page Was Last Updated:  06/07/2005