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The Human Genome Project has taught us a great deal about the different types of variations that exist in our genomes and their distribution across human populations. Humans are 99.9% identical at the DNA level, but the corollary is that at 0.1% of the nucleotide positions, translating to about 4–5 million basepairs on average, we differ[1]. These genetic differences will vary in their effect on human traits according to their location with respect to genes and their impact on the downstream proteins that those genes encode.



The human genome encompasses all of the DNA in an individual. This includes not only the nuclear DNA, but mitochondrial DNA as well. Mitochondrial DNA, as the name implies, is found in the mitochondria of each cell, is inherited only from the mother, and is usually considered separately from nuclear DNA. Nuclear DNA exists in the cell's nucleus, and is therefore found in every cell in the body that has a nucleus (pretty much all cells except red blood cells) and in each of these cells, the DNA is for the most part identical.


The nuclear genome exists not as one long string of 3.2 billion nucleotides, but rather, it is broken into 23 discrete chunks, also known as chromosomes. Humans actually have two genomes, one from each parent, giving a total complement of 46 chromosomes, 22 pair of autosomes and one pair of sex chromosomes (X and Y). The size of DNA is measured in terms of the number of nucleotide base pairs (bp), or quantities of base pairs: kilobases (Kb = 1000 bp) and megabases (Mb = 106 bp).

The classic image of a karyotype (Fig. A1-1) depicts chromosomes as they appear during cell division, at which time the DNA condenses into a form called chromatin. Chromatin is the result of DNA wrapping around proteins, called histones. When in this form, chromosomes can be visualized under a light microscope. At other times, the DNA relaxes, unwinds from the histones, and reveals genes, regions of DNA that code for proteins. Genes in this state are accessible to cellular machinery that reads the DNA message and directs the production of proteins for the cell.

Figure A1-1.

Karyotype depicting chromosomes as they appear during cell division. A normal karyotype has 22 pair of autosomes and one pair of sex chromosomes. This karyotype has sex chromosomes X and Y, indicating a male.

Genes and protein synthesis

While the DNA in every cell of the body is virtually identical, the proteins produced in those cells are not. Each cell is unique and differentiated, based on the ...

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