2.1 Genetics to Genomics
It has been more than 135 years since Gregor Mendel observed that several distinct traits of peas were inherited at statistical rates predicted by the traits of the parents. However it was not until 1944 that inherited traits and deoxyribonucleic acid were linked. DNA contains the biochemical codes for the inheritance that Mendel observed. The DNA associated with a specific trait or function is known as a gene. The entire set of information represented in the DNA is known as the genome. This combines the word âgeneâ with the suffix âomeâ for mass.
DNA is a macromolecule built from repeating subunits (see Fig. 2.1). Each of the subunits contains one of four bases. The âsizeâ of a genome is usually expressed as the number of base pairs (bp) of double-stranded DNA (dsDNA) in an organism. Because the base pairs of dsDNA can be generated from the bases of either of the complementary pieces of single-stranded DNA (ssDNA), the âsizeâ of the genome may also be expressed as the number of bases (b). For instance, the human genome contains about 3 billion base pairs of DNA. Convenient units are thousands of bases (kb) and millions of bases (Mb). Ribonucleic acid is a macromolecule similar to DNA that is also measured in units of bases.
In humans, our DNA is packaged in 24 linear macromolecules of double-stranded DNA and protein known as chromosomes. The chromosomes are usually distributed spatially in the nucleus, but are often referred to as âpairsâ because they physically arrange in pairs during cell division. Each parent contributes to one of the chromosomes in each pair of the child's genome. The different human chromosomes are designated by 1, 2, 3, &, 21, 22, X, and Y. Table 1.2 lists the size in DNA base pairs of the 24 human chromosomes. In normal cells, there are two copies of the numbered chromosomes and these are called the autosomes. The autosomes pair by indicesâa chromosome 19 from the mother pairs with a chromosome 19 from the father. The X- and Y-chromosomes determine sex with the pairs X-to-X and X-to-Y, resulting in female and male progeny, respectively.