DNA
DNA is a molecule that encodes genetic information. It is a long, coiled, double-stranded chain of interlocking base pairs called a double helix. There are four types of bases in DNA: A (adenine), T (thymine), G (guanine), and C (cytosine). The order of the bases in a DNA strand, called the sequence, creates a code for information: the DNA code 'ATC' has a different meaning than the code 'TCA,' and so on. Each cell is equipped with special machinery used to read the sequence and use the information encoded. All the genetic information in an organism is referred to collectively as a 'genome.' One copy of the human genome is about 3 billion bases long.

Genes
A gene is a section of the DNA strand that carries the instructions for a specific function. For example, the 'globin' genes contain instructions for making the hemoglobin protein, which is the protein that allows our blood to carry oxygen throughout the body. Humans have about 50,000 different genes, which work together in complex ways to control much of what our bodies do. While we all have the same genes, there are different versions of many genes, called alleles. For example, while most people have genes, which give them, pigmented (coloured) eyes, there are multiple alleles for specific eye colors. Each person has particular combination of alleles for eye color, for hair color, etc., which makes him or her genetically unique.



Chromosomes
The 3 million bases of the human genome are not all in one continuous strand of DNA. Rather, the human genome is divided into 23 separate pieces of DNA, called chromosomes. Chromosomes are strands of DNA bundled together by proteins. Humans have 22 numbered chromosomes (also called autosomes, and conveniently named 1 to 22) and the X and Y sex chromosomes. A typical cell has 2 copies of each of the numbered chromosomes, one from the mother and one from the father, and two sex chromosomes. Females have two X chromosomes, while males have an X and a Y. This results in a total of 46 chromosomes in each cell.

The Function of Genes
Cells - Cells are the building blocks which make up the tissues of the body. For example, skin is a multilayered sheet of cells, and blood consists of a variety of cells floating in liquid. Inside each cell are the genes which give the cell information about how it is supposed to function. The chromosomes reside in a compartment of each cell called the nucleus.

RNA
Because DNA is stored in the nucleus of the cell, the first step in interpreting the information encoded by a gene is to make a copy of the information that can travel outside the nucleus. The process of making this copy is called transcription. The copy is called a 'messenger RNA' or 'mRNA.' It is called a messenger, because it carries the information in the gene to other parts of the cell, and RNA is an information encoding molecule that is very similar to DNA, except that it is a single strand, and it uses a base called uracil (U) in the place of the thymine (T) in DNA.

Proteins In the case of a few genes, the RNA copy of the gene has a function on its own, usually by folding into a distinctive shape. However, the majority of genes encode instructions for making specific proteins. Proteins are complex chemicals which perform most of the work in a cell (and therefore most of the work in the body). For example, proteins called enzymes direct the building of cell structures, allow our bodies to get energy from the food we eat, and perform many other functions. To make the protein encoded by a gene, the mRNA attaches to special machinery in the cell called a ribosome that can read the genetic code and make a protein, a process called translation.

DNA is being used increasingly as legal evidence, such as in cases of paternity issues. These applications exploit the fact that, with the exception of identical twins, each person is genetically unique. 99.9% of the 3 billion bases of the human genome are the same from one person to the next, giving us all the characteristic structures and behaviors that make us human. The remaining 0.1%, or 1 in 1000 bases, can vary from person to person.



Important definitions
  • Polymorphism - The prefix 'poly' means 'many' and the root 'morph' means 'forms' or 'shapes'. A polymorphism is a place in the human genome where a base can take on multiple forms. For example, a certain base may be A in one person and C in another. The length of some sections of DNA are also polymorphic. For example a particular segment may be 12 bases long in some people and 21 bases in others. The multiple versions of some genes, called alleles, are an example of polymorphisms.
  • Locus - A 'locus' is a unique location on the DNA strand; a way of specifying a base or section of the genome. For example, a gene, which is a section of DNA encoding information for a particular function, can be called a locus. A locus can also refer to a section of DNA which does not have a known function.
  • Genotype - Refers to the distinctive allele or combination of alleles in an individual.

    • When there is a question of whether an individual is the parent of a child, it is possible to use genotyping technology to determine the likelihood of the relationship. Usually, the parent in question is the father, and so the term 'paternity test' is used here, but the same principles would apply to determining maternity.

    As described in the section on 'How genes are inherited', each human has two copies of every chromosome, one from the mother and one from the father. Therefore, for any polymorphic locus, one of the alleles is from the mother and one is from the father. In a paternity test, the child and potential father are genotyped at a number of polymorphic loci. They must have at least one matching allele at all the sites for paternity to be possible. When this is the case, the probability of non-paternity can be calculated by multiplying the allele frequency for the shared allele. For a certain locus, both alleles may match, and it may not be possible to tell which was inherited from the potential parent (although sometimes information from a second, known, parent, may help). If this is the case, that locus cannot be used in the calculation.

    Many genetic conditions and traits are inherited in an autosomal recessive pattern. One example is cystic fibrosis, a condition that primarily involves the lungs (thick mucus makes the lungs susceptible to infection) and the digestive system (enzymes needed to break down fatty foods do not work properly). As discussed in the section on meiosis and mitosis, we have two copies of each gene, one which comes from the mother in her egg and one which comes from the father in his sperm. Similarly, when we have children, we pass on one copy of each of our genes.