THEORY

• Jurisdiction: Australia

DNA structure ................................................................................................ [82.300]

DNA inheritance ............................................................................................ [82.320]

Laws of genetic inheritance

Introduction ................................................................................................... [82.400]

Mendel's first law - The principle of segregation of alleles ......................... [82.420]

Mendel's second law - The principle of independent assortment ............... [82.440]

Exceptions to Mendelian inheritance ............................................................ [82.460]

Genetic variation between individuals .......................................................... [82.480]

Mutations ....................................................................................................... [82.500]

[82.300] DNA structure

The human body is made up of small building blocks known as cells. There are different cell types that carry out various functions throughout the body; for example, muscle cells control movement and red blood cells carry oxygen between the lungs and other cells in the body. Within cells organelles are present that carry out the biological processes required for life. Two organelles of particular interest to forensic biology are the nucleus (a cell core that holds the nuclear DNA) and mitochondria (used in the production of "energy" for the body). Both of these organelles contain their own set of DNA, which can be the target for DNA analysis.

There are hundreds to thousands of copies of mitochondrial DNA in a human cell, and each contains approximately 16,500 base pairs (pieces of information) in a circular configuration. In each cell, there is a single copy of nuclear DNA with approximately 3.2 billion basepairs that are configured in 23 chromosome pairs. Of these 23 chromosome pairs, 22 are not involved in the determination of gender and are called autosomal chromosomes. The final chromosome pair is sex-determining and is typically denoted as "XX" for females or "XY" for males.

Cells within humans are "diploid", which means that there are two copies of each chromosome making up the chromosome pair. During cell division the chromosomes separate from their partner and their information is copied. The now two complete copies of the nuclear DNA are then divided between two "new" cells.

During reproduction single copies of a chromosome from the chromosome pair are passed from the male (in sperm) to the female (in the ovum). These single copy cells are called "haploid" and are known as gametes. They join to form a single diploid cell, which is now a combination of the DNA from mother and father, called a zygote, which is the start of a new

child.

Not all cells within humans contain a nucleus, and, therefore, nuclear DNA. Red blood cells are an example of a cell type that does not contain nuclear DNA. Nuclear DNA that is derived from blood is obtained from the white blood cells in the sample.

DNA molecules store information in much the same way that text on a page in a book conveys information through the order of letters, words and paragraphs. Information in DNA is stored based on the order of nucleotides, genes and chromosomes. Table 1 provides a simple comparison of how information is stored in both printed text and genetic formats.

Table 1: Comparison of printed and genetic information

Printed information Genetic information
Letter	DNA nucleotides
Word	Short DNA sequence
Page	Locus or gene
Chapter	Chromosome
Section	Nucleus
Book	Cell
Library	Body (tissue)

In the library of life, there are approximately three billion letters spread across 46 chromosomal chapters in each human cell that store genetic information for two primary reasons:

(1) so that the DNA molecules can be replicated and passed on as cells divide (ie, new books are written) and
(2) so that cellular machinery known as proteins can be created to provide the building blocks of life (ie, information from the book is put to use).

Information encoded within the DNA structure itself is passed on from generation to generation, with typically one half of a person's DNA information (ie, 23 chapters) coming from their mother and one-half (ie, 23 chapters) coming from their father. This passing of genetic information to the next generation is what enables parentage and other forms of kinship testing to be performed.

The smallest unit in the genetic library is the nucleotide. The nucleotide unit is composed of three parts: a ribose sugar molecule, a phosphate group and the so-called "bases". DNA "bases" come in four flavours: adenine (A), thymine (T), guanine (G) and cytosine (C). Thus, the genetic alphabet only has four letters (A, T, C and G) to convey information rather than the 26 letters (A through Z) used in the English language. However, since...