2.19 Because mutations can affect the functioning and expression of the alleles of genes, resulting in particular traits or characteristics, it is possible to follow the pattern of inheritance of the different alleles of a gene in a family. For most genes, two copies are found in the one individual. If the two copies are the same allele, the individual is said to be homozygous. If there are two different alleles for that gene present, the individual is referred to as heterozygous for that gene. The exceptions to this are traits coded for by genes that are found on the X chromosome, which are discussed below.
Traits that follow a pattern of recessive inheritance
2.20 As noted above, autosomes are the chromosomes that do not determine sex—in humans, this means all of the chromosomes except for the X and Y. Everyone has two copies of the autosomes and therefore two copies of the genes carried on these chromosomes. A recessive trait is one that is expressed only if an individual is homozygous for a mutated copy of that gene—ie, he or she must have two copies of the mutated allele coding for it, one inherited from the mother and one from the father. Two parents who themselves do not express a particular trait nevertheless may have a child with the trait if each parent is a heterozygous carrier for the mutated allele—ie, each parent has one copy of the recessive mutated allele and one copy of a normally functioning allele. Where both parents are carriers, each child has a one-in-four chance of inheriting both abnormal alleles and so developing a clinical disorder (see Figure 7).
2.21 To provide a prosaic example, two brown-eyed parents with recessive alleles for blue eyes can produce a blue-eyed child. Other examples of autosomal recessive (AR) conditions include cystic fibrosis, haemochromatosis, b-thalassaemia, and Tay–Sachs disease (TSD) (see Table 2–1).
Traits that follow a pattern of dominant inheritance
2.22 A dominant trait is one that is manifest when a person has only one mutated allele in a particular gene pair. An affected person may have inherited the mutated allele from either parent or, as the result of a new mutation, may be the first person in the family to have it. The children of persons who have a dominant mutated allele have a one-in-two chance of inheriting that allele and trait (see Figure 8). Examples of autosomal dominant (AD) traits include HD, myotonic dystrophy, hereditary non-polyposis colorectal cancer, Marfan syndrome, familial adenomatous polyposis, and early onset familial Alzheimer’s disease (see Table 2–1, below).
Traits that follow a pattern of X-linked inheritance
2.23 X-linked traits are determined by genes found on the X chromosome. Since males have an X and a Y chromosome, they only have one copy of each of the genes found on the X chromosome and will always express a mutated copy of one of these genes (see Figure 9). Since a woman has two X chromosomes, having a recessive mutated allele on one X chromosome may not cause the trait to be expressed because she will have a normally functioning allele on the other X chromosome. X-linked conditions (XL) caused by recessive genes include haemophilia, Fragile X mental retardation and Duchenne muscular dystrophy (see Table 2–1).
 It should be noted, however, that the development of eye colour is complex and involves more than one gene. For a discussion of the genetics of hair colour, eye colour and other physical characteristics, see R van Oorschot and others, ‘Beyond DNA Databases: Physical Identification Using DNA’ (Paper presented at DNA Evidence: Prosecuting Under the Microscope International Conference, Adelaide, 10 September 2001).