Basic Genetic Mechanisms

In order to be able to give advice about the appearance of inherited disease in future generations, it is essential to have a basic knowledge of the mechanism of genetic transmission.

The nucleus of each cell in the body contains 46 chromosomes arranged as 23 pairs. The twenty-third pair comprises the sex chromosomes (the remainder being known as autosomes). These sex chromosomes are responsible for the transmission of sex characters but also carry a number of other genes unrelated to sex. In a woman, the sex chromosomes are the same length but in a man, one is shorter than the other. The shorter one is known as the "Y" chromosome and the longer one, which is the same as the female sex chromosome, is the "X" chromosome. When the sperm or ova are formed in the body, the pairs of chromosomes separate and the nuclei of the gametes (i.e., sperm or ova) contain only 23 chromosomes. When fertilisation occurs, the 23 chromosomes from each gamete reunite as pairs. Genetic material is thus equally provided from each parent. Genes are discoid elements arranged along the length of a chromosome and each one is known to bear special influence on the development of one or more individual characteristics. Genes are arranged in pairs on adjacent chromosomes. The two genes of the pair can be similar (homozygous) or different (heterozy gous). If different, one can exert an overriding influence and is said to be dominant. The gene that is overridden is said to be recessive.

Genetic disorders can be divided into three broad groups:

• abnormalities of chromosomes - numerical or structural

• abnormalities of individual genes, which are transmitted to offspring

• abnormalities involving the interplay of multiple genes and the environment.

Pathological genes can carry abnormalities, which are transmitted to the offspring in the same way as (other) normal characteristics. In a given individual, the abnormal gene can be recessive and masked by the other one of the pair. The individual would thus not appear to have the disease but could transmit it. There are also some other terms that are important when describing genetic abnormalities: penetrance refers to the proportion of individuals who carry the gene and who express the disease, while expressivity refers to the clinical spectrum of severity of a particular genetic condition. The four important patterns of inheritance are:

• autosomal recessive

• autosomal dominant

• sex-linked recessive

• mitochondrial inheritance.

75% apparently normal Figure 23.1. Recessive inheritance.!

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