Lesson 1: Predisposition, Biological Factors, and the Brain
Genetics
Each of the four main categories of biological factors encompass a number of conditions that affect the quality and function of our bodies. They are not always independent of each other, and they occur in various combinations throughout the general population. The first category, genetics, reminds us that no two people have exactly the same genes - the exception being identical twins. Genes are the long molecules of deoxyribonucleic acid (DNA) that occur at various locations on chromosomes. Each individual has 23 pairs of chromosomes, and each chromosome usually contains over 60 000 genes that contribute to the physical and psychological distinctiveness of individuals. When a baby is conceived, he or she receives 23 chromosomes from the mother and 23 chromosomes from the father. This means, for each characteristic or trait an individual has, there are two sets of instructions. These individual sets of instructions for the same trait are called alleles.
The instructions in the alleles from each parent can be the same or different. For example, one parent may have straight hair and the other parent may have curly hair. The expression of these competing genes is determined by whether the genes or alleles are dominant or recessive. Some genes, like those for curly hair, are described as dominant. When linked with other instructions, they are expressed and the recessive genes are suppressed. A recessive gene (e.g., straight hair) is expressed only when matched with another recessive gene with identical instructions. Biologically speaking, the development of most mental illnesses is associated with recessive genes. If mental illness were the result of dominant genes, the expression in each generation would be practically guaranteed, resulting in a continuing disadvantage.
Many disorders are thought to be the result of a combination of genes instead of a single one. Problems may also arise if a gene is absent rather than present. An important point is that having a gene that has been linked to an illness this does not mean you will certainly acquire the illness. The opposite is also true. If a person does not possess the gene that has been linked to an illness, that person may not be safe from getting the disease. Mental illness is often far too complicated for simple links and associations! Behaviour, unlike physical characteristics (e.g., straight hair versus curly hair), is the result of the interaction among genes, hormones, and the environment.
Problems with entire chromosomes can also result in abnormalities. Humans have 46 chromosomes in all cells except the reproductive (sex) cells. These 46 chromosomes are divided into 23 pairs. Twenty-two of these pairs are called autosomes; they determine general structural and physiological characteristics. The remaining pair of chromosomes are the sex chromosomes. In human females, both of the sex chromosomes are designated X, while in the male, one chromosome is an X and the other is a Y. With each gender, male or female, the mother donates an X to her offspring. The other chromosome comes from the father. It can be either an X or a Y. If it is an X, the offspring is female. If the chromosome is a Y, the offspring is a male. In humans, the male determines the gender of the offspring.
