Genes are organized into chromosomes in the cell nucleus. Genes controlling most characteristics come in pairs, 1 from the father, the other from the mother. Everyone has 22 pairs of chromosomes (called autosomes) bearing these paired genes, in addition to 2 sex chromosomes. Females have 2 X chromosomes; males have an X and a Y chromosome.
Most physical characteristics, many disorders, and some mental abilitiesand aspects of personality are inherited. The inheritance of normal traits and disorders can be divided into those controlled by a single pair of genes on the autosomal chromosomes (unifactorial inheritance, such as eye colour); those controlled by genes on the sex chromosomes (sex-linked inheritance, such as haemophilia); and those controlled by the combination of many genes (multifactorial inheritance, such as height).
Either of the pair of genes controlling a trait may take any of several forms, known as alleles. For example, the genes controlling eye colour exist as 2 main alleles, coding for blue and brown eye colour. The brown allele is dominant over blue in that it “masks” the blue allele, which is called recessive to the brown allele. Only 1 of the pair of genes controlling a trait is passed to a child from each parent. For example, someone with the brown/blue combination for eye colour has a 50 per cent chance of passing on the blue gene, and a 50 per cent chance of passing on the brown gene, to any child. This factor is combined with the gene coming from the other parent, according to dominant or recessive relationships, to determine the child’s eye colour. Certain genetic disorders are also inherited in a unifactorial manner (for example, cystic fibrosis and achondroplasia).
Sex-linked inheritance depends on the 2 sex chromosomes, X and Y. The most obvious example is gender. Male gender is determined by genes on the Y chromosome, which is present only in males. Any faults in a male’s genes on the X chromosome tend to be expressed outwardly because such a fault cannot be masked by the presence of a normal gene on a 2nd X chromosome (as it can in females). Faults in the genes of the X chromosome include those responsible for colour vision deficiency, haemophilia, and other sex-linked inherited disorders, which almost exclusively affect males.
Multifactorial inheritance, along with the effects of environment, may play a part in causing certain disorders, such as diabetes mellitus and neural tube defects.
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