X-linked inheritance occurs when a recessive gene is carried on the X chromosome. Males are more likely to be affected, since the Y chromosome is much smaller than the X and may not have enough genetic information to override the information on the X chromosome, even though it is a recessive gene. Hemophilia is an example of an X-linked disease. Complex traits follow polygenic inheritance, in which many genes are involved in determining the characteristic. Examples of this type of inheritance are intelligence, height, and temperament.
Most chromosomal defects occur during meiosis.
Because these abnormalities involve more DNA than single-gene disorders, they often produce disorders with a multitude of mental and physical symptoms. Down syndrome results when the 21st pair of chromosomes contains extra genetic material. Symptoms include distinct physical features, mental retardation, speech problems, slow motor development, and an impaired immune system. The incidence of Down syndrome increases with maternal age. Sex chromosome disorders usually result in fewer problems than defects of the autosomes and are often not recognized until adolescence.