2.3.2 Why are boys more vulnerable to some conditions?

In some conditions that affect more males than females (such as colourblindness), the explanation has been found to lie in genes on the X chromosome. Most females have two X chromosomes (one inherited from each parent) while most males have an XY combination. This means that if someone should inherit an X-linked gene predisposing to a particular condition, compensation for this will be easier for a female (whose other X chromosome may have a ‘normal’ copy of the gene) than for a male. However, as yet no X-linked genes have been identified in connection with dyslexia, dyspraxia, ADHD or autism. As we shall see later, a combination of many different genes is probably involved in the predisposition to these conditions, and environmental factors are also crucial, because genes alone do not dictate outcome. What the evidence from family studies does suggest is that females may need a higher ‘genetic loading’ (i.e. a stronger ‘family history’) than males for these conditions to be ‘expressed’. Some protective factors therefore seem to be operating in females, but we do not yet know what these are.