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In the diagrams below, D or d represents the defective gene, and N or n represents the normal gene. A mutation does not always result in disease.
Dominant Disorders:
One of the parents has a single defective gene, which dominates it’s normal counterpart. Since offspring inherit half of their genetic material from each parent, there is a 50% risk of inheriting the faulty gene, and therefore the disorder.
Recessive Disorders:
Both parents carry a single defective gene, but also carry a normal gene counterpart. Two defective copies of the gene are necessary to cause inheritance of the disease. Each offspring has a 50% chance of being a carrier, and a 25% chance of inheriting the disorder.
X-Linked Disorders:
Normal females have are XX, and normal males are XY. Women who have a normal gene on one of their X chromosomes are protected from the defective gene on their other X chromosome. However, males lack this protection due to the presence of only one X chromosome. Each male offspring of a mother who carries a defect has a 50% chance of inheriting the defective gene and the disorder. Each female offspring has a 50% chance of being a carrier like her mother. (in the diagram below the X represents the normal gene and the X represents the defective gene)
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