Because human beings have two kind of chromosomes, X chromosome and Y chromosome.Genes are only carried by X chromosome. Females have two X chromosomes. If one X chromosome has genes of hereditary disease and another X chromosome is normal. Normal one will "cover" defective chromosome. Disease symptom will not happen. But males have only one X chromosome and one Y chromosome. Y chromosome can't cover X chromosome. Disease symptom will happen. Therefore, Only females can carry genes of hereditary disease.
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