In the old times, didn't people marry their own siblings?
Yes, the ancient Egyptians would intermarry in order to preserve the 'purity' of the Divinity of Pharaoh's progeny. It was popular to breed with cousins, and siblings. Even in ancient Rome, it was rather popular to marry a cousin or even a sister; the Emperor Nero himself had sexual relations with his own mother and sister. Genetically, breeding with relatives are counter-productive; leading to an increase of homozygous genes; leading to the same allele at the same locus on both members of a chromosome pair and thus creating genetic deleterious defects. It manifests in neurological problems, cardiac and respiratory problems, and manifestation of impotence in male and female offsprings.
The 'purity' that was conceived in incestuous relationships within ancient kingdoms was in all-reality a self-indulged suicide on the said royal dynasty's line. Due to such practices, weak and incompetent rulers were made. Usurping power from an ambitious 'other' was the usual case. As in the case of the end of the old Caeserian line in Rome after the death of Claudius and Nero; and after the death of Rameses II the Great of Egypt, etc.
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