Well, it turns out that Danaus had 50 daughters and Aegyptus had 50 sons and the two agreed to have the cousins marry each other. But Danaus had other plans and instructed all 50 of his daughters to kill their new husbands (sons of Aegyptus) on their wedding night. All but one daughter murdered their husbands. This daughter ended up marrying the son of Aegyptus and from this pair the dynasty came. Zeus had a child with Danae who was the famous Perseus known for killing and decapitating a woman warrior known as a Gorgon when she was sleeping. From Perseus’ bloodline, came the famous Heracles. Refer to Wikipedia for the family tree:
The other 49 daughters went on to marry other men. Some versions of mythology cite them as having to carry jugs of water in Hades. From Wikipedia:
When Aegyptus and his fifty sons arrived to take the Danaides, Danaus gave them, to spare the Argives the pain of a battle. However, he instructed his daughters to kill their husbands on their wedding night. Forty-nine followed through, and subsequently buried the heads of their bridegrooms in Lerna; but one, Hypermnestra, refused because her husband, Lynceus, honored her wish to remain a virgin. Danaus was angry with his disobedient daughter and threw her to the Argive courts. Aphrodite intervened and saved her. Lynceus and Hypermnestra then began a dynasty of Argive kings (theDanaid Dynasty). Some sources relate that Amymone, the “blameless” Danaid,and/or Bryce (Bebryce) also spared their husbands.
Aegyptus, after the death of his sons, escaped to Aroe in Greece and died there. His monument was shown in the temple of Serapis at Patrae.
In some versions, Lynceus later killed Danaus as revenge for the death of his brothers.
The remaining forty-nine Danaides had their grooms chosen by a common mythic competition: a foot-race was held and the order in which the potential Argive grooms finished decided their brides (compare the myth of Atalanta). Two of the grooms were Archander and Architeles, sons of Achaeus: they married Scaea and Automate respectively.
In later accounts, the Danaides were punished in Tartarus by being forced to carry water in a jug to fill a bath without bottom (or with a leak) and thereby wash off their sins, but the bath was never filled because the water was always leaking out.
In Greek mythology, Abas (Ancient Greek: Ἄβας) was the son of Lynceus of the royal family of Argos, and Hypermnestra, the last of the Danaides. Abas himself was the twelfth king of Argos. His name probably derives from a Semitic word for “father”. The name Abantiades (Ἀβαντιάδης) generally signified a descendant of this Abas, but was used especially to designate Perseus, the great-grandson of Abas, and Acrisius, a son of Abas. A female descendant of Abas, as Danaë, was called Abantias.