What happened to the mother? I've had gerbils as well - two pairs. One sand colored and one black and white pair (all of them males). We got the second pair when one in the old pair (Torticollix, because he had a
twisted neck for some reason) died. The other one, who was gaunt and mangy and looked like the kind of person who yells "get off my lawn!" didn't want any companionship and bit the white one rather severely. So we put him in an acquarium of his own, where he lived 'til old age (3 years) and got gaunter and mangier and crotchier by the month. He also got stone deaf, so when the dog was barking like mad at him, he wouldn't notice it if he had his back turned.
The other two lived a more normal gerbal life and passed on more or less simultaneously at the age of 2.