A group of Dutch people born through IVF treatment petitioned a court on Friday seeking DNA tests on the late head of a sperm clinic to determine if he was their father. The 23 parents and children claim that Jan Karbaat, who died last month, may have used his own sperm instead of that of the chosen donor at the fertility clinic he ran close to the port city of Rotterdam. "It's a fundamental right to know where one came from," the lawyer for the families, Tim Bueters, told the Rotterdam court. "It's a question of identity, it helps someone to form their personality," he said, asking the court to order DNA tests to prove whether Karbaat was the father. Karbaat reportedly admitted to having fathered about 60 children in his time at the clinic, which closed in 2009 amid reports of irregularities. Bueters highlighted some inconsistencies, such as that one of his clients had brown eyes when the sperm donor was supposedly blue-eyed, or that another male client physically resembled the doctor. But Lisette de Haan, the lawyer for Karbaat's family, shot back: "There is not the slightest evidence that Mr Karbaat was the donor." She also urged the court to respect the Karbaat family's right to privacy. Karbaat, who billed himself as "a pioneer in the field of fertilisation", even asked in his will that no DNA tests be carried out after his death. But court officials at the request of the families seized some personal objects such as a toothbrush from his home on May 2. DNA tests on these objects would be the preferred option, but the court could also order a test on one of his legitimate children or even call for the body to be exhumed. The court will return its decision on June 2. "As a mother, this judgement won't give me anything," said Esther Heij, one of the plaintiffs, after the hearing. "But I see at home how my son's life has been affected. He was so angry when Karbaat died, and that he was taking this to his grave." On paper, her son and daughter were conceived thanks to the same sperm donor. "Tests are underway, but it's not clear if they really are brother and sister. My daughter looks like Karbaat... tall and thin." Moniek Wassenaar, 36, who also attended the hearing, met the head of the sperm bank in 2010. She recently recounted to Dutch media how he had told her "it was possible I was his biological child." He said he was proud of what he had done, saying "he was in good health and intelligent, so he could share some of his genes with the world. He saw it as something noble. He had no concept of ethics and minimised the impact on the children," she added. Karbaat allegedly falsified his clinic's data, analyses and donor descriptions and exceeded the permitted number of six children per donor.