'Only slight risk' in cousins marrying
A Christchurch geneticist says the risk of defects in the children of cousins who marry is only slight.
Controversy is mounting as Tonga prepares for the marriage of two royal grandchildren to each other.
The No 1 in line for the Tongan throne, Crown Prince Tupouto'a 'Ukukalala, 27, will marry his first cousin, 25-year-old Sinaitakala Fakafanua, 26th in line.
Tonga has declared a national holiday for the lavish wedding on Thursday, but many have expressed concerns over the wedding, including some in the royal family.
A source in the palace household told Fairfax Media that the new king, Tupou VI, opposed the marriage because the relationship was too close.
This year, Princess Frederica Tuita, ninth in line to the throne, attacked the marriage and said the royal system of arranged marriages ''was extremely arrogant and only perpetuated the motive behind social climbers''.
Auckland community leader Will Ilolahia told Radio New Zealand he was worried about the dangers of such close relatives marrying.
However, Christchurch geneticist Dr Alexa Kidd said it was not a ''huge risk''.
Marrying a first cousin only slightly increased a person's chances of inheriting genetic disorders, such as cystic fibrosis or Down syndrome, she said.
''There is a slight risk, but it's not huge - that's why it's legal. Genes come in pairs and marrying a first cousin slightly increases the chances of inheriting rare recessive genes from the family, compared with those who aren't related.''
She said an estimate showed that the statistical risk of having a child with a serious genetic disorders for a couple that were not related was 2 to 2.5 per cent. In first cousins, this rose to 4 to 4.5 per cent.
In spite of the slight risk, Kidd was surprised there had been such a furore over the Tongan royal marriage.
''Many, many first cousins do marry and have successful relationships and families, including those from other royal families. In some cultures it is very common to marry cousins to keep wealth in the family,'' she said.
"However, from a genetic point of view, if you did have to marry within the family it would probably be better to marry a second or third cousin, where there isn't that risk.''
Kidd said first cousins who planned to marry should visit a doctor.
''If two first cousins are in love and want to pursue a relationship I would recommend they see a geneticist and get a full family history mapped to see if there are any genetic disorders that are common,'' she said.