Types of twins

There are two different types of twins:

• monozygotic or identical (MZ)
• dizygotic, fraternal or non-identical (DZ)

MZ twins develop when one egg is fertilised by a single sperm and during the first two weeks after conception, the developing embryo splits into two. As a result, two, genetically identical babies develop.

DZ twins occur when two eggs are released at a single ovulation and are fertilised by two different sperm. These two fertilised eggs then implant independently in the uterus. DZ twins share the same type of genetic relationship as non-twin siblings, hence the term fraternal.

The ATR receives many queries from twins and their families interested in zygosity testing to determine whether a particular twin pair is identical (monozygotic or MZ) or non-identical (dizygotic or DZ). The two most popular methods of acquiring the DNA samples needed to determine zygosity (i.e. whether twins are identical or non-identical) are by collecting blood or cheek cell samples.