Identical Twins and Cousin Marriage: The Intricacies of Genetic and Legal Relationships

When two sets of identical twins get married and have children, the legal and biological aspects of these relationships can become quite complex. While the children would be legally cousins, they would be biologically siblings. This article explores these intricate relationships and provides clarity on the genetic and legal distinctions involved.

Legal vs Biological Relationships

The legal definition of a sibling is based on the relationship recognized by law. In the case of two sets of identical twins who marry and have children, the children would be considered cousins in a legal sense. However, from a biological perspective, the children would be siblings, sharing 100% of their genetic material.

Biological Similarity and Genetic Relationship

Identical twins share 100% of their genetic material, meaning that each twin in one set is as genetically equivalent to the twins in the other set. Therefore, when these identical twins marry and have children, the resulting offspring would inherit the same genetic material from both parents, making them identical to each other in terms of genetics.

Why They Are Considered Siblings

The genetic relationship of the children in this scenario is so close that they would be more genetically similar to each other than typical cousins. Cousins generally share about 25% of their genetic material, while the children of identical twins would share 100% of their genetic material from their identical parent. This similarity can often lead to them being called 'full siblings' despite the legal term of 'cousins'.

Exceptional Cases: Non-Twin Siblings

The situation involving identical twins is no different from if two non-twin siblings married two other non-twin siblings. The children of siblings are considered first cousins, and this does not change if the parents involved are identical twins.

Fraternal Twins and Cousins

If two sets of fraternal twins marry and have children, the children born to each couple would be full siblings. The children from the two sets of parents would be cousins rather than siblings, as fraternal twins are not identical and do not share 100% of their genetic material.

Genetic and Legal Overlap

While the children would be biologically siblings, their legal relationship would be that of cousins. This situation is sometimes referred to as 'double cousins,' as both sets of children share both sets of grandparents. Genetically, however, they are equivalent to half-siblings, as the parents are not identical and thus have different biological relationships.

Historical and Modern Examples

There have been historical and modern cases where the genetic and legal relationships of these children have become contentious. For example, in Oregon, a woman filed a paternity case involving one of two identical twins. The legal confusion over who the biological father was led to complicated situations, such as potential equal division of paternity.

The children of identical twins, while biologically siblings, would be considered legal cousins, although these distinctions can lead to interesting and sometimes challenging legal scenarios. The genetic bond between them is so strong that it can be compared to that of full siblings, while the legal definition is based on the parental marriage.