How are first cousins different from double cousins?

 

You and your sibling share 50% DNA of your mother and father. However, the DNA distribution is not the same in both of you. The distribution of DNA is random. Some of it overlaps, and some are different for both of you. 

Both of your kids will be called first cousins.

Suppose you two get married to individuals belonging to different families, then your kids will be first cousins to each other. They both will share their maternal family and common maternal grandparents. As a result, they will share 12.5% of your parent’s DNA.

Suppose you and your sibling marry siblings that belong to another family, then both of your children would be called double first cousins. They share both sides of the family, paternal and maternal, in the family tree and have two sets of grandparents. Offspring of double first cousins are called double second cousins.

Now you must be wondering why label them differently?

It is all about the distribution of DNA. Your children are genetically much closer to each other than usual first cousins. Measurement of how much individuals are related to each other is called consanguinity. Double first cousins have twice the consanguinity of ordinary cousins because they share ancestors from both sides, paternal and maternal. 

They share 25% of DNA, which is double what first cousins usually share, 12.5%. This is the same amount of DNA that they will share with their grandparents, half-sibling, and an uncle or aunt. Thus, this explains the term Double first cousins. 

Can double first cousins marry each other?

Biologically, it is not advisable because the marriage of first cousins is a high risk for their offspring. It can lead to an increased risk of recessive genetic disorders. It is the same for double first cousins. 

Suppose if mother and father belong to the same community and are cousins. Unfortunately, the mother has a defective gene that can be a cause of illness for the child. Since the father is also part of the same community, it is highly possible that he also has the same defect. In such a scenario, the child gets the defective gene.  

However, if the father comes from a different community, then the child gets a large pool of genes, and the chance of inheriting the problem decreases.

By law, it is prohibited in most of the states in the US. However, a few states still allow it under special circumstances. 

Source: https://ancestryactivatekit.com/blog/how-are-first-cousins-different-from-double-cousins/