Understanding DNA Relationships: Cousins and Siblings Through Unique Ancestry

When exploring the intricacies of family relationships, understanding the level of DNA shared among family members can provide valuable insights. This article delves into the specific genetic connections between two individuals: a person and their first cousin where both the mothers are sisters and the fathers are also cousins. We will break down the precise calculations and explore the nuances of DNA sharing in these unique familial relationships.

Calculating DNA Relationships

First, it's important to understand the basic formula for calculating DNA sharing. The amount of DNA shared typically decreases with the number of steps between the shared common ancestor and the individuals in question. Each step halves the shared DNA percentage. In the case of first cousins, you share 12.5% of your DNA, which is about 15-16% with your unique family structure.

Shared DNA Calculation with First Cousins

If your first cousin is twice the usual relationship, the DNA shared increases. In this specific situation, you would share 25% of your DNA with your first cousin because of the unique combination of shared grandparents through your mothers and cousins through your fathers. Here's the detailed calculation:

Through the maternal side (your mothers are sisters): Your mother Your aunt Your first cousin Through the paternal side (your fathers are cousins): Your father Your grandparent (common ancestor) Your great aunt or uncle (great-grandparent's siblings) Your first cousin once removed Your second cousin

The total steps for the maternal side is 3, which results in a coefficient of relationship of 0.5^3 0.125 or 12.5% DNA. The total steps for the paternal side is 5, which results in a coefficient of relationship of 0.5^5 0.0313 or 3.13% DNA.

Add the two percentages together to determine the total DNA shared:

Total DNA shared: 12.5% (maternal side) 3.13% (paternal side) 15.63%.

Additional DNA Relationships

For half-siblings, the expected DNA shared is around 25%, considering there are 23,000 genes. In this specific case, you would also share with your half-siblings:

25% for half-siblings (common parent) 6.25% for half-first cousins (half-siblings and second cousins) 3.125% for second cousins (through the paternal side)

Adding these percentages together, you would be approximately 34.375% related to your half-siblings.

Genetic Mapping

The amount of centimorgans shared is an essential aspect of understanding genetic relationships. Based on the calculations, you can expect to share approximately 1063 centimorgans with your first cousin. Your genomes would contain 31.25% half-identical segments, sharing one allele, and 68.75% non-identical segments, sharing no allele.

Conclusion

Understanding the unique genetic relationships in your family can help you better comprehend your family tree and genetic makeup. In the complex web of familial relationships, such as where both parents are cousins, it's vital to consider multiple generations and calculate the total amount of shared DNA accurately.

This detailed analysis provides a clear understanding of how DNA is shared among close relatives, particularly in unique family structures.