An understanding of genealogical relationships is necessary before diving into genetically equivalent relationships in your family history. This article will provide an overview of both concepts.
Correctly evaluating shared DNA within the context of genetically equivalent relationships first requires mastery of genealogical relationships. Here we review important genealogical relationships based on some of the pertinent variables. For the sake of simplicity, we limit our analysis to biological relatives and exclude in-law and step relationships.
Immediate Family: These relationships are straightforward: father, mother, sister, brother, son and daughter.
Immediate Family of Ancestors: Your mother’s brother is your maternal uncle. Your father’s sister is your paternal aunt. Your sibling’s child is a niece or nephew. Considering the immediate family members of more distant generations gets more complicated: Your grandfather’s sister is a grand-aunt (sometimes referred to as a great-aunt). The brother of your second great grandmother or (great-great-grandmother) is your second great granduncle (or great-great-granduncle). The relationship to a sibling of an ancestor carries the same prefixes as the relationship to that ancestor. Therefore, a sibling of your third great grandfather will be your third great grand-uncle.
Cousins of the same generation: A first cousin is a child of your aunt or uncle – another grandchild of your common grandparents through a unique descent line. A second cousin is a grandchild of your grand uncle – another great grandchild of your common great grandparents through a unique descent line. If you and a cousin share the same third-great-grandparents, then you are fourth cousins to each other. The number associated with the cousin level is one more than the number of greats in the relationship level of your common ancestors. Therefore, fourth cousins share 3rd great grandparents, fifth cousins share 4th great grandparents and so on. Keep in mind that your relationship to others is dependent on your most recent shared ancestors and not their shared relationships to others. Therefore, you can have third cousins who are in turn, siblings, first cousins or second cousins to each other.
Removed relationships: When you and a relative are not of the same generation, then we describe the relationship by the number of generations removed. Your mother’s first cousin is a first cousin once removed, and you are the generation removed. Your first cousin’s son is also a first cousin once removed, but he is the generation removed. Your grandmother’s second cousin is a second cousin twice removed, and you are two generations removed. Your third cousin’s son is a third cousin once removed, and he is the generation removed. If your head is spinning, consider reviewing our article, “Third Cousins Twice Removed and Consanguinity: Figuring Out How You’re Related to Your Relatives”.
Half relationships: Most relatives share two ancestors in common – a man and a woman who are both members of an ancestral couple. When two individuals share only one common ancestor, we consider them to be half relatives. A son of your mother by a man other than your father is a maternal half-brother. A grandson of your grandfather descended from a woman other than your grandmother is a half first cousin. Note that half relationships only apply if you descend from different partners or spouses of your common ancestor at the generation of your most recent common ancestor. If two of your first cousins were born to your uncle by different spouses, they will still both be first cousins to you even if they are half siblings to each other.
Genetically Equivalent Relationships
Once potential relationships have been estimated, consider genetically equivalent relationships and the genealogical context of a match’s tree. The amount of DNA that two individuals are expected to share with each other depends on the number of generational steps between them and their common ancestor(s). Thus, second cousins and first cousins twice removed are both expected to share similar amounts of DNA since both levels of relationship include six generational steps. In most cases, two genetic cousins will be related through two common ancestors: an ancestral couple. When two individuals only share one ancestor, the amount of DNA they are expected to share in common is cut in half which is equivalent to adding another generational step. As a result, some levels of relationship are genetically equivalent. Half siblings (2 generational steps with one common ancestor) share about 25% of their DNA. An aunt and her nephew (3 generational steps with two common ancestors) also share approximately 25% of their DNA. A grandparent and a grandchild (2 generational steps with one common ancestor – the grandparent) also share approximately 25% of their DNA. A first cousin (four generational steps with two common ancestors) shares about the same amount of DNA as a half-uncle and his half-nephew (three generational steps with one common ancestor). A first cousin once removed (five generational steps with two common ancestors) shares about the same amount as a half first cousin (four generational steps with one common ancestor).
When evaluating the amount of DNA shared with an unknown genetic cousin, consider potential equivalent relationships. When evaluating the amount of DNA shared with known relatives, determine if their amount of shared DNA would be more typical of a half relationship.
Understanding how you relate to others in your family tree can be confusing at times. We’ve created an easy-to-read DNA & Relationship Quick Reference chart to help take the guesswork out of determining family relationships. Download the full-resolution version for free using the button below.
If you need help determing your relationship to a genetic match, or would like help extending your family lines, let Legacy Tree Genealogists provide the research and the extensions to your family tree. Contact us today for a free consultation.
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Paul - Legacy Tree Genealogists Researcher
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How is a person my 4th cousin but not related to my sister.
At some levels of relationship, it is entirely possible to be genealogically related and yet share no DNA with each other. This begins around the level of third cousins. At the level of fourth cousins, around 29% of cousins will share no DNA with each other. The probability of no shared DNA increases the further the relationship is until at the 8th cousin level, there is only a 1% chance that relatives will share DNA. Because you and your sister inherit different portions of your parents’ DNA, and because the DNA you inherit is the basis for the genetic relationships that are determined, it is entirely possible for you to have fourth cousin matches who share DNA with you, but who do not share DNA with your sister.