The Most Recent Common Ancestor (or MRCA for short) represents the closest shared ancestor between two individuals. Closest means the smallest number of generations to the ancestor and thus most recent. If two individuals share an ancestor, they are designated cousins and likely share some DNA from the common ancestor. Any ancestors of the MRCA are shared by the individuals as well but are not the closest.
Often, you share two closest ancestors; that is, common (grand)parents. Thus, the term MRCA is really plural in most cases. When only one MRCA is shared, you are termed half cousins. This term is rarely used but an important distinction in genetic genealogy. It is derived from the half-siblings who only share a single parent; who in this case is designated the MRCA (singular). Corollary: you are not half-cousins if you share both (grand)parents. Thus any other half-relationships between siblings farther down than children of the MRCA does not affect this relationship or its designation.
If the same number of generations occurs up to that common ancestor from each person, then they are direct nth cousins with n being the number of generations to that common ancestor (less one). If the generation count to the common ancestor is different, then they are additionally mth removed cousins, depending on how many generations m they are different by. So 1st Cousins, Once Removed are two generations to the common ancestor(s) for one individual, and three generations for the other.
Full-siblings have both their parents as the MRCA and, as such, can be called 0th cousins (although rarely are so designated except inside computer programs). If only sharing a single parent, they are termed half-siblings. Similarly, Aunts and Uncles are 0th cousins, once removed as the MRCA is the grandparent or parent; respectively. Some languages have additional special terms for close relationships of cousins; sometimes specific to the gender and path to the ancestors. Spouses are only considered, in Genetic Genealogy, when they contribute to the DNA of the descendant. We often use the terms (grand)parents and cousins in a broader sense on this page and elsewhere. Any ancestor above you is a (grand)parent. Any relation down from the MRCA that is not an ancestor is a cousin. Colloquial form restricts the terms parent, grandparent and cousin to specific relations of the actual parent, actual grandparent (no great), and first cousins (no nth, no removed) only. We will not get into colloquial or slang such as "my baby's daddy" here
We will try to qualify when being broader or specific.
Using a measurable Degree of Relationship based on the generation count to the common ancestor is important to understanding the expected strength of a DNA match. The closer each tester is to the MRCA, the more of that ancestors DNA they likely still carry. See our work on Match Codes for ways to designate the relationship and strength in a process-able way on DNA match sites.
In genetic genealogy, they further modify the MRCA term by pre-pending it with the word "Time" to create the term Time to Most Recent Common Ancestor (or TMRCA for short). This is used with DNA analysis tools to report how far back in time a recent common ancestor likely existed and is expressed in years. A conversion of generations to years is usually implied and can be anywhere from 25 to 33 years per generation (4 to 3 generations per century). Sometimes, one will see the abbreviation ybp instead of the term years. In this context, it is essentially the same as the generation count is the root, source value.
See the related Earliest Known Ancestor (EKA) also.
Often, you share two closest ancestors; that is, common (grand)parents. Thus, the term MRCA is really plural in most cases. When only one MRCA is shared, you are termed half cousins. This term is rarely used but an important distinction in genetic genealogy. It is derived from the half-siblings who only share a single parent; who in this case is designated the MRCA (singular). Corollary: you are not half-cousins if you share both (grand)parents. Thus any other half-relationships between siblings farther down than children of the MRCA does not affect this relationship or its designation.
If the same number of generations occurs up to that common ancestor from each person, then they are direct nth cousins with n being the number of generations to that common ancestor (less one). If the generation count to the common ancestor is different, then they are additionally mth removed cousins, depending on how many generations m they are different by. So 1st Cousins, Once Removed are two generations to the common ancestor(s) for one individual, and three generations for the other.
Full-siblings have both their parents as the MRCA and, as such, can be called 0th cousins (although rarely are so designated except inside computer programs). If only sharing a single parent, they are termed half-siblings. Similarly, Aunts and Uncles are 0th cousins, once removed as the MRCA is the grandparent or parent; respectively. Some languages have additional special terms for close relationships of cousins; sometimes specific to the gender and path to the ancestors. Spouses are only considered, in Genetic Genealogy, when they contribute to the DNA of the descendant. We often use the terms (grand)parents and cousins in a broader sense on this page and elsewhere. Any ancestor above you is a (grand)parent. Any relation down from the MRCA that is not an ancestor is a cousin. Colloquial form restricts the terms parent, grandparent and cousin to specific relations of the actual parent, actual grandparent (no great), and first cousins (no nth, no removed) only. We will not get into colloquial or slang such as "my baby's daddy" here
We will try to qualify when being broader or specific.Using a measurable Degree of Relationship based on the generation count to the common ancestor is important to understanding the expected strength of a DNA match. The closer each tester is to the MRCA, the more of that ancestors DNA they likely still carry. See our work on Match Codes for ways to designate the relationship and strength in a process-able way on DNA match sites.
In genetic genealogy, they further modify the MRCA term by pre-pending it with the word "Time" to create the term Time to Most Recent Common Ancestor (or TMRCA for short). This is used with DNA analysis tools to report how far back in time a recent common ancestor likely existed and is expressed in years. A conversion of generations to years is usually implied and can be anywhere from 25 to 33 years per generation (4 to 3 generations per century). Sometimes, one will see the abbreviation ybp instead of the term years. In this context, it is essentially the same as the generation count is the root, source value.
See the related Earliest Known Ancestor (EKA) also.