Category Archives: Relationships

Concepts: What is a Half-Relationship, Like Half First Cousins, Anyway?

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Pretty much everyone knows what a half sibling is – someone who shares one, but not both parents with each other.

If you and your sibling share the same mother, but not the same father (or vice versa), you’re half-siblings. Only one parent is shared between half-siblings.

If you share both parents, you’re full siblings.

Step-Siblings

Step-siblings are often confused and used interchangably with half-siblings, but they aren’t at all the same. And yes, it matters.

If your parent married someone who already had a child, but both you and that child were born to prior (or future) marriages/relationships of your respective parent – you’re step siblings.

In other words, your parents are married to each other, and you may live in the same household, but you don’t share a genetic link with a step-sibling because you don’t share any parent in common.

By way of example, my mom married a man who had a son from a prior marriage, and his son is my stepbrother. The man my mother married is my stepfather.

My mother is my stepbrother’s stepmother.

The real message here, other than clarifying confusing relationship terms that are often used incorrectly, is that:

  • There is a biological relationship between full-siblings and half-siblings
  • There is no biological relationship between step-siblings, unless one exists due to ancestors someplace in the past

I wrote about how much of your ancestors’ DNA you can expect to inherit in the article, Ancestral DNA Percentages – How Much of Them is in You?.

Genetically, Half Versus Full Matters

The amount of autosomal DNA that is expected to be shared between full-siblings and half-siblings differs. Throughout this section, I’m using words like “expected” and “about” because in reality, after parents, “exactly” half of the ancestral DNA in a specific generation does not get passed to the next generation. Random recombination is a factor and therefore, the expected inherited percentages are approximate.

Full siblings share both parents, while half-siblings share only one parent, so full siblings share about 50% of their DNA, while half-siblings share about 25% of their DNA – and only from one parent.

Therefore, every descendant relationship from full or half relationships varies by 50% between the two types of relatoinships.

Half-siblings can be expected to share, on average, half as much autosomal DNA as full siblings – because they only share one parent – not two.

For example, first cousins (1C) share about 12.5% of their DNA, but half first cousins (half 1C) share about 6.25% of their DNA.

In other words, the “half” designation literally means that those two people share half a relationship – one parent (or grandparent, etc.), not both, in the founding generation, and their descendants continue to share half as much DNA as a full relationship in the same generation.

Subsequent Generations

Extending those relationships down the tree generation by generation, we see that in each subsequent generation, the descendants can be expected, on average, to share one-fourth as much DNA with each other as the preceding generation. That’s because two transmission events have taken place, one in Child 1’s line, and one in Child 2’s line.

The same as in full sibling lineages, each subsequent half-sibling descendant generation can also be expected to share one quarter as much autosomal DNA as the preceding one.

“Removed” Relationships

If you encounter a situation where one side of the descendant tree is “longer” than the other by a generation or more, then you’re dealing with a phenomenon known as “removed,” such as first-cousin-once-removed (1C1R), or, in the example above, a third cousin (3C) once removed (1R).

The same concepts still apply. A half 3C1R would share half as much DNA as a 3C1R.

I wrote all about “removed” relationships and their genetic genealogy effects in the article Concepts: What Does a Cousin “Once Removed” Mean.

Summary

In this final chart, I’ve combined the full-sibling and half-sibling charts into one so that you can compare them easily. I’ve also removed the “other” parent that the half-siblings don’t share to conserve space.

I wrote about how much DNA each type of relationship can be expectd to share, both centiMorgans (cMs) and percentages, including relationships not detailed here, such as half-uncles and half-aunts, in the article Shared cM Project 2020 Analysis, Comparison & Handy Reference Charts.

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Concepts: What Does a Cousin “Once Removed” Mean?

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What is a first-cousin-once-removed (1C1R) or a second-cousin-once-removed (2C1R)? In these abbreviations, “R” means “removed.”

Once you understand what “removed” means, it’s really simple, but until then, it’s confusing.

Let’s start with first cousins.

First cousins share common grandparents. Jane and Mark are first cousins. Their parents are siblings.

“Once removed” means one generation offset, or the child of someone in that cousin generation.

Let’s say you’re Jane.

Your child, Jim, is the first cousin once removed to your first cousin, Mark. Said the other way, Mark is a 1C1R to Jim.

Jim and Mark are first cousins once removed.

Flipped around, so that we are comparing Mark’s child instead of Jane’s – Mark’s child, Maurice is a 1C1R to Jane.

Julie and Maurice are both the children of people who are first cousins to each other. They share Steve and Shirley as great-grandparents, so they are second cousins to each other.

Now let’s say that Julie has a son, James.

James and Maurice are second cousins once removed, because they are a generation offset from one another. James is the child of someone who is second cousins with someone else. In this case, James is the son of Julie who is a second cousin (2C) to Maurice.

Now, James has a child, Jill.

Jill and Maurice are second cousins twice removed, because they are another generation offset, or on down their branch of the tree..

Calculating “Removed”

Taking this one step further, Jill is a first-cousin-three-times-removed (1C3R) to Mark.

Cousin relationships are easy to calculate.

Look at the two people you wish to compare. In this case, Jill and Mark.

Find their common generation level. Looking back up Jill’s tree, we find Jane at the same generational level as Mark. Jane and Mark are first cousins (1C). That’s the base relationship before calculating the number of generations your target people, Jill and Mark, are “removed” from each other.

Beginning with the generation below Mark, count how many generations below first cousins Jill is “removed” from Mark.

In this case, in the column at right, you can see that the Julie/Maurice generation is “1”, followed by the James generation at “2” and Jill is generation “3.” Hence, Jill and Mark are 1C3R to each other.

Don’t confused a first-cousin-once-removed (1C3R) with third cousins (3C).

Third cousins would be James and Matthew. Fourth cousins would be Jill and Madison.

It’s easy to calulate the relationship of any two people in your tree.

I wrote about my “chicken scratch” method of calculating relationships in the article, Quick Tip: Calculating Cousin Relationships Easily.

DNA

As you might expect, the amount of expected shared DNA between two people who are second cousins (2C) and second-cousins-once-removed (2C1R) is different, because the DNA has been divided once more in James than it has been in Maurice.

In first-cousins-three-times-removed, the DNA has been divided once again in Jill.

I wrote about DNA and relationship predictions, including half relationships and “removed” relationships in the article, Concepts – Relationship Predications.

You can view expected amounts and ranges of shared DNA for various relationships using DNA Painter’s Shared cM Project tool, here.

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I receive a small contribution when you click on some of the links to vendors in my articles. This does NOT increase your price but helps me keep the lights on and this informational blog free for everyone. Please click on the affiliate links in the articles or to the vendors below if you are purchasing products or DNA testing.

Thank you so much.

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