What does "shared DNA" mean?

[Geno62]

Can someone please reconcile the following statements?

- First cousins, on average, share 12.5% of their DNA.

- Humans and Chimpanzees share 99.8% of their DNA.

TIA!

 

[Peterkin]

When they're looking for family relationships, or crime scene ID, the labs are not checking the entire DNA sequence of the sample; they're testing for specific known human markers, that occur in a particular location on the strand, in a particular order. The genes they're watching for may not even be active or expressed or read by the cell; they just need to be peculiar to an individual. So, that 12% is not of all the person's DNA, but of the specific markers being scrutinized.

(Don't ask me for the mechanism; I'm not current on this science.)

Edited July 17, 2021 by Peterkin
incomplete

[CharonY]

On 7/16/2021 at 6:56 PM, Peterkin said:

When they're looking for family relationships, or crime scene ID, the labs are not checking the entire DNA sequence of the sample; they're testing for specific known human markers, that occur in a particular location on the strand, in a particular order.

While relationships are often analyzed using specific loci, that is likely not what OP is referring. If we talk about cousins the number refers to the averaged total of the total DNA that is being shared. You get ~50% of your DNA from each of your parents, for example. Same goes for a sibling and thus, on average you will have 50% in common with your siblings. There is variability there as you might not get the exact same allele from either parent as your sibling is getting.

The comparison vs chimpanzees is based on an entirely different metric. The 99.8% number is not based on sequence identity (as it would among siblings), but only focuses on genes which we pretty much have the same as chimpanzees. So it is not an apple to apple comparison. I think even then the value is actually slightly lower, because IIRC the 99.8 was based on partial sequences.

[Peterkin]

That's a helpful clarification. Thanks.

[iNow]

6 hours ago, CharonY said:

The comparison vs chimpanzees is based on an entirely different metric. The 99.8% number is not based on sequence identity (as it would among siblings), but only focuses on genes which we pretty much have the same as chimpanzees. So it is not an apple to apple comparison. I think even then the value is actually slightly lower, because IIRC the 99.8 was based on partial sequences.

Indeed. It’s based only on the parts compared, but ignores large sections of genetic code that were simply discarded before comparison began. Watched this video recently with my kid which explains it well