MadSci Network: Genetics

Re: What % of DNA is 'unique' in a child?

Date: Mon Nov 27 23:56:53 2000
Posted By: Christopher Carlson, Ph.D.
Area of science: Genetics
ID: 974831664.Ge

Good question.  The answer depends on what you mean by "unique".  

Humans and other mammals are diploid.  That is, each individual carries 
two copies of each chromosome, one inherited from Mom and one from Dad.  
Thus, you could say that 50% of a child's DNA is identical to Mom's 
because it was inherited from Mom.  The other 50% of the child's DNA comes 
from Dad.  

However, when you look at the sequence of A's T's G's and C's which make 
up DNA, any two copies of the human genome differ at only one position 
every thousand base pairs.  Thus, the chromosomes inherited from Dad are 
only different from Mom's at 1 in 1000 bases, or 0.1%.  If we average the 
copy of the genome from Mom (0% different) and the copy from Dad (0.1% 
different) we can estimate that 0.05% of the kid's genome will differ from 

As an aside, where each copy of the human genome is only 0.1% different 
from any other, the human genome is somewhere between 3 and 5% different 
from a chimps genome, and approximately 20% different from a mouse's 

Anyhow, another way to look at "unique" is to say how many new mutations 
occurred in the copy of the genome inherited from Mom.  The error rate in 
copying the genome from one generation to the next is only about one in a 
billion base pairs.  Because the genome is approximately 3 billion bases 
long, each child will carry 3 new mutations (on average).  Fortunately 
most of these mutations have no detectable effects.  So the copy of the 
genome inherited from Mom will actually be 0.000001% unique.

I hope this helps.


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