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Author Topic: How many genes do we share?  (Read 26965 times)

Katherine Sissons

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How many genes do we share?
« on: 01/06/2011 15:01:03 »
Katherine Sissons  asked the Naked Scientists:
   
Hi Chris,

I was just browsing the internet when I came across a question and answer about the genetics of relatedness on your website here: http://www.thenakedscientists.com/HTML/content/latest-questions/question/919/

The question asks 'If I share 98% of my genes with a chimpanzee and 60% with a banana, how come I only share 50% of my genes with my own daughter?' and the answer seemed to me to be a bit inaccurate.  It might simply be that I am misunderstanding the answer, but I'd like to offer a clearer explanation which I think answers the question. 

We do share at least 98% of our genome with chimpanzees, our closest relative as a species.  We share even more of our DNA with other humans, even the ones we're not related to - let's say 99%.  When people say that you share 50% of your genes with your daughter, they are talking about the percentage of genes which are identical by descent.  This means that a proportion of her genes are identical to your genes, not simply because she is part of the same species as you, but because she got those genes from you.  The two of you will share many many more genes in common, over and above this 50%, but she will have received those genes from her mother, so they are not identical to yours by descent.  Here's another way to think about this:  let's say the two unrelated humans share 99% of their genomes, by virtue of being the same species.  All humans will share these genes - that means that all humans are 99% the same at the level of the gene.

However, the remaining 1% is different and explains variation in characteristics such as hair colour, risk of certain diseases etc.  In the case of your daughter, the two of you will share the same 99% that all humans do.  In addition, of the remaining 1%, 50% of her genes will be the same as yours.  Overall, the two of you share 99.5% of your genes.  The 50% figure refers to the proportion of genes the two of you share over and above the average proportion of genes shared by any two unrelated humans. 

I hope that this will help you to clarify the answer published on the website.

Best wishes,
Katherine Sissons

What do you think?
« Last Edit: 01/06/2011 15:01:03 by _system »

CliffordK

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How many genes do we share?
« Reply #1 on: 01/06/2011 23:24:12 »
With the daughter.

50% of the nuclear genes are paternal genes
50% of the nuclear genes are maternal genes.
100% of the mitochondrial genes are maternal genes.

However, there will inevitably be some genes that are 100% identical between the mother and father, or that either parent will carry pairs of identical genes.

If you look at the proteins or genes involved in DNA replication:
DNA Helicase
DNA Ligase
DNA Polymerase
DNA Primase

Bacteria, Chimps, and Humans all share these basic DNA Replication genes and proteins. 
Two humans may have a 100% match of these basic DNA replication genes.
A human and a chimp have similar genes and proteins, but likely they would vary by a few base pairs.
A human and bacteria would also share similar genes, but they would vary significantly.

Also, note that the humans also have additional proteins and genes used in DNA replication that the bacteria lack including histones and centromeres.

Most DNA fingerprinting is not done in the actual genes, but in uncoded regions of DNA which apparently are not required to produce proteins, and thus have much higher variability.

Genes such as those controlling hair color might vary significantly from individual to individual, although potentially less so in groups with little ethnic/racial variation.  But these variations that we recognize as generating prominent features are very minimal when looking at the DNA as a whole.

 

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