MadSci Network: Genetics
Query:

Re: Do mothers or fathers contribute more genetic code to their offspring?

Date: Wed Sep 8 13:02:33 2004
Posted By: Alex Brands, Post-doc/Fellow, Biological ciences, Lehigh University
Area of science: Genetics
ID: 1093534541.Ge
Message:

Hello AJ,

The answer to your question depends in part on the sex of the child, but in all 
cases 
the difference is very small. 

In any case, both parents contribute a single copy of chromosomes 1 through 
22.  
This adds up to approximately 2.87 billion bases of genetic information from 
each 
parent.  In addition, the mother contributes one X chromosome, which is another 
152 million bases.

The father can contribute either 1 X chromosome (152 million bases), in which 
case 
the child will be a girl, or 1 Y chromosome (51 million bases) in which case 
the 
child will be a boy.  The X and Y chromosomes are known as the sex 
chromosomes.  In total, then, a person’s genome is about 6 billion bases.

The other thing to keep in mind is the structure inside our cells that is 
called the 
mitochondria.  This organelle is responsible for energy production, and is 
inherited 
only from the mother.  Interestingly, the mitochondria has its own DNA, which 
is 
comprised of 16, 569 bases of genetic information.  This is less than .0006% of 
the 
total DNA that the mother contributes.

So, if the father contributes an X chromosome, and the couple has a baby girl, 
the 
mother has contributed about .0006% more DNA than the father, owing to the 
mitochondrial DNA.  If the father contributes a Y chromosome, and the couple 
has a 
boy, the mother has contributed about 101 million bases, or about 3.5% more 
genetic information than the father, due mostly to the difference in size 
between the 
X and Y chromosomes, and just a tiny bit due to the mitochondrial DNA.

For chromosomes 1-22, which account for ~95% of our genetic information, there 
is 
no difference whatsoever between the two scenarios.  

Siblings that have the same mother but different fathers will have the same 
mitochondrial DNA.  This is not the case for siblings that have the same father 
but 
different mothers.  As mentioned above, this accounts for only a tiny fraction 
of the 
total DNA in an individual.

Now let’s consider the X and Y chromosomes.  When the body produces gametes 
(eggs or sperm) through a process called meiosis, the chromosomes go through a 
process called recombination, or crossing over.  Simply put, this is where the 
two 
copies of a chromosome line up and swap genetic code.  We have two copies each 
of chromosomes 1-22.  During meiosis, the two copies of chromosome 1 line up 
and swap code, as do chromosomes 2-22.  In females, the two X chromosomes can 
also do this.  As a result, the X chromosome that is contributed to the 
offspring is 
essentially a mix of the mother’s two X chromosomes.  Therefore, each egg has a 
slightly different X chromosome.

In men, there is only one X chromosome, and only 1 Y chromosome, so neither has 
a partner for recombination.  Each sperm produced by the man should have either 
an X or a Y chromosome.  Since there is very little recombination involving the 
X or Y chromosomes, there is essentially no variation in the X or Y chromosomes 
carried by the sperm.

So, same sex siblings that have the same father have exactly the same Y 
chromosome (if they are male) or almost exactly the same X chromosome from 
their father.   
If they have the same mother, the X chromosome that each has from the mother 
will 
be slightly different. 

Let’s look at the different possible sibling combinations:

1) 2 boys

If they have a different mother. but the same father, their Y chromosomes are 
identical.  That’s 51 million bases.  Their X chromosomes are different.

If they have the same mother, their X chromosomes are 50% the same.  50% of 152 
million bases is 76 million bases.  If they have different fathers, then their 
Y 
chromosomes are different.

Therefore, two boys with the same mother but different father will be slightly 
more 
similar than two boys with the same father but different mother.  An emphasis 
on 
“slightly”…the difference is about 25 million bases, which represents about 
0.4% of 
a person’s total genome.

2) 2 girls

If they have a different mother, but the same father, then the X chromosome 
they 
each got from their father is 100% identical.  That’s 152 million bases.  Their 
other X 
chromosomes, which came from different mothers, are different.

If they have the same mother, but different fathers, then the paternally 
inherited X 
chromosomes will be different.  The maternally inherited X chromosomes will be 
50% identical, so that’s 76 million bases in common.

Therefore, two girls with the same father, but different mother, will be 
slightly more 
similar than two girls with the same mother, but different father.  The 
difference in 
this case is about 76 million bases, or approximately 1.2% of a person’s total 
genome.

3) 1 boy and 1 girl

If they have a different mother, but the same father, then both sex chromosomes 
will 
be different.  The boy inherited a Y chromosome from the father, whereas the 
girl 
inherited an X chromosome from the father.  The mothers are different, so the 
maternally inherited X chromosomes will be different. In this case, both sex 
chromosomes are different.

If they have different fathers, but the same mother, the paternally inherited 
sex 
chromosomes are different.  The maternally inherited X chromosomes will be 50% 
identical (76 million bases in common)

Therefore, a boy and a girl with the same mother, but different fathers will be 
slightly 
more similar than a boy and a girl with the same father, but different 
mothers.  The 
difference in this case is about 76 million bases, which accounts for 
approximately 
1.2% of a person’s total genome.

Alex Brands
Lehigh University





Current Queue | Current Queue for Genetics | Genetics archives

Try the links in the MadSci Library for more information on Genetics.



MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci


MadSci Network, webadmin@www.madsci.org
© 1995-2003. All rights reserved.