Are You Genetically More Like Your Mother or Your Father?

Thursday, January 26, 2017. Author Dr Pleuni Hooijman and Dr Nathan West

Every gene report in the FitnessGenes 'Genetic Results' section is represented by two letters (e.g. RR, RX or XX for the ACTN3 gene result), each representing a variant or variation at a specific region within a given gene, also termed an ‘allele’. The reason why you have two alleles for each gene result, is that you inherited one from each of your biological parents. However, does this mean that we receive exactly half of our DNA from our father and half from our mother?

Well, no. We all actually receive more DNA from our biological mothers, albeit the differences are extremely small. Does this surprise you? And how can this be the case? It comes down to the source and type of DNA.

The X Factor
Most of the cells in our body have one nucleus, although some, like muscle fibers have more than one, and others, like red blood cells, have none. Each nucleus carries genetic information in the form of 23 pairs of chromosomes that are made from DNA (sometimes referred to as "nuclear DNA").
For each pair of chromosomes, we inherit one from each of our parents. For example, your ACTN3 gene found on chromosome 11, could have the X from your mother and the R from your father.

Most of the chromosome pairs are identical in size and the number of genes they contain, except for one: the 23rd pair. This distinctive pair is where the first difference in how much DNA we receive from each parent starts to become apparent. Chromosome 23 is the sex chromosome and helps determines our gender. Females have two copies of the “X” version of the sex chromosome (one from each parent) and hence have a XX pair. Males have one copy of “X” that they inherited from their mother, and one copy of the “Y” sex chromosome from their father. The Y chromosome is much smaller than the X chromosome and therefore also contains fewer genes (78 versus over 1000). Therefore, males can inherit slightly more nuclear DNA from their mother.

So it's only males then?

Again, no. There is another source of DNA found within the cells in our bodies, in addition to the nuclear DNA. DNA is also present in mitochondria, an important cellular compartment (organelle). We’ve written about mitochondria before and how they are our cell powerhouses and regulators of many metabolic processes.

The interesting thing about the DNA in your mitochondria is that it all comes from our mothers. Although both the sperm and egg cells contain mitochondria, shortly after fertilization the mitochondria from the sperm of your father are destroyed, leaving only mitochondria and thus mitochondrial DNA of your mother. Mitochondrial DNA only contains around 37 genes compared to around 20,000 genes in nuclear DNA. Regardless of its size, this still means that both males and females alike will receive slightly more DNA from their mothers compared to their fathers.

When two parents isn't enough...

You may have seen the recent story of a baby boy that has the DNA of three people.  Doctors used pioneering techniques and the two types of DNA discussed earlier.  

The mother had a rare genetic mutation in the mitochondrial DNA of her egg cells that led to several miscarriages. Scientists and doctors came up with a method that would allow the mother to pass on her nuclear DNA, but not her faulty mitochondrial DNA. 

They did this by using a donated egg from a second woman.  The nucleus and therefore nuclear DNA of the second women was removed from the egg and was replaced by the mothers. This new egg was then fertilized using the father's sperm before being implanted back into the mother. The resulting baby boy was therefore composed of nuclear DNA from its mother and father (23 chromosomes each) and mitochondrial DNA from the women who donated her egg.

Nuclear DNA vs Mitochondrial DNA

At FitnessGenes we only test nuclear DNA and not mitochondrial DNA as this is more easily extracted and is also where the majority of genetic information can be found (20,000 vs. 37 genes). Another reason is there are many mitochondria within each single cell. So if a change occurs in the DNA of one mitochondria, it may not be seen in all other mitochondria (there can be a mosaic pattern of expression of a certain genetic variant). Therefore, if you take a cell sample and test for the mitochondrial DNA, it is very hard to predict where and what the effects of genetic variations are.

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