How much DNA do we share?

The next time you refer to someone as monkeying around you may not be that far from the truth. Chimpanzees are our closest relative as a species and we share at least 98% of our genome with them. Our feline friends have 90% of homologous genes with us, with dogs it is 82%, 80% with cows, 69% with rats and 67% with mice [1]. By virtue of being the same species, all humans share 99% of their genome, which means that all humans are 99% genetically similar.

Our bodies are made up of 3 billion genetic building blocks, or base pairs. Of those 3 billion base pairs, only a tiny amount are unique to us. The remaining 1% of genes that are different explain the variations in characteristics, such as colour of hair, eyes and skin, and also risk for certain diseases.

When undertaking a paternity test, DNA samples from family members are analysed. AlphaBiolabs examines short tandem repeat (STR) markers that are specific locations on chromosomes made up of sequences of repeated DNA. These STR markers are highly variable in length between individuals. Each individual has two copies of each STR marker, known as alleles: one is inherited from the father and the other from the mother. By looking at these specific DNA markers in each sample, it is possible to identify which half of the child’s DNA is inherited from the mother and which half is from the father.

AlphaBiolabs tests 24 STR markers including two sex markers when undertaking DNA testing. The two alleles observed at each STR marker are compared between the tested individuals.

When testing a biological parent, both child and the parent will share 50% of these identical sections of DNA at each marker. Identical twins share the same DNA profile. The average percentage of DNA shared for different types of relationships is shown in Table 1.

RelationshipAverage % DNA shared
Identical twin100%
Parent/child, full siblings50%
Grandparent/grandchild, aunt/uncle and niece/nephew, half siblings25%
First cousin12.5%

Table 1. Average DNA shared by pairs of relatives

This is where the confusion arises. A chimpanzee shares 98% of our genome, but your own child only 50%? How does that work? The 50% figure actually refers to the proportion of DNA that a parent and child share over and above the average proportion of DNA shared by any two unrelated humans. The explanation is that a paternity test only analyses the percentage of DNA that is identical by descent. This means that a proportion of the child’s DNA is identical to the parent’s DNA, not just because he or she is the same species, but because the genes came from the parent. However, the parent and child will share more DNA in common over and above this 50%. So, a parent and child will share the same 99% that all humans do. And of the remaining 1%, 50% of the DNA will be identical. So, I guess you could say that a parent and child will share 99.5%.

So why is human and chimpanzee DNA so similar? It is thought that these two species are so closely related because they descended from a single ancestor species 6 or 7,000,000 years ago. As they evolved, their DNA changed as it was passed from generation to generation. It is these DNA changes that account for the differences between human and chimp appearance and behaviour.

For information on any of our DNA testing services, please contact AlphaBiolabs on 0333 600 1300; email: info@alphabiolabs.com; www.alphabiolabs.com

[1] Initial sequence and comparative analysis of the cat genome. Genome Res 2007(17):1675–1689.