A new study reveals how Neanderthals and modern humans have incompatibilities when it comes to interbreeding between the two species, suggesting why modern humans only carry about four percent of Neanderthal DNA.

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In this new analysis of the Neanderthal Y chromosome, which is a set of genes that is inherited by sons from their fathers, it shows how this is almost non-existent in modern human populations all over the world.

Scientists from Stanford University in California discovered key differences in the immunity genes of the Neanderthal Y chromosome that are linked to miscarriages.

This four percent Neanderthal DNA that remained in modern humans is the result of the breeding of two distinct human populations from 50,000 years ago which are the Homo sapiens who migrated out of Africa that began to explore and colonize Europe and Asia.

These new findings reveal that the Neanderthal Y chromosome is a distinct chromosome that can be detected in any human today. According to co-author of the study, Carlos Bustamante from Stanford University, California, the presence of any Neanderthal Y chromosome DNA is almost non-existent, but it does not mean that it is totally extinct in all the human samples we have tested, but it is highly possible.

Researchers believe that it is also possible that the Neanderthal Y chromosomes have been in circulation among the modern human gene pool but disappeared during the course of thousands of years.

Another theory states that Neanderthals' and Homo sapiens' genes are not compatible with each other where researchers found evidence to support this. There are seven genes in the Neanderthal Y chromosome that are different since they are part of the immune system. Three of these are known as H-Y genes or minor histocompatibility antigens, which are similar to those that are checked upon when doctors screen organ donors and recipients for compatible immune profiles.

Since these Neanderthal genes are located within the Y chromosome, these are only present in males. The theory would be, female immune systems might attack a male fetus that carries these Neanderthal genes during pregnancy. This might result in consistently miscarrying male babies with Neanderthal Y chromosomes leading to its non-existence in modern humans.

This new study is published in the American Journal of Human Genetics.