Why are some people born males and others are born females? It’s a question scientists have been trying to figure out for more than a century. Despite extensive efforts to get an answer, there’s still a large knowledge gap, according to new research.

Biologists from Lund University in Sweden reviewed research conducted over the past 100 years on sex chromosomes to understand where our current knowledge of the subject stands.

Read: Why Are There Only Two Sexes? Evolutionary Theories Behind The Idea Of Two Genders, Instead Of 1 or 4

Upon reviewing more than 100 scientific articles and papers, the researchers conclude two tracks have been most used in research since the late 19th century: empirical studies and various theories. They suggest more studies that focus on ecology should be conducted to expand on our knowledge of sex chromosomes and the genome. A genome is a organism's complete set of genetic information, including more than 3 million DNA base pairs.

“Additional approaches in research will probably lead to a more detailed understanding of the development of sex chromosomes and, in turn, of how females and males evolve,” senior lecturer Jessica Abbott said in a statement.

Read: Male Flies Don't Compete With Brothers For Female Attention, Leading To Gentler Handling Of Females

In their paper, published in the journal Proceedings of Royal Society B, Abbott and her colleagues discuss how the study of sex chromosome evolution has primarily been focused on genetics. They believe looking at how the environment affects the development and evolution of sex chromosomes can advance the research. Factors they suggest looking at include food access and how age differences can play a role within a population.

The authors also found there’s a need for the development of new theories.“There have been relatively few major advances since the ground-breaking work of the 1970s and 1980s,” their paper reads. They suggest ecology be more thoroughly investigated theoretically. Lastly, they recommend using new technologies to generate more data. This data can play a role in testing some of the current hypotheses and developing new theoretical frameworks.

It’s understood that female and male bodies function very different from one another, despite having the same genome. This is seen in regards to reproduction. "There is a form of genetic conflict between the sexes - a conflict in the genome itself - which we know little about,” said Lund University professor Bengt Hansson.

Similar to humans, most other organisms have also adopted two-sex reproduction. If you're interested in learning more about some of the theories behind why there's only two sexes, check out Medical Daily's "Why Are There Only Two Sexes? Evolutionary Theories Behind The Idea Of Two Genders, Instead Of 1 Or 4."

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