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Understanding Genetic Traits
Have you ever asked yourself, why do men have nipples? When you really think about it, male nipples do not serve an actual function, as opposed to female nipples, which produce milk for a mother's offspring. In regard to male vs. female sexuality, men and women have different sexual parts and organs, so why would both men and women have nipples?
Let's start with simple genetics. A baby receives a copy of every gene from the father and a copy of every gene from the mother. Thus, if parents have a boy, he has both sets of his parents' genes.
However, males and females can have different traits as well. Biology Professor Andrew M. Simons of Carleton University provides the example of birds. Female and male birds have different colors and plumage; therefore, we can tell the difference between a male or female bird (Scientific American).
This poses the question: If traits are inherited from both the mother and the father, why should there be such a difference between a male and female?
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Uncoupling, Genetic Correlation & Natural Selection
One explanation is that during the transfer of the genes, the genetic code of the child is "uncoupled," rather de-linked, in the process, and goes on to develop in the context in which the genes find themselves. In other words, Professor Simons explains uncoupling like so: "This happens if a trait is influenced by different genes in males and females, if it is under control of genes located on sex chromosomes, or if gene expression has evolved to be dependent on context (whether genes find themselves within a male or a female genome)." The link to this Scientific American article is included below.
Genetic correlation, which is the shared genetic basis of two traits (such as nipples), occurs. It is called an evolutionary "default" when males and females shares the same traits due to genetic correlations. Thus, men acquire the trait of nipples due to this default.
In regard to evolution, there is a process called natural selection. During this process, animals with the most beneficial traits suited to their environment will survive and those traits will be passed on to their offspring. For nipples, it works the same way. As Simons reports, "the presence of nipples in males is probably best explained as a genetic correlation that persists through lack of selection against them, rather than selection for them."
Sometimes natural selection makes sense and sometimes it doesn't. It seems that men have nipples due to an evolutionary default in the natural selection process. So to answer the age-old question, "why do men have nipples," the simple answer is because women do.
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Reference & Image Credits
Simons, Andrew M. "Why do men have nipples?" Scientific American, September 17, 2003 -- scientificamerican.com/article.cfm?id=why-do-men-have-nipples
Photo 1 (boy): Arvind Balaraman / FreeDigitalPhotos.net -- freedigitalphotos.net/images/view_photog.php?photogid=1058
Photo 2 (breastfeeding): Ken Hammond / USDA -- http://en.wikipedia.org/wiki/File:Breastfeeding_infant.jpg