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What is inbreeding? -

Inbreeding is defined as the mating of organisms closely related by ancestry. In this instance, we are looking at human inbreeding.

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Contrary to procreating -

Inbreeding goes against the biological aim of procreating, which is the mixing of DNA.

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The role DNA plays -

Human DNA is arranged into 23 pairs of chromosomes. Within each chromosome there are hundreds of thousands of genes.

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The human gene -

Each gene has two copies known as alleles, or matching genes; one from our biological mother, one from our biological father.

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What do genes determine? -

Genes determine every aspect of our appearance. They contain the information that decides a person's specific physical and biological traits, like hair color, eye color, and blood type. In other words, a gene is the basic physical and functional unit of heredity.

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Gene categories -

These genes fall into two categories: dominant and recessive.

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Dominant vs. recessive -

What's the difference between dominant and recessive genes? If one of the genes in the pair is dominant, then the result is you gain the trait of the dominant gene. However, for traits that originate from the recessive gene, you need both genes to be recessive. Here are some examples.

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Eye color -

Remember, genes contain the information that determines a person's specific physical and biological traits. So for example, the gene for brown eyes is dominant and so having just one of these in a pair will result in your eyes being brown. However, the gene for blue eyes is recessive so you'll need both of them to get blue eyes.

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Hair type -

Similarly, genetic factors appear to play a major role in determining hair texture—straight, wavy, or curly—and the thickness of individual strands of hair.

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Where can it go wrong? -

Certain congenital defects and genetic diseases are carried by recessive genes. These disorders include cystic fibrosis, sickle cell anemia, and Tay-Sachs disease (a rare, inherited disorder that is characterized by neurological problems caused by the death of nerve cells, or neurons).

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Similar DNA -

Blood-relative mating partners have similar DNA. Therefore the likelihood of them carrying the same recessive gene is greatly increased.

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Effects of inbreeding -

That being the case, inbreeding escalates the chances of being born with such conditions.

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Consanguineous marriage -

In fact, according to a study published by the National Library of Medicine, the rate of near natal and childhood death increases if the child comes from a first cousin union, a so-called consanguineous marriage.

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Costs of human inbreeding -

As previously outlined, the odds of inheriting rare genetic diseases goes up exponentially among children who are the result of human inbreeding.

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Inbred children -

Inbred children often display decreased cognitive abilities and muscular function, reduced height and lung function, and are at greater risk from diseases in general.

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Mental and physical problems -

Research has indicated that the effects of inbreeding in humans continue into adulthood, further promoting physical and mental disorders.

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Effect on fertility -

Inbreeding has been found to decrease fertility. Fetuses produced by inbreeding also face a greater risk of spontaneous terminations due to inherent complications in development.

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Premature birth -

Consanguineous parents also possess a high risk of premature birth and producing underweight and undersized infants.

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Additional problems -

Genetic disorders due to inbreeding include blindness, hearing loss, neonatal diabetes, and limb malformations.

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Schizophrenia -

Consanguinity has even been suggested as a risk factor for the development of schizophrenia in offspring.

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A custom practiced for millennia -

Inbreeding has probably been practiced for millennia, geneticists believe. For thousands of years our ancestors, including Neanderthals,  lived in small, isolated populations, leaving them severely inbred, according to genetic analysis published by New Scientist.

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Commonplace in antiquity -

Inbreeding was certainly commonplace in antiquity. In the case of the pharaohs of ancient Egypt for example, the custom not only kept traits within a bloodline, it prevented another family marrying in and lining up to take the throne. In fact, inbreeding propagated power.

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Intermarriage -

Royal intermarriage was often practiced among European royal families, usually for interests of state. Likewise, inter-nobility union was used as a method of forming political alliances among elites. Pictured is Philip II of Spain and his wife, Mary I of England, who were first cousins once removed.

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The Hapsburg lip/jaw -

So commonplace was intermarriage within the House of Hapsburg that the Hapsburg lip (or the Hapsburg jaw, as it's also known) was often cited as an ill-effect. The condition is today known as prognathism, an extension or bulging out (protrusion) of the lower jaw (mandible). In this 17th-century portrait of Charles II of Spain, the disorder is clearly evident.

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Out with inbreeding -

Today, cultural taboos, parental education, and greater awareness of inbreeding consequences have played significant roles in minimizing inbreeding frequencies in areas like Europe. But the custom is still practiced in other regions of the world.

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Global consanguinity rates -

It's estimated that one billion of the current global population live in communities with a preference for consanguineous marriage.

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Socially acceptable -

Consanguinity is a deeply rooted social trend among one-fifth of the world population mostly residing in the Middle East, West Asia, and North Africa.

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More public awareness -

But even in these regions there is mounting public awareness on prevention of congenital and genetic disorders in offspring due to inbreeding.

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Counseling on consanguinity -

And evidence suggests that an increasing number of couples contemplating marriage and reproduction in highly consanguineous communities are seeking counseling on consanguinity.

See also: Pregnancy myths people still believe

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