Many such single-gene defects can decrease the fitness of affected people and are therefore present in the population in lower frequencies compared to what would be expected based on simple probabilistic calculations.

An affected person usually has unaffected parents who each carry a single copy of the mutated gene and are referred to as genetic carriers.

[23] Two unaffected people who each carry one copy of the mutated gene have a 25% risk with each pregnancy of having a child affected by the disorder.

[24][25] Some autosomal recessive disorders are common because, in the past, carrying one of the faulty genes led to a slight protection against an infectious disease or toxin such as tuberculosis or malaria.

Exceptions to this finding are extremely rare cases in which boys with Klinefelter syndrome (44+xxy) also inherit an X-linked dominant condition and exhibit symptoms more similar to those of a female in terms of disease severity.

The sons of a man with an X-linked dominant disorder will all be unaffected (since they receive their father's Y chromosome), but his daughters will all inherit the condition.

A woman with an X-linked dominant disorder has a 50% chance of having an affected foetus with each pregnancy, although in cases such as incontinentia pigmenti, only female offspring are generally viable.

The sons of a man with an X-linked recessive disorder will not be affected (since they receive their father's Y chromosome), but his daughters will be carriers of one copy of the mutated gene.

X-linked recessive conditions can sometimes manifest in females due to skewed X-inactivation or monosomy X (Turner syndrome).

More simply, this means that Y-linked disorders in humans can only be passed from men to their sons; females can never be affected because they do not possess Y-allosomes.

[citation needed] Y-linked disorders are exceedingly rare but the most well-known examples typically cause infertility.

Because only egg cells contribute mitochondria to the developing embryo, only mothers (who are affected) can pass on mitochondrial DNA conditions to their children.

[32] Genetic disorders may also be complex, multifactorial, or polygenic, meaning they are likely associated with the effects of multiple genes in combination with lifestyles and environmental factors.

Studies that aim to identify the cause of complex disorders can use several methodological approaches to determine genotype–phenotype associations.

One method, the genotype-first approach, starts by identifying genetic variants within patients and then determining the associated clinical manifestations.

This is opposed to the more traditional phenotype-first approach, and may identify causal factors that have previously been obscured by clinical heterogeneity, penetrance, and expressivity.

Most genetic disorders are diagnosed pre-birth, at birth, or during early childhood however some, such as Huntington's disease, can escape detection until the patient begins exhibiting symptoms well into adulthood.

Many genetic disorders affect stages of development, such as Down syndrome, while others result in purely physical symptoms such as muscular dystrophy.

[39] Despite this, most treatment options revolve around treating the symptoms of the disorders in an attempt to improve patient quality of life.

[5] The earliest known genetic condition in a hominid was in the fossil species Paranthropus robustus, with over a third of individuals displaying amelogenesis imperfecta.