Question: How many variations of a gene does a person normally carry?
About the article "Nucleic acid sequence", section "Genetic testing".

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A common kind of call is Voice Carry Over (VCO).
VCO allows a person who is Hard-of-Hearing or Deaf and does speak to use one's voice while receiving responses from a person who is Hearing via the operator's typed text.
There are many variations of VCO, including 2-Line VCO and VCO with privacy. 
Normally, a person inherits two copies of the gene that produces beta-globin, a protein needed to produce normal haemoglobin (hemoglobin A, genotype AA).
A person with sickle cell trait inherits one normal gene and one abnormal gene encoding haemoglobin S (haemoglobin genotype AS). 
Knocking out the activity of a gene provides information about what that gene normally does.
Humans share many genes with mice.
Consequently, observing the characteristics of knockout mice gives researchers information that can be used to better understand how a similar gene may cause or contribute to disease in humans.
Examples of research in which knockout mice have been useful include studying and modeling different kinds of cancer, obesity, heart disease, diabetes, arthritis, substance abuse, anxiety, aging and Parkinson's disease.
Knockout mice also offer a biological and scientific context in which drugs and other therapies can be developed and tested.
Millions of knockout mice are used in experiments each year. 
Many screening variations have been devised to elucidate a gene that leads to a mutant phenotype of interest. 
A genetic carrier (or just carrier), is a person or other organism that has inherited a genetic trait or mutation but does not display that trait or show symptoms of the disease.
Carriers are, however, able to pass the gene onto their offspring, who may then express the gene.
This phenomenon is a direct result of the recessive nature of many genes.
For example, the daughters of Queen Victoria, Princesses Alice and Beatrix, were carriers of the X-linked hemophilia gene (an abnormal allele of a gene, necessary to produce one of the blood clotting factors).
Both had children who continued to pass on the gene to succeeding generations of the royal houses of Spain and Russia, into which they married.
Males who carried the altered gene had hemophilia, while females simply passed it to nearly half of their children.
Up to 1 in 25 individuals of Northern European ancestry may be considered carriers of mutations (CFTR genes) that can lead to cystic fibrosis.
The disease appears only when two of these carriers have children, as each pregnancy between them will have a 25% chance of producing a child with the disease.
However, it is also thought that carriers may be more resistant to diarrhea during typhoid fever or cholera, and are therefore not truly asymptomatic.
This resistance leads to increased survival of the carriers, thereby increasing the frequency of the altered genes in the population.
Genetic carrier testing can be used to tell if a person carries one or more mutations of the CF gene and how many copies of each mutation.
The test looks at a person’s DNA (genetic material), which is taken from cells in a blood sample or from cells that are gently scraped from inside the mouth.
Although only about 1 of every 3,000 Caucasian newborns has CF, there are more than 1,000 known mutations of the gene that causes CF.
Current tests look for the most common mutations.
The mutations screened by the test, vary according to a person's race or ethnic group, or by the occurrence of CF already in the family.
More than 10 million Americans, including 1 in 29 Caucasian Americans, are carriers of one mutation of the CF gene.
In other races or ethnicities, 1 in 46 Hispanic Americans, 1 in 65 African Americans, and 1 in 90 Asian Americans carry a mutation of the CF gene.