Inheritance of single-gene disorders

[Vania]

Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body.
Chromosomes are made of a very long strand of DNA and contain many genes (hundreds to thousands). Except for certain cells (for example, sperm and egg cells), every normal human cell contains 23 pairs of chromosomes. There are 22 pairs of nonsex (autosomal) chromosomes and one pair of sex chromosomes, for a total of 46 chromosomes. Normally, each pair consists of one chromosome from the mother and one from the father.
The sex chromosomes determine whether a fetus becomes male or female. A male has one X and one Y sex chromosome. The X comes from his mother and the Y comes from his father. A female has two X chromosomes. One X comes from her mother and the other X comes from her father.
The traits (any gene-determined characteristic, such as eye color) produced by a gene can be characterized as

Dominant
Recessive

Dominant traits are expressed when even one copy of the gene for that trait is present.
Recessive traits carried on autosomal chromosomes can be expressed only when two copies of the gene for that trait are present because the corresponding gene on the paired chromosome that is not for the trait is usually expressed instead. People with one copy of an abnormal gene for a recessive trait (and who thus do not have the disorder) are called carriers.
With codominant traits, both copies of a gene are expressed to some extent. An example of a codominant trait is blood type. If a person has one gene coding for blood type A and one gene coding for blood type B, the person has both A and B blood types expressed (blood type AB).
An X-linked (sex-linked) gene is one that is carried on an X chromosome. X-linking also determines expression. Among males, almost all genes on the X chromosome, whether the trait is dominant or recessive, are expressed because there is no paired gene to offset their expression.