fundamentals
Inheritance of Single Gene Disorders
Introduction
 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 or code for functional RNA molecules. Chromosomes are made of a very long strand of DNA and contain many 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 traitsare expressed when even one copy of the gene for that trait is present. Recessive traitscarried on autosomal chromosomes can be expressed only when two copies of the gene for that trait are present, one on each of the pair of chromosomes. People with one copy of an abnormal gene for a recessive trait (and who thus do not have the disorder) are called carriers. Withcodominant 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. 