Inheritance which follows the laws which Mendel proposed. A parent has two copies of a gene at a particular locus, called alleles - one or other of these alleles is transmitted into an individual egg or sperm.
This is called the law of segregation, and is why there is a 1 in 2 probability that an offspring will inherit a particular allele of a particular pair of alleles. Also, considering more than one pair of alleles, the two alleles of pair A will be inherited independently from the two alleles of pair B if they are on a different chromosomes. This is the law of independent assortment and is one of the reasons explaining why some brothers and sisters can have completely different physical characteristics!
Disorders inherited in this way are called Mendelian, monogenic or single gene disorders and are caused by mutations that occur in the DNA sequence of a single gene. More than 6,000 human diseases are caused by single gene disorders. They are all individually rare but occur in about 1.4% of the population as a whole and cause a high rate of morbidity and mortality.
Single-gene disorders are inherited in recognizable patterns and are classified according to how they are transmitted from one generation to the next. If the mutation is on an autosome, the condition is said to be inherited in an autosomal pattern. If the mutation is on the X-chromosome, the condition is said to be inherited in an X-linked manner. Autosomal dominant, autosomal recessive and X-linked recessive conditions are the most common. X-linked dominant conditions also occur, but are rare.