The foundational principles of Mendelian inheritance are directly applicable to human genetics, explaining the transmission patterns of thousands of traits, including many inherited diseases. A single gene disorder (also known as a monogenic disorder) is a condition primarily caused by a mutation (alteration) in a single gene.

1. Autosomal Dominant Disorders:
2. Mechanism: Only one copy of the altered gene (on an autosome, a non-sex chromosome) is sufficient to cause the disorder. The affected individual usually has one mutated allele and one normal allele.
3. Inheritance Pattern: Affected individuals typically have an affected parent. The disorder does not skip generations. Males and females are affected equally. There is a 50% chance for an affected parent to pass the disorder to each child.
4. Example: Huntington's Disease. A progressive neurodegenerative disorder. If an individual inherits one copy of the dominant mutated huntingtin gene (e.g., 'H'), they will develop the disease, even if their other allele is normal ('h').

1. Autosomal Recessive Disorders:
2. Mechanism: Two copies of the altered gene (on an autosome) are required for an individual to be affected. Individuals with only one copy of the altered gene are called carriers; they are typically asymptomatic but can pass the altered gene to their offspring.
3. Inheritance Pattern: Affected individuals typically have unaffected parents (who are both carriers). The disorder often appears to "skip" generations. Males and females are affected equally. If both parents are carriers, there is a 25% chance for each child to be affected.
4. Example: Cystic Fibrosis (CF). A disorder affecting mucus and sweat glands. Caused by mutations in the CFTR gene. An individual must inherit two copies of the mutated recessive allele (e.g., 'ff') to have the disease. Carriers are 'Ff'.

1. X-Linked Recessive Disorders:
2. Mechanism: Caused by mutations on the X chromosome. Males have one X and one Y chromosome (XY); females have two X chromosomes (XX).
3. Inheritance Pattern: Affects males much more frequently and severely than females, as males only have one X chromosome. If a male inherits a recessive allele on his single X, he will express the trait. Females generally need two copies of the recessive allele to be affected, and if they have one normal allele, they are typically carriers with normal phenotype. Affected fathers cannot pass X-linked traits to their sons. Affected mothers pass the trait to all their sons.
4. Example: Red-Green Color Blindness. A common X-linked recessive disorder. Let X_C be the normal allele and X_c be the colorblind allele.

The concept of complementation is a powerful analytical tool in genetics, especially relevant in human genetics, to determine if two distinct mutations that produce the same abnormal phenotype are actually located in the same gene or in different genes. It provides insights into the genetic basis of complex traits and diseases.

• Core Principle: Complementation occurs when two individuals, both exhibiting the same recessive phenotype (meaning they are both homozygous for a recessive mutation causing that phenotype), produce phenotypically normal offspring when mated (or when their cells are fused). This outcome signifies that their respective mutations are located in different genes.