**Question:** The following are autosomal dominant disorders except

A. X-linked recessive disorders
B. Mitochondrial disorders
C. Autosomal recessive disorders
D. Inheritance through the placenta (e.g., viral infections)

**Core Concept:** Autosomal dominant disorders are genetic conditions that are inherited from one parent with a normal copy of the gene and one mutated copy. These disorders affect individuals with two copies of the same sex chromosome (autosomes) and are inherited in a 1:1 ratio (i.e., offspring have a 50% chance of inheriting the disorder). The correct answer should be a type of inheritance pattern that does not fit this description.

**Why the Correct Answer is Right:** Mitochondrial disorders are inherited through the maternal or paternal mitochondria and not the autosomal chromosomes. Therefore, they are classified as mitochondrial disorders and not autosomal dominant disorders.

**Why Each Wrong Option is Incorrect:**

A. X-linked recessive disorders are inherited through the X chromosome, not autosomes. They are typically associated with half of the male population (XY) being affected, while females (XX) have a lower risk but can also be affected.

B. Mitochondrial disorders are not autosomal dominant disorders but rather a type of inherited disorder due to genetic material within mitochondria (mtDNA) inherited from the mother.

C. Autosomal recessive disorders are inherited through autosomes and involve the presence of two mutated alleles. Unlike autosomal dominant disorders, autosomal recessive disorders affect both males and females equally.

D. Inheritance through the placenta refers to disorders that are transmitted directly from the mother to the child via the placenta, such as viral infections. These are not classified as autosomal dominant disorders.

**Clinical Pearl:** Understanding the inheritance patterns of genetic disorders is crucial for diagnosing and counseling families about risk. It is essential to know the mode of inheritance (autosomal dominant, autosomal recessive, X-linked, mitochondrial, or placentally inherited) to predict the probability of inheritance in offspring and counsel families appropriately.