Autosomal Inheritance

Autosomal Dominant:

• Affected offspring only need to have one affected parent (heterozygous individuals will still express the trait due to dominant masking)
• The condition appears in every generation when analyzing the pedigree.
• A dominant affected individual can have one or two dominant alleles (AA or Aa).

Autosomal Recessive:

• In order for offspring to be affected, both of its parents must AT LEAST be carriers (heterozygous OR homozygous recessive).
• In other words, a recessive affected individual MUST have two recessive alleles (aa).
• The condition may not appear in every generation.
• Note that individuals who are carriers do not exhibit symptoms.

Consider the following Pedigree:

• Affected individuals are only observed in one generation, so it is unlikely that it is dominant. It must be recessive.
• Furthermore, BOTH a male and a female are affected, so it must be autosomal.
• Therefore, the mode of inheritance for this pedigree is autosomal recessive.

Sex-Linked Inheritance

Traits must be carried by either the X or Y chromosome (sex chromosomes).

• X-linked inheritance is the most common type of sex-linked inheritance, because the X chromosome contains about 5 times more genes than the Y chromosome.
• Recessive X-linked inheritance is always expressed in males (who have only one X) and in females ONLY if they inherit two copies of the mutated gene.
• Dominant X-linked inheritance will be expressed in males and in females who inherit EVEN ONE copy of the mutated gene.

The following diagram effectively displays the outcomes of X-linked Recessive Inheritance:

Written by Josephine Ankomah