The mode of inheritance is typically applied to Mendelian or Monogenic disorders. It is quite complex to establish an individual’s risk of inheriting a complex or multifactorial disorder. Monogenic conditions can be categorized depending on their transfer from one generation to the next.
· Autosomal Dominant conditions. are passed down from an affected parent with a 50% risk for each child. For example, Myotonic dystrophy, Huntington’s disease, Polycystic kidney disease (PKD), Marfan syndrome, Neurofibromatosis, . Autosomal dominant disorders can also result from new (de novo) variants that may arise during the formation of egg or sperm cells or in early embryonic development.
Autosomal Recessive conditions require the inheritance of two abnormal copies of the gene. The statistical risk for each child of carrier parents is as follows:
25% probability that two abnormal copies of the gene are inherited (child is affected)
50% probability that one normal and one abnormal copy of the gene are inherited (child is a carrier just like his parents)
25% probability that two normal copies of the gene are inherited (child is unaffected by the condition).
· X-Linked Dominant conditions result from genes located on the X chromosome. Generally, men are more seriously affected by such disorders. The possibility of transmission of the X-linked dominant disorder varies in males and females. The trait never gets transmitted to the sons by a male and all daughters would be affected. There is a 50% probability for an affected female of having an affected child in each pregnancy.
· X-Linked Recessive conditions result from genetic modifications on the X chromosome. Men are more often affected in comparison to females. An affected female has a 50% probability of having an affected son or carrier daughter. An affected male has a 50% probability of having a carrier daughter but cannot pass the condition on to his sons. Examples include Duchenne Muscular Dystrophy, Hemophilia A, and Lesch-Nyhan syndrome.
· Y-linked conditions are only transmitted from males to their sons (100% probability), as women can never be affected since they lack the Y chromosome.