Most are familiar with simple Mendelian inheritance: One copy, or allele, of a particular gene comes from your mother, and the other from your father. If one gene is defective, usually, if you have an effective copy from the other parent, you’re asymptomatic - and it doesn’t matter which allele comes from which parent. But what happens when Mendelian genetics gets thrown to the winds?
Prader-Willi Syndrome and Angelman Syndrome are twin examples of this phenomenon in action. Contrary to normal Mendelian, these two follow quite a strange pattern of inheritance: If the defective gene comes from the father, the offspring has Prader-Willi Syndrome, and the maternal allele is silenced; and if the defective gene comes from the mother, the child has Angelman Syndrome, and the paternal allele is silenced. These two syndromes are classic examples of genomic imprinting, and come with varying sets of symptoms and severity.
Prader-Willi Syndrome tends to be more severe than Angelman Syndrome, and is characterised by low muscle tone, short stature, incomplete sexual development, cognitive disabilities, problem behaviours, and a chronic feeling of hunger that can lead to excessive eating and life-threatening obesity. Angelman Syndrome is characterised by developmental impairment, speech delay, balance disorder, and “behavioural uniqueness” - usually apparent through frequent laughter or smiling and an overall excessively happy demeanor.
Images above show sperm, top, and an egg being fertilised, bottom.