To the uninitiated, genetics may sound simple. You have a particular trait because your parents had it. Real-world genetics, however, is anything but simple.
To assert that a child has a trait (blond hair, asthma, a predilection for being overweight) because one of the parents had it is to presume that there is only one mode of inheritance. Unfortunately for the lay person (but fortunately for the employment of geneticists and genetic counsellors), traits can be inherited in many different ways.
Perhaps the simplest mode of inheritance is what is known as “autosomal dominant”. “Autosomes” constitute all chromosomes in a cell with the exception of the sex chromosomes, X and Y. A trait which is “autosomal” has its gene located on a non-sex chromosome. “Dominant” means exactly what the term conjures up: the presence of this trait will gag the expression of another. For example, Huntington’s disease is caused by…
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