X-linked Inheritance

Everybody has a paired combination of X and Y chromosomes, with women being XX and men being XY.  Moms can only contribute an X chromosome, while dads can contribute either an X or Y, resulting in a girl or boy baby respectively.

The X chromosome can have dominant and recessive genes, and those manifest themselves in different ways between males and females.  A recessive gene (a) will manifest its characteristics in men since they have just one copy of the X chromosome.  Depending on whether a woman’s other X chromosome has a dominant or recessive version of that gene, the result is the woman is either a carrier of the characteristic (Aa combo) or manifests that characteristic (aa combo).

Dominant genes from a mom will manifest themselves in either her sons or daughters.  The boys will manifest the characteristic since there’s only one X chromosome in their genome.  Women will show the characteristic even if her accompanying chromosome has a recessive version of the gene since the dominant gene will ‘overrule’ the recessive gene…a woman would have either an AA or Aa gene combination, both of which will result in the appearance of the characteristic. 

There are lots of nuances with how strongly a recessive or dominant gene will influence the physical outcome, along with the complexities of blending with the dad’s version of the gene, so please forgive this simplified explanation of genetic inheritance for the purposes of this post.

X was a hard letter!