Pedigrees/X linked Dominant

[MedPR]

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This TBR explanation says that the key to identifying an X-linked dominant is to find the pedigree in which all daughters and none of the sons of males who carry the trait are affected. Why is this? Do boys always get their X from the mother?

 

[chiddler]

This TBR explanation says that the key to identifying an X-linked dominant is to find the pedigree in which all daughters and none of the sons of males who carry the trait are affected. Why is this? Do boys always get their X from the mother?

Mother has XX. So her gametes are either X, or X. Father is XY, so gametes are X or Y. Therefore, for a male to form, X from mom Y from dad. The X is always from mom.

Furthermore, any X sex linked diseases are always expressed with boys because it is the only chromosome present so there is no homologous chromosome present to fix the disease by complementation.

 

[Ssina]

I think they say it that way so you can ID it faster on the exam from a pedigree since diseases fall into 4 categories... E.g. recessive skips generations while dominant doesn't.

and when it comes to X dominant... I look to the dad.. If he has it then all daughters will have it... If mom has it then only half of children (boys or girls) will have it.

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[SLC]

It's explained pretty well above, but to simplify it (remember, simple is good).

A male has an XY pair of sex chromosomes. Female has XX.

If a male is conceived, the Y has to come from the dad, since the mom has no Y to give.

Therefore If the male has the disease he passes on only his disease free Y chromosome to his sons (no disease) and only his diseased X chromosome to his daughters (disease). Therefore the daughters are affected, and the sons are free from the disease.

It's as simple as that!