Fragile X syndrome is X-linked dominant inheritance associated with a trinucleotide repeat in FMR1 (FA2020 p62).
Even if you did not know this fact, fragile X syndrome (a disease affecting the X chromosome) should be X-liked of some sort.
Knowing the disease is X-linked, you can get to the answer because in the couple, the father is the only one with a family history of the disease. Whether the disease was X-linked recessive or X-linked dominant, if the father had the diseased X-chromosome he would have the disease (as males only have one X chromosome). Since the father does not have the disease, he does not have the chromosome and thus cannot pass it on. Thus, the chance of their child having the disease is 0%
the_enigma28Any reason why II-2 & II-3 females are not shown to be affected??+3
yhm17Possibly due to X-inactivation leading to mosaicism so they wouldn't demonstrate the phenotype. +14
specialist_jelloMy thought process was trinucleotide repeat diseases show anticipation
Stem says "SIMILAR EXPANSION" so the number of repeats cant be same generations later.+2
hivwizardYeah i figured that the chances of the next generation having the same repeat expansion as generation 0 would be really low +1
submitted by โcassdawg(1781)
Fragile X syndrome is X-linked dominant inheritance associated with a trinucleotide repeat in FMR1 (FA2020 p62).
Even if you did not know this fact, fragile X syndrome (a disease affecting the X chromosome) should be X-liked of some sort.
Knowing the disease is X-linked, you can get to the answer because in the couple, the father is the only one with a family history of the disease. Whether the disease was X-linked recessive or X-linked dominant, if the father had the diseased X-chromosome he would have the disease (as males only have one X chromosome). Since the father does not have the disease, he does not have the chromosome and thus cannot pass it on. Thus, the chance of their child having the disease is 0%