(Solved): A pedigree for fragile X syndrome is shown below. Recall that that this neurological disease is cau ...

A pedigree for fragile $X$ syndrome is shown below. Recall that that this neurological disease is caused by expansion of a trinucleotide repeat in the $5^{?}$ UTR of the FMR-1 gene, which is on the $X$ chromosome. Why can males inherit a disease allele from unaffected mothers? (Check all reasons.) (b) A fragile $X$ pedigree Unaffected Affected Heterozygous or hemizygous for pre-mutation allele Check All That Apply The spontaneous mutation rate for trinucieotide repeat genes is much higher than the normal mutation frequency of genes without trinucleotide repeats. The mothers of the affected sons in this pedigree had pre-mutation alleles whose trinucleotide repeat regions are larger than normal and thus have a high chance of expanding to become disease olleles. Females cannot have fragile $X$ syndrome even if they are homozygous for disease alleles. In the brain cells of females, the trinucleotide repeat regions always contract so that disease alleles never exist in the female brain. Females heterozygous for a disease allele and a normal FMR-f allele can show symptoms of fragile $X$ syndrome.