Practice question

X chromosome transmission shows characteristic patterns: sons receive X from mother, daughters receive X from both parents. STR allele unexplained by maternal or paternal haplotypes suggests new mutation altering repeat number rather than SNP interference or VNTR artifact. Microsatellite mutation rate is elevated due to replication slippage, generating novel alleles in germline. Maternal inheritance explains expected allele but not unexpected size variant. Therefore appearance of atypical X-linked STR allele most plausibly results from de novo mutation expanding or contracting repeat tract within X locus.