Which disorder is associated with CGG trinucleotide expansion on the X chromosome, affecting the FMR1 gene and neural development?

Which disorder is associated with CGG trinucleotide expansion on the X chromosome, affecting the FMR1 gene and neural development?

• A. Fragile X syndrome
• B. Red-green color blindness
• C. Phenylketonuria (PKU)
• D. Lactose tolerance

Answer: (A)

This question tests recognition of a trinucleotide repeat expansion on the X chromosome that silences a neural development gene. Fragile X syndrome results from a CGG repeat expansion in the FMR1 gene on the X chromosome. When the CGG repeats exceed about 200, the FMR1 promoter becomes hypermethylated, the gene is silenced, and the essential FMRP protein is lost. Without FMRP, synaptic development and plasticity are disrupted, leading to intellectual disability, developmental delays, and often autistic-like features. It’s inherited in an X-linked pattern, with males typically more severely affected. Other options don’t fit because they involve different genes and mechanisms: red-green color blindness is due to mutations in X-linked opsin genes; phenylketonuria is autosomal recessive caused by PAH gene mutations; lactose tolerance relates to lactase persistence regulation, not the FMR1 gene or CGG repeats.

This question tests recognition of a trinucleotide repeat expansion on the X chromosome that silences a neural development gene. Fragile X syndrome results from a CGG repeat expansion in the FMR1 gene on the X chromosome. When the CGG repeats exceed about 200, the FMR1 promoter becomes hypermethylated, the gene is silenced, and the essential FMRP protein is lost. Without FMRP, synaptic development and plasticity are disrupted, leading to intellectual disability, developmental delays, and often autistic-like features. It’s inherited in an X-linked pattern, with males typically more severely affected.

Other options don’t fit because they involve different genes and mechanisms: red-green color blindness is due to mutations in X-linked opsin genes; phenylketonuria is autosomal recessive caused by PAH gene mutations; lactose tolerance relates to lactase persistence regulation, not the FMR1 gene or CGG repeats.