The mode of inheritance for the trait – Fragile X syndrome is

The correct answer is X-linked dominant.

Fragile X syndrome is a genetic disorder characterized by mild-to-moderate intellectual disability and cognitive impairment. The mode of inheritance for this condition is explained step-by-step below:

1. Genetic Basis: Fragile X syndrome is caused by the expansion of a CGG trinucleotide repeat in the FMR1 gene. This gene is located on the X chromosome.

2. X-Linked Inheritance: Because the causative gene (FMR1) is located on the X chromosome, the inheritance pattern of the trait is X-linked. This means the gene is passed down on sex chromosomes rather than autosomes (non-sex chromosomes).

3. Dominant Mode of Action: The trait is classified as dominant because a single mutated copy of the gene on the X chromosome is sufficient to cause the clinical phenotype in both males and females. Although females have two X chromosomes, having a mutation in just one of them can cause the syndrome. However, due to X-inactivation in females, their symptoms are often milder compared to males (who have only one X chromosome and are therefore hemizygous).

Consequently, Fragile X syndrome is inherited in an X-linked dominant pattern.