Cause of developmental delay.

FMR1 gene is on X chromosome, obviously, and is a trinucleotide repeat disorder (along with Friedrich’s ataxia, myotonic dystrophy, Huntington disease etc), so inheritance is interesting.

Dads can carry gene, but only pass it on to their daughters (who will all get it).

Mums will carry gene on 1 chromosome, so sons and daughters can both get it, but 50:50 chance.

As with other trinucleotide repeat disorders, gene expands with each generation, so risk of disease increases from 1 generation to the next, and this is somewhat predictable: intermediate gene (so 45-54 copies) won’t expand to cause disease (200+ copies) in 1 generation, but premutation gene (55-199) copies probably will.

Features:

• Moderately severe learning disability
• Facial features – long face, midface hypoplasia, large lips and jaw, small ears
• Macro-orchidism

Females less severely affected, of course.