Later, a few have pronounced laughing that is truly paroxysmal or contagious, but more often, happy grimacing and a happy disposition are the predominant behaviors. Newborns appear to be physically well formed, but by 12 months of age some show a deceleration of cranial growth which may represent relative or absolute microcephaly. Most AS individuals have head circumferences less than the 25th percentile by age 3 years, often accompanied by a flattened occiput. Average height is lower than the mean for normal children but most AS children will plot within 2 SD of the mean. During infancy weight gain may be slow due to feeding problems but by early childhood most AS children appear to have near normal subcutaneous fat.
Obesity is rare but by late childhood some increased weight gain can occur.
Hypopigmentation is seen in AS and in most of these individuals is thought to be due to deletions of the P gene. It appears to code for a tyrosine transporter protein, presumably located within the melanosome membrane and crucial in cell membrane transport of tyrosine, an important precursor in melanin synthesis. Deletions of the P allele in AS appear to result in skin and ocular hypopigmentation. Such children typically have blond or sand-colored hair, light blue iris color and fair skin complexion and can be suspected of having a form of albinism. Classes of AS that do not involve deletion of the P gene would not be expected to show hypopigmentation but apparent hypopigmentation has been noted in non-deletion cases, suggesting that additional factors, other than deletion of the P allele, may also be operative.