The presence of a single transverse palmar crease can be, but is not always, a symptom associated with abnormal medical conditions, such as Fetal alcohol syndrome, or with genetic chromosomal abnormalities, including Down syndrome (chromosome 21), cri du chat syndrome (chromosome 5), Noonan syndrome (chromosome 12), Patau syndrome (chromosome 13), Edward's syndrome (chromosome 18), and Aarskog-Scott syndrome (X-linked recessive). Bilateral or unilateral single palmar creases are also associated with aberrations on chromosome 9. Also sometimes found on hand of affected side of patients with Poland Syndrome.
Males are twice as likely as females to have this characteristic, and it tends to run in families. In its non-symptomatic form, it is more common among Asians and native Americans than among other populations, and in some families there is a tendency to inherit the condition unilaterally, that is, on one hand only. wikipedia