Here is from Wikipedia:
Single palmar creases are less common than two palmar creases, however 10% of the population have one palmar crease on one hand and 5% have one palmar crease on both hands. It is sometimes associated with Down's syndrome, though other physical symptoms would be manifest with this relatively rare 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.
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), Klinefelter syndrome, Noonan syndrome (chromosome 12), Patau syndrome (chromosome 13), Edward's syndrome (chromosome 18), and Aarskog-Scott syndrome (X-linked recessive). A unilateral single palmar crease was also reported in a case of chromosome 9 mutation causing multiple basal cell nevus syndrome and Robinow/brachydactyly 1 syndrome . It is also sometimes found on the hand of the affected side of patients with Poland Syndrome and craniosynostosis.
Although all of that sounds daunting, it did say it can be associated with something like these but is not always associated with it. In your shoes, I would have your child evaluated. Don't just struggle along with this alone or try to diagnose by doing tests on the Internet. See your pediatrician and ask about having your daughter assessed. You need to know if the difficulties are from something that can be named or if she is just being difficult.
thank you, i have a appointment to take her to the doctor this week so hopefully they can put us on the track to some assessments.