VIKANES, A et al. 2010. Recurrence of hyperemesis gravidarum across generations: population based cohort study. British Medical Journal 340:c2050. Vikanes 2010. Recurrence of HG across generations: cohort study This article is open access. The full text pdf can be downloaded from the link.
A review of literature relating to genetic factors can be seen at http://www.pregnancysicknesssupport.org.uk/healthcare-professionals/literature-review/genetic-factors-in-relation-to-nvp/