 genetic improvement in traits associated with seasonal breeding in sheep is challenging because these traits have low heritabilities , are generally not expressed until late in life , are commonly recorded only in females , and are expressed only in some lambing seasons and management systems . 
 detection of quantitative trait loci and their use in marker - assisted selection could therefore substantially enhance selection responses . 
 a population of sheep with an extended breeding season was developed through selection for fertility in spring matings and provides opportunities for further study of candidate genes influencing seasonal breeding . in particular , the melatonin receptor 1a gene is polymorphic in many sheep breeds and appears to influence a number of seasonal reproductive responses . 
 in addition , a variety of clock genes have been identified in laboratory mammals and shown to influence biological rhythms . 
 mutations in these clock genes have been identified and shown to influence circadian periodicities and reproductive patterns in golden hamster and mouse . in sheep , 
 expression of clock genes in the suprachaismatic nucleus and pars tuberalis ( pt ) suggests that " calendar " cells in the ovine pt play a role in maintaining circannual rhythms . 
 thus the various clock genes represent potentially important candidate genes that may be involved in control of seasonal breeding . 
None