DNA POLYMORPHISMS IN THE CHICKEN GROWTH-HORMONE GENE - RESPONSE TO SELECTION FOR DISEASE RESISTANCE AND ASSOCIATION WITH EGG-PRODUCTION

Analysis of the growth hormone (GH) gene in 12 strains of White Leghor n chickens revealed restriction fragment length polymorphisms (RFLPs) at three MspI sites and at a SacI site. Based on linkage disequilibriu m analysis, they gave rise to eight different alleles (i.e, combinatio ns of RFLPs), with five occurring at frequencies above 5% in at least one strain. Pairs of GH-RFLPs were at near maximal linkage disequilibr ium, suggesting either a lack of recombination or the presence of sele ction pressure during evolution of the GH gene. Allele frequencies wer e determined in 12 non-inbred strains derived from three different gen etic bases. These strains had been selected either for an array of egg production traits, resistance to Marek's disease or resistance to avi an leukosis. Selection for disease resistance was consistently correla ted with an increase in the frequency of one of the alleles. One strai n segregated for only two alleles, which differed by three RFLPs. Anal ysis of variance in this strain indicated that the GH allele co-select ed with resistance was associated with a delayed onset of ovulation bu t a higher persistency of ovulation as age progressed, resulting in an overall increase of egg production by 15% (age at first egg to 497 da ys). The resistance-associated GH allele was dominant for the onset of ovulation and recessive for the persistency of egg production. There was no significant effect of the GH genotype on juvenile body weight, egg weight or egg specific gravity.