Autosomal and X-linked additive genetic effects on body weight, body measurements and efficiency-related traits in sheep

Abstract

This study aimed to measure the contribution of autosomal and X chromosome to the phenotypic variation of growth traits including birth weight (BW), weaning weight (WW) 6-month weight (W6) and pre- and post-weaning average daily gains (ADGa, ADGb), body measurements including body length (BL), height at wither (HW), height at back (HB), heart girth (HG) and scrotal circumference (SC) as well as efficiency related traits including pre- and post-weaning Kleiber ratio (KRa, KRb), pre- and post-weaning efficiency of growth (EFa, EFb) and pre- and post-weaning relative growth rate (RGRa, RGRb) in Makooei sheep. To this end, REML procedure fitting a series of 4 uni-variate animal models including various combinations of autosomal, X-linked and maternal effects were fitted to estimate variance components and corresponding genetic parameters. Bi-variate analyses were also applied to estimate (co)variances between traits. Estimates of autosomal heritability (ha2) for growth traits ranged from 0.09 (ADGb) to 0.29 (BW). For body measurements, ha2 was in the range between 0.01 (HW) and 0.42 (SC) and for efficiency related traits it was in the range between 0.08 (KRb and RGRb) to 0.13 (KRa). Except for body measurements, other traits studied were affected by maternal effects. The effects of X-linked loci emerged after weaning in a way that for average daily gain and efficiency-related traits measured during post-weaning growth phase, the estimates of the X-linked heritability (hs2) were comparable to autosomal heritability and ranged between 0.04 (KRb) to 0.07 (EFb). The highest hs2 were observed for height of animals measured at back and wither as 0.12 and 0.13, much higher than corresponding ha2 as 0.01 and 0.04, respectively. The autosomal genetic correlation ranged between 0.05 (EFb-HW) to 0.99 (HW-HB). The X-liked genetic correlation was in the range from 0.16 (RGRb-ADGb) to 0.99 (HW-HB) which showed that a part of genetic (co)variance between studied traits originated from activity of loci on X chromosome. The correlation between breeding values including and excluding X-linked effects decreased as the X-linked genetic effects increased. The lowest correlation was observed for height at wither as 0.85 indicating the importance of including X-linked genetic effects in the procedure of genetic evaluation. It was concluded that in order to increase the accuracy of genetic evaluation, separating X-linked effects from autosomal effects is necessary especially for traits which affected strongly by X-linked loci.