|Silva Del Rio, Noelia - UNIV OF WISCONSIN-MADISON|
|Fricke, Paul - UNIV OF WISCONSIN-MADISON|
Submitted to: Journal of Dairy Science
Publication Type: Abstract Only
Publication Acceptance Date: March 10, 2008
Publication Date: July 7, 2008
Citation: Silva Del Rio, N., Broderick, G.A., Fricke, P.M. 2008. Mathematical simulation to assess the validity of Bonnier's equation for estimating the frequency of monozygous twinning in a population of Holstein cattle [abstract]. Journal of Dairy Science. 91(suppl. 1):241. Technical Abstract: Twin calving records (n = 96,069) collected from 1996 to 2004 were extracted from Minnesota Dairy Herd Improvement archives to estimate the incidence of monozygous (MZ) twinning in a population of Holstein cattle and to evaluate how varying the twin sex ratio and frequency of same-sex twins affects MZ estimates made, using Bonnier’s equation. Bonnier’s equation: m=2npq-n2/2pq(n-n2), estimates the proportion of MZ twins among same-sex twins (m) based on total opposite-sexed twin pairs (n2) and the observed proportions of male (p) and female (q=1-p) calves among all twin births. Bonnier’s equation assumes the sex of one twin is independent of the other; therefore, similar proportions of same- and opposite-sex twin pairs would be expected in the absence of MZ twinning. We hypothesized a dramatic decrease in Bonnier’s estimate of MZ twinning if same-sex twins comprise a smaller proportion of a population than expected. Based on our study population, 56.4% of twin pairs were same-sex (30.1% MM; 26.3% FF) and 51.9% of twin calves were male, resulting in an estimated MZ twin frequency of 11.6% using Bonnier’s equation. The estimates of MZ twinning were calculated by simulating a reduction of same-sex twins of 5% (54.2% same-sex twins) or 10% (52.0% same-sex twins), whereas the proportion of male calves born as twins was 51.9% (observed) or simulated to be 50%. Based on our study population, the estimates of MZ twinning were greater than expected, based on observed outcomes of MZ twinning (Silva del Rio et al., Therio. 66:1292;2006). We concluded that slight changes in the percentage of same-sex twins in a study population dramatically affect MZ estimates using Bonnier’s equation, whereas the percentage of male calves born as twins has a minimal impact. Thus, if factors other than MZ twinning affect the proportion of same-sex twins in a study population, Bonnier’s equation will inaccurately estimate the frequency of MZ twins.