Silva Del Rio, Noelia  UNIV OF WISCONSINMADISON  
Broderick, Glen  
Fricke, Paul  UNIV OF WISCONSINMADISON 
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 samesex twins affects MZ estimates made, using Bonnier’s equation. Bonnier’s equation: m=2npqn2/2pq(nn2), estimates the proportion of MZ twins among samesex twins (m) based on total oppositesexed twin pairs (n2) and the observed proportions of male (p) and female (q=1p) 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 oppositesex twin pairs would be expected in the absence of MZ twinning. We hypothesized a dramatic decrease in Bonnier’s estimate of MZ twinning if samesex twins comprise a smaller proportion of a population than expected. Based on our study population, 56.4% of twin pairs were samesex (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 samesex twins of 5% (54.2% samesex twins) or 10% (52.0% samesex 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 samesex 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 samesex twins in a study population, Bonnier’s equation will inaccurately estimate the frequency of MZ twins.
