Graphical approach to evaluate genetic estimates of calf survival

Genetic variation and resemblance among relatives are fundamentals of quantitative genetics. Our purpose was to identify bulls with a bimodal pattern of inheritance in the quest for new discoveries about the inheritance of calf survival. A bimodal pattern of inheritance for calf survival was identified in sons of Holstein bulls. A bimodal pattern of inheritance indicates 2 groups of sons resulting from an allele effect, a grandsire effect, or some other common factor. Different combinations (AA, Aa, aa) of 2 alleles at a locus cause varying phenotypes to be expressed. Bulls that are heterozygous for loci affecting reproductive performance may have a bimodal pattern of inheritance if the difference in effect of the 2 alleles is large. If the bimodal pattern is caused by an allele effect, then molecular markers can be identified for use in marker-assisted selection breeding programs. Data on predicted transmitting ability for perinatal survival for the first parity of 8,678 sons of 599 sires were collected from 1984 through 1997 from the National Association of Animal Breeders calving ease database, which included 7 Midwestern states. Sixteen bulls were identified with a potential bimodal pattern of inheritance because they had 2 distinct groups of sons. The 2 groups of sons were separated by calculating the coefficient of variation for each possible combination of sons; the combination that gave the smallest coefficient of variation difference between the 2 groups was considered the correct distribution of the sons into those groups. Bulls with a bimodal distribution were analyzed to determine the distribution of the grandsons among the maternal grandsires (MGS) of the 2 groups of the bimodal distribution. The bimodal distribution may be a result of heterozygous sires or MGS that are homozygous for low or high survival. If the bimodal distribution is caused by a MGS effect, then marker-assisted selection can still be used by evaluating the MGS instead of the sires.