Author
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Riley, David |
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CROCKETT, J - UNIVERSITY OF FLORIDA |
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Submitted to: Florida Cattleman
Publication Type: Trade Journal Publication Acceptance Date: 2/7/2006 Publication Date: 3/1/2006 Citation: Riley, D.G., Crockett, J.R. 2006. Heterosis Retention and the Dominance Model in Florida Beef Research. Florida Cattleman. 70(6):42,44,46. Interpretive Summary: Technical Abstract: The heterosis retained and expressed by crossbred cattle for a variety of traits has been and still is an appropriate area for genetic research. It has been studied in detail in British and European cattle since the 1980s in Nebraska. Results involving Brahman and other breeds adapted to the tropics and subtropics are scarce and, as yet, inconclusive. This issue has strong potential to affect the profit of cow-calf producer as much or more than many other issues. Heterosis is also known as hybrid vigor. Heterosis is the difference (usually the superiority) between the performance of crossbred cattle and the average of the purebreds that make up the crossbred. Heterosis is measured for individual traits, and it is either discussed in trait units (e.g., 50 lb) or as a percentage of the purebred average. Heterosis is especially beneficial for traits that are difficult or time-consuming to improve through selection, such as reproductive traits, calf survival, and longevity traits. Many other traits respond to heterosis, but not to the extent of these. Generally, breeds that are extremely different have larger heterosis expressed than breeds that are relatively similar to each other. Heterosis is simple to employ; crossbred matings are made, particularly, crossbred females are used. Dominance Model of Heterosis Expression The expression of heterosis is believed to be mostly determined by dominance effects at many genes. This model (a model is an approach for understanding) assumes several things, the most practical being that heterosis expression is proportional to breed heterozygosity. A gene (a gene is also frequently referred to as a locus) is breed heterozygous if its two copies (these copies are usually called alleles; they are sometimes different forms or versions [but sometimes the same] of the same gene) are from different breeds. Every bovine cell has two approximately six foot lengths of DNA; this entire complement of DNA is called a genome. Both of these strings have every bovine gene. One of these DNA strings is of paternal origin (sperm) and one is of maternal origin (egg). For example, when a Hereford bull and Brahman cow are mated, the calf resulting from this mating will have one string of DNA that is entirely Hereford in origin and one that is entirely Brahman in origin. If we aligned these two strings of DNA where we could view the different alleles for each gene or locus directly across from each other, there would be one allele of Hereford origin and one of Brahman origin at every point in the genome (100% breed heterozygosity). This first cross (F1) animal expresses the maximum heterosis for cattle traits; in fact, F1 performance determines the heterosis for a trait. The performance of crossbred animals other than F1 animals can be predicted similarly. For example, if a female produced in the above cross (Hereford bull X Brahman cow) is mated to a Hereford bull (back cross), the resulting calf has one string of DNA entirely of Hereford (paternal) origin. However the string provided from the F1 cow has undergone recombination (exchange of segments which occurs during meiosis), which makes it a mixture of the strings from her sire and dam. We don’t know exactly what fraction of that recombined string of DNA (egg) is Hereford or Brahman, but statistically, our best estimate is ½ Hereford and ½ Brahman. If we again align the two strings from cells in the ¾ Hereford ¼ Brahman calf, we would see that about ½ of the genome has one Hereford allele and one Brahman allele; the remainder would have Hereford alleles on both strings, which would contribute nothing to heterosis. The ¾ Hereford ¼ Brahman calf is therefore 50% breed heterozygous, and according to the dominance model, we predict that it will express or retain ½ of the heterosis of the F1. This retained heterosis is usually discussed as a |