Quantitative trait loci affecting egg quality -- albumen thickness, blood and meat spots, fishy taint and shell durability -- were identified through the mapping of the chicken genome. The results of a recent doctoral study enhance the efficiency of chicken breeding.
In her doctoral study, MTT Research Scientist Mervi Honkatukia charted chromosomal areas of the genotype that affect egg albumen quality. One of the areas was divided into two distinct parts, both of which had an impact on albumen quality.
"Thick albumen is considered to be a sign of freshness. Biochemical reactions cause albumen thinning as the egg ages, but sometimes even the albumen of fresh eggs is thin. This is a hereditary trait," Honkatukia says.
Pores or fractures in the egg shell may allow microbes to enter the egg. The yolk is susceptible to contamination; the albumen 'anchors' the yolk in place at the centre of the egg to prevent contact with the shell.
Spots eliminated through breeding
Blood and meat spots are internal quality defects sometimes found in eggs. Although they do not involve a risk to human health, they may slightly raise the egg infection risk and lower the reproductive performance of eggs for hatching. Spots are primarily an aesthetic problem.
The study was the first reported study to employ gene mapping tools to blood and meat spots. It was discovered that there is a link between susceptibility to blood and meat spots and a specific region of chromosome Z; the candidate gene ZO-2 located in the region and the gene's internal regulation factors appeared to have an impact on the prevalence of blood and meat spots in different populations.
"Although the quantitative trait loci only explain around two per cent of phenotypic variance, any genetic progress achieved through breeding is permanent and cumulative, in other words, produces a compound impact from generation to generation. The results can be used in breeding selection to root out undesirable traits," the research scientist summarises.
In addition to genotype, the living conditions of chickens also affect the frequency of blood and meat spots: an incorrect diet, temperature fluctuations and sudden noises increase the occurrence of spots.
Fishy taint is a gene defect
Honkatukia's third research area was the fishy-taint problem, which a quality issue triggered by feed made from rapeseed, for example. A 2005 study by MTT used gene mapping tools to identify the gene defect causing the quality-reducing odour defect. Honkatukia's dissertation expands on that study.
"The simple feather test we developed can be used to determine whether a specimen has a gene defect and whether an unaffected specimen is carrying a gene defect. The results have been patented; the breeding company owning the patent, Lohmann Tierzucht GmbH, has also applied the results to practice."
Information on genotype and DNA markers enable the identification of hereditary traits at an early stage, long before their manifestation. This enables increased breeding efficiency. Gene markers are also helpful when a specific trait is gender-confined or can only be assessed for one gender. This makes it easier to root out quality defects.
"The reduction in the number of discarded eggs achieved through gene testing improves the profitability of egg producers. Those who stand to gain the most out of all this are quality-conscious consumers," Honkatukia concludes.
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