Long-Term Observations of Egg Size Inheritance in Closed Coturnix Quail Breeding Lines
Bryant's Roost Research Report
Version 1.0 – July 2026
Disclaimer
This working paper documents observations collected during a six-year closed breeding program maintained by Bryant's Roost. The hypotheses discussed are intended to compare these observations with existing scientific literature and to identify questions for future research. This report has not undergone peer review and should not be interpreted as establishing a causal genetic mechanism.
Abstract
Egg size is one of the most important selection traits in Coturnix quail. Larger eggs generally produce larger chicks, making egg size an important consideration for many breeders. While previous research has shown that egg size is highly heritable and controlled by multiple genes, there is little published information describing how egg size changes within long-term, closed breeding lines.
This paper summarizes observations from a six-year breeding program involving Egyptian and Pharaoh Coturnix quail. The Egyptian line originated from recessive birds identified within an established Pharaoh population. Once identified, both lines were maintained as closed breeding populations without outcrossing. Birds were managed under consistent nutrition, housing, and incubation practices throughout the project.
After six years of selection, the Pharaoh line has continued to produce hatchable eggs averaging 14–18 grams, while the Egyptian line consistently produces hatchable eggs averaging 12–14 grams, despite similar adult body size. Additional reciprocal crosses between unrelated White and Falb Fee lines also produced consistent differences in egg size depending on the direction of the cross.
Current research on heritability, quantitative trait loci, artificial selection, and parent-of-origin effects is reviewed as possible explanations for these observations. Although no conclusions regarding causation can be made, the consistency of these breeding lines over multiple generations suggests that additional research into the genetic control of egg size in Coturnix quail is warranted
Introduction
Egg size is one of the most economically and biologically important production traits in poultry. In addition to influencing marketability, egg weight is associated with chick weight at hatch, early growth, and overall reproductive efficiency. Because of these relationships, egg size has long been a target of selection in commercial breeding programs as well as among hobby and exhibition breeders.
Previous research has demonstrated that egg weight is a highly heritable quantitative trait influenced by numerous genes rather than a single locus (Narinc et al., 2015; Haqani et al., 2021). Previous research has demonstrated that egg weight is a highly heritable quantitative trait influenced by numerous genes rather than a single locus. Artificial selection experiments have demonstrated rapid responses to selection for egg size; however, at least one long-term study reported that continued selection for larger eggs eventually plateaued despite ongoing selection pressure, while selection for smaller eggs continued over successive generations (Pick et al., 2016).
Despite extensive research demonstrating the polygenic nature of egg size, relatively little information exists regarding long-term reciprocal breeding experiments conducted within closed Coturnix quail populations (Haqani et al., 2021).Likewise, relatively little information exists describing how parent-of-origin effects, sex-linked inheritance, or other genetic mechanisms influence egg size in long-term closed Coturnix breeding populations..
Over the past six years, Bryant's Roost has maintained several closed Coturnix breeding lines under standardized management, including independently selected Pharaoh, Egyptian, White, and Falb Fee populations. Selection within these lines has consistently favored larger body size, larger hatchable egg size, fertility, hatchability, and breed-specific characteristics. During routine breeding and selection, consistent differences in egg size were observed between these established lines. Reciprocal breeding experiments performed for unrelated breeding objectives also produced repeatable patterns in daughter egg size that appeared to differ according to sire line.
To the author's knowledge, relatively few published reports describe reciprocal breeding observations collected over multiple generations within closed Coturnix breeding populations (de Koning et al., 2004) maintained under standardized management conditions. The purpose of this report is to document these observations, compare them with existing scientific literature, and discuss several possible biological explanations that may serve as hypotheses for future investigation.
Because these observations were collected during routine operation of a closed breeding program rather than a controlled experimental trial, statistical analyses were not performed. The objective of this report is to document repeatable observations and compare them with the current scientific literature
Materials and Methods
The observations presented in this paper are based on a closed Coturnix quail breeding program maintained by Bryant's Roost over a six-year period. The foundation stock originated from a local breeder and consisted primarily of standard Pharaoh Coturnix with mixed genetics. During the early stages of selection, an Egyptian phenotype was identified within the original Pharaoh population. Rather than allowing the recessive gene to remain within the breeding population, separate selection programs were established. One program focused on developing a true-breeding Egyptian line, while the second focused on maintaining a distinct Pharaoh line free of the Egyptian mutation. Both breeding lines have since been maintained as separate populations through selective breeding.
No outside birds have been introduced into these breeding populations since the original foundation stock was established. This closed breeding system was maintained to preserve line integrity and reduce the introduction of unknown genetic variation from unrelated breeding programs.
During the initial development of the Egyptian and Pharaoh lines, breeders were maintained in individual breeding groups to establish genetically consistent populations and verify inheritance patterns. Once the lines consistently bred true, the breeding program transitioned to structured family breeding groups. These family groups continue to allow pedigree-based selection while reducing the accumulation of inbreeding and preserving the long-term genetic stability of each line.
Throughout the observation period, birds were managed under consistent husbandry practices. Adult breeders were housed indoors in an insulated barn in wire breeding cages with continuous access to feed and fresh water. Ventilation was provided by an exhaust fan sized to maintain acceptable air quality by removing ammonia while allowing fresh air exchange through windows and doors.
Breeders were maintained on a consistent nutritional program consisting of Kalmbach 17% Layer Pellets and Show Pro 18% Layer Mash offered free choice throughout the breeding season.
Eggs were incubated under standardized conditions throughout the breeding program. Although several incubator models were used during the early years of the project, all observations reported in this paper were produced using Hatching Time cabinet incubators and hatchers operated under consistent incubation protocols.
Selection within each breeding line emphasized overall breeder performance rather than plumage phenotype alone. Selection criteria included fertility, hatchability, body size, hatchable egg size, vigor, production characteristics, temperament, and the ability of each line to consistently reproduce its defining genetic traits across successive generations.. Reciprocal crosses discussed in this report were originally performed for unrelated breeding objectives. The resulting differences in daughter egg size were recognized during routine evaluation of breeding performance and subsequently examined in the context of the present study.
Unlike experimental breeding programs based on distinct generations, selection within these lines was continuous. Eggs were hatched throughout the breeding season, and replacement breeders were introduced as superior individuals were identified rather than waiting for an entire generation to be replaced. Selection decisions were based on the performance of individual birds rather than generation turnover, allowing desirable genetic combinations to be retained in the breeding population while inferior breeders were replaced as improved individuals became available. As a result, breeding groups frequently consisted of birds of varying ages while maintaining structured family relationships. This management approach more closely reflects practical selection methods used by many poultry breeders and allowed continuous genetic improvement while preserving line integrity.
Observations
Observation 1: Reciprocal Cross Between the Egyptian and Pharaoh Lines
A reciprocal breeding system was used involving the established Egyptian and Pharaoh breeding lines. The objective of this cross was to produce sex-linked replacement females for the Egyptian breeding program rather than to investigate egg size inheritance. An Egyptian male was mated to Pharaoh females. Because the Egyptian mutation is sex-linked, the resulting offspring could be sexed at hatch based on feather color. Female offspring displaying the Egyptian phenotype were retained as replacement breeders within the established Egyptian breeding program. Male offspring from this reciprocal cross were not incorporated into the Egyptian breeding line. Replacement males continued to originate exclusively from the established Egyptian line.
At the time the Egyptian breeding line was established, no individual egg weight records were maintained. Consequently, baseline egg size data from the original founder birds are unavailable.
Following more than six years of independent selection, the two breeding lines continued to demonstrate consistent differences in egg size. Hens from the established Egyptian line produced eggs averaging approximately 12–14 g, whereas hens from the established Pharaoh line routinely produced hatchable eggs ranging from 14–18 g.
Repeated use of this reciprocal cross successfully produced replacement Egyptian females for incorporation into the established Egyptian breeding line. Despite the continual introduction of these females over multiple years of selection, the average egg size of the Egyptian line remained consistently within the 12–14 g range.
Observation 2: Reciprocal Cross Between the White and Falb Fee Lines
The White breeding line was maintained independently following acquisition and consistently produced extra-large hatching eggs. The Falb Fee breeding line, while selected for large body size, consistently produced smaller eggs averaging approximately 12–14 g.
Two reciprocal crosses were evaluated. The reciprocal crosses were originally performed to evaluate inheritance of the Pearl phenotype and to develop future breeding stock. Egg size observations were recorded during routine evaluation of the resulting breeding lines.
In the first cross, White males were mated to Falb Fee females. The resulting heterozygous Pearl offspring consistently produced large eggs comparable to those of the White breeding line.
In the reciprocal cross, Falb Fee males were mated to White females. The resulting heterozygous Pearl offspring consistently produced smaller eggs comparable to those of the Falb Fee breeding line.
The reciprocal crosses therefore produced heterozygous Pearl offspring in both directions of the cross, yet average egg size consistently differed depending upon which breeding line served as the sire. These reciprocal crosses were maintained for approximately one year before the breeding program producing smaller eggs was discontinued because the desired increase in hatchable egg size was not achieved.
Results
Following six years of independent selection, the Pharaoh and Egyptian breeding lines demonstrated consistent differences in average egg size despite sharing a common foundation population. The Pharaoh breeding line continued to produce extra-large hatching eggs ranging from approximately 14–18 g, whereas the Egyptian breeding line consistently produced eggs averaging 12–14 g.
During the same period, body size within the Egyptian breeding line increased through selection and became comparable to that of the Pharaoh breeding line. Despite this convergence in body size, no corresponding increase in average egg weight was observed. Furthermore, the Egyptian breeding line continued to be maintained through replacement females originating from Pharaoh hens in the reciprocal breeding program, yet the average egg size remained consistent within the Egyptian line.
A second reciprocal breeding experiment was conducted using established White and Falb Fee breeding lines. White hens consistently produced extra-large eggs, whereas the Falb Fee line consistently produced eggs averaging approximately 12–14 g despite being selected for large body size.
When White males were mated to Falb Fee females, the resulting heterozygous Pearl females consistently produced large eggs comparable to those of the White breeding line.
Conversely, when Falb Fee males were mated to White females, the resulting heterozygous Pearl females consistently produced smaller eggs comparable to those of the Falb Fee breeding line.
Both reciprocal crosses produced heterozygous Pearl offspring; however, average egg size differed depending upon the sire line. This difference remained consistent throughout approximately one year of observation. Due to continued production of smaller eggs, the heterozygous Pearl line originating from Falb Fee sires was removed from the breeding program.
Limitations
The observations presented in this report originated from a practical breeding program rather than a controlled experimental study. Individual egg weights from the original Egyptian foundation birds were not recorded, preventing direct comparison with subsequent generations. Statistical analyses were not performed, and no molecular genetic testing was conducted. While the observed patterns have remained consistent across multiple generations and reciprocal breeding experiments, the present observations alone cannot establish which genetic mechanism, or combination of mechanisms, is responsible for the observed patterns.". Controlled breeding studies, quantitative egg weight measurements, and molecular analyses will be required to distinguish among the proposed hypotheses.
Discussion
The most consistent observation throughout this breeding program has been the persistence of smaller egg size within the Egyptian line despite six years of selection for larger hatchable eggs. During the same period, body size increased substantially until it became comparable to the Pharaoh line. This suggests that selection for increased body size alone was insufficient to increase egg size within the established Egyptian breeding population.
The work by Pick et al. (2016) provides one possible explanation for these observations. Their research demonstrated that selection for larger egg size eventually reached a plateau, while selection for smaller eggs continued across successive generations. This interpretation is also consistent with QTL mapping studies in Japanese quail demonstrating that egg size is influenced by numerous loci distributed across multiple chromosomes rather than a single gene (Haqani et al., 2021). If the original Egyptian birds selected to establish the line happened to carry a combination of alleles associated with moderate egg size, it is possible that six years of breeding within a closed population has fixed those allele combinations. If that is the case, selecting the largest eggs from within the Egyptian line may never produce eggs comparable to the Pharaoh line. An outcross to another established large-egg line may be required to introduce additional genetic variation.
That explanation seems reasonable for the Egyptian and Pharaoh lines. However, it does not fully explain what was observed in the reciprocal White and Falb Fee crosses. The reciprocal White and Falb Fee crosses appear less consistent with a simple selection plateau.
When White males were bred to Falb Fee females, the resulting heterozygous Pearl daughters consistently laid large eggs similar to the White line. When the cross was reversed, using Falb Fee males over White females, the heterozygous Pearl daughters consistently laid smaller eggs similar to the Falb Fee line. These observations remained consistent throughout approximately one year of production.
This raises another question. Female birds inherit their single Z chromosome from their father. If some of the quantitative trait loci influencing egg size are located on the Z chromosome, daughters would inherit those particular alleles exclusively from the sire. Parent-of-origin effects have also been reported for several production traits in chickens (de Koning et al., 2004), suggesting that inheritance of complex traits may involve mechanisms beyond simple additive genetics. I have not found published work specifically examining these possibilities in long-term reciprocal Coturnixbreeding programs, but they offer possible explanations for the patterns observed here.
It is equally possible that the observations result from founder effects, fixation of different polygenic combinations, dominance, epistatic interactions, or other genetic mechanisms that have not yet been identified. At this point, I cannot determine which explanation is correct. Whether these observations ultimately prove to represent founder effects, parent-of-origin effects, sex-linked inheritance, or another genetic mechanism entirely, they have remained remarkably consistent throughout this breeding program. My goal in publishing these observations is simply to document them so they can be compared with future breeding experiments. I hope they encourage additional breeders and researchers to investigate the inheritance of egg size in Coturnix quail.
Conclusion
The purpose of this paper was not to prove a new genetic mechanism, but to document observations made during more than six years of maintaining closed Coturnix breeding lines. Current research demonstrates that egg size is a highly heritable, polygenic trait and that selection for increased egg size may eventually reach a plateau. Those findings provide one possible explanation for the differences that developed between the Pharaoh and Egyptian breeding lines.
However, the reciprocal breeding experiments described in this paper raise additional questions that are not easily explained by the current literature. In two independent breeding projects, daughter egg size consistently appeared to correspond with the sire line, suggesting that paternal inheritance may play a greater role than expected. Whether these observations are the result of allele fixation within closed populations, Z-linked quantitative trait loci, epistatic interactions, or another genetic mechanism remains unknown.
As breeders, we often make observations long before we understand the biology behind them. My intent is simply to document what has been consistently observed in my breeding program and compare those observations with the available scientific literature. I hope this report encourages additional reciprocal breeding experiments, more detailed record keeping, and further research into the inheritance of egg size in Coturnix quail.This report represents the beginning of that investigation rather than its conclusion. As additional breeding data and published research become available, these observations and the hypotheses discussed here will continue to be refined
References
de Koning, D. J., Haley, C. S., Windsor, D., Hocking, P. M., Griffin, H., Morris, A., Vincent, J., & Burt, D. W. (2004). Quantitative trait loci with parent-of-origin effects in chicken. Genetics Research. https://doi.org/10.1017/S0016672304006950
Original publication:
https://www.cambridge.org/core/services/aop-cambridge-core/content/view/D92AC27876020304B3EE3E8CE9C68E25/S0016672304006950a.pdf
Poultry Nerds Research Review:
https://www.poultrynerdspodcast.com/research-library/parent-of-origin-effects-chicken-genetics
Haqani, M. I., et al. (2021). Mapping of quantitative trait loci controlling egg-quality and production traits in Japanese quail (Coturnix japonica) using restriction-site associated DNA sequencing. Genes, 12(5), 735. https://doi.org/10.3390/genes12050735
Original publication:
https://www.mdpi.com/2073-4425/12/5/735
Poultry Nerds Research Review:
https://www.poultrynerdspodcast.com/research-library/egg-size-genetics-japanese-quail
Narinc, D., Aygun, A., Karaman, E., & Aksoy, T. (2015). Egg shell quality in Japanese quail: Characteristics, heritabilities and genetic and phenotypic relationships. Animal, 9(7), 1091–1096. https://doi.org/10.1017/S1751731115000506
Original publication:
https://www.sciencedirect.com/science/article/pii/S1751731115000506
Poultry Nerds Research Review:
https://www.poultrynerdspodcast.com/research-library/eggshell-quality-genetics-japanese-quail
Pick, J. L., Hutter, P., & Tschirren, B. (2016). Artificial selection reveals the energetic expense of producing larger eggs. Heredity, 116, 542–549. https://doi.org/10.1038/hdy.2016.16
Original publication:
https://www.nature.com/articles/hdy201616
Open-access version:
https://pmc.ncbi.nlm.nih.gov/articles/PMC4868267/
Poultry Nerds Research Review:
https://www.poultrynerdspodcast.com/research-library/artificial-selection-egg-size-japanese-quail
Companion Reading
Poultry Nerds Research Library. Artificial Selection and Egg Size Evolution: Why Larger Eggs May Reach a Genetic Plateau.
Poultry Nerds Research Library. Eggshell Quality Genetics in Japanese Quail.
Poultry Nerds Research Library. Egg Size Genetics in Japanese Quail: Understanding QTL and Egg Quality Research.
Poultry Nerds Research Library. Parent-of-Origin Effects in Chicken Genetics.
Suggested Citation
· Bryant, J. (2026). Egg Size Persistence in Closed Egyptian Coturnix Breeding Lines: Observations from a Six-Year Breeding Program. Bryant's Roost Research Report BRRR-001. Bryant's Roost. https://www.bryantsroost.com