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Shell Quality in Poultry as Seen From the Breeder’s Viewpoint
Shell Quality in Poultry as Seen From the Breeder's Viewpoint 3. Heritabilities: Expected versus Accomplished Response W. F. van TIJEN Spelderbalt Institute for Poultry Research, Beekbergen, The Netherlands (Received for publication September 7, 1976)
INTRODUCTION A selection experiment with two strains of chickens, each belonging to a different breed showed that it was possible to improve the shell quality by breeding, but that this gain was secured at the cost of a part of the productivity (van Tijen, 1973, 1977a). After three years of selection, the four lines acquired were crossed in all possible combinations and four pure lines and 12 crosses were obtained. Variance analyses which were carried out for the different traits showed that in general the effect of the selection expressed itself in the same way in the strains as in the crosses (van Tijen, 1977b). The third paper in this series presents estimations of heritability for the various traits with some comments. Moreover, the response to be expected will be calculated and will be compared with the actual gain made.
be the best method of improving shell quality in the material under investigation. As far as known to the author, the only paper published dealing with a comparison of realized and expected response for various traits is that of Goher and McGibbon (1974). MATERIAL AND METHODS The selection experiment was carried out with a White Leghorn and a Rhode Island Red strain (van Tijen, 1973, 1977a). The heritability estimates over the years 1967—1970 were obtained by means of a variance analysis (Lerner, 1958; Falconer, 1960). The classification was hierarchical with the individual daughters as the smallest variable unit. The final analysis took the following shape:
Source
SQ
Components of the M.SQ
Between sires
MSS
a* + KCTJ + nka*
Between mothers (within sires)
MSj
a* + K a d
Between daughters (within mothers, within sires)
MS D
oL
LITERATURE Estimates of heritability with regard to various shell quality traits have been published by numerous authors. For summaries of these see Kinney (1969) and van Tijen and Kuit (1970). Rodda (1972) estimated the relative selection efficiencies expected by basing the selection on early shell thickness with the object of improving this trait at a later stage in the laying period. His estimates were larger than one. This indicates that early selection would
The effect of unequal number of dams per sire and daughters per dam was corrected (King
1121
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 18, 2015
ABSTRACT A White Leghorn and a Rhode Island Red strain were each divided into two genetically equal parts which for three generations were selected for productivity and shell quality respectively. Heritabilities for the various traits were estimated. The results obtained agreed well with those of other workers. No specific trend as a result of the selection was detected and the size of the standard deviation seemed to be fairly constant. The 5% confidence intervals calculated showed that the heritabilities were estimated to one decimal point (10%) precision. The expected responses—estimated from selection pressures and heritabilities—were compared with the actual gains. In general the agreement between prediction and realized result appeared to be satisfactory, an exception being found in the case of the productivity traits—number of eggs and production percentage—and internal quality. An explanation for this discrepancy is sought in the interaction of genotype and environment. Poultry Science 56:1121-1126, 1977
1122
VANTIJEN
and Henderson, 1954). The mean squares were equated to the expected values. The necessary variance components were estimated and in their turn used for the estimation of the heritabilities.
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The response R to be expected as a result of a certain selection pressure is closely connected with the heritability. This relation is represented by the formula R = h^ in which S stands for selection differential: being the difference between the population mean and the mean of the selected portion. The mothers are selected on the basis of their own performance. The response to be expected can be obtained with the help of the above formula. The sires however have been selected on the basis of the performance of their full sisters and the expected gain is found by using the heritability of the means of the groups of full sisters (h p ), instead of h?. These heritabilities are related by the following formula: 1 + (n-l)t
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in which n is the average number of full sib families, r is 0.5 and t is the intraclass correlation of the phenotypic values of the full sibs within a family. As a trend over the years seemed to be absent, we were justified in averaging the estimates over the three years concerned. These averages were used to predict the response to be expected as a result of the selection of the sires (Rj = hfSj) and of the dams (R2 = h;S2)- The average selection differential per year has been approximated by means of a regression line b y x (y = cumulative selection differential, x = time). The mean of Rl and R2 was calculated to obtain the expected progress per year during the three
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EGG SHELL QUALITY
1123
years of selection. The average realized response per year was also approximated by calculating the regression. Here y stands for the response and x for the time. The expectation was finally compared with the gain per year actually made. RESULTS AND DISCUSSION
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Downloaded from http://ps.oxfordjournals.org/ at NERL on May 18, 2015
a. The heritabilities. These were estimated for the following traits: number of eggs and production percentage at the ages of 38 and 72 weeks respectively; egg weight deformation, shell thickness, specific gravity and height of thick albumen at 30 and at 60 weeks. As has been stated above, no specific trend in the course of the years—either upward or downward—was noted. The averages over the four years are given in Table 1. The figures are in agreement with those of other workers: the heritability for number of eggs and production percentage is low and that for egg weight is high. The estimates for the egg quality traits lie between these two. The heritability for internal quality is in general somewhat higher than that for the shell quality traits. It would be outside the scope of this paper to report the estimates that were obtained via sire- and dam component separately. A comparison of these could given information on whether or not sex-linked c.q. maternal effects are present. A high hg could point in the direction of the presence of sex-linked effects. A high h j on the other hand indicates the possible influence of maternal effects. In both cases the full sib estimation of the heritability is over-estimated. In order to get an impression of these differences, Table 2 has been arranged. In this table the size of the differences between hf and h j has been given in the form of plus or minus signs. Each plus sign indicates that \\\ is 0.1 larger, each minus sign that it is 0.1 smaller than h j . In scrutinizing this table, it seems that maternal effects have their influence in the case of the productivity traits while one could speculate on the presence of sex-linked effects when looking at the shell quality traits. For the other traits both influences seem to be absent. Table 1 also gives the estimates of the standard deviations—in absolute and in relative terms. The absolute size seems to be very constant (0.04—0.05). Expressed as a percentage the standard deviation is large when the heritability
1124
VAN TIJEN TABLE 3.—A comparison of the averaged realized and expected response per year after three years of selection on shell quality and productivity. (The figures in brackets indicate the correlation between cumulative response and time) Lines RS
Trait
RP
WS
WP
eggs ) f response r J
0.7 1.4(0.58)
1.1 3.7(0.92)
0.1 1.0(0.68)
2.4 2.0(0.76)
Production expected .. . realized
percentage ) r )> response
0.4 0.3(0.24)
0.7 1.7(0.58)
0.0 1.3(0.52)
1.3 2.1(0.79)
0.2 -0.2(0.26)
1.5 1.2(0.92)
0.1 0.5(0.59)
1.2 1.2(0.89)
Deformation expected 1 .. , ( response realized > r
-0.6 -1.6(-0.99)
-0.1 -0.7(-0.72)
-0.5 -1.1 (-0.98)
-0.1 -0.6(-0.86)
Shell thickness expected 1 realized f resPonse
0.8 1.0(0.92)
0.3 0.2(0.58)
1.0 1.3(0.98)
0.3 0.7(0.99)
Specific gravity expected 1 . . . f response r realized )
1.2 1.8(0.88)
0.2 0.1(0.13)
1.5 1.6(0.92)
0.2 0.0(-0.04)
Height of thick white expected 1 realized f resPonse
-1.7 -0.2(-0.02)
24.2 17.5(0.84)
-7.4 15.8(-0.90)
Egg weight expected ) realized (
res
Ponse
13.4 20.8(0.94)
TABLE 4.—A comparison of the averaged realized and expected response per year after the elimination of the environmental effects Rhode Island Reds
Trait Number of eggs expected ( realized J
P-S response
Production percentage expected ) res onse realized } P
P-S
Egg weight expected ) realized 1
P-S
Deformation expected 1 realized f Shell thickness expected ) ,. . ;( realized Specific gravity expected I realized f
White Leghorns
res
P
onse
0.4 2.3
2.3 1.0
0.3 1.4
1.3 0.8
1.3 1.4
1.1 0.7
-0.5 -0.9
-0.4 -0.5
0.5 0.8
0.7 0.6
1.0 1.7
1.3 1.6
25.9
20.8 5.0
S-P res
Ponse S-P
response r S-P res
P
onse
Height of thick white expected ) res onse realized > P
P-S 17.7
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 18, 2015
Number of expected ... realized
EGG SHELL QUALITY
Number oi eggs Production p«rcentag« Egg weight Deformation Shell thickness Specific
gravity
30 60 30 60 30 60 30 60 30 60 30 60
Height of t h i c k albumen
30 60
Number of eggs
30 60 30 60
Production
percentage
Egg weight Deformation Shell thickness Specific
gravity
30 60
Number of eggs
30 60 30 60
Production percentage
Deformation
S p e c i f i c gravityHeight of t h i c k albumen Humber of eggs Production percentage
30 60 30 60 30 60 30 60 _ 30 6030 6030 60.
Egg weight
30 60.
Deformation
30 6030 60_ 30 60. 30 .'0
Specific gravity Height of t h i c k albumen
•
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_r—^
30 60 30 60 30 60 30 60
Height of t h i c k albumen
Egg weight
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relation. In the S-lines the correlation for the shell quality traits and in the P-lines that for production and egg weight is very close. In general the coefficient is lower in the case of those traits for which the respective lines have not been selected. In view of these data, it seems justified to more or less consider the P-lines as a control for the S-lines and, vice versa. Thus, the differences may be considered as an approximation of the genetic effects, the environmental influence having been eliminated to a high extent. With the above statement in mind, the data in Table 4 have been obtained by subtraction. In interpreting the results one has to bear in mind that the predictions are derived from the selection pressures exercised and the heritabilities. The latter are estimates which from the nature of the case are afflicted with an error. We would be inclined to comment upon the data of this table as follows: As far as the productivity traits are concerned, there seems to be a difference between the two strains for number of eggs as well as for production percentage. Although there is a positive response in both cases, the realization seems to be less than the prediction in the Rhode Island Reds, while the reverse is true in the White Leghorns. For egg weight the realization agrees well with the production. The same can be said of the shell quality traits: here also is a fairly close agreement between the progress realized and that which had been predicted. For internal quality, like productivity traits, there seems to be a difference between the two strains: in both of them a positive response was predicted. In the Rhode Island Reds, however, there is a fair agreement between prediction and realization, while in the White Leghorns the expected response seems to be considerably larger than the one actually realized. In general it may be stated that the agreement between prediction and realization is satisfactory, an exception being formed by the productivity traits and the internal quality. We would be inclined to seek an explanation for the discrepancies in the interactions of genotype and environment.
——
•—•
H,P,
FIG. 1. Heritabilities of four strains of laying stock with their 5% confidence intervals.
REFERENCES Falconer, D. S., 1972. Introduction to Quantitative Genetics. Oliver & Boyd, Edinburgh and London. Goher, N. E., and W. H. McGibbon, 1974. Evaluation of selection methods in a poultry breeding pro-
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 18, 2015
is low (number of eggs and production percentage) and small when it is high (egg weight). Assuming a normal distribution, 5% confidence intervals have been calculated for each heritability. These are represented in Fig. 1. The graph shows that the heritabilities in this material have been estimated with 10% precision. b. Expected versus realized response. Table 3 gives a comparison of the expected and the realized average response per year during the three years of selection for the various traits. The figures show that for some traits there is a good agreement between expectation and realization, while for others this is not the case. It is difficult to comment on these data since no actual randombred control line, against which the selected line could be measured, was involved in this experiment. Table 3 also gives the correlation coefficients between cumulative response and time, thus giving an impression of the closeness of the
1125
1126
VAN TIJEN
gram. 2. Correlated responses to selection for part-record rate of lay with and without full sibbing. Poultry Sci. 53:1928. King, S. C , and C. R. Henderson, 1954. Variance components in heritability studies. Poultry Sci. 33:155-169. Kinney, T. B. Jr., 1969. A summary of reported estimates of heritability and of genetic and phenotypic correlations. U.S. Dept. of Agric. Agricultural, Washington, Handbook No. 363. Lerner, M., 1958. The Genetic Basis of Selection. John Wiley and Sons Inc., New York. Chapman and Hall Ltd., London.
Rodda, D. D., 1972. Breeding for late eggshell quality in the domestic hen. Brit. Poultry Sci. 13:45—60. Tijen, W. F. van, 1977a. Shell quality in poultry, as seen from the breeder's viewpoint. 1. Improvement reached after four years of selection and the effect on productivity. Poultry Sci. 56:1107-1114. Tijen, W. F. van, 1977b. Shell quality in poultry, as seen from the breeder's viewpoint. 2. Search for heterosis effects. Poultry Sci. 56:1115-1120. Tijen, W. F. van, and A. R. Kuit, 1970. The heritability of characteristics of egg quality, their mutual correlation and the relationship with productivity. Arch. Gefliigelk. 34:201-210.
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 18, 2015
INTRODUCTION A selection experiment with two strains of chickens, each belonging to a different breed showed that it was possible to improve the shell quality by breeding, but that this gain was secured at the cost of a part of the productivity (van Tijen, 1973, 1977a). After three years of selection, the four lines acquired were crossed in all possible combinations and four pure lines and 12 crosses were obtained. Variance analyses which were carried out for the different traits showed that in general the effect of the selection expressed itself in the same way in the strains as in the crosses (van Tijen, 1977b). The third paper in this series presents estimations of heritability for the various traits with some comments. Moreover, the response to be expected will be calculated and will be compared with the actual gain made.
be the best method of improving shell quality in the material under investigation. As far as known to the author, the only paper published dealing with a comparison of realized and expected response for various traits is that of Goher and McGibbon (1974). MATERIAL AND METHODS The selection experiment was carried out with a White Leghorn and a Rhode Island Red strain (van Tijen, 1973, 1977a). The heritability estimates over the years 1967—1970 were obtained by means of a variance analysis (Lerner, 1958; Falconer, 1960). The classification was hierarchical with the individual daughters as the smallest variable unit. The final analysis took the following shape:
Source
SQ
Components of the M.SQ
Between sires
MSS
a* + KCTJ + nka*
Between mothers (within sires)
MSj
a* + K a d
Between daughters (within mothers, within sires)
MS D
oL
LITERATURE Estimates of heritability with regard to various shell quality traits have been published by numerous authors. For summaries of these see Kinney (1969) and van Tijen and Kuit (1970). Rodda (1972) estimated the relative selection efficiencies expected by basing the selection on early shell thickness with the object of improving this trait at a later stage in the laying period. His estimates were larger than one. This indicates that early selection would
The effect of unequal number of dams per sire and daughters per dam was corrected (King
1121
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 18, 2015
ABSTRACT A White Leghorn and a Rhode Island Red strain were each divided into two genetically equal parts which for three generations were selected for productivity and shell quality respectively. Heritabilities for the various traits were estimated. The results obtained agreed well with those of other workers. No specific trend as a result of the selection was detected and the size of the standard deviation seemed to be fairly constant. The 5% confidence intervals calculated showed that the heritabilities were estimated to one decimal point (10%) precision. The expected responses—estimated from selection pressures and heritabilities—were compared with the actual gains. In general the agreement between prediction and realized result appeared to be satisfactory, an exception being found in the case of the productivity traits—number of eggs and production percentage—and internal quality. An explanation for this discrepancy is sought in the interaction of genotype and environment. Poultry Science 56:1121-1126, 1977
1122
VANTIJEN
and Henderson, 1954). The mean squares were equated to the expected values. The necessary variance components were estimated and in their turn used for the estimation of the heritabilities.
| |
00 00
1> 00
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O •
0 0 0 0
O 0 O 0
0 O 0
m m O O O O
0 in m 0 O
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0 in
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•
*
•
*
O O
4o<
(hs2 =
2
2
o s + °d + ° P
"2
2
© O
^
m
*-i
1-1
«-H
* •* o o
U
4od ha =
H CO N 00 w
2
2
m^-
00 N
mm
r-i 00
11
The response R to be expected as a result of a certain selection pressure is closely connected with the heritability. This relation is represented by the formula R = h^ in which S stands for selection differential: being the difference between the population mean and the mean of the selected portion. The mothers are selected on the basis of their own performance. The response to be expected can be obtained with the help of the above formula. The sires however have been selected on the basis of the performance of their full sisters and the expected gain is found by using the heritability of the means of the groups of full sisters (h p ), instead of h?. These heritabilities are related by the following formula: 1 + (n-l)t
Tt-ri
mm
vooo
TJ-*3-
o 6
11
M t-i
0 0
do
^
1
0 0 (so
00 t^ i-< m
d d d d Mm
00 N
0 0 O O o o do d 6 d d m Tf rn ^-
?! .?*
do
o o o o ^- in d 0 d d o o d d in m in
.5-2
THW
O CO O 00
TJ-
0 0
d d
0 0
r^ r*
t^ t^
i^ r^
o o O O
^f- in O O O O
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*t
HO\
00 ^O
*o *© vo *o *© *o do o d d '
0 0
in in O O O O
do
3-8 rfirfi O O
hf
in which n is the average number of full sib families, r is 0.5 and t is the intraclass correlation of the phenotypic values of the full sibs within a family. As a trend over the years seemed to be absent, we were justified in averaging the estimates over the three years concerned. These averages were used to predict the response to be expected as a result of the selection of the sires (Rj = hfSj) and of the dams (R2 = h;S2)- The average selection differential per year has been approximated by means of a regression line b y x (y = cumulative selection differential, x = time). The mean of Rl and R2 was calculated to obtain the expected progress per year during the three
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H
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 18, 2015
hf + h d
in O O
TJ-
0
do do do
°s+ °d + °p
h? =
rfO O TJ-
o o 0 d do'
EGG SHELL QUALITY
1123
years of selection. The average realized response per year was also approximated by calculating the regression. Here y stands for the response and x for the time. The expectation was finally compared with the gain per year actually made. RESULTS AND DISCUSSION
5 *-»
+ x& +
w O
8 c _0 y 3 -U O
a*g v> a. <£ a05 tf S £
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 18, 2015
a. The heritabilities. These were estimated for the following traits: number of eggs and production percentage at the ages of 38 and 72 weeks respectively; egg weight deformation, shell thickness, specific gravity and height of thick albumen at 30 and at 60 weeks. As has been stated above, no specific trend in the course of the years—either upward or downward—was noted. The averages over the four years are given in Table 1. The figures are in agreement with those of other workers: the heritability for number of eggs and production percentage is low and that for egg weight is high. The estimates for the egg quality traits lie between these two. The heritability for internal quality is in general somewhat higher than that for the shell quality traits. It would be outside the scope of this paper to report the estimates that were obtained via sire- and dam component separately. A comparison of these could given information on whether or not sex-linked c.q. maternal effects are present. A high hg could point in the direction of the presence of sex-linked effects. A high h j on the other hand indicates the possible influence of maternal effects. In both cases the full sib estimation of the heritability is over-estimated. In order to get an impression of these differences, Table 2 has been arranged. In this table the size of the differences between hf and h j has been given in the form of plus or minus signs. Each plus sign indicates that \\\ is 0.1 larger, each minus sign that it is 0.1 smaller than h j . In scrutinizing this table, it seems that maternal effects have their influence in the case of the productivity traits while one could speculate on the presence of sex-linked effects when looking at the shell quality traits. For the other traits both influences seem to be absent. Table 1 also gives the estimates of the standard deviations—in absolute and in relative terms. The absolute size seems to be very constant (0.04—0.05). Expressed as a percentage the standard deviation is large when the heritability
1124
VAN TIJEN TABLE 3.—A comparison of the averaged realized and expected response per year after three years of selection on shell quality and productivity. (The figures in brackets indicate the correlation between cumulative response and time) Lines RS
Trait
RP
WS
WP
eggs ) f response r J
0.7 1.4(0.58)
1.1 3.7(0.92)
0.1 1.0(0.68)
2.4 2.0(0.76)
Production expected .. . realized
percentage ) r )> response
0.4 0.3(0.24)
0.7 1.7(0.58)
0.0 1.3(0.52)
1.3 2.1(0.79)
0.2 -0.2(0.26)
1.5 1.2(0.92)
0.1 0.5(0.59)
1.2 1.2(0.89)
Deformation expected 1 .. , ( response realized > r
-0.6 -1.6(-0.99)
-0.1 -0.7(-0.72)
-0.5 -1.1 (-0.98)
-0.1 -0.6(-0.86)
Shell thickness expected 1 realized f resPonse
0.8 1.0(0.92)
0.3 0.2(0.58)
1.0 1.3(0.98)
0.3 0.7(0.99)
Specific gravity expected 1 . . . f response r realized )
1.2 1.8(0.88)
0.2 0.1(0.13)
1.5 1.6(0.92)
0.2 0.0(-0.04)
Height of thick white expected 1 realized f resPonse
-1.7 -0.2(-0.02)
24.2 17.5(0.84)
-7.4 15.8(-0.90)
Egg weight expected ) realized (
res
Ponse
13.4 20.8(0.94)
TABLE 4.—A comparison of the averaged realized and expected response per year after the elimination of the environmental effects Rhode Island Reds
Trait Number of eggs expected ( realized J
P-S response
Production percentage expected ) res onse realized } P
P-S
Egg weight expected ) realized 1
P-S
Deformation expected 1 realized f Shell thickness expected ) ,. . ;( realized Specific gravity expected I realized f
White Leghorns
res
P
onse
0.4 2.3
2.3 1.0
0.3 1.4
1.3 0.8
1.3 1.4
1.1 0.7
-0.5 -0.9
-0.4 -0.5
0.5 0.8
0.7 0.6
1.0 1.7
1.3 1.6
25.9
20.8 5.0
S-P res
Ponse S-P
response r S-P res
P
onse
Height of thick white expected ) res onse realized > P
P-S 17.7
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 18, 2015
Number of expected ... realized
EGG SHELL QUALITY
Number oi eggs Production p«rcentag« Egg weight Deformation Shell thickness Specific
gravity
30 60 30 60 30 60 30 60 30 60 30 60
Height of t h i c k albumen
30 60
Number of eggs
30 60 30 60
Production
percentage
Egg weight Deformation Shell thickness Specific
gravity
30 60
Number of eggs
30 60 30 60
Production percentage
Deformation
S p e c i f i c gravityHeight of t h i c k albumen Humber of eggs Production percentage
30 60 30 60 30 60 30 60 _ 30 6030 6030 60.
Egg weight
30 60.
Deformation
30 6030 60_ 30 60. 30 .'0
Specific gravity Height of t h i c k albumen
•
•
~__ ~~ —'
U.S.
_r—^
30 60 30 60 30 60 30 60
Height of t h i c k albumen
Egg weight
—
i
,
•—• .
.
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i
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. "' M.S.
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relation. In the S-lines the correlation for the shell quality traits and in the P-lines that for production and egg weight is very close. In general the coefficient is lower in the case of those traits for which the respective lines have not been selected. In view of these data, it seems justified to more or less consider the P-lines as a control for the S-lines and, vice versa. Thus, the differences may be considered as an approximation of the genetic effects, the environmental influence having been eliminated to a high extent. With the above statement in mind, the data in Table 4 have been obtained by subtraction. In interpreting the results one has to bear in mind that the predictions are derived from the selection pressures exercised and the heritabilities. The latter are estimates which from the nature of the case are afflicted with an error. We would be inclined to comment upon the data of this table as follows: As far as the productivity traits are concerned, there seems to be a difference between the two strains for number of eggs as well as for production percentage. Although there is a positive response in both cases, the realization seems to be less than the prediction in the Rhode Island Reds, while the reverse is true in the White Leghorns. For egg weight the realization agrees well with the production. The same can be said of the shell quality traits: here also is a fairly close agreement between the progress realized and that which had been predicted. For internal quality, like productivity traits, there seems to be a difference between the two strains: in both of them a positive response was predicted. In the Rhode Island Reds, however, there is a fair agreement between prediction and realization, while in the White Leghorns the expected response seems to be considerably larger than the one actually realized. In general it may be stated that the agreement between prediction and realization is satisfactory, an exception being formed by the productivity traits and the internal quality. We would be inclined to seek an explanation for the discrepancies in the interactions of genotype and environment.
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H,P,
FIG. 1. Heritabilities of four strains of laying stock with their 5% confidence intervals.
REFERENCES Falconer, D. S., 1972. Introduction to Quantitative Genetics. Oliver & Boyd, Edinburgh and London. Goher, N. E., and W. H. McGibbon, 1974. Evaluation of selection methods in a poultry breeding pro-
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is low (number of eggs and production percentage) and small when it is high (egg weight). Assuming a normal distribution, 5% confidence intervals have been calculated for each heritability. These are represented in Fig. 1. The graph shows that the heritabilities in this material have been estimated with 10% precision. b. Expected versus realized response. Table 3 gives a comparison of the expected and the realized average response per year during the three years of selection for the various traits. The figures show that for some traits there is a good agreement between expectation and realization, while for others this is not the case. It is difficult to comment on these data since no actual randombred control line, against which the selected line could be measured, was involved in this experiment. Table 3 also gives the correlation coefficients between cumulative response and time, thus giving an impression of the closeness of the
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gram. 2. Correlated responses to selection for part-record rate of lay with and without full sibbing. Poultry Sci. 53:1928. King, S. C , and C. R. Henderson, 1954. Variance components in heritability studies. Poultry Sci. 33:155-169. Kinney, T. B. Jr., 1969. A summary of reported estimates of heritability and of genetic and phenotypic correlations. U.S. Dept. of Agric. Agricultural, Washington, Handbook No. 363. Lerner, M., 1958. The Genetic Basis of Selection. John Wiley and Sons Inc., New York. Chapman and Hall Ltd., London.
Rodda, D. D., 1972. Breeding for late eggshell quality in the domestic hen. Brit. Poultry Sci. 13:45—60. Tijen, W. F. van, 1977a. Shell quality in poultry, as seen from the breeder's viewpoint. 1. Improvement reached after four years of selection and the effect on productivity. Poultry Sci. 56:1107-1114. Tijen, W. F. van, 1977b. Shell quality in poultry, as seen from the breeder's viewpoint. 2. Search for heterosis effects. Poultry Sci. 56:1115-1120. Tijen, W. F. van, and A. R. Kuit, 1970. The heritability of characteristics of egg quality, their mutual correlation and the relationship with productivity. Arch. Gefliigelk. 34:201-210.
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