Changes to the Poll Dorset and Dorset Horn Breeding Evaluation
(Part 1 – Genetic Parameters) - January 2014
Summary
EBLEX recently funded a review of the genetic parameters used in the Dorset
breeding evaluation. Improvements have been made to the way EBVs are
calculated, particularly the Maternal Ability EBV. Breeders will notice changes in the
scaling of a number of EBVs and the ranking of certain animals.
Due to the number of changes incorporated into this evaluation, breeders are
strongly advised to use the new values when assessing sheep and not to make
direct comparison to their rating in previous evaluations.
A technical note is provided here for those breeders wishing to know more about
these changes.
Introduction
Estimated Breeding Values are calculated using knowledge of the heritability of
different traits (the degree to which variation between animals is influenced by their
genes) and the relationships (correlations) between these traits. Periodically these
“genetic parameters” are reviewed to ensure that they are appropriate for the
sheep/breed being analysed.
In 2014 EBLEX funded SRUC to review the genetic parameters used in the
evaluation of Dorset sheep.
Work by SRUC has developed:
 A new set of heritabilities and genetic correlations using a dataset of Dorset
records for animals born between 2003 and 2012.
 A better way to calculate EBVs for Maternal Ability and Litter Size (Born)
 Two new EBVs for Birth Weight and Litter Size (Reared) {Not yet published}
How have heritability values changed?
The table below shows the new heritability values that are being used in the Dorset
evaluation.
Estimates of heritability (h2) for Dorset Sheep
Trait
Birth Weight (New Trait)
8 Week Weight
Maternal Ability
Litter Size (Born)
Litter Size (Reared) (New Trait)
Scan Weight
Old h2
0.21
0.10
0.07
0.36
New h2
0.20
0.11
0.15
0.12
0.06
0.27
Change
Lower
Higher
Higher
Lower
Muscle Depth
Fat Depth
Mature Size
0.31
0.22
0.35
0.29
0.26
0.25
Similar
Similar
Lower
How do I interpret heritability values?
The heritability value indicates the proportion of the variation between animals that
can be explained by their genes (or you could think of it as the degree to which an
animal’s appearance is influenced by its genes).
If a growth trait has a heritability of 0.4 then 40% of the liveweight variation between
animals would be due genetic differences between them and 60% would be due to
environmental influences. Health and reproduction traits tend to have a low
heritability – but this doesn’t mean they are not worth assessing; even a small
change in these traits can be highly profitable.
What do these changes mean?
The most striking change is that the heritability of 8 Week Weight has fallen, whilst
that of Maternal Ability has risen. This could be interpreted as the evaluation doing a
better job of attributing lamb weight gain to the genetic potential of the ewe to
produce milk and care for her lamb (where appropriate) and no longer confounding
this influence with the lamb’s genetic ability to grow. EBVs will be re-scaled to take
into account these changes.
How have the genetic correlations changed?
The old and new genetic correlations are shown in the tables below.
Old genetic correlations between traits
8 Week Weight
Scan Weight
Muscle Depth
Fat Depth
Maternal Ability
Mature Size
Litter Size Born
8 WK
SWT
MD
FD
MAT
MS
LSB
8 WK SWT MD
0.90 0.38
0.46
FD MAT MS LSB
0.46 0.20 0.35 -0.22
0.31 0.40 0.35 -0.15
-0.01 0.38 0.00 -0.05
0.40 0.00 -0.16
0.33 -0.18
0
New genetic correlations between traits
BWT
Birth Weight
8 Week Weight
Maternal Ability
Scan Weight
BWT
8 WK
MAT
SWT
8 WK
0.50
MAT
-0.29
-0.22
SWT
0.27
0.82
0.33
MD
0.27
0.31
0.20
0.46
FD
0.10
0.19
0.23
0.31
MS
0.01
0.40
0.40
0.48
LSB
0.25
0.41
-0.42
0.04
LSR
0.25
0.19
-0.12
0.07
Muscle Depth
Fat Depth
Mature Size
Litter Size Born
Litter Size Reared
MD
FD
MS
LSB
LSR
0.18
-0.02
0.49
0.04
-0.02
0.19
How do I interpret genetic correlations?
The size of the correlation indicates how closely related traits are to one another. A
correlation of 0 indicates “no relationship”, 0.90 would be a very strong, positive
relationship between traits and -0.90 would indicate a strong negative relationship.
In this case the strongest genetic relationship is between 8 Week Weight and Scan
Weight (0.82). This is unsurprising as the genes that control growth rate to 8 weeks
of age are also likely to influence growth rate in later life.
What do these changes mean?
The predicted genetic relationship between growth and carcase traits has changed
very little, although the relationship between Fat Depth and Muscle Depth is now
positive (in line with observations in other breeds). Litter Size now has a positive
relationship with most performance traits (previous analyses were assuming a
slightly negative relationship).
The Maternal Ability EBV has a negative correlation with 8 Week Weight. This is in
line with the antagonistic relationship often observed between carcase traits and
maternal traits. It then tends to be positively correlated with other traits assessed
later in life.
Dorset Test Runs
SRUC have completed a series of test runs using over a quarter of a million Dorset
animal records to determine the impact of these changes.
This table shows the average EBV for animals in each analysis.
 The correlation between results in the two analyses was high for most traits
(0.92-0.99).
 The correlation of 0.73 reported for Maternal Ability EBV was lower (as
expected) and reflects the improvements that have been made in the way the
trait is assessed. Breeders will notice changes in the ranking of animals for
Maternal Ability.
 The low correlation (0.67) for Mature Size EBV is due to the trait being poorly
recorded and strongly correlated with the Maternal Ability EBV (which has
changed considerably- as explained above).
0.10
0.02
0.05
0.38

On average index values have fallen by around 15 index points as a result of
these changes.
8 Week Weight EBV
Scan Weight EBV
Muscle Depth EBV
Fat Depth EBV
Maternal Ability EBV
Mature Size EBV
Litter Size Born EBV
Terminal Sire Index*
Maternal Index*
Old Run
Average Standard
Value
Deviation
0.76
1.41
1.52
2.85
0.40
0.80
0.07
0.28
0.21
0.56
0.18
0.73
0.01
0.07
111
72
102
59
New Run
Average
Standard
value
Deviation
0.45
0.86
1.37
2.48
0.41
0.77
0.07
0.30
0.42
0.87
0.65
1.05
0.01
0.10
98
66
88
66
Correlation
between two
sets of results
0.91
0.99
0.97
0.95
0.73
0.67
0.92
* These are the old index values – which are to be re-modelled to reduce the index of
animals with a low Fat Depth EBV.
Conclusions
This work represents a significant update of Signet’s Dorset breeding evaluation.
These changes should be viewed as enhancements, which will ultimately lead to
more stable and more accurate EBVs.
Now that this work has been completed, SRUC have worked on changes to the two
breeding indexes used by Dorset breeders. One aspect of these changes is the
application of a new penalty to reduce the index of animals with a very low Fat Depth
EBV.