Download Strategies for getting rid of horn genes in poll-Merino

Strategies for getting rid of horn genes in poll-Merino flocks
Julius van der Werf, Sheep CRC Program Leader – Genetics & Genomics (December 2012)
A gene is a bit of DNA that affects a trait and each gene has two alleles, one from mum and one
from dad. The development of horns in sheep appears to be controlled by a single gene for which
there is good DNA marker. There are three possible combination of the alleles H – for horns and P
for polled: HH, HP or PP. Because the horned allele is recessive, horned animals must have both
alleles for horns – i.e. the HH genotype. Polled animals can have a genotype of either PP or PH.
The test for the horn gene is based on a marker that is close to the gene that provides information
on the combination of alleles. Based on our observations in the Information Nucleus we can predict
whether an animal is likely to have horns or is polled, based on the marker genotype – PP, PH or
HH. This information allows us to predict whether the progeny of a genotyped ram are more or less
likely to have horns when mated to ewes with different genotypes. (See Table 1).
Table 1
Summary of sire breeding values for horn status and the probability of progeny
phenotypes resulting from different P, H status of parents.
Outcome
Male progeny
Female progeny
Horned
Polled
PP sire
3%
82%
1%
92%
PH sire
3%
65%
1%
87%
HH sire
4%
49%
1%
82%
PP sire
3%
49%
1%
82%
PH sire
37%
27%
5%
50%
HH sire
70%
6%
8%
18%
Horned
Polled
When mated to PP ‘polled’ dams
When mated to HH ‘horned’ dams
Estimates are based on 2,300 Merino progeny in the Sheep CRC Information Nucleus (October 2011). Note
these figures do not add to 100% because animals with knobs and scurs are not counted as either polled or
horned.
If you occasionally have horned sheep in your poll flock, the flock is carrying the horned allele.
Since the horned allele is recessive you need two copies of it to show horns. The frequency of the
horn allele can be quite high even though you may not very often see horns. Monitoring a flock for
the number of male animals with horns provides a good estimate the frequency of the horn allele
and the proportion of rams that are carriers of the horn allele. This is summarised in Table 2.
Table 2
Allele frequency, carrier rams and horn and polled status of male progeny
Horn allele
frequency
%
50
20
10
Carrier rams
%
50
32
18
Progeny phenotype frequencies
%horn
21
4
1
% scurs & knobs
27
17
11
1
%poll
52
79
88
We can see that even if we have only 1% of males with horns, the allele frequency is still around
10%. This means that in 10% of matings a sire or dam could pass on a horn allele to its offspring
and 18% of polled rams being used will be carriers of the horn allele.
The new genomic test for the horn gene means that, in poll flocks, we can avoid breeding from
rams that are carriers. Simply testing rams the before using them, and only using PP rams, rapidly
decreases the horn allele frequency as illustrated in the examples below.
Three strategies and their impact
Consider a poll flock with 1000 ewes, using 25 rams each year. Each year 10 of those are replaced
by young rams and 410 new ewe replacements are needed. The example below shows the
outcomes for three strategies in a flock where the horn allele frequency is 10%, i.e. 1% of the rams
has horns.
1) Do nothing, no testing and accept some horned rams
2) Test all rams before using them
3) Test all new rams as well as all new dams
In the first strategy (blue line), the allele frequency is not expected to change much. Even if you
avoid breeding from rams with horns, you will still use poll rams that are carriers of the horn allele
(18% of all rams) but you won’t know that they carry the horned allele.
In the second strategy (green line) you test all rams in the first year and only new rams in following
years. The allele frequency drops off quickly in the first years and then continues to drop with
ongoing testing of new rams.
In the third strategy (purple line) you test a lot of replacement animals (ewes and rams), about 420
each year, but the allele frequency will drop faster and will be eliminated within 8 years.
The following graph (Fig 1) shows how quickly the allele frequency drops off for the 3 strategies
and Figure 2 shows the number of horned rams born each year.
12%
horn allele frequncy
10%
8%
random rams
6%
test all rams
4%
test all rams&ewes
2%
0%
0
5
10
15
20
25
years
Fig 1. Change in horn allele frequency with time in response to different strategies for
using the genomic test for poll.
2
7.0
6.0
nr of horn rams
5.0
4.0
random rams
3.0
test all rams
2.0
test all rams&ewes
1.0
0.0
-1.0
0
5
10
15
20
25
years
Fig 2. Change in the number of male progeny with horns over time in response to different
strategies for using the genomic test for poll in a 1000 ewe flock.
Conclusions
A strategy where all rams are initially tested, and then in subsequent years all new rams are tested
is effective in reducing the allele frequency and the incidence of horns. Given that the cost of
testing is $17/test and there is a price premium of between $100 to $200 per ram for polled
compared to horned rams, testing rams before they enter a poll flock appears to be a very costeffective strategy for breeders and ram buyers wanting polled Merinos.
While testing both ewes and rams (Strategy 3) reduces the horn allele frequency more quickly,
than just testing rams, the cost of testing is much higher and may not be worth the additional
investment.
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