Name: ____________________
Period: ______ Date: ________
Horn and Scur Inheritance in Cattle
Horns:
In beef cattle of European descent being polled, or naturally born hornless, is a
characteristic that is determined by one gene. Cattle inherit one polled gene from
their bull (father) and one from their cow (mother); the polled gene is dominant.
Genotype
Genotype Shorthand
Phenotype
Horns or No Horns
Homozygous Dominant
Heterozygous
Homozygous Recessive
Advantages of Polled Cattle:
Horned cattle cause significant economic loss to the industry through bruising during
transport and at feedlots. They also pose a danger to handlers. For these reasons markets
are placing increasing pressure on producers to produce cattle without horns. The pressure
is particularly strong from the live export and feedlot trades. This means that producers
must either dehorn or breed polled cattle. Otherwise they face ever increasing penalties
when selling horned cattle. In the worst case scenario, they may not be able to sell horned
cattle at all.
Breeding polled cattle has several advantages over dehorning or tipping the horns of
horned cattle. These advantages include avoiding particular costs or risks such as:
 labor associated with dehorning or tipping
 infection to wound sites
 blowfly strike to wound sites
 the need for chemicals to prevent or treat infection and/or fly strike
 reduced growth rates while wounds are healing
Polled Inheritance Problems:
1. A homozygous polled bull mates with a heterozygous polled heifer. What is the probability that
they will have a polled calf?
♂
♀
2. A homozygous non-polled bull mates with a heterozygous polled heifer. What is the probability
that they will have a polled calf?
♂
♀
3. A heterozygous bull mates with a heterozygous polled heifer. What is the probability that they
will have a non-polled calf?
♂
♀
4. A horned bull mates with a polled cow whose last calf was horned. What is the probability that
they will have a polled calf?
♂
♀
5. A polled bull mates with a polled cow; both are known carriers for the horn trait. What is the
probability that they will have a polled calf?
♂
♀
Scurs:
Scurs are incompletely developed horn-like growths that are generally loose
and moveable under the skin because they are not attached to the skull.
They can range in size from small scab like growths to occasionally almost as
large as horns. The genes for scurs and horns are not linked; the presence
or absence of horns has no effect on the transmission of the scur gene. Not
all horned cattle carry the gene for scurs and not all polled cattle lack the
scur gene.
The scur gene in cattle is sex-linked recessive. Because the scur gene is located on the X chromosome
heifers and cows will have two copies of the gene and bulls will have one copy of the gene.
Genotype
Female Genotype
Phenotype
Male Genotype
Phenotype
Homozygous Dominant
Heterozygous
Homozygous Recessive
6. A recessive bull mates with a homozygous dominant heifer. What is the probability that they will
have a female calf with scurs?
♂
♀
7. A recessive bull mates with a carrier cow. What is the probability that they will have a female
calf with scurs?
♂
♀
8. A bull without scurs mates with a cow that has scurs. What is the probability that they will have
a male calf with scurs?
♂
♀
9. A dominant bull mates with a heterozygous cow. What is the probability that they will have a
female calf without scurs?
♂
♀
African Horn Gene:
Cattle with Zebru ancestry, like the Brahman and Santa Gertrudis, have one additional
gene, the African Horn gene that can affect the inheritance of horns in cattle. Like the
Scur gene the African Horn Gene is sex-linked recessive and carried on the X
chromosome.
Genotype
Female Genotype
Phenotype
Male Genotype
Phenotype
Homozygous Dominant
Heterozygous
Homozygous Recessive
10. A recessive bull mates with a horned heifer. What is the probability that they will have a female
calf without horns?
♂
♀
11. A horned bull mates with a carrier cow. What is the probability that they will have a male calf
with horns?
♂
♀
12. A bull without horns mates with a hornless cow that has previously had horned calves. What is
the probability that they will have a male calf without horns?
♂
♀
13. A recessive bull mates with a horned cow. What is the probability that they will have a female
calf without horns?
♂
♀
Application:
14. What genotype bull do you need to breed with a heterozygous polled heifer to produce all polled
offspring?
15. If you were a cattle breeder why would you be more likely to castrate a Brahman calf with horns
than one without?
16. Explain how a bull and a cow that are polled without scurs could produce a calf that has scurs.
17. Explain how a bull that you know is homozygous dominant polled who is bred with a cow that is
homozygous dominant polled can produce a male calf that is horned.
18. Explain how a bull that you know is homozygous dominant polled bred with a cow that is
homozygous dominant polled can produce a female calf that is horned.
Name: ____________________
Period: ______ Date: ________
Horn and Scur Inheritance in Cattle
Horns:
In beef cattle of European descent being polled, or naturally born hornless, is a
characteristic that is determined by one gene. Cattle inherit one polled gene from
their bull (father) and one from their cow (mother); the polled gene is dominant.
Genotype
Homozygous Dominant
Heterozygous
Homozygous Recessive
Genotype Shorthand
AA
Aa
aa
Phenotype
Polled
Polled
Non-polled
Horns or No Horns
No Horns
No Horns
Horns
Advantages of Polled Cattle:
Horned cattle cause significant economic loss to the industry through bruising during
transport and at feedlots. They also pose a danger to handlers. For these reasons markets
are placing increasing pressure on producers to produce cattle without horns. The pressure
is particularly strong from the live export and feedlot trades. This means that producers
must either dehorn or breed polled cattle. Otherwise they face ever increasing penalties
when selling horned cattle. In the worst case scenario, they may not be able to sell horned
cattle at all.
Breeding polled cattle has several advantages over dehorning or tipping the horns of
horned cattle. These advantages include avoiding particular costs or risks such as:
 labor associated with dehorning or tipping
 infection to wound sites
 blowfly strike to wound sites
 the need for chemicals to prevent or treat infection and/or fly strike
 reduced growth rates while wounds are healing
Polled Inheritance Problems:
19. A homozygous polled bull mates with a heterozygous polled heifer. What is the probability that
they will have a polled calf?
♂
A
A
♀
A
AA
AA
a
Aa
Aa
All (100%) of the offspring should be polled (AA, Aa).
20. A homozygous non-polled bull mates with a heterozygous polled heifer. What is the probability
that they will have a polled calf?
♂
a
A
♀
A
Aa
Aa
a
aa
Aa
Half (50%) of the offspring should be polled (Aa).
21. A heterozygous bull mates with a heterozygous polled heifer. What is the probability that they
will have a non-polled calf?
♂
A
A
♀
A
AA
Aa
a
Aa
Aa
One quarter (25%) of the offspring should be non-polled (aa).
22. A horned bull mates with a polled cow whose last calf was horned. What is the probability that
they will have a polled calf?
♂
a
a
♀
A
Aa
Aa
a
aa
aa
Half (50%) of the offspring should be polled (Aa).
23. A polled bull mates with a polled cow; both are known carriers for the horn trait. What is the
probability that they will have a polled calf?
♂
A
a
♀
A
AA
Aa
a
Aa
aa
Three quarters (75%) of the offspring should be polled (AA or Aa).
Scurs:
Scurs are incompletely developed horn-like growths that are generally loose
and moveable under the skin because they are not attached to the skull.
They can range in size from small scab like growths to occasionally almost as
large as horns. The genes for scurs and horns are not linked; the presence
or absence of horns has no effect on the transmission of the scur gene. Not
all horned cattle carry the gene for scurs and not all polled cattle lack the
scur gene.
The scur gene in cattle is sex-linked recessive. Because the scur gene is located on the X chromosome
heifers and cows will have two copies of the gene and bulls will have one copy of the gene.
Genotype
Homozygous Dominant
Heterozygous
Homozygous Recessive
Female Genotype
XBXB
XBXb
XbXb
Phenotype
♀ No Scurs
♀ No Scurs
♀ Scurs
Male Genotype
XBY
--XbY
Phenotype
♂ No Scurs
--♂ Scurs
24. A recessive bull mates with a homozygous dominant heifer. What is the probability that they will
have a female calf with scurs?
♂
Xb
Y
♀
XB
XBXb XBY
XB
XBXb XBY
None (0%) of the female offspring should have scurs.
25. A recessive bull mates with a carrier cow. What is the probability that they will have a female
calf with scurs?
♂
Xb
Y
♀
XB
XBXb XBY
Xb
XbXb XbY
Half (50%) of the female offspring should have scurs (XbXb).
26. A bull without scurs mates with a cow that has scurs. What is the probability that they will have
a male calf with scurs?
♂
XB
Y
♀
Xb
XBXb XbY
Xb
XBXb XbY
All (100%) of the male offspring should have scurs (XbY).
27. A dominant bull mates with a heterozygous cow. What is the probability that they will have a
female calf without scurs?
♂
XB
Y
♀
XB
XBXB XBY
Xb
XBXb XbY
All (100%) of the female offspring should have not have scurs.
African Horn Gene:
Cattle with Zebru ancestry, like the Brahman and Santa Gertrudis, have one additional
gene, the African Horn gene that can affect the inheritance of horns in cattle. Like the
Scur gene the African Horn Gene is sex-linked recessive and carried on the X
chromosome.
Genotype
Homozygous Dominant
Heterozygous
Homozygous Recessive
Female Genotype
X DX D
X DX d
XdXd
Phenotype
♀ No Horns
♀ No Horns
♀ Horns
Male Genotype
X DY
--XdY
Phenotype
♂ No Horns
--♂ Horns
28. A recessive bull mates with a horned heifer. What is the probability that they will have a female
calf without horns?
♂
Xd
Y
♀
Xd
XdXd XdY
Xd
XdXd XdY
None (0%) of the female offspring should be without horns.
29. A horned bull mates with a carrier cow. What is the probability that they will have a male calf
with horns?
♂
Xd
Y
♀
XD
X DX d X DY
Xd
XdXd XdY
Half (50%) of the male offspring should have horns (XbY).
30. A bull without horns mates with a hornless cow that has previously had horned calves. What is
the probability that they will have a male calf without horns?
♂
XD
Y
♀
XD
X DX D X DY
Xd
XDXd XdY
Half (50%) of the male offspring should not have horns (XDY).
31. A recessive bull mates with a horned cow. What is the probability that they will have a female
calf without horns?
♂
XD
Y
♀
XD
X DX D X DY
Xd
XDXd XdY
All (100%) of the female offspring should not have horns (XDXD and XDXd).
Application:
32. What genotype bull do you need to breed with a heterozygous polled heifer to produce all polled
offspring?
AA because the female is Aa and all the offspring need at least 1 dominant trait.
33. If you were a cattle breeder why would you be more likely to castrate a Brahman calf with horns
than one without?
Brahman can carry the African Horned gene. Males with the gene (recessive) have horns. Calves
without horns do not have to be dehorned, $ and increases the risk of infection, so the males who
genetically lack horns are preferable.
34. Explain how a bull and a cow that are polled without scurs could produce a calf that has scurs.
Both parents are heterozygous and they carry the scur gene. Calf inherits at least one dominant
polled gene so it doesn’t have horns BUT it has to inherit a recessive scur gene - the calf has to
be a male – Y chromosome from bull; recessive scur gene from heterozygous cow – males can’t be
heterozygous (sex-linked trait) and the presence of 1 gene results in scurs in males.
35. Explain how a bull that you know is homozygous dominant polled who is bred with a cow that is
homozygous dominant polled can produce a male calf that is horned.
Cow carries the African Horned gene – she gives the X chromosome and the bull gives the Y.
36. Explain how a bull that you know is homozygous dominant polled bred with a cow that is
homozygous dominant polled can produce a female calf that is horned.
Cow and Bull are both carriers of the African Horned gene – they both give recessive genes on
the X chromosome.