Double muscling or muscular hypertrophy is an inherited condition in cattle, characterised by hyperplasia (increase in number) and, to a lesser extent, hypertrophy (enlargement) of muscle fibres.

Characteristics of the Double Muscling Syndrome:


Double-muscled animals are characterized by an increase in muscle mass of about 20%, due to general skeletal muscle hyperplasia and, to a lesser extent, hypertrophy.
This relative increase in the number of muscle fibres (hyperplasia) occurs during intra-uterine development, such that double-muscled cattle possess nearly twice the number of muscle fibres at birth as do normal cattle.
The muscles of double-muscled cattle also have a significantly reduced amount of connective tissue (collagen).
Not only is collagen reduced in amount, but it is structurally different to normal collagen in that it has a lower proportion of stable, non-reducible, cross-links.
Muscular hypertrophy and hyperplasia is not uniform throughout the beast, being minimal around the neck and increasing as one moves to the hindquarters where it is maximal. This distribution results in the caecases of double-muscled animals having a higher proportion of "expensive" cuts of meat relative to carcases of normal cattle.


The bone mass of double-muscled cattle tends to be around 10% less than that of normal cattle. This is primarily due to their long bones being shorter, more slender, and of lower density.
This reduced bone mass results in a significantly higher muscle : bone ratio in double-muscled cattle.


Double-muscled cattle exhibit hypodevelopment of their fatty tissues. This is due to a reduction in the volume of fat cells rather than to a reduction in their numbers.
Not only is the total fat content reduced, but its composition is different, with double-muscled animals having a much higher percentage of polyunsaturated fats (11% compared with 5% in normal cattle).


During forced exercise, double-muscled cattle show signs of fatigue faster than normal cattle. This is thought to be due to a reduced capacity for aerobic metabolic activity by the exercising muscles.
Double-muscled cattle tend to have a reduced tolerance for heat stress. This is thought to be due to the increased heat production associated with their increased muscle mass.

Reproduction and Growth:

The syndrome of double muscling is associated with a number of reproductive problems. In the case of those animals where the syndrome is fully expressed there may be:
..........(i).....delays in puberty.
..........(ii)... reduced fertility due to an increased incidence of mortality in double-muscled embryos,
..........(iii).. increased incidence of dystocia,
..........(iv)...reduced milk production,
..........(v)....increased calf mortality.

Most double-muscled calves tend to have higher birth weights and higher pre-weaning growth rates than their normal contemporaries. Post-weaning, however, their growth rate tends to fall behind that of their normal contemporaries; this appears to be due to a lower feed intake.
If muscle weight gain per unit energy intake is taken into account, double-muscled cattle have better feed efficiency than normal cattle.


When compared with normals, the carcases of double-muscled cattle have many desirable characteristics:

Higher dressing percentage:
The carcases of double-muscled cattle dress out at between 65 and 70 percent. This is due to a combination of:
..........(i).....increased muscle mass,
..........(ii)....reduced body fat,
..........(iii)...reduced bone mass, and
..........(iv)...smaller internal organs.

Higher proportion of "expensive" cuts of meat:
This is due to the non-uniform distribution of the muscular hypertrophy and hyperplasia which is found in double-muscled cattle.

Reduced fat content with a higher proportion of polyunsaturated fats:
(See above)

Better meat quality:
Meat from double-muscled cattle is significantly more tender than that from normal cattle. Much of this is thought to be due to its lower collagen content and to the fact that what collagen is present is not as tough due to its lower proportion of stable, non-reducible, cross-links.

The significance for producers is that double-muscled animals produce a higher proportion of desirable cuts of lean meat with greater efficiency than do comparable, conventional cattle. For consumers, this meat is more tender and, being lean and having a higher polyunsaturated fat content, conforms more closely with current nutritional guidelines than meat from normal animals.



Curly calf syndrome is a lethal genetic defect that has been identified in beef cattle. Calves are stillborn and have a twisted or curved spine and extended and contracted limbs, hence the term “curly calf syndrome.” This is a lethal genetic defect scientifically known as  Arthrogryposis Multiplex (AM) The tern AM is a Greek derivation which means curved or hooked joints. This congenital defect was discovered in the Angus  breed and genetically traced to a popular certain bull (GAR Precision 1680) in that breed. The mode of inheritance for this condition is a simple recessive gene. This is a pattern of inheritance is similar to coat colour for black and red in cattle or horned or polled. Animals with only one copy of the AM gene and one copy of the normal gene appear normal and are known as carriers. Carrier animals are normal, perform as good as non-carriers, but when mated to a carrier female can there is a 25% chance for the offspring to be affected. 50% of the offspring can be carriers for AM while there is a 25% chance for “clean” offspring. The condition (curly calf syndrome) can  only be expressed when the individual is homozygous for the AM gene ( both genes are present in the affected calf) for the trait.

A DNA based test has been developed to identify individuals that have one copy or no copies of the AM gene. Animals testing free of the AM gene are designated as AMF (AM Free). Not all animals need to be tested, only those that have ancestors in their pedigree that are known carriers would be necessary to eliminate the chance of the occurrence of curly calves in a breeding program.

    Images of affected calves:


3.  Polled, Scurred and Horned


There are 3 different sets of genes that determine whether an animal will be polled, scurred or horned. Two of them can cause horns, and the other causes scurs. One of the horn genes, and the scur gene are both sex-linked, meaning that the genes are linked with the genes that determine the sex of an animal, and they express themselves differently in females than they do in males.

The first gene I will discuss is commonly found in Bos taurus cattle, that is the European and British breeds. This is the Polled gene, P and p. P is polled and p is horned. This gene is NOT sex-linked, and expresses itself in the same way in both male and female animals.

P is dominant, which means that as long as it is present the animal will be polled. PP and Pp animals are both polled in appearance, however Pp animals may pass on either P or p to their offspring. pp animals, are horned in appearance, and will always pass on the p gene to their offspring.

PP = Homozygous - Animal is Polled

Pp = Heterozygous - Animal is Polled

pp = Homozygous - Animal is Horned

A homozygous horned animal, in this animal the horns are present. Note the shape of the poll.

Another Homozygous horned animal, this animal has been dehorned, but again notice the shape of the poll.

This animal is heterozygous polled. She carries the horn gene, but since polled is dominant the animal is polled in appearance. Note the shape of the poll.

And finally, a Homozygous polled animal. Again note the shape of the poll.

As a side note, this gene is one that is known to mutate fairly frequently, which may result in the occaisonal polled animal being born to a horned animal.

The other horn (African horn gene) gene is known in Bos indicus animals, or breeds of Zebu type animals. Brahman animals may carry this gene. This gene IS sex-linked, and expresses iself differently in females than it does in males. It is written as Af and af. In females, Af is recessive, and requires that there be two copies of Af for the animal to be horned. However in males the gene is dominant, and only one copy of the gene is necessary for the trait to express.

In males:
AfAf = Homozygous - The animal is Horned

Afaf = Heterozygous - The animal is Horned

afaf = Homozygous - The animal is Polled

In females:
AfAf = Homozygous - The animal is Horned

Afaf = Heterozygous - The animal is Polled

afaf = Homozygous - The animal is Polled

A horned female will always give birth to a horned male, however, her female calves may or may not be horned, depending on what gene was passed on from the sire.

The final option for horn like growths is the scurred gene. Scurs are a hornlike growth that are loosely attached to the head. This gene, is thought to work the same way as the Af gene. With the belief that for a male that is heterozygous for the Sc gene to express scurs, he needs to also carry a copy of the p gene. Sc is recessive to the p gene, where a horned animal may be scurred, but they will not be expressed.

So in males:
ScSc = Homozygous - Animal is Scurred

Scsc = Heterozygous - Animal is Scurred, although it is believed that HE must also carry a copy of p

scsc = Homozygous - Animal is Polled

In females:
ScSc = Homozygous - Animal is SCURRED

Scsc = Heterozygous - Animal is Polled

scsc = Homozygous - Animal is Polled

Again, a Homozygous female should always have a scurred male calf, and her female calves may or may not be scurred, depending on the sire.

In the case of these three genes an animal may possibly carry or even be homozygous for the horn gene, the African horn gene and the scur gene.