## 4 The Use of Myostatin Genetics to Increase Meat Yield in Cattle
### The MSTN c.821 Deletion  
A second well-researched variant form of the myostatin gene is the _MSTN_ c.821(del11) allele (a deletion of 11 nucleotides in the coding DNA sequence from positions 821-831 of _MSTN_). A double-muscled phenotype can be found for many breeds that carry this allele, and it is commonly found as two copies (or in the homozygous form) in Belgium Blue cattle [25]. It can be found in many other breeds including Angus and Angus-cross cattle [31]. In some reports, it has been referred to as the'myostatin blocker gene'.  
Following its discovery, the c.821(del11) allele has been widely studied. For example, it was reported that tenderness and ease of meat fragmentation were significantly higher in myostatin carrier carcasses compared to non-carrier carcasses [32].  
In a study undertaken by the Beef CRC (Cooperative Research Centre for Beef Genetic Technologies 200-52012) in Australia, it was reported that as a percentage of cold carcass weight, Angus steers with a single copy of c.821(del11) had an increase in retail yield (67% versus. 63%) with less bone (18% versus 19%) and less fat trim (15% versus. 18%), when compared to non-carrier steers [33]. There was also an increase in eye muscle area(85 cm\({}^{2}\) vs. 73 cm\({}^{2}\)), but there was no difference in marbling scores or rib fat depths. The authors concluded that a breeding programme that utilised a single copy of the 'non-functional' myostatin gene would significantly increase retail beef yield.  
A study at the Roslin Institute in Scotland examined the carcass characteristics of Angus-sired cattle that were heterozygous for _MSTN_ c.821(del11), comparing them with non-carrier animals [34]. The allele was revealed to significantly increase carcass weights, eye muscle area, and phenotypical muscle expression, but it reportedly had no effect on marbling, or other fat traits.  
It was revealed that _MSTN_ c.821(del11) allele carriers not only have a higher carcass meat yield, but also have higher yields of expensive cuts of meat. Bulls with the allele have a dressing out percentage average of 70%, when compared to non-carrier genotypes with 64% [35]. This is because of the increased meat content of the carrier carcasses (76% vs. 65% on average) and, hence, total meat is 28% more for the double-muscled animals.  
In Senepol cattle, animals heterozygous for _MSTN_ c.821(del11) presented a larger area of _Longissimus dorsi_ muscle, while their carcasses were leaner with less subcutaneous and intramuscular fat compared to the ones with no copy of the mutation [36]. In Angus cattle, it was revealed that c.821(del11) reduced fatness and increased EMA by 2.8-3.6 cm\({}^{2}\)[37], while another study revealed that net feed intake declined for feedlot finished Angus cattle with one copy of c.821(del11), but that muscling increased, as did the dressing out percentage, retail meat yield, and retail meat-to-bone ratio. The carrier steers had less fat across all carcass fat measurements [38].  
In another study, it was revealed that cattle with a single copy of the c.821(del11) allele, when compared to non-carrier cattle, had similar qualities for marbling, shear force (SF), meat colour, and pH, and that the mutation did not appear to reduce meat quality in Angus steers [39]. They also found the inclusion of one copy of the _MSTN_ c.821(del11) mutation can increase retail meat yield and maintain qualities valued by consumers, such as improved tenderness, juiciness, flavour, and satisfaction.  
This table provides a brief overview of the effects of the _MSTN_ c.821(del11) in different cattle breeds emphasizing the impact of this allele on muscle development, carcass characteristics, and meat yield.