A Note on Cattle Genetics and Beef Tenderness – Another Reason Belted Galloway Cattle Make Wonderful Beef

The entire genetic makeup, or genome, of cattle is stored in chromosomes of which each cell in the animal has 60 in number; 29 pairs of autosomes and 1 pair of sex chromosomes. Each chromosome is made of DNA composed of molecules containing nitrogenous bases combined with sugar and phosphate molecules called nucleotides (adenine, cytosine, guanine and thymine – abbreviated A, C, G, and T respectively) linked in sequence to make the DNA molecule. The specific sequence of these nucleotides determines the function of each portion of the DNA molecule. Specific sequences occurring along the DNA strand contain all the instructions necessary for making a protein. These sequences are called genes and their location on the DNA molecule is called a genetic “locus”. It is possible for the DNA sequence at a genetic locus to differ among individuals. The viable DNA code sequences that occur at any genetic locus are called alleles. An animal’s genotype for a gene is the set of alleles it happens to possess. In a cattle, each cell has two copies of each chromosome, therfore two alleles make up the individual animal’s genotype. Since the sequence of DNA molecules encodes for a specific sequence of amino acids in the resulting protein, differences in genetic sequences can result in an alteration in the amount of protein produced or in the production of proteins with altered function. This can then result in cattle with altered appearance or altered performance (referred to as a phenotype).

Allelic variation can be recessive, meaning that a calf must inherit the same allele (the same sequence) from the sire and dam before there is an effect on appearance or performance. Additive allelic variation means that an animal inheriting different alleles from each parent has a resultant phenotype intermediate between those found in animals carrying indentical copies of the alleles. Dominant alleles mean that the presence of one allele is sufficient to result in the appearance or performance trait of interest. Sites (loci) on the genome that frequently show different alleles in a given population are termed polymorphic sites

Recently scientists have started to identify regions of DNA that associate with economically relevant production traits in cattle. Several of these regions are significantly polymorphic. Tests have been developed to identify the sequence differences at these sites and therefore identify an animal that is carrying a segment of their DNA that is more or less associated with a desired production trait. The allelic differences at these sites are referred to as genetic markers and the ability to select animals with desired genetic markers for breeding is referred to as marker-assisted selection. In the last issue of the U.S. Beltie News we discussed two types of genetic markers microsatellites and single nucleotide polymorphisms (SNPs – pronounced “snips”). In the instance of microsatellites stretches of DNA will contain simple repeated sequences (eg. CACACACACACACA…….) These short tandem repeats vary in the length of the repeated segment so individuals can vary in the length of these repeats at any locus. Single nucleotide polymorphisms are variation at a site of a single nucleotide base between the two strands of DNA.

The term genotyping is used to describe the process of using laboratory methods to determine which genetic markers an individual animal carries. If the animal gets identical marker alleles from each parent the animal is said to have a homozygous genotype at that locus. If the alleles differ, the animal is said to be heterozygous at that locus.

As discussed in the previous issue of the newsletter, genotyping using microsatellites and SNPs can be used for parentage determination and for the identification of animals carrying desired phenotypic traits. The commonly evaluated economically relevant traits for cattle production include birth weight, weaning weight, ADG, reproduction, milk production, and carcass quality traits such as tenderness, marbling, back fat thickness, and rib eye area. Most of these traits are complex and are controlled by many genes as well as the animals environment; nutrition, stress, disease, etc.

EPD Based Selection vs. DNA Based Selection

Expected progeny differences (EPDs) are useful for breeding decisions since they provide estimates of the genetic value of an animal as a parent. Differences in EPDs between two individuals of the same breed predict differences in performance between their future offspring when each individual animal is mated to animals of the same average genetic merit. EPDs are commonly calculated for birth, growth, maternal, and carcass traits and are reported in the same units of measurement as the trait (normally pounds). EPD values may be directly compared only between animals of the same breed. (see: http://www.ext.vt.edu/pubs/beef/400-804/400-804.html) It is thought that when an animal has an EPD above average for a given trait, the animal has inherited a higher proportion of alleles for genes that favorably affect the trait. Thus selection based on EPD is designed to increase the number of favorable alleles and animal has without actually knowing which specific genes are involved.

With DNA based selection of breeding animals there is exact knowledge of which chromosomal locations are associated with improvement in a given trait and this knowledge forms the basis of genetic tests. Selection then becomes focused on know genetic marker alleles at specific loci to make genetic improvement in the trait of interest. (see: this document)

Marker-assisted selection is the process of using genotyping to assist in the selection of breeding stock to produce the next generation of a genetic improvement program. Thus instead of using only a traditional or EPD selection program to increase the incidence of favorable alleles for desired traits in the offspring, genotyping is used to assist in the selection of those favorable alleles.

It is important to recognize that currently available markers for many complex traits are associated with only one of the many genes that contribute to the trait. As mentioned above, other genes and the production environment will influence whether the offspring actually display the desired trait. As a consequence, marker-assisted selection should be seen as an adjunct to, and not a replacement for, observational data and EPD.

The potential benefits from the use of marker-assisted selection are the greatest for traits where (1):

1. Observational data are poor predictors of the heritability of the trait.
2. The trait is difficult or expensive to measure (e.g., Disease resistance).
3. The trait cannot be measured until the animal has already contributed to the next generation (e.g., Carcass analysis).
4. The traits are not routinely measured (e.g., Tenderness).
5. Observational data cannot segregate the trait of interest from an undesired trait (e.g., Genetic markers that can segregate marbling from an increase in back fat thickness).
   

The use of marker assisted selection can reduce the number of years it takes to introduce phenotypic improvement in cattle by selecting for cattle that carry two copies of the marker of interest (homozygous marker for the trait) and against those animals that carry no copies of the marker. Since all of the offspring from a homozygous parent will inherit a single copy of the marker of interest, continuous use of homozygous sires for 4 generations should result in approximately 90% of the resultant herd carrying two copies of the marker.

Using Marker-Assisted Selection for Carcass Traits

Traditional carcass quality traits including marbling, tenderness and carcass yield have been measured at slaughter, thus obviating the use of the involved animal in the breeding program. Means to predict these traits in live animals have been one of the major goals of beef cattle breeders. Although ultrasound assessment has added greatly to the assessment of an individual live animal, it does not necessarily predict the ability of the animal to pass those traits on to the next generation. Consequently, extensive research has led to the identification of genetic markers that associate with carcass quality traits.

Several markers for carcass quality traits are now commercially available.

The remainder of this article will focus on markers that select for beef tenderness.

Eating satisfaction from beef results from the interaction of tenderness, juiciness and flavor. Of these beef tenderness is the most sought after and least consistent attribute available to consumers. Beef tenderness can be based on; species (Bos Taurus – European cattle - are thought to produce more tender meat than Bos indicus cattle-zebu or “humped” cattle); pre-mortem nutrition, handling and slaughter technique; and animal genetics. The genetically controlled variation in beef tenderness can be expressed pre-mortem or post-mortem and is currently thought to be due to variations in three genes. These are myostatin, calpain and calpastatin. The genetic effects of myostatin are expressed pre-mortem. The genetic variant that produces a non-functional myostatin gene, has the largest pre-mortem effect on beef tenderness of any single genetic feature investigated to date. Polymorphisms in the gene myostatin are responsible for “double muscling” which has been noted to be associated with increased tenderness particularly in Piedmontese and Belgian Blue cattle. This increase in tenderness is thought to be related to a decrease in connective tissue in the muscle. The other two genes shown to be related to beef tenderness are active in the post-mortem events associated with aging of beef. These post-mortem events are thought to be responsible for 90% of the changes responsible for the development of tender beef. The mechanism of increasing tenderness associated with beef storage at refrigerated temperatures has been shown to be related to two proteins; the ?-calpain protease and to its inhibitor calpastatin. The ?-calpain protease is an enzyme that catalyzes the degradation of key myofibrillar and associated proteins. These are structural proteins in the beef muscle. More post-mortem activity of this enzyme leads to increased breakdown of muscle structural protein and therefore increase beef tenderness. Calpastatin is an inhibitor of the ?-calpain protease and regulates 60% of the tenderness effect of aging. Increased activity of this inhibitor protein blocks the action of the ?-calpain protease, thus less muscle breakdown occurs during the aging process and the beef is less tender. (Figure 1)

Figure 1

Genotyping for two genetic variants in the gene coding for the ?-calpain protease and one in the gene coding for calpastatin has become commercially available. The presence of these genetic variants has been correlated with Warner-Bratzler shear force (WBSF) measurements which are currently the best objective measurements of beef tenderness. The more the WBSF is reduced the more tender the beef. The variants in the gene coding for the ?-calpain protease correlating with increased tenderness result in a more active enzyme. The variants in the gene coding for calpastatin that correlate with increased tenderness result in a less active inhibitor. This relationship is shown in Table 1.

Table 1

This table gives the genotype and estimated change in W-B shear force measurement from the NBCI trials of the Merial Igenity product. A similar product with slightly different genotypes is available from Bovigen. An Igenity tenderness score has been designated to facilitate an understanding of the relationship between genotype and shear force. Studies of the Igenity tenderness genotypes from 171 Belted Galloway cattle showed that these cattle score better for tenderness than other cattle breeds. These data show that 71.1% of Belted Galloway cattle score 6 or above on the Igenity tenderness scale (10 is the most tender) versus 41.1% of the 1600 reference cattle tested. ( Figure 2) This difference is primarily due to the fact that 99% of the Belted Galloway cattle tested had a genetic variant of the gene coding for calpastatin that resulted in markedly decreased activity of this inhibitor of the ?-calpain protease. This is an extremely unusual finding among breeds of cattle and bodes well for the marketability of our breed.