In 1866, Gregor Mendel published results from studies investigating the genetic material of life. Through the use of peas in crossbreeding experiments, Mendel reported the inheritance of characteristics or traits occurred via units (which would later be described as genes). For a refresher on nucleotides, DNA, and genes please see the first article in our series: “Genetics and Genomics: An Introduction.” The objective of this article is to provide information on the breadth of genomics in humans and animals.
When sperm and oocytes are produced, each may not carry the exact same DNA sequence, i.e., a polymorphism (poly=many, morph=form) may occur which involves one of two or more variants of a particular DNA sequence. The most common polymorphism involves variation at a single nucleotide, or base pair. This variation is called a single nucleotide polymorphism, or SNP (pronounced “snip”), and may serve as a marker for a variety of genes. Although the DNA sequence of any two people (or animals) is approximately 99% identical, a single nucleotide change in a single gene can lead to a genetic disease.
Scientists are currently studying how SNPs in the genome are associated with disease, production traits, and fertility in livestock. In humans, scientists are also studying SNPs associated with disease, while commercial human genomic testing has focused on health traits and ancestry.
The field of genomics encompasses DNA sequencing, genetic mapping and analysis of the complete genome of an organism, including organizing the results in databases. The Human Genome Project, a collaborative effort of scientists from the USA, UK, France, Japan, Germany, and China was completed in 2003 at a cost of 2.7 billion dollars. This milestone occurred 50 years after Watson and Crick described the structure of DNA as a double helix. In 2009, after six years of research by 300 scientists from 25 countries, the bovine genome was decoded at a cost of 53 million dollars.
Genomic information is freely available from the National Center for Biotechnology Information, National Library of Medicine. In addition to human (Homo sapiens) and cattle (Bos taurus and Bos indicus), the list of species sequenced is exhaustive and includes the cat (Felis catus), dog (Canis lupus familiaris), horse (Equus caballus), and sheep (Ovis aries).
The human and bovine genome each contain approximately 3 billion base pairs, which are found in the 23 and 30 pairs of chromosomes within the nucleus of cells in the human and bovine, respectively. Each human and bovine chromosome contains hundreds to thousands of genes, which provide the instructions for making proteins. The bovine genome has approximately 22,000 genes, of which 80% are the same as human genes.
Cattle convert forage into energy-dense milk and muscle, providing a significant source of nutrition and economic livelihood to humans since domestication occurred nearly 10,000 years ago. Studying the bovine genome may help researchers to enhance the nutritional value of beef and dairy products, and develop cattle that are more fertile and disease resistant. A current commercial genomic test includes a variety of health traits (mastitis, metritis, lameness, retained placenta, ketosis, and displaced abomasum) and estimates differences in expected lifetime profit associated with risk of disease in cows.
Genomic analyses have been used to study similar diseases in animals and humans. A genome-wide association analysis revealed a gene mutation in canine degenerative myelopathy that resembles amyotrophic lateral sclerosis (Lou Gehrig’s disease) in humans, while a gene has been identified as a risk variant for cleft lip and palate in both dogs and humans.
Human genomic testing companies have genotyped 8 to 10 million people worldwide, with the focus being on health traits and ancestry. In contrast, greater than 2.25 million genotypes are represented in the Council on Dairy Cattle Breeding database, with the majority of genotyped animals being Holsteins (87%). Further, the majority (87%) of genotyped cattle are female. Dr. Chad Dechow, Associate Professor of Dairy Cattle Genetics at Penn State University, argues it is not surprising more females than males have been genotyped given the majority of the dairy cattle population is female. Further, Dr. Dechow notes the high proportion of females genotyped provides evidence the majority of genotyping is being done for herd management as opposed to marketing.
The development of this document was made possible by USDA NIFA AFRI grant No. 2013-68004-20365.
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