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CRC FOR BEEF GENETIC TECHNOLOGIES
FACT SHEET
Understanding DNA Technology
FAST FACTS
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Most living cells contain a nucleus with chromosomes made up of DNA
DNA provides instructions to living cells and determines their form and function
The sequence of base pairs in DNA is the basis of DNA technology for genetic evaluation
Modern technologies allow detailed examination of the DNA structure of individual animals, allowing more accurate
estimates of genetic merit for complex traits
Some traits (e.g. coat colour, horns and some genetic disorders) are determined by a single gene or very few genes
Most complex production traits of economic importance are controlled by a very large number (hundreds or thousands) of genes
What is the role of DNA?
Animals and plants are made up of cells. Most cells have a nucleus,
which contains the chromosomes that hold the genetic blueprint for
all living cells. Chromosomes are composed of deoxyribose nucleic
acid, or DNA. One copy of each chromosome pair is inherited from
each parent.
The characteristics of all living organisms are determined by
information contained within the DNA. Animals that have different
characteristics have different DNA sequences.
DNA looks something like a long ladder twisted into a helix, or coil.
The “sides” of the ladder are formed by a backbone of sugar and
phosphate molecules, and the “rungs” consist of bases joined weakly
in the middle as shown in Figure 1. The bases are the key to DNA
functioning as a source of genetic information.
There are four different bases: A – Adenine, T – Thyamine, G- Guanine,
C – Cytosine. In the double helix structure of a DNA molecule G always
partners C, and A always partners T to form pairs of bases. There
are approximately 3 billion base pairs in bovine DNA. The sequence
of these base pairs contains the ‘message’ in DNA. Sequence
differences form the basis for differences between animals in their
genetic merit.
When the sequence differs at a single nucleotide it is called a single
nucleotide polymorphism (abbreviated to SNP—pronounced snip—
shown in Figure 2). When the sequence occurs as a repeat of two
or three base pairs—for example, CACACACACACA—it is called a
microsatellite.
Cow
Cow
Figure 1: Chromosomes are located within the nucleus of cells.
Chromosomes comprise DNA molecules.
Understanding DNA Technology Fact Sheet
Figure 2: Cow 1 differs from cow 2 at a single
nucleotide location (SNP).
CRC FOR BEEF GENETIC TECHNOLOGIES
What is a Gene?
A gene is a specific sequence of base pairs at a particular location on the
chromosome. It codes for a specific product that generally has an effect on cell
function. It is estimated there are between 22,000 and 28,000 genes in a beef or
dairy animal. The genes are separated on the chromosome by DNA sequences that
do not code for a specific product but do form part of the animal’s genetic makeup.
These sequences are also referred to here as genes.
DNA terminology
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For each gene, there may be two or more forms (known as alleles) that can result in
different observable phenotypes (measurements of traits in animals). For example,
different alleles for one gene influencing coat colour can code for red or black
pigmentation.
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What is a Gene or DNA Marker?
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A gene or DNA marker is a detectable, heritable base pair sequence at a known
location that is associated with differences between animals. Mutations,
microsatellites and SNPs are all terms that describe a particular type of gene
marker, which differ in their frequency of occurrence and their usefulness for genetic
testing. Microsatellites, for example, occur less frequently than SNPs but have many
variants, which make them useful for parentage testing.
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Some gene markers, such as those for coat colour, directly identify the genes
influencing the trait of interest. Other gene markers, such as those for productive
traits, indirectly identify the genes influencing the trait of interest because they are
located in close proximity and tend to be inherited together: the closer the proximity
to the actual gene causing the differences, the stronger the observed association
(Figure 3). Genetic shuffling that occurs between one generation and the next or
across different breeds can cause the associations between markers and the genes
influencing production traits to be broken. This is one of the reasons that marker
tests need to be recalibrated over time and across breeds.
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As SNPs are abundant in the genome, they are ideal indirect markers for production
traits. Each production trait is influenced by many genes, so inferring genetic merit for
a particular trait requires the accumulated effects from many markers. As most gene
markers are not within the gene that influences the trait of interest, the strength of
the association between the marker and the gene will vary and therefore the value,
or informative power of the marker will also vary.
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Value of markers
Trait – Attribute or characteristic of
animals that can be measured and
improved genetically (for example, growth
rate, fertility, carcass or meat quality)
Genetic Merit – potential performance
of an animal for a particular trait, as
determined by its genetic make-up
Genotype – the full genetic makeup
of an organism including dominant
and recessive alleles that may not be
expressed
Phenotype
–
the
observable
characteristics or traits of an organism
that result from the interaction between
the organism’s genotype and the
environment
Locus – the specific location of a gene or
DNA sequence on a chromosome
Allele – one form of a gene or a genetic
locus
Polymorphism – the existence of more
than one form or allele at a specific
genetic locus
Homozygous – refers to a specific locus
where the same allele was inherited from
each parent
Heterozygous – refers to a specific locus
where a different allele was inherited
from each parent
Dominant and recessive – describes the
mode of inheritance of a gene and the
interaction of alleles. When an animal is
heterozygous, the dominant allele will be
expressed and the recessive allele will be
invisible in the phenotype (see factsheet:
Utilising DNA to change type traits for
more information).
Carrier – refers to a heterozygote animal
that is carrying a recessive allele that can
be passed on to progeny
Gene of interest
Marker A
Value
aaaa
Gene of interest
Marker A
Value
aa
Figure 3: Markers that are located closer to the gene will be of
greater value than those that are further removed
Understanding DNA Technology Fact Sheet
Further reading
The Beef CRC website (http://www.beefcrc.com.au/)
and the SBTS/TBTS webinar series (http://sbts.une.
edu.au/Webinars/webinars.html) have additional
information on understanding DNA technology and its
applications in beef cattle.
CRC FOR BEEF GENETIC TECHNOLOGIES