Transcript beef cattle genetics - Michigan State University

BEEF CATTLE GENETICS
By
David R. Hawkins
Michigan State University
Genetic Principles
• Cattle have 30 pair of chromosomes in the
nucleus of each cell.
• 29 pairs are similar and the remaining pair
are the sex chromosomes which can be
either XX (female) or XY (male).
• Regions on each chromosome called “loci”
contain the genetic information “genes”
which control the expression of a trait.
Genetic Principles
• Since the chromosomes and loci occur in pairs, so
do the genes.
• Genes on homologous chromosomes are called
alleles.
• If the alleles are similar the animal is
“homozygous”, but if they are different, it is
“heterozygous”.
• “Genotype” refers to the genetic makeup of an
animal while “phenotype” refers to what we see or
measure.
Qualitative vs. Quantitative
• Qualitative traits are those that are
controlled by 1 or 2 gene pairs and tend to
be distinct in their phenotype. Examples of
qualitative traits are coat color, polled vs.
horned, dwarfism, etc.
• Quantitative traits tend to be influenced by
several gene pairs. Examples include
production traits like weaning weight, milk
production, carcass traits, etc.
Response to Selection
• Five factors influence the rate with which
populations can be changed by selection.
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Heritability
Selection Differential
Genetic Correlation
Generation Interval
No. of traits for which one selects
Heritability
• Heritability is the proportion of the differences
that we observe between animals that can be
transmitted to their progeny.
• Heritability (h2) is usually expressed as a %
between 0% and 100%.
• In general, fertility traits have low heritability,
growth traits are medium to high and carcass traits
are highly heritable.
Selection Differential
• Selection differential (SD) is the difference
between the selected animals and the
average of the population from which they
were selected.
• If a selected bull had a weaning weight of
500 lb. and his herd average was 450 lb.,
the selection differential would be 50 lb.
(500 – 450 = 50)
Improvement per Generation
• This can be calculated by multiplying h2 x
SD.
• If the heritability were 30% and the
selection differential were 50 lb. then the
improvement per generation would be (.3 x
50 lb. = 15 lb.)
Generation Interval
• Generation Interval (GI) is the average age
of the parents when their progeny are born.
• In beef cattle herds the average generation
interval is 5 to 6 years.
• Theoretically, it could be as short as 2 years,
but one would have to mate yearling bulls
to yearling heifers and replace the parents
each year, which is not practical.
Genetic Correlation
• This refers to the situation where different
traits are influenced by the same genes.
• For example, weaning weight and yearling
weight are genetically correlated.
• Genetic correlations can be either positive
or negative.
• If traits are correlated, selection in one trait
will result in change in the other trait.
Multiple Trait Selection
• Selection for more than one trait, reduces
the amount of progress one achieves in one
trait alone.
• Change occurs at a rate of 1/square root of
n, where n = number of traits being
selected.
Types of Selection
• Tandem is selection for one trait at a time.
It is usually not very effective.
• Independent culling level establishes
minimum or maximum values that are
acceptable for each trait.
• Selection index provides a means of
including several traits and different
economic weighting factors for each trait.
Pedigree
• A pedigree is a written record of ancestry
that is usually maintained by the respective
breed association.
• Performance pedigrees contain a large
amount of information including name,
birth date, identification, owner, parentage,
performance, EPDs, etc.
Expected Progeny Differences
• Abbreviated as EPD
• Can be calculated for any trait that can be
measured quantitatively.
• Expressed in same units as measured (lb.,
cm., sqin., etc.)
• Are computed by the respective breed
associations.
EPDs continued
• Calculations are possible because of A. I. which
permits sires to be used in more than one herd.
• High speed computers can quickly solve the
mathematical equations.
• Information from several sources (pedigree,
individual performance, progeny data, etc.) are
combined to arrive at a single value for each trait.
• EPDs change over time as more data are included
in the analyses.
Accuracy
• Accuracy is a measure of how much an
EPD might change or reliability.
• Accuracies range from “0” to “1.00”, and
they increase as an animal ages.
• Pedigree information results in an accuracy
of .05 to .10. Individual performance will
increase accuracy to about .35 and progeny
data will increase accuracy to .99.
EPDs continued
• EPDs can be used to compare animals across a
breed in the same analysis.
• Breed average for each trait is not “0”.
• Many breeders are using breed percentiles to
determine where an animal ranks in the breed.
• EPDs are an important tool for selection but
should not be the sole criteria for evaluating a
potential breeding animal.
Steps in Using EPDs
1. Obtain a sire summary
2. Establish selection goals based on your
cow herd, breeding program and resources
3. Prioritize traits
4. Find the breed’s average EPD values and
percentiles
5. Determine an acceptable range of EPDs
for your herd
Steps in Using EPDs
6. Establish some trade-offs that you are
willing to make
7. Keep records of how bull’s perform in
your herd and how their EPDs correlate
with actual progeny data
8. Reevaluate your goals and decide which
sires to use for the next mating season
SUMMARY
1. Decide which traits are economically
important.
2. Focus on traits with moderate to high
heritability.
3. Be aware of genetic antagonisms.
4. Be objective in your evaluation.
5. Set realistic and obtainable goals.
SUMMARY continued
6. Consider a selection index for multiple trait
selection.
7. Use proven sires with high accuracy.
8. Use all emerging technologies.
9. Breed cattle that are functionally sound for
your environment.
SUMMARY continued
10. Develop a long range plan and follow it.
11. Listen to your customers.
12. Continually reevaluate your plan and
make adjustments as needed.