Transcript Agricultural Genetics

AGRICULTURAL
GENETICS
By C Kohn
Waterford Dept of Agricultural Sciences
Locker and Heritability
• Locker the Steer was a good example of heritability.
• Locker was very similar to his mother, grandmother, and
great-grandmother and resembled them ways that
included…
• Appearance – he looked like they did (color, face, etc.)
• Structure – his skeletal frame was very similar
• Attitude – he behaved in similar ways and had
the same kinds of “quirks”, like opening gates
and loving chin rubs
• Did he do this because of genetics, or
because of Mr. Kohn?
• Which things are genetic? Were they
completely genetic, or a mix of how he
was raised and the genes he had? TPS
Genetic Inheritance Overview
• So how do traits get passed from parents to offspring?
• Each parent provides one copy from each pair of
chromosomes (humans have 23 pairs of chromosomes).
• 23 chromosomes come from your mother and 23 from your father).
• Each chromosome has a portion of all of the genes in the
body
• All chromosomes together carry all the genes of the body.
• Each cell’s nucleus contains the same chromosomes that
every other cell in the body has
• This is due to the fact that every cell came from the original zygote
– first cell.
Inheritance Review
• Because each offspring has two copies of each
chromosome (maternal and paternal), they will have at
least two copies of every gene.
• The gene, or stretch of DNA, will code for the proteins responsible
for every trait in the body.
• As we’ve previously discussed, one copy of a gene affects
another copy of the same gene
• For example, Mr. Kohn has a gene for blue eyes and a gene for
brown eyes; however, only one of the genes is visible as the
phenotype.
• In cattle, ‘no horns’ is dominant to ‘horns’. If a cow is heterozygous
for horns, they won’t have any (even though they have a gene for
horns) – the horn gene is recessive to the polled (no horns) gene.
Darwin and Mendel
• The 19th century work of Charles Darwin and Gregor Mendel
paved the way for a genetic revolution in agriculture.
• Darwin demonstrated how a species can change as a result of either
natural or artificial selection.
• Mendel demonstrated inheritance patterns shown by Punnett squares.
• Watson and Crick’s discovery of DNA further paved the way for
dramatic increases in ag productivity through biotechnology.
• Knowing that species can change, and knowing how they
change generation to generation, was pivotal in revolutionizing
the agriculture industry.
Early Ag Genetics
• Prior to 1900, most agricultural breeding was the result of
trial and error, with little understanding of how or why it
worked.
• Cattle are a classic example; all major American breeds of cattle are
originally from Europe.
• A breed is a specific variety of a species of domesticated
animal with similar traits and qualities.
• All breeds of cattle came from the ancient Taurine species of
cattle. Thousands of years ago,
early civilizations began to trap
and tame these animals.
• Having a source of meat at all times
provided more stability to early
civilizations.
Origins of Cattle Breeds
• After they were domesticated, early cattle breeding occurred in
isolated places.
• Because early peoples had little communication with each other and
rarely strayed from their groups
• Over time, widely different kinds of cattle in an area would
become less diverse as some traits were selected more than
others.
• With little outside interference,
the cattle in one village would
look more like each other.
• They would also look less like
cattle from other communities.
• Because of this breeds with
specific characteristics
developed.
Why breeds of cattle are different.
• Different climates and environments selected for different
traits.
• Cold harsh weather necessitated selection for hardiness and
survivability.
• Lush pastures provided opportunity to select for high production
levels.
• Scarcity of feed on the other had forced locals to select for efficient
grazing.
• The environment of each portion
of Europe resulted in different
kinds of cattle.
• Dairy cattle show some of
the most distinct differences.
Breeds of Dairy Cattle
• With its lush pastures and fertile cropland, Holland
created the largest and most productive breed of dairy
cattle – the Holstein.
Breeds of Dairy Cattle
• The inhabitants of the islands in the English Channel
selected for cattle that were small, efficient, and produced
rich milk that was high in butterfat for their cheese and
butter production. This led to the rise of the Jersey breed,
among others.
Breeds of Dairy Cattle
• The mountains of Switzerland created the need for strong,
hardy cattle that could withstand the cold winters and
rocky terrain. These needs led Swiss farmers to select for
the traits common to the Brown Swiss breed.
Breeds of Dairy Cattle
• The desire for a multi-purpose animal in England led to
the rise of the Milking Shorthorn breed – an early breed
that could be used as both a milking animal as well as a
meat animal.
Other Dairy Breeds
• Other dairy breeds include –
• Guernsey – high quality grazing animals
• Ayrshire – deep red cattle
• Red and White Holsteins – identical to regular Holstein cattle
except that they have a recessive red color.
• Due to their initial isolation, different regions of Europe
produced their own “brands” of cattle unique to the needs
of the region in which they were developed.
• These breeds of cattle were created not by survival of the
fittest individuals for their environment (as in natural
selection) but by artificial selection –
• Artificial Selection: the selective breeding of plants and animals to
enhance specific qualities or traits (such as milk production).
Selection for Traits
• How does someone “select” for a trait?
• Simple: imagine you have two cows, Anna and Bessy.
• If Anna has better production than Bessy, you are probably going to
keep Anna for breeding and sell Bessy for meat.
• The same would be true for bulls: if Charles the Bull has
calves with really good production and Darryl does not,
Charles will be kept to breed Anna and Darryl will be sold
for meat.
• By selecting both Anna the Cow and Charles the Bull, you
would hopefully produce a second generation of cattle
with a greater milk production.
• Over years and years, you will slowly increase the
average milk production of every cow by selecting
the best animals to breed.
Breed Associations
• As the benefits of selective breeding became better
understood, breed associations began to form
• Their mission was to support farmers as they developed better,
more productive breeds.
• They kept track of the performance of different bulls by ‘scoring’
their offspring for different traits.
• Organizations such as the National Holstein Association
and the American Jersey Cattle Association provided
farmers with data on different bulls.
• This data allowed individual farmers to select bulls with traits they
needed for their herd.
• For example, if you cows generally milk very
well but have very poor feet and legs, you
would pay special attention to bulls who have
offspring with very good feet and legs.
Dr. Stephen Babcock
• A major Wisconsin contribution to this process was
made by Dr. Stephen Babcock of the University of
Wisconsin.
• Prior to Babcock’s work, there was no reliable way to
test the quality of milk from each cow and from each
farm.
• This meant that dishonest grocers, dairies, or farmers could
dilute their milk with water in order to increase the weight.
• It also meant that it was hard to tell if a cow was producing
high or low quality milk (quality of milk is mostly determined
by the amount of butterfat in the milk).
• Babcock’s test not only helped prevent adulteration of
milk but provided a basis by which to improve dairy
genetics.
• It was also the first world-renown scientific achievement at
UW-Madison, putting Wisconsin on the map of the scientific
world and spawning a rich tradition of excellence in
scientific research.
Artificial Insemination (AI)
• The development of artificial insemination (AI) was also critical,
particularly in the dairy industry.
• AI is a process in which semen is collected from a bull, packaged into straws,
and frozen and shipped to individual farms so that it can be artificially inserted
into a cow’s reproductive tract.
• This is different from natural insemination (when a bull directly breeds a cow.)
• AI enables one bull to inseminate thousands of cows, reducing the
need for bulls on every farm.
• Instead of only having access to bulls in the area, you could improve your
herd’s genetics by introducing high quality genetics from across the world.
• The use of AI required farmers
to understand the principles of
inheritance.
• Inheritance is the
measure of how efficiently a
trait can be passed from
generation to generation.
Heritability
• Heritability is a measure of how likely offspring are to
inherit the phenotypes of their parents
• In other words, it is a way to determine the impact of both the
genotype of the parents as well as the impact of the environment
and care on the phenotype of the offspring
• For example: if we have a cow that produces a lot of high-
quality milk, what is the likelihood that she will have
daughters that also produce large amounts of high-quality
milk?
• This question is incredibly important
because it determines the amount of
money a farmer will make more so
than anything else.
• TPS
Heritability Measurements
• The heritability value (or h2 value) is a measure of how
likely a phenotypic trait is caused by the genes of the
parents vs. the environment in which the animal lives.
• For example, you can see the value for milk yield for
Holsteins is 0.30.
• This means that 30% of milk production is because of genetics, and
70% is because of their environment and care (diet, nutrition,
health, weather, etc.).
• Productive life is only 13% affected by genetics (and 87% by how
well you care for your animals).
How Heritability is Used
• Heritability tells a producer how much they can improve
aspects of their herd through genetics and bull selection.
• A trait with heritability over 40% (or 0.40) would be a trait that
could more easily be changed through genetic selection.
• For example, you can easily improve the quality of the milk you
produce through a selection of bulls that typically have daughters with
high butterfat content).
• On the other hand, some traits are affected little, if at all, by
genetics.
• E.g. the length of the life of a cow has a very low h2 value, meaning
that how well you care for the animal means far more than the
genotype of that animal’s parents.
• A h2 value under 15%, or 0.15, means that genetics really do not play
a major role in that particular trait.
How Herd Genetics Change
• Improving the overall genetics of a farmer’s herd involves 4 key
factors:
• Accuracy of Selection: rate of choosing only highest genetic-quality
animals
• Selection Intensity: Proportion of herd animals kept for breeding each
generation
• Genetic Variation – how varied your herd animals are for each trait
• Generation Interval – the herd’s rate of reproduction
• To obtain faster genetic change, you need excellent animal
genetics, highly inheritable traits, and rapid maturation and
reproduction.
Galton’s Law
• What can make genetic improvement difficult is a
phenomenon known as Galton’s Law.
• Galton’s Law states that the more extreme a trait, the less
likely the offspring of the parent’s with that trait are to
acquire it at the same intensity.
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5.5
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4.5
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3.5
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Offspring
Parent
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Galton’s Law
• For example, in 2010 a cow in Sheboygan County set a
new world record for most amount of milk produced in a
single year at 72,170 lbs.
• Even though this animal must have exceptionally superior
genetics, it is exceedingly unlikely that her daughters will
produce as much milk as her.
• In all likelihood, her daughters will produce more milk than average,
but not as much as their mother.
• As a trait becomes exceptionally high or low, it’s likelihood
of being expressed in the same way by the offspring is
less and less.
• Phenotypic traits tend to stay near the average for a population.