Year 10 Science
Selective Breeding Information Sheet
Selective breeding is the process of breeding plants and animals for particular
genetic traits (features, characteristics or phenotypes). Selection is done by
choosing parent organisms with desirable phenotypic features or
characteristics and mating them together. Typically, organisms that are
selectively bred are domesticated, and the breeding is done by professional
breeders or farmers. Bred animals are known as breeds, while bred plants are
known as varieties, cultigens, or cultivars. The term selective breeding is also
called artificial selection.
Selective breeding is considered a natural process as there is still the chance
that traits or features controlled by recessive genes rather than by dominant
genes will appear in the offspring and therefore not all of the offspring will
necessarily be better, bigger or stronger than the parent breeding stock.
Techniques such as genetic engineering, cloning and genetic modification
take this randomness of offspring away so are not considered as natural or
safe.
Charles Darwin discussed how selective breeding had been successful in
producing change over time in his book, Origin of Species. The first chapter of
the book discusses selective breeding and domestication of such animals as
pigeons, dogs and cattle. Selective breeding was used by Darwin as a
springboard to introduce the theory of natural selection, and to support the
idea of evolution.
Animal breeding
Animal breeding begins with breeding or stud stock (a group of animals used
for the purpose of planned
breeding). When breeders are
looking to breed animals, they
look for certain valuable traits in
the breeding stock for a certain
purpose such as with chickens, a
breeder may look for the traits of
good egg production and high
meat content or for horses that
can run fast and are disease free.
The breeder has to study different
breeds and types of chickens and
analyze what can be expected
from a certain set of
characteristics before he or she
starts breeding them. Therefore,
when purchasing initial breeding stock, the breeder seeks a group of birds
that will most closely fit the purpose intended.
Purebred breeding aims to establish and maintain stable traits that animals
will pass to the next generation. By "breeding the best to the best," employing
a certain degree of inbreeding, considerable culling, and selection for
"superior" qualities, one could develop a bloodline superior in certain respects
to the original base stock. Such animals can be recorded with a breed
registry, the organization that maintains pedigrees and/or stud books. Singletrait breeding (breeding for only one trait over all others), can be problematic.
In one case roosters bred for fast growth or heavy muscles did not know how
to perform typical rooster courtship dances, which alienated the roosters from
hens and led the roosters to kill the hens after reproducing with them.
Techniques used in animal breeding to ensure the right male fertilizes the
right female can be as simple as locking the two dogs together in a cage,
putting the bull and cows in the same paddock or using artificial insemination,
where semen is collected from a bull by a vet at one farm, taken to another
farm in frozen straws stored in liquid nitrogen and introduced into the cows
uterus at another farm using a large hypodermic needle.
Plant breeding
Plant breeding has been used for
thousands of years, and began with the
domestication of wild plants into uniform
and predictable agricultural varieties..
Gregor Mendel’s experiments with red
and white sweet peas are an example
of selective breeding of plants.
High-yielding varieties have been
particularly important in agriculture,
such as; rice, wheat and corn. Selective
breeding is generally easier in plants
than animals for a number of reasons,
including; that plants don’t move, it is
easier to recognise when plants are
receptive to pollen, it is easier to isolate
individual plants or parts of plants and
controlled pollination only requires a
paint brush and a few plastic bags.
Fish and shellfish (aquaculture) breeding
Selective breeding in aquaculture holds high potential for the genetic
improvement of fish and shellfish. Aquaculture species are reared for
particular traits such as growth rate, survival rate, meat quality, resistance to
diseases, age at sexual maturation, shell traits like shell size, shell colour, etc.
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Growth rate – growth rate is normally measured as either body weight
or body length. This trait is of great economic importance for all
aquaculture species as faster growth rate speeds up the turnover of
production.
Survival rate – survival rate may take into account the degrees of
resistance to diseases. This may also see the stress response as fish
under stress are highly vulnerable to diseases.
Meat quality – the quality of fish is of great economic importance in the
market. Fish quality usually takes into account size, meatiness, and
percentage of fat, colour of flesh, taste, shape of the body, ideal oil and
omega-3 content .
Age at sexual maturation- The age of maturity in aquaculture species is
another very important for farmers as during early maturation the
species divert all their energy to gonad production affecting growth and
meat production and are more susceptible to health problems.
Rainbow trout (S. gairdneri) was
reported to show large improvements
in growth rate after 7-10 generations
of selection. Growth gains by 30%
could be achieved by selectively
breeding rainbow trout for three
generations. In Japan, high
resistance to disease in rainbow trout
has been achieved by selectively
breeding the stock. Resistant strains
were found to have an average
mortality of 4.3% whereas 96.1%
mortality was observed in a highly
sensitive strain.
Selection for live weight of Pacific oysters showed improvements ranging from
0.4% to 25.6% compared to the wild stock. Sydney-rock oysters (Saccostrea
commercialis) showed a 4% increase after one generation and a 15%
increase after two generations. Chilean oysters (Ostrea chilensis), selected
for improvement in live weight and shell length showed a 10-13% gain in one
generation.