SELECTIVE BREEDING
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Selective
Breeding and natural selection are similar because both involve selection of which animals or plants survive to breed.
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Selective
breeding and natural selection are different because In selective breeding the farmer / breeder does the selection. In
natural selection it is the survival of the fittest in a habitat that leads to
selection.
Method of Selective Breeding in Plants :
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Hybrid G was produced by selective
breeding. Individual plants from pure lines of A and B were selected (for size
of cobs) and crossed to produce hybrid E.
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Similarly, individual plants from
pure lines of C and D were selected and crossed to produce hybrid F.
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Plants from hybrids E and F were
then selected for their cob size, and crossed to produce hybrid G.
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Crossing would be done by transfer
of pollen from anthers to stigmas of plants.
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Cob G is larger, it has more seeds
and the seeds are more even in size.
Note : To show genetic differences between
hybrid C and hybrid G, the sequence of genes are different.
Aims of selectively
breeding plants :
·
Plants have resistance
to disease, so they are not killed by fungi, bacteria, etc.
·
Plants are better suited
to climate, so can grow well in a particular location.
·
Plants have a better
balance of nutrients (sugar, starch, protein, oil) to produce more nutritious
food, or have a high vitamin content etc.
·
Plants that give high
yield and have crops with bigger grains
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Plants that grow
rapidly giving an early crop.
·
Drought
resistance
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Plants with strong
stems to help the plant survive the wind or heavy rain
Method of Selective Breeding in Animals :
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Choose a cow with the best
characteristics and give hormone / FSH injections to cause multiple ovulations.
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Collect ova and use IVF to fertilise
with sperm collected from a bull with the best characteristics.
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Separate cells of embryos that
develop and produce large numbers of embryos.
·
Screen for sex (males) and implant
into surrogate mother cows.
Note : To choose
cows for selective breeding in a dairy farm, milk yield and milk to feed
conversion rate are important.
MICROPROPAGATION :
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Very small pieces
of the plant called explants are obtained.
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Explants are
trimmed to about 0.5 – 1 mm.
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Explants are
grown in vitro in sterile agar medium, containing nutrients and hormones.
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Auxins cause
growth of callus and roots.
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When the explants
have grown, they are transferred to compost in a greenhouse.
Advantages :
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Plants can be
grown rapidly in large numbers as only a few cells are needed.
·
Plants can be produced at any time
of year since grown inside.
·
Large numbers of plants can be
stored easily until needed.
·
All are
genetically identical and have desired characteristics. For example,
Disadvantages :
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All plants have same genes, so all are susceptible to same diseases /
could all be affected at same time.
Note : All plants produced by micropropagation are
genetically identical as all have same genes since they are produced
by mitosis from cells of the same parent plant
CLONING :
Clone : An organism arising from asexual reproduction from a
single parent cell, with identical genetic makeup (genotype) to parent.
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Diploid nucleus from the udder cell of an adult sheep is
obtained.
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An ovum is
enucleated.
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The diploid
nucleus is inserted into the ovum by a shock / spark of electricity, thus, in
Vitro fertilisation occurs.
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Embryo is
implanted in the uterus of a surrogate mother.
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For example,
Dolly
Disadvantages :
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Cloned animals,
like ‘Dolly’, may have genetic defects
This video, which is the first 15 minutes of an early 1990s BBC documentary, has much information that people studying this topic may find very useful. One of the people who explains some of the major consequences of selective breeding in it is a chicken farmer who supplies chickens to supermarkets;
ReplyDeletehttps://www.youtube.com/watch?v=HoOW1z85Qrs