Selective Breeding
• Definition
• Selective breeding is also known as artificial selection. It is
humans selecting desirable traits in a species and choosing
which individuals breed in order to increase these traits in
the species.
• All crops and domesticated animals today are a product of
selective breeding.
Selective breeding v’s natural selection
• Natural selection is certain individuals of a species being
“fitter”, possessing alleles which make them more successful
in their environment. These individuals survive longer and
produce more offspring. Therefore more of their alleles are
passed on to the next generation and over time these alleles
become more common in the gene pool
• In selective breeding humans determine which phenotypes,
and therefore genotypes are desirable in the species. They
then increase the alleles for these traits through breeding
programmes. Often the species produced would be unable to
survive in the wild
Selective breeding – nothing new
• Agriculture began 10,000yrs ago. Humans selected wild
varieties of plants and animals with the traits they desired
and began selective breeding to increase these desired traits;
e.g. to produce bigger, easier to harvest grains with a greater
yield, to breed animals that were the most docile and easy to
handle.
From Biozone powerpoint ‘cultural evolution’
From Biozone powerpoint ‘cultural evolution’
Selective breeding animals
• Belgium Blue cattle – cattle bred to produce a huge amount
of muscle (meat). Produced by selecting individuals with a
mutation in myostatin gene which results in the production of
an increased number of muscle fibres (hyperplasia)
• Excess muscle growth begins in utero so calves have to be
born by caesarean section.
• Fertilisation is almost always by artificial insemination,
meaning that sperm can be shipped across continents and
only the ‘best’ bulls are selected to breed.
Selective breeding -methods
• Inbreeding is reproduction from the mating of parents who
are closely related genetically.
• Livestock breeders often practice controlled breeding to
eliminate undesirable characteristics within a population,
which is also coupled with culling of what is considered unfit
offspring, especially when trying to establish a new and
desirable trait in the stock.
• Repeated test crosses are often used in order to produce
pure breeding individuals
Modern corn
Ancient corn
from Peru
(~4000 yrs old)
Choosing only the best corn plants for seeds results
in better crops over a long time.
Polyploidy
• Polyploidy is a mutation that occurs during meiosis and
results in multiple sets of chromosomes (3n/4n etc)
• Polyploidy is usually fatal in animal species, but frequently
occurs in plants.
• Polyploid plants have bigger fruits and grains and infertile
polyploid are seedless. These traits are selected for in
selective breeding programmes
• Polyploidy can be induced in plants using a chemical called
colchine. This is used to produce bigger, stronger polyploid
plants and to make fertile polyploid plants.
From Biozone Powerpoint ‘Mutations’
How To Make A Fertile Polyploid Hybrid
To produce a tetraploid plant, the alkaloid colchicine is applied to the terminal bud of
a branch. All the cells in the developing branch will be tetraploid (4n) with four sets of
chromosomes. This includes cells of the stem, leaves, flowers and fruit. Gametes (egg
and sperm) produced by a flower on this tetraploid branch will be diploid (2n) with
two sets of chromosomes. A flower on the normal diploid (2n) branch will produce
haploid (n) gametes containing one set of chromosomes.
http://waynesword.palomar.edu/hybrids1.htm
How Colchicine induces polyploidy
The original mother cell is diploid (2n). During anaphase the chromatids separate and
move to opposite ends of the cell. Colchicine causes the dissolution (depolymerization)
of protein microtubules which make up the mitotic spindle in dividing cells. This leaves
the cell with twice as many single chromosomes (four sets rather than two). When this
cell divides, each of the two daughter cells will have fours sets of chromosomes, a total
of eight chomosomes per cell. [Note: Spindle poisons such as colchicine are used to
prevent tumor cells from dividing in certain chemotherapy treatments.]
http://waynesword.palomar.edu/hybrids1.htm
Genome analysis
• Genome analysis is determining the locus (position
on the chromosome) and base sequence of all an
organisms genes.
• Chromosome mapping determines on which
chromosome and at which locus a gene occurs.
• DNA sequencing determines the exact base
sequence of each gene, it can be used to distinguish
between different alleles.
• Genome analysis is used in selective breeding to
determine if an individual has a specific, desired
allele and to select individuals for breeding
programmes based on their alleles
Genome analysis and selective breeding examples
• Genome analysis of kiwifruit is being used to selectively breed
new, trademarked varieties of fruit with characteristics such
as disease resistance.
Source: http://www.plantandfood.co.nz/page/our-research/breedinggenomics/
• Sheep in NZ are being selectively bred to be immune to facial
eczema, a fungal disease that can destroy whole flocks.
Genome analysis of sheep was carried out and individuals
immune to the disease were selected for a breeding
programme. Source:Ag research NZ
Other Applications of Selective
breeding
• Breeding programmes for endangered species, may involve
genome analysis and selection of the least genetically related
individuals to breed (to maintain genetic diversity in the
species).
• Selective breeding programmes have resulted in higher yields
and better disease resistance in aquaculture species, such as
salmon.
Implications of selective breeding
• We are concerned with the biological implications of
selective breeding that may impact on :
1. Ecosystems
2. Genetic biodiversity
3. Health or survival of individuals
4. Survival of populations
5. Evolution of populations
Brainstorm some possible (general) implications of
selective breeding for each of these.