KSW – BIOLOGY_ MODULE 2
WHAT IS SELECTIVE BREEDING?
 Selection of animals and plants for particular characteristics
over many generations.
 e.g. cattle for beef or milk, chickens for egg production,
maize for crop production, apple trees for fruit taste or
production, disease resistant crops
 Selective breeding breeds out unwanted alleles from the
population, deliberately restricting the variation
 Over generations of breeding out variation it leads to a
population where all animals have the originally desired
qualities but massively reduced genetic diversity.
Disadvantages of Selective Breeding
Inbreeding depression
• Breeders of animals and plants in today's world are looking to produce organisms that
will possess desirable characteristics, such as high crop yields, resistance to disease,
high growth rate. These observable characteristics will benefit the organism and
species in the long term.
HOWEVER
•
Selective breeding of particular genes runs the risk of losing some of the other genes
from the gene pool altogether, which is IRREVERSIBLE.
•
This is IN-BREEDING DEPRESSION, where the exclusivity of the advantageous genes
mean that some other less desirable genes are phased out.
•
In the long term, it actually provides an advantage if organisms have dissimilar pairs of
genes for any hereditary characteristic.
Genetic diversity in the long term is reduced, because many organisms end up with
similar genetic material due breeding with each other constantly.
•
KSW – BIOLOGY_ MODULE 2
EXAMPLE:
• With Crufts behind them for another year, breeders and judges are now preparing for
smaller dog shows all over the UK. Many of these rely on the strict selective breeding of
pedigree dogs.
• This can lead to chronic genetic problems and diseases, the animal could get sick easier.
• Genetic engineering borderlines on many moral issues, particularly involving religion,
which questions whether man has the right to manipulate the laws and course of
nature.
• For show animals such as some dogs, selective breeding is used quite strictly by
breeders aiming at a certain look which is set down as the breed - so there are serious
moral questions to it too.
• Too much close interbreeding to obtain certain cosmetic traits can also result in the
amplifying of inherent illnesses
• The more that inbreeding is used to get rid of undesirable traits or to fix a desirable
trait, the more likely it is that individuals will also inherit the same set of genes for the
immune system from both parents, and be born with less vigorous immune systems.
• The immune system problem is compounded over successive generations as the
animals become genetically more uniform (like the cheetah). According to one theory,
immunodeficiency may be caused by a simple lack of different pairs of genes that
control the immune system.
• This is why random-bred cats are generally so robust.
More examples:
1. Bloodhounds develop loose skin around their eyes, increasing the potential unwanted
small objects getting stuck in their eyes
2. Centuries of selective breeding has led to the creation of many different breeds of
animal all inheriting very similar characteristics to the previous generation, which may
include genetic diseases. (Many King Charles Cavaliers develop heart murmurs and
other heart problems, cruelly cutting their lives short)
3. Bulldogs develop oversized heads which result in many having to be born through
caesarean section
4. Pekingese dogs have very short faces, making it hard for them to breath or smell
5.