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Choosing a breeding animal
selected by determining which genes will be passed on to the next generation
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Selective breeding
- different breeds of domestic animals have been developed this way
- inbreeding to fix desirable characteristics
- - creates homozygosity for good or bad
- - also produces undesirable characteristis
- -- eg unwanted coat color or something dangerous to health and welfare of animal
- -- ethical considerations?
- - new mutations - eg loss of top coat in Rex cats (considered desirable)
- - some compromise the health of the animal
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Removing genes that cause abnormalities from breeding lines
- difficult to do
- small number of animals in breed, gene present in large proportion of this breed
- - culling all carriers wil deplete breed numbers
- - reduces gene pool and genetic variation
- selecting for some caracteristics and against others
- - difficult to do if genes are linked
- - don't always know which are linked
- using a few animals to produce the next generation will increase inbreeding reulting in:
- - increased homozygosity
- - inbreeding depression
- - reduced fertility
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Methods of selection
- progeny test
- performance test
- molecular genetic examination
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Progeny test
- determines what genes an animal is carrying by test mating and evaluating offspring
- gives you lots of extra puppies
- backcross to recessive gene
- - mating is carried out and the offspring (progeny) are examined (tested)
- - useful for revealing carriers of recessive gene
- - useful for identifying genes responsible for sex-limited characteristics
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Limitations of progeny testing
- breeding from animals with anomalies (ethical issues?)
- lethal genes result in the death of the embryo in utero - eg MM dominant manx
- impossible to look for carriers of such genes by looking for affected progeny as they are never born:
- - by laproscopy
- - kill the mother
- abnormally small litter size may be due to embryonic death as a result of homozygosity for a lethal gene
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Performance test
- detemines which characteristics are governed by dominant or co-dominant genes
- - because phenotype reflects genotype
- based upon phenotype or performance of animal
- - what does the animal look like?
- best for characteristics determined by single genes and dominant genes
- - easier to detect dominant than recessive genes
- remove unwanted dominant genes by simply not breeding that animal
- also provides extra puppies
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Limitations of performance testing
- recessive genes
- - does not work well for anomalies caused by recessive genes
- - phenotype only reveals presence of recessive genes in homozygous state
- - cannot completely eliminate recessive genes unless...
- -- ... heterozygous carrier animals can be detected and eliminated
- polygeneic and multifactorial characteristics
- - difficult to eliminate condition by looking at phenotype
- age or onset of gene expression
- - gene is only expressed after animal is old enough to breed
- - animal can be examined when young and appear free from anomaly, yet develop it later in life
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Performance testing of siblings
- best to use brothers and sisters - full siblings - have 50% of their genes the same
- refrain from breeding littermates
- not all unaffected siblings will be carriers of the recessive allele
- can also use half siblings
- - one male mated to a number of different females - all progeny are half siblings
- - do not have as many genes in common as full siblings, so reduces accuracy of test
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Pedigree analysis and ancestor evaluation
- inherited from ancestor with anomaly
- not always an accurate method:
- - not all animals whose ancestors are affected will inherit the deleterious gene
- may miss breeding of good genes
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Polygeneic characteristics
polygenic traits are caused by a number of genes
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Multifactorial characteristics
multifactorial characteristics are caused by an interaction of genes and the environment
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Molecular genetic examination
- eliminates test matings
- identifies certain individual genes in an animal
- DNA testing - more sensitive, much quicker
- gene probe
- - used when the gene that causes anomaly is identified and sequenced
- - test is for the gene itself
- marker DNA
- - exact gene is not known, but it is known that gene is linked to another gene or a certain DNA sequence
- - either the linked gene or linked DNA sequence can be identified
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Polymerase Chain Reaction (PCR)
- creates millions of copies of the sample of DNA
- - can use small samples of DNA for analysis
- using molecular genetic techniques, animal is classified as:
- - clear
- - heterozygous
- - homozygous for the desired gene
- unwanted gene is removed from the population without culling too many animals
- lessens the risk of reducing the gene pool to where the more serious recessive genes are revealed
- limited in that it can only test for conditions caused by single genes
- - not for polygenic conditions such as hip dysplasia
- molecular testing in dogs more advanced than in cats
- - new gene testing for polycystic kidney disease (PKD) in Persian cats
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Performance tests
- physical exam or any non-genetic testing
- hip dysplasia
- elbow dysplasia
- Sheep Dog Society eye scheme
- polycystic kidney disease (PKD)
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Molecular genetic screening (DNA testing)
- canine leukocyte adhesion deficiency
- congenital stationary night blindness in Briards
- copper toxicosis in Bedlington Terriers
- fucosidosis in English Springer Spaniels - nervous disorder
- phosphofructokinase deficiency in English Springer Spaniel - also American Cocker Spaniel
- progressive retinal atrophy (PRA) in Miniature Long-haired Dachshunds
- progressive retinal atrophy (PRA) in Irish Setters
- pyruvate kinase deficiency in West Highland White Terriers
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