1. Choosing a breeding animal
    selected by determining which genes will be passed on to the next generation
  2. 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
  3. 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
  4. Methods of selection
    • progeny test
    • performance test
    • molecular genetic examination
  5. 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
  6. 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
  7. 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
  8. 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
  9. 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
  10. 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
  11. Polygeneic characteristics
    polygenic traits are caused by a number of genes
  12. Multifactorial characteristics
    multifactorial characteristics are caused by an interaction of genes and the environment
  13. 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
  14. 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
  15. Performance tests
    • physical exam or any non-genetic testing
    • hip dysplasia
    • elbow dysplasia
    • Sheep Dog Society eye scheme
    • polycystic kidney disease (PKD)
  16. 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
Card Set
vet tech theriogenology chapter 8 selection and eradication schemes set