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2011-10-26 22:38:33
vet tech theriogenology chapter inbreeding crossbreeding heterosis set

vet tech theriogenology chapter 7 inbreeding, crossbreeding, heterosis set
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  1. Inbreeding
    • breeding between two related individuals
    • reduces gene pool
    • reduces heterozygosity, increases homozygosity because...
    • ... an animal is more likely to receive the same allele from both mom and dad, so 2 of same gene
    • increase in homozygosity reveals recessive genes because...
    • ... recessive alleles in the genome are only epressed when they occur in the homozygous state
    • inbreeding reveals the presence of recessive genes because they are expressed in the phenotype
    • related animals more likely to have some alleles that are the same than non-related
    • done when breeders are selecting for a particular characteristic and want to fix that characteristic in future populations
    • the less the relationship, the less the chance of homozygosity at any locus
  2. Gene fixing / Breeding true
    • increase in homozygosity fixes genes, animal is more likely to breed true because...
    • ... when animal is homozygous it has 2 copies of the same allele to pass on at a particular locus
    • a lot of the time this is bad
    • as likely to fix "bad" recessive genes as good ones
    • many recessive genes are "bad"
    • "bad" carrier animal is less able to live or breed (ie less fit)
    • unrelated individuals generally have different "bad" alleles so offspring are not homozygous
  3. What affects the outcome of inbreeding?
    • the degree of relationship of the animals
    • how many "bad" alleles the related animals have in their genome - don't know until breed
    • whether the related animals have the same "bad" alleles
  4. Reasons for inbreeding
    • to fix a desirable characteristic
    • - 50/50 chance of getting a "bad" characteristic
    • to detect whether one of the parents is the carrier of a recessive gene (eg deformity, illness)
    • laboratory animals are deliberately inbred so that lines of animals are genetically the same
    • - eg nude mice, immunocompromised mice
  5. Inbreeding depression
    • describes the general reduction in vitality and viability of many inbred animals
    • occurs because of increase in homozygosiy and expression of recessive genes that are deleterious
    • most marked in fertility and progeny survival - inbred animals are often sterile
    • main reason why inbreeding is considered a bad thing
  6. Inbreeding coefficient of an individual
    the probability that the two genes present at a locus in that individual are identical by descent
  7. Mating of relatives (inbreeding) results in...
    • decrease in performance (inbreeding depression)
    • - especially traits associated with viability and/or reproductive ability
    • as level of inbreeding increases, so does incidence of:
    • - embryonic mortality - spontaneous abortion
    • - stillbirths
    • - single-gene recessive disorders
    • decrease in genetic variance
  8. Line breeding
    • form of inbreeding
    • use of an individual more than once in a pedigree - eg show champion
    • breeders attempting to make their animals as genetically similar to the champion ancestor as possible
    • will also created inbreeding depression wth a reduction in fertility and viability
  9. Cross breeding
    breeding of unrelated individuals
  10. Effects of cross breeding
    • opposite of inbreeding
    • increases heterozygosity
    • - unrelated individuals not likely to have same alleles, offspring will be heteroygous at many loci
    • masking of recessive genes
    • - when there is heterozygosity at a gene locus, no recessive genes are expressed
    • reduces the likelihood of breeding true - due to heterozygosity at many loci
    • increases fertility and viability of offspring
    • - gives hybrid vigor or heterosis
  11. Hybrid vigor
    • arises because of masking of recessive genes which depress viability in the homozygous state
    • animals with hybrid vigor have a superior performance compared to their parents
    • most obvious when two inbred (but different) lines of animals are mated
    • each inbred line will show inbreeding depression because of homozygosity for different genes
    • offspring will be heterozygous for all those genes
    • - deleterious genes will be masked, offspring will show increased hybrid vigor