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Repro & Genetics 1-6
Repro & Genetics
| Question | Answer |
|---|---|
| genetic variation | differences arise from DNA sequence variation eg coat colour, disease resistance |
| gene | unit of inheritance |
| allele | alternate form of a gene |
| locus | physical location of a gene on a chromosome |
| DNA | genetic material |
| phenotype | observable characteristic |
| genotype | genetic makeup |
| trait | measurable or observable characteristic |
| Modes of inheritance | dominant vs recessive alleles |
| Simply inherited traits | characteristics controlled by a single gene little environmental influence eg, coat colour red or black |
| Polygenic inheritance | a single trait is controlled by two or more genes results in a range of phenotypes |
| Major genes | have a large effect on quantitative traits eg, MSTN in cattle affects muscle growth and "double muscling" |
| Germplasm | genetic resources eg, semen, embryo, animals |
| Seedstock | animals bred for breeding |
| population | group of inter-mating animals |
| line | related group within a breed |
| breeding value | genetic merit |
| heritability | relationship between genotype and phenotype |
| aim of selection | increase frequency of desirable genes |
| natural selection | survival of the fittest organisms with traits better suited to their environment are more likely to survive and reproduce - passing on their genes |
| artificial selection | replacement selection/culling |
| selection | does not create genes, only change the frequency of expression |
| selection for multiple traits | most systems require multi-trait selection selecting for more traits reduces selection pressure per trait |
| Three main methods of selection for multiple traits | Tandem selection Independent culling levels Selection Index (most efficient) |
| Selection Index | combines multiple traits into a single value weighting factors based on economic importance allows for maximum genetic progress |
| Gene frequency | proportion of alleles in a population |
| Australian Selection Indexes | ABI-Angus breeding index Domestic index Heavy Grain index Heavy Grass index |
| Mating systems | methods of assigning males to females after selection change genotypic frequencies, mean and variance |
| Gene frequencies only change via: | mutation migration genetic drift selection |
| Random mating | mates are chosen at random |
| assortative mating | includes negative and positive assortative mating based on phenotype |
| Positive assortative mating | best to best, worst to worst increases variation |
| Negative assortative mating | best to worst used for corrective mating reduces variation |
| Inbreeding | includes mild inbreeding (cousins) and intense inbreeding (sire-daughter, sibs, etc) |
| Genetic effects of inbreeding | increases homozygosity decreases heterozygosity doesn't change gene frequency can create sub-populations (lines) |
| Phenotypic effects of inbreeding | exposes recessive defects (does not create new defects) |
| Inbreeding depression | lower fertility reduced reproductive performance reduced rates of survival |
| Linebreeding | mild inbreeding to maintain relationship to a superior ancestor often used around an elite sire can be used to detect/eliminate defects |
| inbreeding coefficient | measures degree of inbreeding range: 0-1 |
| Outbreeding | mating individuals less closely related than average population |
| Types of outbreeding | mating between unrelated animals of the same breed species crossing crossing between breeds |
| effects of outbreeding | increased heterozygosity increases population mean reduces expression of genetic defects |
| Outcrossing (within breed) | mating unrelated animals within a breed common in seedstock to maintain performance and minimise inbreeding avoids most recessive defects occurring |
| crossbreeding | mating different breeds |
| advantages of crossbreeding | breed complementarity heterosis/hybrid vigour |
| breed complementarity | combining strengths of two breeds eg. merinos x white Suffolk (finer wool quality and better meat quality) |
| Heterosis | crossbreds exceed parent breed average especially strong in reproductive and survival traits includes maternal heterosis and individual heterosis |
| Types of crossbreeding | single cross two breed rotational cross three breed rotational cross terminal sire cross |
| Single cross | simple two breed cross replacement animals purchased externally |
| Single cross: advantages and disadvantages | Pro: high heterosis Cons: no maternal heterosis, disease risk from externally purchased stock |
| Two breed rotational cross | uses two breeds in alternating generations |
| Two breed rotational cross: pros and cons | Pros: maternal and individual heterosis, produce own replacements, lower disease risk |
| Three breed rotational cross | uses three breeds heifers always bred to sires least related to them |
| Three breed rotational cross: pros and cons | Pros: greater maternal heterosis, produce own replacements, suitable for larger operations Cons: requires 3 pastures, more management |
| Terminal sire cross | combination of rotation and terminal mating young females enter rotation; older females go to terminal pasture all terminal offspring marked |
| Terminal sire cross: pros and cons | Pros: highest hybrid vigour, produces own replacements , efficient use of crossbreds Cons: high management requirements, requires three sire breeds |
| Aim of selection | increase the population mean for traits in the breeding objective |
| Steps of a breeding program | define breeding goal select criteria design breeding program implement and monitor |
| Selection differential | measures superiority of selected parents |
| What does heritability do? | partitions phenotypic superiority into genetic vs environmental |
| male selection differential | typically much higher than females (smaller proportion selected) |
| Section differential and intensity | SD increases when a smaller proportion of animals are selected Selection intensity is used when phenotypic variation is known higher selection intensity = greater genetic gain |
| Generation Internal (GI) | average age of parents when offspring are born |
| GI species differences | shortest GI in chickens and pigs Longest in horses bulls typically have shorter GI than cows because they are replaced sooner |
| Genetic Variation | σG determines how much gain is possible Greater genetic variation = greater response to selection hard to change genetically but can be maintained through diversity |
| Reproductive technologies | used to speed genetic gain by increasing selection intensity and reducing GI |
| Reproductive technologies examples | AI MOET JIVET sexed semen cloning transgenic techniques |
| Accuracy of selection (rGP) | correlation between true and estimated breeding value |
| Accuracy of selection is increased by: | environmental control adjustment factors reducing σE in heritability formula Using information from relatives Measuring genetically correlated traits Progeny testing (most accurate) |
| Genetic Correlations | selection for one trait can change another is traits are genetically correlated used to improve hard to measure traits such as feed efficiency via correlated traits like ADG |
| Aids to selection | individual selection repeated records pedigree selection progeny testing |
| Individual selection | based on own performance very effective for highly heritable traits limited for sex-limited, carcass or late-expressing traits |
| Repeated records | improve accuracy for traits with temporary environmental effects |
| Pedigree selection | uses ancestors performance useful for late-expressing or sex-limited traits |
| Progeny testing | most accurate method of estimating breeding value high accuracy even when heritability is low costly, slow, increases generational interval |
Created by:
sakelleher29
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