Outbreeding Flashcards
What is outbreeding?
mating of individuals less closely related genetically than the average of the population
Where is outbreeding extensively used?
meat and animal production
What are the 4 types of outbreeding?
within breed, grading up, crossbreeding, and species crossing
What is Within Breed outbreeding?
crossing inbred lines
What is Within Breed outbreeding often used for?
poultry
How does Within Breed outbreeding occur in poultry?
- there are several strains/lines of one breed
- within each line there is intense selection for economically important traits
- over time they become genetically distinct
What is Grading Up also called?
topcrossing
What is Grading Up outbreeding?
mating purebred males (that are superior) to non-purebred females (grade)
What is Grading Up outbreeding attempting to do?
create a purebred population
What is Crossbreeding outbreeding?
crossing different breeds
What is Species Crossing outbreeding?
crossing closely related species
What are examples of Species Crossing?
Horse x Donkey, Cattle x Bison, Bos Taurus x Bos Indicus, Angus x Brahman
Why are angus and brahman bred together?
combines growth rate, carcass attributes of Angus w/ hardiness and adaptability to hot humid conditions of Brahman
Who first bred angus and brahman?
USDA in coastal Louisiana
What does outbreeding do genetically?
increase heterozygosity
How does outbreeding affect deleterious recessives?
keeps them in heterozygous form, so they are not expressed (masked)
Does outbreeding eliminate deleterious recessive alleles?
no, it perpetuates them by masking their expression
How does outbreeding do phenotypically?
heterosis or hybrid vigor
What is heterosis?
the degree of superiority of the outbred offspring for a particular trait as compared to the average of the parental line or breed
What is observed heterosis the net effect of?
such influences at all loci affecting the trait
What is heterosis the difference between?
the average of all crossbreds, in comparison to the average of all purebreds
How do you calculate heterosis?
mean of F1 offspring - mean of parent breeds
How do you calculate percent heterosis?
(mean of F1 offspring - mean of parent breeds) / mean of parent breeds x100
Why isn’t crossbreeding used for dairy cattle?
No breed, when crossed with Holstein, will produce a F1 generation crossbred cow whose fluid milk yield exceeds that of straightbred Holstein
Does crossbreeding always improve the efficiency of production?
no
How is crossbreeding used for meat production?
crossbreds often don’t exceed the better purebred parent for individual traits such as survival, growth rate feed efficiency, and carcass merit, BUT heterosis for all the traits contributing to overall population efficiency will result in a greater net economic merit
What is breed complementation?
combining the desirable characteristic (traits) of 2 or more breeds (or lines)
What is an example of where breed complementation is used?
commercial beef cattle producers
What are angus characteristics?
moderate mature size, good fertility, milk production higher than Hereford, muscle is more marbled after a standard time on feed
What is necessary for carcass to grade choice under USDA?
marbling/intramuscular fat
What does the degree of heterosis and complementarity depend on?
- genetic relationship of parents
- heritability and ability to measure trait
How closely related should parents be to increase heterosis?
more diverse they are the more chance of heterosis
What kind of relationship do heterosis and heritability have?
inverse
What specific industries use outbreeding?
- commercial cow/calf
- swine
- poultry (meat breeds)
- sheep (meat breeds)
What is the goal of commercial cow/calf?
produce 1 calf a year
Why is outbreeding used in commercial cow/calf?
take advantage of heterosis because fertility is a lowly heritable trait
Why are swine outbred?
maximize reproduction and growth (maternal lines)
How are swine outbred?
extensive crossing of inbred lines (reciprocal recurrent selection?)
What is reciprocal recurrent selection?
system of selection for increasing the combining ability of two (or more) lines (or breeds) that have demonstrated from past crosses that they develop well
What are the two types of reciprocal reccurent selection?
- general combining ability (GCA)
- specific combining ability (SCA)
What is general combining ability?
evaluating how one breed will combine with several other breeds in a series of crossing (cross one as male and then reverse and use other as male and then evaluate that too)
What is specific combining ability?
evaluating how two breeds will combine
How does reciprocal recurrent selection develop superior inbred lines?
crossing more genetically diverse lines of two breeds=superior stock
What is the net result of Reciprocal Recurrent Selection?
- development of genetically diverse inbred lines
- better performing crossed lines would express greater heterosis
- those inbred lines could then be crossed with inbred lines of other breeds to generate more heterosis
Why are meat poultry breeds outbred?
growth advantages
How are dairy cattle bred?
outcrossed within breed but rarely between breeds
Why are dairy cattle rarely bred (not about a specific breed)?
economics-established very specific parameters for value of product
What is days open?
a reproductive trait measured in dairy, meaning days not pregnant
What is desirable days open?
60-90 days, don’t want to exceed 90
Is days open an inheritable trait?
yes
What does hybrid vigor increase?
fertility and health/survivability
What does dairy(?) crossbreeding result in?
hybrid vigor, reduce inbreeding, breed complementation
In what livestock is limited crossbreeding used in?
- dairy cattle
- poultry (laying hens)
- wool sheep
What kind of breeding is used for laying hens?
extensive line crossing
Why are laying hens not crossbred?
crossing a layer and broiler won’t meat either objective, it is not sustainable
Why are wool sheep not crossbred?
little response to heterosis, heritability of most wool traits is moderate to high (when heritability is high, heterosis is low)
When is crossbreeding best in terms of heritability?
when heritability is low
How is heritability represented?
h^2
What things must be considered before you attempt systems of Crossbreeding?
- Define objectives
- Breeds
- Herd size
- Implementation cost versus Expected Return
- Nothing is Free
How are objectives defined?
- traits of interest (level of heterosis)
- complementarity achievable
What must be met in terms of Breeds for crossbreeding to occur?
- compatibility of breeds using GCA test
- availability of quality animals (want to be superior)
- continued supply
What does continued supply mean?
source for replacements
What herd size is wanted for crossbreeding?
the larger the better, but is more complex because need more land, tags, etc
What is implementation of cost versus expected return?
would the return justify all the effort
What is the significance of nothing being free?
increased performance will require greater availability of feed and nutrients
What are maternal swine breeds?
larger litter size, produce more milk, better mothering ability
What are paternal swine breeds?
faster growth, better meat
What are examples of maternal swine breeds?
Yorkshire, Landrace, and Chester White
What are examples of paternal swine breeds?
Duroc, Hampshire, and Spots
What is a reciprocal cross?
test the reverse sex of the direct cross. If direct cross is a male Angus x female Hereford, reciprocal cross is female Angus x male Hereford
Does the performance of reciprocal crosses make any diffference?
yes
