Week 7 Mating system - inbreeding and crossbreeding Flashcards
What are the mating strategies on animal performance
Random mating
Assortative mating
What are the mating strategies based on pedigree relationship
- inbreeding
- linebreeding
- outbreeding
- Crossbreeding
What is a random mating
Mating system in which mates are chosen at random. This is done after selecting the best males and females.
What is assortative mating
Mated according to their phenotype
There are positive and negative assortative mating
What is positive assortative mating
Mate individuals with the similar phenotypes, Mating the best to the best. Worst to the worst
What is the effect of positive assortative mating
Increase the amount of variation in the population if we keep both extremes
What is negative assortative mating
Mating individual with unlike phenotype.
Mate the best to the worst, can be calling corrective mating and compensatory mating (remove symptoms of one kind.
What is inbreeding
Mating of the animals related to each other.
What kind of inbreeding stretchiest are there?
Mild inbreeding and intense inbreeding
What is mild inbreeding
Involves mating individuals that are not closely related, such as cousins
What is intense inbreeding
Mating individuals that are closely related such as sibs, sires and daughters, dams and sons
What is the effect of inbreeding
Increase the genotypic frequency
What is line inbreeding
Decrease variation within each line
What is the phenotypic effect of inbreeding
Uncovers undesirable recessive genes.
-Since inbreeding increases homozygosity, it tends to uncover lethal genes and detrimental genes that have been hidden in the heterozygotes (in the carriers)
Inbreeding tend to see more animals with genetic defects
Does inbreeding cause genetic defects
No, it does not create defective genes but uncovers the genes that causes defects that have been hidden in the heterozygotes.
It also uncovers detrimental genes that reduce performance (genes for slow growth)
The phenotypic mean of the population will usually decrease for most traits when inbreeding occurs
What is the main issue with inbreeding
Inbreeding depression
What is inbreeding depression
The reduction in the population mean that occurs with increase inbreeding.
Reduce performance in fertility.
Why use inbreeding
A superior phenotype you want to maintain in the line.
Maintaining a high proportion of the genes of an outstanding ancestor in the herd or flock.
-Inbreeding can also used to help detect and eliminate undesirable recessive genes. (Using sire x daughter mating as a test for undesirable recessive genes .
Increase prepotency
What is prepotency
Is the tendency of a parent, most often the sire, to stamp his characteristics on his offspring. It will be more uniform, producing lesser variation of gametes
What is the causes of inbreeding depression
If gene action is entirely additive inbreeding will not cause a reduction in the population mean. If the gene is control by the dominant or the epistasis gene, it will not be inherited to the next generation.
As well as overdominance and epistasis
What is over dominance
Refers to the situation in which the heterozygotes is superior to either f the homozygotes.
What is additive gene,
A phenotype that is express based on two or more genes combination
What is epistasis
Depends on maintaining certain favourable combinations of alleles at different loci, inbreeding changes the combination of alleles and case a decline in the population
What are the degrees of inbreeding, from the most intense to the lease
- Selfing
- full-sib mating or parent-offsprings mating (most intense form of inbreeding in animals)
- Half-sib, Grandparent-grandoffspringm uncle-niece. (1 generation apart
- Mating of first cousins
What is Selfing
Mate a plant to itself
How do you know the intensity of inbreeding
Inbreeding coefficient, F.
F represents the increase in homozygosity resulting from mating individuals that are more closely related than the average of the population
What are the two most common forms of outbreeding
Outcrossing
Crossbreeding
What is outcrossing
The mating unrelated individuals within a breed
What is Crossbreeding
The mating of animals from different breeds
What are the effects of outbreeding
Reduce homozygosity, increase heterozygosity, decrease prepotency (because it decrease the homozygous loci frequency), increase mean population
Increases the phenotypic uniformity in the first cross between inbred lines
Who uses outcrossing most commonly
Seedstock producers
What are the effect of outcrossing
Continually selecting the best available unrelated sires for use in herd has been responsible for most of the changes occurred in breeds of livestock
Maintain high levels of performance in herds and flocks
Relatively few of the undesirable recessive genes being uncovered
Who uses Crossbreeding more commonly
Commercial producers
What is a breed
A race of animals within a species that usually have a common origin and similar identifying. Characteristics
What are the major reason for Crossbreeding
Breed complementarity
Heterosis or hybrid vigor
What is breed complementarity
An improvement in the overall performance of crossbred offspring resulting from crossing breed of different but complementary biological types
What is heterosis
An increase in the performance of hybrids over that of purebreds, most noticeably in traits like fertility and survivability
What does breed complementation involves
Arrays of traits, crossing maternal breed(those excels in maternal traits of fertility, freedom from dystocia, etc)
With paternal breed (excel in paternal traits such as rate and efficiency of gain, meat quality etc)
How can heterosis be useful?
The crossbreds need to be better than either parental breed
What is the heterosis that we get in the crossbred dams called
Maternal heterosis
Genetic reasons for heterosis
Dominance
- Good traits tend to be dominant and crossbred get the good traits from each breed.
Over-dominance
- refers to the situation in which the heterozygotes is superior to either of the homozygotes
- Crossbreeding increase heterozygosity and the frequency of heterozygotes in the population, therefore the mean of the population will increase.
Epistasis
- the expression of the genes at one locus depends on the alleles present at one or more other loci.
- crossing different breed or lines of animals favourable combinations of genes at different loci may be established( may get favourable epistasis combination of genes)
What is the criteria useful for evaluating different Crossbreeding systems
-Merit of component breeds
-Hybrid Vigor
-Breed complementarity
Consistency of performance
-Replacement of considerations
-Simplicity
-Accuracy of genetic predictions
What are the crossbreeding system
- Single cross between 2 breeds
- Two breeds rotational cross
- Three breed rotational cross
- Terminal sire crossbreeding
What is Single Cross between 2 Breeds
One type of crossbreeding system simply involves a single cross between two breeds
• Mate breed A males to breed B females
• For replacement animals, buy outside purebred A males and purebred B females
– Do not keep any AB crossbred progeny as replacements
The Disadvantages of Single Cross between 2 Breeds
Disadvantages
– Whereas this crossbreeding system gives heterosis
in the offspring, do not get any maternal heterosis from using crossbred dams
– This crossbreeding system only uses 2 breeds, so can not get the good traits from any other breeds
– Buying outside animals may increase the probability of introducing diseases into your herd
– Dependency on other producers for good animals to bring into your herd.
What is Two-Breed Rotational Cross
There are two breeding pastures
– Replacement females sired by breed A bulls go into
pasture 2 and are mated to breed B bulls for the
remainder of their lifetime
– Replacement females sired by breed B bulls go into
pasture 1 and are mated to breed A bulls for the rest of
their lifetime
• After a period of time, the calves from cows in pasture 1 will stabilize at about 2/3 breed A and 1/3 breed B
– 2/3 of their genes will be derived from breed A and 1/3 will be derived from breed B
• Calves in pasture 2 will eventually stabilize at 2/3 breed B and 1/3 breed A
– 2/3 of their genes will be derived from breed B and 1/3 will be derived from breed A
what is the advantages of Two-Breed Rotational Cross
• The producer can use his own replacement females
• Minimizes the possibility of diseases being introduced into the herd
– We get maternal heterosis from using crossbred females, as well as heterosis in the progeny
– It takes into consideration heterosis for fertility and milk production in the crossbred cow, heterosis for livability of the crossbred calf, and heterosis for growth rate in the calf
What is Three-Breed Rotational Cross
• Follows the same pattern as in the two-breed rotational cross, except that a third breed is added
• Replacement females sired by breed A bulls go into pasture 2 and are mated to breed B bulls for the rest of their lifetime
• Heifers sired by breed B bulls go into pasture 3 and are mated to breed C bulls for the rest of their lifetime
• Heifers sired by breed C bulls go into pasture 1 and are
mated to breed A bulls for the rest of their lifetime
what are the advantages of Three-Breed Rotational Cross
• The producer has complete control over selection and
management of replacement females, since he produces his own replacements instead of buying them
• Get heterosis in both the crossbred progeny and in the crossbred dams
• Gives more maternal heterosis than a two-breed rotation
• A three-breed rotation gives 87% as much maternal heterosis as in the F1 female
What is Terminal Crossbreeding System
• This system is also called a rota-terminal crossing system
• 3 breeding pastures and 3 sire breeds are required
• Rotational part of system - Heifers sired by breed A bulls are mated to breed B bulls and vice versa
• Terminal cross - older and poorer females from pastures 1 and 2 are taken to pasture 3 where they are
mated to breed C bulls and all offspring marketed
• The breeds used in the rotational portion of the system should be
comparable in size and milk production to minimize difficult births and to stabilise nutrition and management requirements in breeding pastures 1 and 2
• The rotational portion would involve the youngest cows in the herd
– the yearling heifers, 2-yr olds, 3-yr olds, and maybe the 4-yr olds
• Cows are not mated to the terminal sire breed until they are 4-yr-old
or older when there is less likelihood of calving difficulty
• The terminal sire breed would be one that has good growth and carcass traits, since all of the progeny from the terminal cross are marketed
Why are heifers from the terminal sire breed not kept as replacement for Terminal Crossbreeding System
because they are larger in size, have higher energy
requirements, and require more feed. Also, they are out of the older and poorer
cows
What happen to Steers and poorer heifers for terminal crossbreeding system
not kept for replacements would also be marketed from
the rotational portion of the system
What are the advantages for Terminal Crossbreeding System
• This system allows the producer to produce all of his own replacement females
• It uses crossbred dams
• Get individual heterosis in the crossbred offspring and
maternal heterosis from the crossbred mothers
• This system allows us to take advantage of size
complementarity, we can mate smaller dams to a larger breed of sire
– This system allows us to utilize heterosis, breed complementation, and size complementarity
Crossbreeding Systems in Practice
- Modifications or combinations of one or more of these systems will probably be necessary to fit each individual operation in terms of facilities, management, labour availability, and market
- A long-term plan should be well thought out before starting a crossbreeding program, crossbreeding will often require more labour and management decisions
- Crossbreeding is not a cure for poor management and unproductive animals
- Selection of breeding males and females is just as important as in any other breeding program
- In order to maximize the performance of the animals in our herd or flock, we need to use selection in conjunction with crossbreeding
