Download Udspaltning af den recessive q = 0,01 og p = 0,99 f(rr) = q2 = 0,012

Chapter 9: Inbreeding and
cross-breeding
• Effect of inbreeding on individual and
population level
• Effect of cross-breeding
• Minimum inbreeding systems
• Population structure
Inbreeding depression
• decrease in the level of the production traits due to
inbreeding
• Genes with dominance have most effect on the
heterozygotes
• Increased level of homozygosity is the under laying
factor which causes the inbreeding depression
Genotype
AA
Frequency p2 + pqF
Aa
2pq - 2pqF
aa
q2 + pqF
Linearity of the inbreeding
depression
Linear fall in daily slaughter gain in swine
Hybrid Vigour
• Increase in the level of production traits, due to
crossing, is called hybrid vigour or heterosis
• Genes with dominance causes most effect on the
heterozygotes
• Increased heterozygosity is the under laying factor
which causes hybrid vigour
Effect of crossing
• Cannot be predicted
• Trial and error principle
• Good crossing-combinations can be
repeated again and again
Crossing systems
Two-way, three-way, four-way, back, or
rotation crossing
Effect of crossing: Example
Calculation of hybrid vigour in
sows:
Example litter size
• Landrace  Yorkshire  11
• Pure bred  10
Hybrid vigour = (11 - 10)/10 = 10 %
Crossing demands a surplus
of females
• An RDM or SDM cow bears in average only 1.1
to 1.2 heifer calf, which is sufficient to maintain
the pure bred population.
• Therefore, crossing production is not possible in
these breeds, if pure breeding is desirable in the
entire population
Negative recombination effects
in F2
Biochemistry in a two-gene system, which can be actual in F2
individuals after a crossing of aabb x AABB
Breeding systems, summary
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Inbreeding
Pure breeding
Line Breeding
Crossing
Minimum inbreeding systems
• Pedigree
• Selected animals gets the same number of offspring
• Circular mating systems
Pedigrees
• Mating of non-related
• (far out related) individuals
• Ca. 3-4 generations
• Necessary to maintain pedigree in pure
breeding
All individuals gets two
offspring
Distribution of offspring after random mating
Circular mating systems
Breeding pyramid
Gene flow between the different levels of herds