1. Dominance, mating and crossbreeding
Chapter 10

2. Mating Mating is pairing selected sires and dams
= comes after selection
Mating determines how selected alleles combine within individuals
Benefit from non-additive genetic effects

3. Mating strategies Mating within populations Mating between populations
Which individuals to mate together?
Mating between populations
= crossbreeding
Which lines to cross with each other?

4. Mating within populations
Affects genetic make-up of next generation of selection candidates
Does not affect long-term genetic improvement

5. Major mating strategies
Random mating
Assortative mating
Mating based on family relationships
Mate selection

6. Random mating = Absence of any specific mating strategy
Mates chosen at random within selected sires and dams
NOT same as random selection!

7. Assortative mating Mating based on phenotypes or EBVs Two types:
Positive assortative mating
Negative assortative mating

8. Positive assortative mating
= Mating of similar individuals
Progeny?
More extreme
 Increases genetic variance (temporarily)
Practical use limited
Also need to mate poorer animals with other poorer animals!

9. Negative assortative mating
= Mating of dissimilar individuals
Progeny?
More intermediate
 Decreases genetic variance (temporarily)
Compensatory/corrective mating
Aims to correct faults of parents in offspring
Dairy cattle

10. stierenkaarten

11. Mating on family relationships
Preferential mating of relatives
= Inbreeding
Avoidance of mating of relatives

12. Minimum kinship matingx
Avoiding mating of relatives
Calculate coefficient of kinship for all possible combinations of sires and dams
Choose matings to minimise average coefficient of kinship
Postpones occurrence of inbreeding
Reduces long-term rate of inbreeding

13. Minimize kinship mating in practicex
In real life, minimum kinship mating is complicated but can be done
Many breeding programmes use some sort of mate selection to restrict kinship
- avoid matings between close relatives
Full sibs
Half sibs
cousins

14. Mate selection Combines selection and mating in a single step
Calculate expected progeny performance for every possible mating of candidates
Do matings with highest expected progeny performance
Utilises non-additive genetic effects
but you have to know them.....

15. Summary: Mating within…
…populations means deciding which sire to mate with which dam
You can benefit from
Corrections in phenotype
non-additive genetic effects
Main use: avoid highly inbred offspring
full sibs
half sibs
cousins

16. Mating between populations
Used to produce final production animals
Pure production strategy, no effect on breeding population

17. Crossbreeding
Breeding system where sires and dams originate from different lines
Lines within breed
Different breeds
Many ways to combine lines
Benefit as much as possible of heterosis
Breed complementarity
Common in pigs, chicken and beef cattle

18. A tropical breed: Nelore
short hair, skin folds, long ears, no body fat reserves, long legs

19. A temperate breed: Red Agnus
Coarse hair, subcutaneous fat, short stature

20. corssbred: productive in tropical climate, high tick resistance!X

21. Crossbreeding in dairy cattle
Crossbred (F1)between indigenous cattle and Holsteins perform well in a number of developing countries
Should crossbreeding be implemented?
Consider population with F1-cows

22. Crossbreeding in dairy cattle
Large pure-bred cow population needed for the production of F1-replacements.
Bottleneck:
Reproductive rate males: no problem
Reproductive rate females: very low
1 offspring per year
4 offspring during life time

23. 100,000 F1 animals (Zebu*Holstein)
Parental breeds:
Holsteins: imported (semen)
Zebu: local pure-bred cow population
Replacements for F1 population?
Every year 25% of cows (F1 and purebred) replaced
25,000 F1-heifers needed each year.
50,000 Zebu cows need to be inseminated with Holstein semen.
50,000 Zebu cows need to be inseminated with Zebu semen for maintaining Zebu population

24. Crossbreeding between breeds: concluding remarks
In developing countries:
Quick fix: improved performance,
No long term genetic improvement!
Who maintains the local breed?
Only when the local breed serves a purpose!

25. Time for a break

26. Crossbreeding: Atlantic Salmon
25 ♀-Gaspe 25 ♀- Mowi ♀-Laks
Milt 15 ♂
Gaspe (CD)
Mowi (NO)
Laks (UK)
eggs
eggs
eggs
eggs
eggs
eggs
eggs
eggs
eggs
♂- ♀ GA LA MO
GAGA
GALA
GAMO
LAGA
LALA
LAMO
MOLA
MOMO
Crosses:
15/05/2018

27. Cross Breeding: salmon
Ranking for GF3 from smolt to 4.5 kg
The results of the SP tests are used to direct the crosses in future spawning's for production.
15/05/2018

28. Heterosis Crossbred offspring is better than parent average
Also called hybrid vigour (plant breeding)
Caused by dominance (and epistasis)
“Specific combining ability of lines”
Can be expressed in trait units
HF1 = µF1 – ½(µsire + µdam)
Usually expressed as percentage
HF1 = 100% x (crossbred mean – parent mean) (parent mean)

29. Example: Heterosis in salmon
Harvest weight
Mowi-line: harvest weight = 3.8 kg
Laks-line: harvest weight = 4.0 kg
Mowi x Laks F1: harvest weight = 4.1kg
What is the heterosis?
HF1 = ½( ) = 0.2 kg
HF1 % = 0.2 kg / 3.9 kg x 100% = 5.1%

30. Genetic basis of heterosis
Crossbreeding leads to more heterozygotes if there is a difference in allele frequency ()
Line1 = MM, line 2 = LL,  = 1
F1 = ML = maximum heterosis
1-locus: d > 0  F1 will be better than parent average = ML > (MM + LL)/2

31. Example: Fillet% in Salmon
Line 1, p(M) = 0.8; Line 2, p(M) = 0.3
 = = 0.5
GMM = 60%, GML = 64%, GLL = 65%
o = (65+60)/2 = 62.5%
d = = 1.5%
HF1 = d x 2 (eq 10.10) = 1.5 x 0.52 = fillet%

32. Heterosis: two types
Direct or individual heterosis
Maternal heterosis

33. Crossbreeding: two-way scheme
dam line
Breeding line D C
sire line
Breeding line C
Selection on reproduction
Selection on growth and meat quality
(♂ & ♀!)
CxD
F1-offspring
Production Population

34. Two-way system More uniform F1 production population
Breed/line complementarity
Heterosis in (CxD) progeny
Note: F1 animals are not used for pure line breeding!
 All selection within pure lines D and C

35. Two-way system Heterosis for growth (CxD) progeny? Direct: growth
Maternal: --
Hgr = 4%
Gr (C) = 700 g/d; Gr (D) = 680 g/d
Gr (CxD)= 1.04 x ( )/2 = 718 g/d

36. Two-way system Heterosis for litter size in (CxD) progeny?
Direct: vitality
Maternal: --!
Hgr = 4% (vitality)
nr Pl (C) = 10; nr Pl (D) = 11
Nr Pl (CxD)= 1.04 x (11) = Pl

37. Crossbreeding: three-way scheme
Selection & Replacement
Selection & Replacement
Selection & Replacement
Line A
Line C
Selection on reproduction
Line D
F1 sow
C×D
Selection on growth and meat quality
Fattening pig
A × (C × D)

38. Three-way system: heterosis
Direct: growth in (A x (CxD)) pigs
Mean parental lines?
(750 + ½( ))/2
1.04 x 720 = 749 g/d
Heterosis CxD is not heritable!!

39. Three-way system: heterosis
What about litter size?
Direct: 4% (vitality of piglets)
Maternal: 6 % (nr of piglets born)
Mean litter size
A: 8
C: 10
D: 11

40. Three-way system: heterosis
Litter size: parent mean
CxD !
(10+11)/2 = 10.5
Maternal H: 0.06 x 10.5
Direct H: 0.04 x 10.5
Sum: = PL

41. Three-way system
Uniform end product (A x (CxD)); breed complementarity
Heterosis in (A x (CxD)) pigs
Maternal heterosis in (CxD) sows
 better reproduction
Protection of genetic material

42. What is the breeding goal?
Selection & Replacement
Selection & Replacement
Selection & Replacement
Info
Info
Line A
Line C
Line D
F1 sow
C×D
Selection on growth & litter size
Selection on growth, backfat and meat%
Fattening pig
A × (C × D)

43. Summary: Crossbreeding…
… = production strategy
Crossbred individuals are (usually) not parents
Crossbreeding itself does not contribute to G
Aims to benefit from heterosis & line complementarity
Genetic improvement within pure lines
Should aim at crossbred performance