Diapositive 1

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Course Genetics and Breeding (12226)
Faculty of Veterinary Medicine An-najah National
University Instructor Dr. Jihad Abdallah Mating
Systems
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Outbreeding Systems

• Outbreeding (or outcrossing) Is the mating of
  individuals whose relationship is less than the
  average relationship in the herd or population.
• As a mating strategey, outbreeding more commonly
  refers to crossbreeding or linecrossing
• Crossbreeding mating of sires of one breed or a
  combination of breeds with dams from another
  breed or combination of breeds
• Linecrossing mating of sires of one line or a
  combination of lines with dams from another line
  or combination of lines

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Crossbreeding

• In crossbreeding we look for breed
  complementarity ? each breed is good for traits
  or characteristics different from the other breed
• Maternal breed excellent in maternal traits
  (fertility, mothering ability, milk production,
  etc)
• Paternal breed excellent in paternal traits
  (growth rate, meat quality and carcass yield)
• Breed complementarity is achieved in terminal
  sire crossbreeding ? systems in which maternal
  breed females are mated to paternal-breed males
  to produce progeny desired from market standpoint.

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Terminal Sire Crossbreeding
Sire breed x Dam
breed A
B
AB (Crossbred progeny)
All progeny including females are sold in the
market for slaughter
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• The main objective of crossbreeding is to obtain
  hybrid vigor or heterosis.
• Hybrid vigor (HV) an increase in the performance
  of crossbred progeny over purebred progeny in
  traits affected by non-additive factors
  (dominance, overdominance and epistasis). It is
  an increase in gene combination value (GCV) that
  results from the increase in heterozygous
  genotypes due to crossbreeding.

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Measuring Hybrid vigor
Average performance of crossbred offspring
Average performance of parental breeds (or lines)
HV can be expressed in basis
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• Example Crossbreeding between two dairy breeds
• Average milk production per year of breed A is
  8000 kg
• Average milk production per year of breed B is
  6000 kg
• Average of crossbred cows is 7500 kg.
• Calculate the resulting hybrid vigor on
  percentage basis.

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Composite (Synthetic) breeds

• A composite or synthetic breed is a breed made up
  of various proportions of two or more breeds ?
  designed to benefit from hybrid vigor.

A X B
AB X C
ABC X ABC
Breed of various proportions of A, B, and C
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Backcross

• Backcross is when the resulting hybrid offspring
  are mated back to one of the two parental breeds
  (or lines).

A X B
A X AB ? called backcross 1 (BC1)
A X AAB ? called backcross2 (BC2)
The parental breed or line to which we repeatedly
cross the hybrid offspring is called recurrent
parent breed (or line)
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Retained Hybrid Vigor

• Backcrossing causes a reduction in the F1 hybrid
  vigor.
• Retained Hybrid Vigor (RHV) is the proportion of
  the F1 hybrid vigor which remains in later
  generations (generations subsequent to the F1)

n is the number of breeds involved in the
cross PSi the proportion of breed i in
sires PDi the proportion of breed i in dams
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• Example1 Calculate the of retained hybrid
  vigor from the following cross
• (A X B) X (C X (A X B) )

Female breed 0.25A
0.25B 0.5C
A
0.5 A
0.5 B
Male breed
0 C
RHV 1-(0.5x0.250.5x0.25 0x0.5)X100 75
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• Example2 Calculate the of retained hybrid
  vigor from the following cross
• C X (A X B)

Female breed 0.5A
0.5B 0C
A
0 A
0 B
Male breed
1 C
RHV 1-(0x0.50x0.5 1x0)X100 100
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Rotational Crossbreeding Systems

• A crossbreeding system in which females are
  rotated among sire breeds such that they are
  mated to sires whose breed composition is most
  different from their breed composition.
• Such system is designed to maintain high levels
  of HV.
• The rotation could be over space or over time

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Two-breed spatial rotational crossbreeding system
Males of breed B X
Males of breed A X
Females of higher proportion of breed A
Females of higher proportion of breed B
Females of higher proportion of breed B are moved
and mated with males from breed A and the
resulting females (which now contain higher
proportion of breed A) are moved and mated with
males from breed B
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Three-breed spatial rotational crossbreeding
system
Males of breed A X
Males of breed B X
Females of higher proportion of breed A
Females of higher proportion of breed C
Males of breed C X
Females of higher proportion of breed B are moved
and mated with males from breed C
Females of higher proportion of breed C are moved
and mated with males from breed A
Females of higher proportion of breed B
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(No Transcript)
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Predicting equilibrium HV in rotational
crossbreeding systems
Where n is the number of breeds in the system
Example1 two-breed system
Exampl2 three-breed system
4-breed system ? RHV93.3
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Grading up

• Is the mating of males of high genetic merit with
  females of low genetic merit from the same breed
  or from a different breed.
• Mainly used to upgrade local females with males
  from foreign breeds.
• The hybrid females resulting from the cross are
  mated back to purebred males from the foreign
  breed.

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Foreign breed x Local breed F
L
F X 1/2F 1/2L
F X 3/4F 1/4L
F X 7/8F 1/8L
15/16F 1/16L
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Inbreeding

• Is the mating of individuals more related than
  the average of the population.
• May result in inbreeding depression.
• Inbreeding depression is the opposite of hybrid
  vigor. It is a decrease in performance of inbred
  individuals for traits affected by dominance and
  epistasis like fertility and survival traits. It
  is a manifestation of poor gene combination
  value which results from increased homozygosity.
• But well designed inbreeding could be beneficial.

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Linebreeding

• Is a mild form of inbreeding in which mating is
  designed to maintain a high degree of
  relationship to an outstanding ancestor or group
  of ancestors without causing high levels of
  inbreeding.

A PGS
PGD MGS
MGD S
D X
rAX 0.50 FX 0.125
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Advantages of Linebreeding

• Increase uniformity (formation of inbred lines)
• Increase the opportunity for making hybrid vigor
  by crossing inbred lines.
• Maintain or increase number of individuals with
  high relationship to an outstanding ancestor
  (maintain and spread of excellent genes)

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Linecrossing

• Mating of sires of one line with dams of another
  line.
• Line A X Line B
• AB