Hybrid Vigor Or Heterosis
Genetics (10201232)
Faculty of Agriculture and Veterinary Medicine An-najah National University
Instructor: Dr. Jihad Abdallah Topic 14: Mating Systems
1 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: 1. Crossbreeding: mating of sires of one breed or a combination of breeds with dams from another breed or combination of breeds 2. Linecrossing: mating of sires of one line or a combination of lines with dams from another line or combination of lines
2 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.
3 Terminal Sire Crossbreeding
Sire breed x Dam breed A B
AB (Crossbred progeny)
All progeny including females are sold in the market for slaughter
4 • 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.
5 Measuring Hybrid vigor
HV PF1 PP
Average performance of Average performance of parental crossbred offspring breeds (or lines) HV can be expressed in % basis:
P P %HV F1 P X 100% PP
6 • 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.
7400 7000 %HV X 100% 5.7% 7000
7 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
8 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)” 9 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 %RHV 1 P P X100% Si Di i1 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
10 • Example1: Calculate the % of retained hybrid vigor from the following cross: (A X B) X (C X (A X B) )
Female breed A 0.25A 0.25B 0.5C
0.5 A 0.125
0.125 Male breed 0.5 B 0 0 C
%RHV= [1-(0.5x0.25+0.5x0.25 +0x0.5)]X100%= 75%
11 • Example2: Calculate the % of retained hybrid vigor from the following cross: C X (A X B)
Female breed A 0.5A 0.5B 0C
0 A 0
0 Male breed 0 B 0 1 C
%RHV= [1-(0x0.5+0x0.5 +1x0)]X100%= 100%
12 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
13 Two-breed spatial rotational crossbreeding system
Males of breed B Males of breed A X X
Females of higher Females of higher proportion of breed A 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
14 Three-breed spatial rotational crossbreeding system Males of breed A Males of breed B X X
Females of higher Females of higher proportion of breed A proportion of breed C Males of breed C X Females of higher proportion Females of higher of breed B are moved and proportion of breed C are mated with males from breed moved and mated with Females of higher C males from breed A proportion of breed B
15 Two-breed rotational Three-breed system rotational system Gen. Breed of %RHV Breed of %RHV sire sire 1 A 100 A 100 2 B 50 B 100 3 A 75 C 75 4 B 63 A 88 5 A 69 B 87 6 B 66 C 84 7 A 67 A 86 16 Predicting equilibrium HV in rotational crossbreeding systems
n (2 2) Where n is the number of breeds in %RHV X100% (2n 1) the system Example1: two-breed system: (22 2) %RHV X100% 66.7% (22 1)
Exampl2: three-breed system: (23 2) %RHV X100% 85.7% (23 1)
4-breed system %RHV=93.3%
17 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.
18 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
19 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.
20 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
rAX = 0.50 X FX = 0.125
21 Advantages of Linebreeding
1. Increase uniformity (formation of inbred lines) 2. Increase the opportunity for making hybrid vigor by crossing inbred lines. 3. Maintain or increase number of individuals with high relationship to an outstanding ancestor (maintain and spread of excellent genes)
22 Linecrossing
• Mating of sires of one line with dams of another line. • Line A X Line B
AB
23