Crossbreeding (2)
Lecture 20 Introduction to Breeding and Genetics GENE 251/351 School of Environment and Rural Science (Genetics) Crossbreeding: Overview
reasons for crossbreeding understanding crossbreeding effects predicting crossbreeding effects Crossbreeding Effects
• Additive effects of the genes of breeds
– Direct Additive breed effects – Maternal Additive breed effects
• Non-Additive effects of genes of different breeds
– Direct heterosis – Maternal heterosis Predicting Crossbred Performance
• Additive direct breed effects • Additive maternal breed effects
• Proportional to breed proportion of animal / dam
• Direct heterosis • Maternal heterosis
• Proportional to heterozygosity of animal / dam Crossbreeding Examples
2-Breed Cross 3-Breed Cross Rotational Cross x
x x
x x
x
x Predicting Additive Breed Effects
Rotational Cross 2-Breed Cross 3-Breed Cross x x x
x x
x
Direct 50% A, 50% B; Direct: 50%A, 25%B; 25%C; Maternal 100% B x Maternal: 50%B; 50%C
Rotating: 67%A; 33%B & vv Predicting Non-Additive breed effects
Rotational Cross 2-Breed Cross 3-Breed Cross x
x x x x x
x
Heterosis 100% Heterosis 100% Heterosis 67% No Maternal heterosis Maternal heterosis 100% Maternal heterosis 67% Rotational Cross x A x B
B x AB x 1 3 A x 4 A 4 B
5 B x A 3 B x 8 8
A x 5 A1 1 B 16 16 x
1 2 2 A x 3 A 3 B ... giving 3 heterosis
x B x 2 A 1 B ... giving 2 heterosis 3 3 3 Crossbreeding structures
• 2-Breed Cross • 3-Breed Cross • Rotational cross x x
x x
x 100% heterosis x 50% -100% heterosis
x
• Synthetics or Composites 67% heterosis Generally 50% heterosis Crossbreeding: More ‘structure’ gives more merit ...
In general ... x The shorter the breed pedigree back to purebred parents: x
• the more heterosis can be expressed (e.g. F1 > F2) • the more sire-dam complementarity can be expressed
BUT: The more expensive the operation is to run (need more purebred parents) Crossbreeding:
Pig and poultry Cattle and sheep
Sire line Dam line Sire line Dam line
Crossbreds Crossbreds
With more reproductive species, it is relatively cheaper to maintain purebred lines
Poultry > Pigs > > Sheep > Cattle Crossbreeding: Specialized lines and crossbreeding or dual purpose breeds?
Simple example: Assumptions:
Males slaughtered Females produce wool
A crossbreeding system is more profitable, it exploits sire-line and dam-line complementation Which crossing system to adopt?
PUREBREED when no cross is better. F CROSS 1 when direct heterosis is important. 3 BREED CROSS when both direct and maternal heterosis are important. 4 BREED CROSS when paternal heterosis is important as well. BACKCROSS when only 2 good parental breeds are available and/or when direct heterosis is not important. ROTATIONAL CROSSES when females are too expensive to either buy in or to produce in the same enterprise. OPEN OR CLOSED COMPOSITE when both males and females are too expensive. A few initial well judged importations establish the synthetic (or 'composite'), and it can then either be closed (which helps to establish a breed 'type'), or left open to occasional well judged importations.
Patterns of use of crossbreeding
Industry Fecundity Typical crossing systems
Poultry highest 4-breedcrosses Pigs 3-breed crosses;back crosses Meat sheep 3-breedcrosses Wool Sheep purebred* Dairy purebred* Temperate Beef rotations;composites Tropical Beef lowest composites
*Wool sheep and dairy industries are exceptions due to availability of an outstanding pure breed in each. Crossbreeding parameters...
• Direct additive breed effects Ad – Additive effects of purebreeds. – For yearling weight, they relate to the ability to grow quickly.
• Maternal additive breed effects Am – Additive effects of purebreeds as expressed by the dams – They relate to milk production and rearing ability.
• Direct dominance effect Dd – The effect of heterosis in crossbred individuals
• Maternal dominance effect Dm – The effect of heterosis due to crossbreeding in the dam A simple example
Mean Yearling Wt. of progeny. Breed of Cow
1 2
1 294 309 Breed of Bull 2 304 279
A simple example
Direct breed effects A Mean Yearling Wt. d of progeny. Breed of Cow
1 2 Maternal breed effects Am 1 294 309 Direct heterosis D Breed of Bull d 2 304 279
Maternal heterosis Dm
Breed difference = Mean Breed 1 – Mean Breed 2 = 294 - 279
= (Ad1 + Am1) – (Ad2 + A m2)
i.e. contains direct PLUS maternal breed difference
A simple example
Mean Yearling Wt. of progeny. Breed of Cow
Can we estimate maternal effect separately? 1 2
1 294 309 Breed of Bull 2 304 279 yes
Estimate difference between 1 x 1 and 1 x 2?
No, this contains also heterosis and part of the direct breed difference
Estimate difference between 1 x 2 and 2 x 1?
Yes, this contains only maternal effect difference -- why? -- A simple example
Mean Yearling Wt. of progeny. Breed of Cow
Maternal breed difference = 304 – 309 = 5 1 2
1 294 309 Am1 = - 2.5 Breed of Bull 2 304 279 Am2 = + 2.5
Direct Breed effect
Mean Breed 1 = (Ad1 + Am1) = Ad1 – 2.5 = 294 Ad1 = 296.5
Mean Breed 2 = (Ad2 + Am2 ) = Ad2 + 2.5 = 279 Ad1 = 276.5
Ad1 – Ad2 = 20 = Direct breed difference
A simple example
Mean Yearling Wt. of progeny. Breed of Cow Heterosis 1 2 1 294 309 Breed of Bull Mean of F1 crosses – Mean of Purebred (Parental) lines 2 304 279 = 306.5 – 286.5 = 20
Maternal Heterosis ?
Need both crossbred and purebred mothers Estimating crossbreeding parameters from coefficients
Cross Ad1 Ad2 Am1 Am2 Dd Dm Merit 296.5 276.5 -2.5 +2.5 ? estimated 20 Pure1 1 0 1 0 0 0 294
Pure2 0 1 0 1 0 0 279
1 x 2 .5 .5 0 1 1 0 309 observed
2 x 1 .5 .5 1 0 1 0 304
1 x (12) ? ? ? ? ? ? ? Predicting new crosses
Cross Ad1 Ad2 Am1 Am2 Dd Dm Merit 296.5 276.5 -2.5 +2.5 20 ? Pure1 1 0 1 0 0 0 294
Pure2 0 1 0 1 0 0 279
1 x 2 .5 .5 0 1 1 0 309
2 x 1 .5 .5 1 0 1 0 304
1 x (12) .75 .25 .5 .5 .5 1 Summary
• Crossbreeding structures differ in
– Breed composition – Amount of heterosis – Practical aspects
• Can estimate crossbred parameters by writing out the coefficients of the effects for different crosses