3. Outcrossing
Mating of unrelated animals within a breed. It usually applies only
to matings within a purebred but show no relationship for at least 4
to 6 of its previous generations.
Outcrossing within a herd by use of selected sires is also called as
selective breeding.
Effective for characters governed by additive effect of genes having
high heritability.
The outcrossing is practised to exploit intra herd variability.
4. Uses of outcrossing
It can be practised in a purebred herd/flock when there is lack of
selection response due to decrease in genetic variability.
To reduce inbreeding in a closed population.
To introduce new genes with reference to colour, horn type etc.
Highly effective for characters under the control of genes with
additive effects.
It is the effective system for genetic improvement, if carefully
combined with selection.
6. Top crossing
Mating of a inbred males of a certain family to females of another
family or non-inbred population of the same breed.
A top cross is made when a breeder goes back to the original genetic
source of the breed or strain for some new genetic material for further
genetic improvement of the breed / strain.
e.g.
1. Angus breeders from America return to Scottland to buy a stud sire to
top cross their Angus breed.
2. Indian Jersey is top crossed with newly imported Jersey.
8. Line crossing
Crossing of inbred lines within a specific breed.
Takes advantage of both increased homozygosity and
increased heterozygosity
1. Incrossing : Mating between inbred lines within a breed
2. Incross breeding : Mating between inbred lines of different
breeds.
10. Strain crossing
Mating between strains within the same breed.
e.g. Crossing of different White Leghorn Strains.
WLH strains A (England) x WLH strain B (Australia)
WLH strain crossbred AB
12. Grading or Grading-up
Breed substitution or breed replacement
Grading is the continuous use of sires of one pure breed on the
females of non-descript animals and its female progenies
generation after generation.
The objective is to change the genetic makeup of a non-descript
(mixed population/mongrel/Scrup) to a ‘pure bred’ –To raise the
performance of ND to the level of purebred quickly
13.
14.
15. Merits
It is followed to increase the number of purebred
population, if they are relatively scare or new to that area.
To introduce a new gene or desirable quantitative traits in a
population
For grading-up programme minimum number of bulls / sire
is sufficient or even frozen semen straws are enough.
16. Disadvantages
This process requires 4-5 generations/ several years
The first cross usually shows marked improvement in productivity
over the original female stock.
Due to increase in the level of exotic inheritance - poor in
adaptation to local environment
It can be practised after fixing the target for level of exotic
inheritance based on the level of performance. In dairy cattle the
level of Jersey / H.F. inheritance should not exceeds 50 – 62.5%.
21. Crossbreeding
The mating of animals from different established
breeds is called crossbreeding. The progeny produced is
called crossbred.
Purposes:
To exploit hybrid vigour or heterosis
Complementarity
Development of new breeds or synthetics.
32. Criss-crossing
It is similar to back crossing except that
both the parental breeds (P1 and P2) are used
alternately in each generation
33.
34. Three-way cross or triple cross
In this system of cross breeding three
breeds are used (A, B, C). First generation
crossbred female (AB) are crossed with males
of third breed/line.
35.
36. Four breed crosses or Double two
breed crosses or Four-way cross
Four breeds are used. In this four-way
cross, the crossbred progeny from two
separate two-way crosses are mated to
produce commercial progeny called „double
hybrids‟ (AB.CD).
37.
38.
39. Rotational crossing
The males of two or three breeds are used in
regular sequence (rotation) in successive generations
on crossbred females of the previous generations. Thus
it is called as rotational crossing which may involve
two or three breeds.
In pig breeding, this method have been used widely
with different breeds for the production of market
animals / hybrids.
50. The mating of unrelated individuals is called as outbreeding
Outbreeding is the mating of animals which are distinctly less
closely related to each other than the average of the population i.e.
the individuals that have no common ancestors in the proceeding 4
to 6 generations of their pedigree.
Outbreeding increases the heterozygosity and variability of the
population.
The general effects of outbreeding are the opposite of those of
inbreeding.
Outbreeding
53. Heterosis
G.H.Shull
Heterosis is a phenomenon in which progeny of crosses
between purebred populations or inbred lines exceed the
average of the two parental populations.
Heterosis is the superiority of the outbred animals (F1) over
the average of their parents in individual merit.
54. HV is measured as the difference between the average performance of
crossbreds and the average performance of their purebred parent lines or
breeds, mathematically,
HV = MF1 – MP
Where HV = hybrid vigor measured in units of a trait
MF1 = the average performance of crossbreds
MP = the average performance of both parent lines = MP1 – MP2
2
Where MP1 = the average performance of the first parent line
MP2 = the average performance of the second parent line
Hybrid vigor is often expressed on a percentage basis
%HV = MF1 – MP x 100
MP
55. For example, if 21 day litter weight average 98 lb for purebred
pigs of breed A, 106 lb for purebreds of breed B, and 113 lb for
F1 A x B cross pigs, then
Mp = Mp1+Mp2
2
= 98 + 106
2
=102 lb
and %HF1 = MF1 – MP
MP
= 113 - 102 x 100
102
= 10.8 %
57. Here is another example, if milk production for breed A
averages 12,000 lb, for breed B 18,000, and for F1 A x B
crosses 16,000 lb then
Mp = Mp1+Mp2
2
= 12,000 + 18,000
2
=15,000 lb
and %HF1 = MF1 – MP
MP
= 16,000 - 15,000 x 100
15,000
= 6.7 %
