Download systems of breeding

Phenotype: The joint action of all the genes and of the environment expressed by an
animal.
Genotype: The genes which contribute to the phenotype make up of the animal
Alleles: the different forms of genes are called alleles
Homozygous: When the same allele is present on each member of a pair of
homologous chromosomes, the individual animal is said to be homozygous for that
allele.
Heterozygous: If the two alleles of the pair are different in kind, the individual is said
to be heterozygous.
Epistasis: Although different genes segregate independently, it is possible for a
gene at one locus to influence the expression of a gene at a totally different locus.
Heterosis or Hybrid vigor:
Crosses of animals from different strains or lines of the same breed, from different
breeds or from different species often results in offspring whose level of production is
above that of the average of the parental types. Heterosis or hybrid vigor is the name
given to the increased vigor of the offspring over that of parents when unrelated
individuals are mated.
The increased productivity may be due to improvement in one or more such things as
increased fertility, a faster growth rate, greater milk production, increased pre and
post natal viability, improved mothering ability and egg production in poultry.
Estimation of Heterosis for economic trait:
Percentage heterosis: Mean F1 of offspring-Means of parents breed
--------------------------------------------------------Mean of the parent breed
E.g suppose the average litter size at weaning in swine is 7 for breed A and 8
for breed B and 8.5 in litter of F1.
Average litter size of parent breed would be (7+8)/2=7.5 piglet
Percent heterosis: (8.5-7.5)/7.5x100=13.33%
BREEDING SYSTEMS:
In developing a suitable breeding systems for a specific region first the environmental
constraints have to be identified. Secondly the available genetic resources should be
evaluated. In choosing a breeding strategy there are three major options:
- Improvement within local breeds
- Crossbreeding local breeds with exotic breeds.
- Importation of exotic breeds for purebreeding.
Table 1. illustrates the options in developing a breeding strategy. All breeding
systems involve selection in one form or another.
Table 1. Choices in developing a breeding strategy for a specific region.
In prospective
environment is exotic
cross breed better than
local population
Selection in local
population
Yes
Which exotic breed is
best?
How much exotic
blood is desirable?
100%
Is heterosis
important?
No
Replace in local
population
Selection in new
population
Yes
New blood formation
Is a systemic cross
breeding programme
possible
Yes
Selection in new
breed
Selection in parent
population
SYSTEMS OF BREEDING
The aim of the breeders, is to evolve outstanding and improved types of animals,
which can render better services to man. Selection and system of breeding constitute
the only tools available to the breeder for improvement of animals, since new genes
cannot be created, though they can recombine into more desirable groupings.
Systems of breeding have been broadly divided as under:
1.
Inbreeding: Breeding of the related animals
2.
Outbreeding: Breeding of the unrelated animals.
INBREEDING
This means the mating of related individuals. Each animal has two parents, four
grandparents, eight great grandparents biologically and so on, having 1,024
ancestors in the 10th generation and 10,48,576 ancestors in the 20th generation.
Relationship, therefore, becomes a vague term and must be specified. So in order to
be more specific we should say that inbreeding involves the mating of related
individuals within 4 – 6 generations. It has also been defined as the mating of the
more closely related individuals than the average of the population.
Inbreeding can again be divided into following groups:
Close breeding
Sire to daughter
Son to Dam
Full brother and sister
Line breeding
Half brother and sister or
Mating of animals more distantly
related, e.g., cousin mating
Inbreeding
Close Breeding
This means the mating of full sister to full brother or sire to his daughter or dam to her
son. These types of mating should be used only when both parents are outstanding
individuals, and then only at increased risk of bringing undesirable recessive genes
into homozygous forms in the progeny.
Advantages of Close Breeding
1. It increases the degree of homozygosity within animal making the resulting
offspring pure or homozygous in a large proportion of their gene pairs than in
the case of line bred or outcross animal. In so doing the less desirable
recessive genes are brought into light so that they can be more readily culled.
Thus inbreeding is the surest and quickest method of fixing and perpetuating a
desirable character or group of characters.
2. If carried out for a period of time, it tends to create lines or strains of animals
that are uniform in type and other characters.
3. It keeps the relationship to a desirable ancestor highest.
4. Because of the greater homozygosity, the selected inbred animals are more
homozygous for desirable genes(genes which are dominant) and they
therefore transmit these genes with greater uniformity.
5. The best of the inbred animals are likely to give superior results in outcrosses.
Disadvantages
1.
The undesirable characteristics are intensified in the progeny if
unfavourable genes segregation occurs.
2.
It has been observed that the progeny becomes more susceptible to
diseases.
3.
Breeding problems and reproductive failure usually increase.
4.
It is difficult to find out the stage of breeding at which it should be
discontinued in order to avoid the bad effects of the system.
Line Breeding
This means the mating of animals of wider degrees of relationship than those
selected for close breeding. It promotes uniformity in the character. Homozygosity is
not reached so quickly as in close breeding. Neither desirable nr harmful characters
are developed so quickly. It is a slowed method for the fixation of hereditary
outstanding bull or cow and the progeny is mentioned as being line bred to certain
ancestors.
Advantages of Line Breeding
1.
Increases uniformity
2.
The dangers involved in close breeding can be reduced.
Disadvantages of Line Breeding
1.
The chief danger in line breeding is that the breeder will select the animal
for pedigree giving no consideration to real individual merit. This may in
some cases result in a few generations, which receive no benefits from
selection.
Consequences of Inbreeding in General
The effect of both close breeding and line breeding are similar. The only difference is
in the degree of their intensity. They are more intense in close breeding and less so
in line breeding. These may be described as follows:
1.
It increases homozygosity (like alleles) and decreases heterozygosity
(dissimilar genes) and hence favours the development of genetic uniformity
amongst the animals.
2.
It is the best method of getting true strain from unknown stocks as it sorts
out the characters in the homozygous condition and thus help in the selection
of the desirable and culling of the undesirable individuals. Strains which breed
true are not obtained in animals, however, and in plants are obtained only
when close inbreeding is accompanied by intense selection.
3.
The outward effects of inbreeding may include the following:
a)
Effects on growth rate.
With the exception of a few cases a reduction in the reproductive efficiency
has been noted. The studies on rats on the other hand have shown that they
withstand inbreeding (up to 100 generations over 40 years) without any
apparent bad effect on growth rate. The reproductive rate of rats is very high,
however, relative to farm stock.
b)
Effect on reproductive performance.
In almost all the cases a reduction in the reproductive efficiency has been
noted. It may delay testicular development and puberty, reduce
gametogenesis or increase the embryonic death rate.
c)
Effect on vigour.
It has been noted that the death rates among the inbred groups of animals
are higher than those in outbred ones. They are also adversely affected by
environmental conditions. They have higher percentages of Runs (animals
which are undesirable due to variety of reasons).
d)
Effect on production.
Productive traits usually show moderate decrease with increases inbreeding.
e)
Appearance of lethal and abnormalities.
Hereditary abnormalities or lethal factors are likely to appear more often in the
inbred animals than in the outbred ones.
Of the two components of inbreeding, i.e. close breeding and line breeding, it is safer
to go in for the latter one. A few recommendations have also been made with regard
to them. It is recommended for the seed stock breeds in the following situation:
•
Better than average herds
•
If the owner is well informed regarding both its possibilities and pitfalls.
•
Herds with two or more sires to keep the inbreeding level under control
and not allowing it to rise to dangerous levels.
•
For some experimental purpose
It is further recommended that the herds of the following types should never be
selected for breeding:
•
The grade or commercial herds
•
Poor herds below average
•
Herds with one sire and where the owner has got no
knowledge about genetics.
Inbreeding is useful if we want to evolve a more uniform line in any animal species.
Out-breeding
Out-breeding is the breeding of unrelated animals and this involves the following
types of breeding:
1.
Out-crossing
2.
Cross breeding
3.
Species Hybridization
4.
Grading-up
1. Criss-crossing
2. Triple crossing
3. Back crossing
Out-Crossing
It consists in the practice of mating of unrelated purebred animals within the same
breed. The animals mated have no common ancestors on either side of their
pedigree up to 4 – 6 generations and the offspring of such a mating is known as the
out-cross.
Advantages
1.
2.
3.
This method is highly effective for characters that are largely under the
control of genes with additive effects, e.g., milk production, growth rate in beef
cattle, etc.
It is an effective system for genetic improvement if carefully combined with
selection.
It is the best method for most herds.
Cross breeding
It is the mating of animals of different breeds. It is generally used where the crossed
progeny is directly marketed and are not needed for breeding and further
multiplications. It has become quite common in pigs and in the production of hybrid
chickens. With beef cattle also it is practised to a certain extent. Cross breeding for
milk production has been tried with varying degrees of success. It is generally used
for the production of new breeds.
Cross breeding is carried out with two purposes:
1. For commercial production of market animal.
2. To obtain foundation animal for new breed.
Aim of Crossbreeding:
The aims of crossbreeding are to use the best sires available regardless of breed and
to gain hybrid vigour in the offspring and possibly more economical milk production.
1. To increase productivity over straight breeds because of the resulting hybrid
vigour or heterosis.
2. 2. Produce commercial animals with desired combination of trait not available
in any one breed.
3. Produce foundation stock for developing new breeds. It is generally used for
the production of new breeds.
Advantages of Crossbreeding
1.
It is valuable as a means of introducing desirable characters into a breed in
which they have not existed formerly.
2.
It serves a good purpose in evolving a new breed owing to the fact that it
disturbs the balance and brings about recombination in the germ plasm to
cause variations. Selection can then fix the favourable variations in the
population.
3.
It is an extremely handy tool to study the behaviour of characteristics in
hereditary transmission.
4. The crossbred animals usually exhibit an accelerated growth and vigour or
heterosis, which means the blending of desirable dominant genes from two
breeds in the first generation. Such animals are more thrifty than either of the
parents; they grow rapidly, produce more milk, wool, eggs, etc., than would be
expected from their pedigree. The productive traits usually show the greatest
improvement from cross breeding.
Disadvantages:
1. The breeding merit of crossbred animals may be slightly reduced because of
heterozygous native of their genetic composition.
2. Crossbreeding destroys combinations and breaksup established character in
strain or breed.
Methods of Cross- breeding
1. Criss-crossing.
When the two breeds are crossed alternatively, the method is known as crisscrossing. This method is proposed for utilising heterosis in both dams and progeny.
Breed A females are crossed with breed B sires. The crossbred females are mated
back to sires of breed A and so on. In this system the cross breeds soon come to
have about 2/3 of their inheritance from the breed of their immediate sire with 1/3
from the breed being used.
A
B
A 50
B50
A
A 75
B 25
B
A 37.5
B 62.5
A
A 86.5
B 31.5
2.
Rotational/Triple crossing:
In this system three breeds are crossed in a rotational manner. It is also known as
rotational crossing.
Three breeds are used in this system. The females of crosses are used on sire of
pure breeds in rotation. The crossbreds will son come to have 4/7 of inheritance of
the breed of the immediate sire, 2/7 from the breed of maternal grand-sire and 1/7 of
the hereditary material of the other pure breed.
A
A 50
B 50
B
C
A 25
B 25
C 50
A
A 62.5
B 12.5
C 25
B
A 31.25
B 56.25
C12.5
C
A 15.63
B 28.13
C 56.25
A
A 57.82
B 14.07
C 28.13
B
4.
Back Crossing.
A 28.91
B 57.04
C 14.07
Back crossing in mating of a crossbred animal back to one of the pure parent races,
which were used to produce it. It is commonly used in genetic studies, but not widely
used by breeders. When one of the parents possesses all or most of the received
traits, the back cross permits a surer analysis of the genetic situation than F1 does.
Annexe IIIc: Schematic presentation of back crossing of Jatsham with Siri Bull
Mithun (Bos frontalis) X Thrabum (Bos indicus)
Male Jatsha
Female Jatsham hybrid
Siri bull X Jatsham
Yangkum
Yangku
Siri bull X Yangkum
Doebum
Doeb (Male)
Siri bull X Doebum
Doethram
Doethra
Siri bull X Doethram
Datum
Data (Male)
Siri bull X Datum
Nublang
Thrabum
Species Hybridisation
By crossing two different species sometimes we get good virile individuals. The
mule is a good example of a commercially important species hybrid.
Jack (male ass)
x
Mare (female horse)
= Mule
Stallion (male horse)
x
Jannet (female ass)
= Hinny.
Zebra(male Zebra)
x
Mare(female horse)
=Zebroid
Cattle( male of cattle)
x
Yak(female)
= Zoo and Zoom
Mules are nearly always sterile as far as is yet known. Hinny is generally inferior
to mule as a work animal. It is also sterile.
Heterosis or Hybrid Vigour
Heterosis or Hybrid vigour is a phenomenon in which the crosses of unrelated
individuals often result in progeny with increased vigour much above their
parents. The progeny may be from the crossing of strains, breeds, varieties, or
species. One of the explanations for this increased vigour is that genes
favourable to reproduction are usually dominant over their opposites. As a
species or breed develops, it becomes homozygous for some dominant genes,
but some undesirable genes are also present at high frequencies. When one
breed is crossed with the other, one parent supplies a favourable dominant gene
to offset the recessive one supplied by the other and vice versa.
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e.g.
Breed
I
II
AA
Bb
CC
dd
aa
BB
CC
DD
Cross I X II
Aa
Bb
Cc
Dd
The offspring, therefore, has a larger number of loci with dominant genotypes
than does either parent and it likely to be more vigorous.
Another explanation for hybrid vigour is over-dominance, where the
heterozygous condition (Aa) is superior to either homozygous condition (aa or
AA)
Heterosis is employed to produce commercial stocks with high individual merit.
The successful exploitation of heterosis depends upon how superior the crosses
are over the pure bred, and the cost of replacement of pure bred stock. For these
reasons sit is more commonly practised in poultry, swine, and sheep among
which fertility is high and the cost of replacement of pure bred stock is likely to be
low. There is no advantage to cross breeding, however, if one parent line of
breed is better than the crossbred progeny
Grading Up
Grading is the practice of breeding sires of a given breed to non-descript females
and their offspring for generation after generation.
Explanation of Grading
It is evident from the above definition that grading up is the successive use of
purebred bulls of a certain breed of non-purebred herds. The continued use of
good purebred sires for only a few generations are all that are required to bring
the herd to the point at which it has all the appearance, actions and practical
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value of pure breeds. The following table shows the rapidity with which purebred
sires will change the genetic composition of the non-descript one.
Offspring
Per cent replaced
st
1 Generation
nd
2 Generation
rd
3 Generation
th
4 Generation
th
5 Generation
th
6 Generation
th
7 Generation
50
75
87.5
93.75
96.87
98.44
99.22
Per cent non-descript
50
25
12.5
6.25
3.13
1.56
0.78
From the above table, we can see that the offspring come closer to a 100 per
cent improved breed, as we go on grading. Grading is a process by which a few
purebred sires can rather quickly transform a non-descript population into a
group of pure breeds. The grading process does not create anything new but it
may transfer the good qualities of an improved breed.
For grading up programme it is advisable to use a breed that has thrived well
under local conditions. Otherwise graded animals may not adapt themselves to
the local environment.
Advantages of Grading
th
1.
Purebreds can be obtained just after a few generation (after 7 to 8th
generations).
2.
The start can be made with a little money in comparison to the purchase
of an entire herd of purebreds.
3.
It helps to prove the potentialities of the sire and adds to its market value
4.
It is a good start for new breeders who can slowly change over to pure
breed systems.
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Limitations of Grading.
1. Pure breeds are not always better than grad or country animals for the use
to be made of them
2. Pure breed stocks, which give good results in one set of environmental
conditions, do not always give favourable results in some different
environmental set-up. The purebred dairy cattle from temperate zones often
degenerate when used in tropical areas. Moreover, their offspring fails to
show vigour and constitution for high reproduction. To make grading
successful, the pure breed must have the ability to perform under the
environmental set-up where their offspring is going to perform.
New breed formation: The aim is to combine the high production potential of
exotic breeds with the adaptability of local breeds e.g trihybrid, tetrahybrid.
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