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Genetics and Breeding:

Konsep Dasar, Interval generasi

dan Respon Seleksi

Dr. G. Ciptadi

PENGERTIAN DASAR SELEKSI

DEFINISI SELEKSI :

1.Perbaikan mutu genetik ternak melalui perubahan rata-rata fenotip populasi

agar lebih menguntungkan secara ekonomis

2. Memilih ternak untuk bibit pada generasi berikutnya

CARA :

1. Eksploitasi variabilitas genetik/fenotip

Antar bangsa/populasi

Dalam bangsa /populasi

TAHAPAN:

1. Pemilihan/penentuan tujuan

2. 2. Pemilihan metode seleksi

TUJUAN :

Sebuah karakter/kombinasi beberapa karakter yang ingin diperbaiki

(tidak harus dapat diukur pada individu yang bersangkutan)

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DASAR SELEKSI: INFORMASI

PERFORMANS• Individual selection: selection of individuals based on their

own phenotypic values (also called mass selection).

• Family selection: whole families are selected or rejected based according to the phenotypic mean of the family (we select the family with the largest mean). This method is preferred when the heritability of the trait is low. But the efficacy of this method is reduced when the environmental variation common to the members of the same family is large.

• Sib-selection and progeny testing are family-selection methods.

• Within-family selection: the best individual in each family is selected. The main case in which this method has an advantage is when there is a large common environmental variance.

Seleksi berdasar Sumber Informasi:

1. Seleksi individu: catatan individu ybs untuk evaluasi ybs

- pada sapi perah/potong : kurang tepat meramal produksi

(perlu h2 tinggi untuk seleksi efektif)

- estimasi produksi individu : dasar NILAI PEMULIAAN (NP)

NP : h2 ((Performan Indv- Rata-rata Performans Pop)

2. Seleksi pedegree (silsilah):

dasar catatan tetua/nenek moyang

dipengaruhi derajat inbreeding (jauh dekat kekerabatannya

3. Uji Kolateral:

Berdasarkan catatan anggota keluarga

Ketepatan tergantung h2, Ripitabilitas.

4Uji Progeni: - . pada pejantan dan informasi prod anak

Dasar : anak adalah contoh acak genotip orang tua

Contoh : sapi 4.5 – 5.0 tahun.

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Selection Response

• Selection response is the expected rate genetic change (genetic progress or

improvement) which results from selection.

• The effectiveness of selection is measured by the rate of genetic change

achieved.

• Selection intensity (i) is the selection differential

expressed in standard deviations.

• Selection differential (S): the difference between the

mean selection criterion of selected animals and the

mean selection criterion of all potential parents.

• Selection criterion (SC): the information on which we

base selection (breeding value, phenotypic value, etc).

6Point of truncation

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Selection intensity increases as the proportion selected is

reduced

8

9

Per year

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With phenotypic selection (individual selection or

mass selection) the formula for selection response

can be expressed as:

L

Sh

L

ih

L

hiG PA

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Note that with phenotypic selection, accuracy = h

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Generation Interval

• Generation interval is the amount of time

required to replace one generation with the next.

• More practical definition: the average age of

parents when their offspring are born.

• (rataan umur tetua pada saat anak-anak lahir,

kelahiran 1,2,3, dst)

• As the generation interval increases the rate of

genetic change is reduced.

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Average generation interval

Species Male Female

Cattle 3 to 4 4.5 to 6

Sheep 2 to 3 4 to 4.5

Swine 1.5 to 2 1.5 to 3

Horses 8 to 12 8 to 12

Chickens 1 to 1.5 1 to 1.5 .

lahir kawin beranak

1 2

12 bln 1 bln

9 bln

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Selection intensity, generation interval and the additive variance may

differ between males and females.

This should be accounted for in the equation for selection response

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EXAMPLE 1: Suppose we select for six-month weight in lambs based

on phenotypic values. If the average of all candidates for selection is 32

kg, the phenotypic standard deviation is 4 kg, the generation interval in

sheep is three years and the heritability of six-month weight is 0.40,

suppose also that the average of selected parents is 42 kg. Find:

1. The intensity of selection

2. The expected selection response per year.

Solution:

yearkgL

ShG

Si

P

/33.13

)3242(40.0

5.24

)3242(

2

This corresponds to an improvement at a rate 4.16% per year

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EXAMPLE 2: Phenotypic selection for yearling weight in beef cattle.

Bulls: keep the top 3%

Cows: keep the top 50%

Suppose heritability is 0.40, generation interval is 5 years for bulls and 6.5 years

for cows and the additive genetic variance is 1225 in both sexes. Find the

expected selection response per year.

yearlbG

haccuracy

/9.55.65

)35)(80.0(63.0)35)(27.2(63.0

63.040.02

i = 2.27 for males and 0.80 for females

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Example: suppose we are selecting dairy bulls for improving

milk yield using progeny testing and we are using the top 5% of

bulls with average accuracy of 0.90. What is the expected genetic

change in fat% given that the genetic correlation between milk

yield and fat% is –0.50, the heritability of fat% is 0.50 and the

phenotypic variance for fat% is 0.80. Suppose also that the

generation interval is 6 years.

yearG

Lhi

XY

YYAx Y

/%098.06

)632.0)(06.2)(90.0)(50.0(

6,632.0)80.0)(50.0(,06.2

/

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Selection for multiple traits

• So far we have dealt with selection for a single

traits. However, in practice we may want to

improve several traits.

• There are three methods of selection for multiple

traits:

1. Tandem selection

2. Independent culling levels

3. Economic selection index

METODE SELEKSI:

1. Tandem : secara berurutan

cara : 1 sifat selesai diteruskan sifat kedua

waktu yang diperlukan panjang

untuk sifat yang tidak berkorelasi positif

2. Independen Culling Level:

ditentukan batas produksi tertentu, jika kurang dari batas minimal

maka dilakukan culling/afkir

kekurangan: tidak ada kompensasi untuk sifat-2 yang unggul

3. Indeks : metode terbaik

menghitung indeks beberapa sifat yang diseleksi

perlu banyak data

Hasil : berupa NILAI SKOR/INDEKS, yang digunakan untuk RANKING

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Tandem Selection

• In this method we begin by selection for one trait,

after several years we start selecting for the second

trait and so forth until the desired level of

improvement is reached .

• This method is not efficient:

- it takes long time

- if unfavorable genetic correlations exist between

selected traits, selection for one will negatively affect

one or more of the other traits.

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Independent culling levels

• A standard (a culling level) is set for each trait.

• Selected animals should meet the standard for each

trait.

• Any animal which does not meet the standard of any

one of the traits is culled even if it is excellent for one

or more of the traits.

• Faster than the tandem method, but it is very strict

(may cause culling of excellent animals for one trait

which do not meet the standards for other traits).

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Selection Index

• Combines information on several traits in a

single index using weighting factors.

nnPbPbPbI .............2211

Where :

- b1, …………,bn are weighting factors (partial regression coefficients)

- P1: information on trait 1

.

.

.

- Pn: information on trait n

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Advantages

• More efficient than tandem selection and independent

culling methods.

• It takes into account the genetic correlations between

traits.

• It takes into account the economic value for each

trait.

• An animal’s excellence in one trait can compensate

for its weakness in another trait.

Kemajuan Genetik Karena seleksi

berdasarkan nilai intensitas seleksi

Contoh: Sapi

Populasi sapi lahir/thn = 8000 ekor, sex rasio 1 : 1

Jika diseleksi (kawin alam ) = 3000 pejantan

Proporsi terseleksi = 3000/4000 = 0,7

Intensitas seleksi (i =tabel) = 0.42

Atau (program breeding), diseleksi 1000 pejantan

Proporsi terseleksi = 1000/4000 = 0.25

Intensitas seleksi (i =tabel) = 1. 271

Jika dg IB, misalnya hanya perlu 4 pejantan

Proporsi terseleksi = 4/4000 = 0.001

Intensitas seleksi ( i =tabel) = 3.37

Respon Seleksi (R) = i.h2 .DS

Progres genetik :Hubungan Seleksi dengan nilai H

dan h2

Reproduksi Pertumbuhan Karkas

1. h2 (heritabilitas) Jelek (0.0 –

0.15)

Sedang (0,2 –

0.4)

Tinggi (0.5 –

0.70)

Progres genetik

(seleksi)

Rendah Sedang Tinggi

2. H (efek heterosis) Tinggi ( 10 –

20 %)

Sedang ( 5 – 8

%)

0 %

Perbaikan genetik

(persilangan)

Tinggi Sedang 0

Catatan:

1. Progress genetik =h2 x DS

2. Respon seleksi = R = ih2DS

3. Interval Generasi (I): Sex, Spesies berbeda