Konsep Dasar, Interval generasi dan Respon Seleksi

Transkripsi

1 Genetics and Breeding: Konsep Dasar, Interval generasi dan Respon Seleksi Dr. G. Ciptadi 1

2 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 Pemilihan metode seleksi TUJUAN : Sebuah karakter/kombinasi beberapa karakter yang ingin diperbaiki (tidak harus dapat diukur pada individu yang bersangkutan)

3 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. 3

4 Seleksi berdasar Sumber Informasi: 1. Seleksi individu: catatan individu ybs untuk evaluasi ybs - pada sapi perah/potong : kurang tepat meramal produksi (perlu h 2 tinggi untuk seleksi efektif) - estimasi produksi individu : dasar NILAI PEMULIAAN (NP) NP : h 2 ( (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 tahun.

5 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). 5

6 Point of truncation 6

7 Selection intensity increases as the proportion selected is reduced 7

8 8

9 Per year 9

10 With phenotypic selection (individual selection or mass selection) the formula for selection response can be expressed as: G hi A h 2 i P h 2 S L L L Note that with phenotypic selection, accuracy = h 10

11 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. 11

12 Average generation interval Species Male Female Cattle 3 to to 6 Sheep 2 to 3 4 to 4.5 Swine 1.5 to to 3 Horses 8 to 12 8 to 12 Chickens 1 to to bln 12 bln 1 bln 1 2 lahir kawin beranak 12

13 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 13

14 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: S i P 2 h S G L (42 32) (42 32) kg/ year This corresponds to an improvement at a rate 4.16% per year 14

15 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. i = 2.27 for males and 0.80 for females accuracy h G 0.63(2.27)(35) 0.63(0.80)(35) lb / year 15

16 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 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 Suppose also that the generation interval is 6 years. i x G 2.06, Y / X A Y h 2 Y 2 Y (0.50)(0.80) 0.632, ( 0.50)(0.90)(2.06)(0.632) 0.098% / 6 L year 6 16

17 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 17

18 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

19 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. 19

20 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). 20

21 Selection Index Combines information on several traits in a single index using weighting factors. I b P b P b n P n Where : - b 1,,bn are weighting factors (partial regression coefficients) - P 1 : information on trait P n : information on trait n 21

22 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. 22

23 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) = Jika dg IB, misalnya hanya perlu 4 pejantan Proporsi terseleksi = 4/4000 = Intensitas seleksi ( i =tabel) = 3.37 Respon Seleksi (R) = i.h 2.DS

24 Progres genetik :Hubungan Seleksi dengan nilai H dan h 2 Reproduksi Pertumbuhan Karkas 1. h 2 (heritabilitas) Jelek ( ) Sedang (0,2 0.4) Tinggi ( ) Progres genetik (seleksi) Rendah Sedang Tinggi 2. H (efek heterosis) Tinggi ( %) Sedang ( 5 8 %) 0 % Perbaikan genetik (persilangan) Tinggi Sedang 0 Catatan: 1. Progress genetik =h 2 x DS 2. Respon seleksi = R = ih2ds 3. Interval Generasi (I): Sex, Spesies berbeda