Scientific Farm Animal Production

Biology

Quiz 13 :

Genetic Change Through Selection

Quiz 13 :

Genetic Change Through Selection

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Demonstrate the calculation of the number of gametes and genotypes arising from various numbers of heterozygous gene pairs.
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The genomes of mammals are composed of above 30,000 to 40,000 genes. The economically valuable traits in farm animals include milk production, egg production and growth rate and carcass composition. These characteristics are controlled by huge number of gene pairs and it is necessary to know the traits are inherited involving one or two pairs of genes.
Consider an example of 20 pairs of heterozygous genes (1 gene pair on each pair of 20 chromosomes) affects yearling weight in sheep, cattle or horses. The estimated number of genetically different gametes is given below.
img Each pair of heterozygous genes results in three different genetic combinations like AA, Aa, and aa. Two pairs of heterozygous genes result in 9 different genetic combinations.

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Demonstrate the calculation of 205-day adjusted weaning weights.
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Environmental effects or influences are removed by the use of adjusted records. For example, the weaning weight can be adjusted according to the age of calf and the age of dam. The formula used for the computation of adjusted weaning weight or 205 day weight for the beef cattle is given below:
img img Adjustments used:
img Adjusted weaning weight for bull A:
img Adjusted weaning weight for bull B:
img The interpretation describes about bulls A and B weaned at 205 days of age and the dams had been matured cows between the ages 5 and 10 then the weights would be estimated as
img respectively.

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Explain the importance of selection differential, generation interval, and heritability to generating change in a specific trait.
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Selection differential is sometimes knows as reach and it is the superiority of the selected animals compared to herd average. To improve the weaning weight of the beef cattle, Heifers and bulls are selected that have the weaning weight above the herd average. For example, the average weaning weight of a selected replacement Heifer is 480lb in a herd averaging 440lb. The selection differential for Heifers would be 40lb and this is due to genetic differences and differences caused by the environment.
Generation interval is the average age of parents during the birth of offspring. It is calculated by adding the average age of all breeding females to the average age of all breeding males and divided by 2.
img The generation interval is approximately two years in swine, 3 to 4 years in dairy cattle and 5 to 6 years in beef cattle. In poultry industry rapid speed of genetic change is evaluated for rapid generation turnover.
Heritability is the estimate of percent of total phenotypic variation caused due to breeding value. Heritability is also defined as part of selection differential inherited from parent to offspring. The heritability is said to be high and the performance of progeny for a trait is a good estimate than the performance of the parent. Realized heritability is the part actually obtained and compared to that was attempted in selection.

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Describe the importance of the development of breeds to the livestock industry.
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What are the forces that drive selection?
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Use the bell curve to describe genetic variation.
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Discuss examples of change in livestock species resulting from selection.
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Compare the role of elite seedstock, multiplier seedstock, and commercial breeders.
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Demonstrate the genetic change formula.
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Discuss the advantage of using quantitative approaches to selection such as EPDs.
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Compare the heritability of various traits.
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Compare the various selection methods.
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Explain the impact of simultaneous selection for multiple traits on the speed of genetic change.
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Describe the genetic model and its components.
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