Download Answers: Chapter 13 – Genetic Change Through Selection (Thomas

Answers: Chapter 13 – Genetic Change Through Selection (Thomas Field)
Definitions for Key Terms
Quantitative Traits:
Genetic traits that can be objectively measured; the observations usually exist along a
continuum. Controlled by many gene pairs.
Qualitative Traits:
Genetic traits that that are descriptively or subjectively measured and would include hair color,
horned vs. polled, etc. Controlled by few, if not just one gene pair(s).
Phenotype:
Observation of measurement of each trait. Variation of phenotype exists within the trait due to
Genotype and Environment.
Genotype:
Result of both the cumulative effects of the animal’s individual genes for the trait and the effect
of the gene combinations.
Environmental Effects: Summation of all non-genetic influences
Breeding Value:
Total of all the independent genetic effects in an animal on a given trait in mathematical terms.
Responds to genetic selection.
Nonadditive Value:
Portion of genotype that is attributed to the gene combinations unique to a particular animal.
Genetic combinations are reestablished in each successive generation; therefore non-additive
value does not pass from generation to generation. Only accessed via choice of mating system
(cross breeding)
Known Effects:
Type of environmental effect that has an average effect on individuals in a specific category.
(Age, age of dam, gender). Calves born earlier in the calving season and thus older at time of
weaning typically weigh more than their younger counterparts.
Unknown Effects:
Type of environmental effect that is random in nature and specific to an individual phenotype.
Difficult to account for; but breeders can use management to minimize their impacts.
Heritability:
Primary method of making genetic improvement in traits controlled by many pairs of genes.
Describes the percent of total phenotypic variation (phenotypic differences) that is due to
breeding value. Also defined as that portion of the selection differential that is passed from
parent to offspring. Measure determined by studying populations ; is not specific to individuals.
May differ from breed to breed, and environment to environment.
Selection:
Differential reproduction – where some animals are prevented from reproducing, while allowing
other animal to become parents of numerous offspring. Factors of selection include: selection
differential, heritability, and generation interval.
Selection Differential: Also called Reach. Superiority (or inferiority) of the selected animals compared to the herd
average. (If the average weaning weight of selected replacement heifers is 480 lbs in a herd
averaging 440 lbs, then the selection differential for the heifers would be 40 lbs. Part of the 40 lb
difference is due to genetic differences, the remaining part is due to differences caused by the
environment)
Reach:
Another name for Selection Differential
Realized Heritability:
Portion of heritability actually obtained compared to what was attempted in selection.
Genetic Change:
Calculated by the following formula:
Genetic Change per year = Heritability x Selection Differential
Generation Interval
Generation Interval:
Average age of the parents when the offspring are born. Calculated by adding the average age
of all breeding females to the average age of all breeding males and dividing by two.
Approximately 2 years in swine, 3-4 years in dairy cattle, 5-6 years in beef cattle.
Sex-Limited Traits:
Selecting for a trait that is not expressed by that particular sex. Example: selecting for milk
production in dairy industry because bulls don’t express that trait.
Contemporaries:
Animals of similar breed type, raised in a similar environment, and of the same age
Tandem:
Selecting for more than one trait at a time. Effective if the situation calls for rapid change in a
single, highly heritable trait. Rather ineffective if selection is for more than 2 traits, or if the
desirable aspect of one trait is associated with the undesirable aspect of another trait.
Independent Culling
Level:
Selection Index:
Best Linear Unbiased
Prediction (BLUP):
Establishes minimum culling levels for each trait in the selection program. 2nd most effective
type of selection – but THE MOST Prevalent. Most useful when number of traits being
considered is small AND when only a small percentage of offspring is needed to replace the
parents.
Method that recognizes the value of multiple traits and places an economic weighting on the
traits of importance. Allows an overall ranking of the animals from best to worst – utilizing a
highly objective approach. Most effective system but the most difficult to develop.
Disadvantages include: shifts in economic value of traits over time and potential failure to
identify functional defects and weaknesses.
Allows field data to be utilized in computation of valid breeding values that could be used to
compare animals across herds. Used to create national sire or animal evaluation systems –
usually sponsored by Breed associations or large seedstock companies.
Review Questions
1. Phenotype = Genotype +
a. Phenotype = Genotype + Environment
2. Phenotypic variation that is due to differences in an animal’s genotype is called
a. Breeding Value
b. Non Additive Value
3. What is the term for superiority (or inferiority) of animals selected to be parents compared to the average of the
herd?
a. Selection Differential
4. The amount of genetic change made in a herd of animals during a year can be predicted by knowing what 3
variables?
a. Heritability
b. Selection Differential
c. Generation Interval
5. What is the term for the average age of parents when their offspring are born?
a. Generation Interval
6. What is a sex-limited trait?
a. Milk Production
7. Why are EPDs considered to be the most useful selection tools?
a. Because they are associated with performance data and help producers make profit-oriented decisions.
8. Which selection procedure involves selection for only one trait at a time and is least effective?
a. Tandem
9. Which selection procedure involved ranking each individual animal against all others in the herd for the traits
being selected and then obtaining a total score of the ranks for each animal, and is most effective?
a. Selection Index
10. What are the limitations to making selection decisions based on visual appeal?
a. Producers have no idea about the actual genetic potential of the animal
b. Producers can make measurable changes when using genetic selection tools – these changes cannot be
measured when only using visual appeal.
c. Visual appeal does not take into consideration genetic traits, the heritability of those traits, and how
improving those traits can improve profitability.