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Stutts - ANSC 4395 Breeding & Genetics - Test 3 Exam
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Stutts - ANSC 4395 Breeding & Genetics - Test 3 Exam
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Stutts - ANSC 4395 Breeding &Genetics - Test 3 Exam
Simply-Inherited Traits - -traits affected by only a few genes
qualitative or categorical
affected little by environment
-Quantitative - -measured with numbers
-Polygenic Traits - -Affected by many genes with no gene having an
overriding influence.
- EX: Growth rate, milk production, ribeye area
Typically quantitative or continuous in expression.
- Phenotypes are usually described by numbers
Greatly influenced by environment
-Examples of Simply-Inherited Traits - -Coat color, horns
-Examples of Polygenic Traits - -Weaning weights, milk yield, REA... CANT
DO TEST MATINGS FOR THIS TYPE OF TRAIT
-Threshold Traits - -Polygenic traits that have categorical phenotypes
- Pregnancy, dystocia, gait
- Influenced by many genes but have an either/or result
-Which traits are more important? - -Polygenic traits, traits that determine
profitability and productivity
EX) Growth rate, fertility, milk production
-Niche markets - -markets more sensitive to simply-inherited traits
-Simply Inherited - -Genetic Defects
-Common characteristics of Polygenic and Simply Inherited - -1. Both are
subject to the same Mendelian Mechanisms: Law of Segregation and
Independent Assortment
2. Affected by dominance and epistasis
3. Principles of selection and mating apply to both
- Attempt to increase frequency of desirable alleles
(More difficult to do with polygenic traits)
-Function of the Number of genes involved - -- The more genes involved,
the more DIFFICULT it is to observe the effects of individual genes, and
therefore the less specific info we have about those genes
, - When a few or one gene affects a trait, the effects of those genes are well
understood (Exact genotype may not be known, but a probable genotype
may be identified.)
-Test Matings - -Matings designed to reveal the genotype of an individual
for a small number of loci.
- used on simply inherited traits
- Have to characterize the net effect (breeding values, expected progeny
differences)
-Why test matings cannot be done on polygenic traits - -Polygenic traits
have so many different genes affecting them
- Cannot observe the effects of specific genes
-An individual that is Aa will produce gametes - -- Half will contain A, other
half will contain a
- When populations are small, there may not be exactly half of each
-Three types of test matings to determine if an individual is heterozygous - -
1. Mating to recessive
2. Mating to heterozygous
3. Mating sire to daughters (last resort)
-Method with the highest probability of detection - -Mating to recessive
- Probability of siring a recessive offspring is 1/2
-If the condition is deleterious - -- May be hard to find homozygous
recessive
- Use known heterozygotes
- Probability of a recessive offspring is 1/4 or the probability of the offspring
showing the dominant phenotype is 3/4
-If heterozygous females are hard to find, then mate sire to daughters - --
Assume the daughter's dams do not carry the recessive gene
- Probability that a daughter carries the recessive gene is 1/2 if sire is
heterozygous
-The probability of offspring showing the recessive phenotype is 1/8 - -- 1/4
is probability of homozygous recessive from mating heterozygotes
- 1/2 comes from half of the daughters of a heterozygous male are expected
to carry the recessive gene
- Probability of dominant phenotype from this mating is 7/8
-The probability of a heterozygous male having all normal offspring ______
rapidly as the number of matings _______ - -diminishes ; increases
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