AB&G - End-of-year Exam Revision Flashcards
What is the equation for the annual response to selection?
R = phenotypic SD * selection intensity * heritability / generation interval
How would you calculate the correlated response on trait 2 by selecting on trait 1? How would you adapt that if you were asked to use DNA tests as the correlated trait (e.g. the DNA test accounts for 50% of the genetic variation)?
CR2,1 = genetic variation * selection intensity (trait 1) * phenotypic SD (trait 2) * sqrt(heritability of trait 1 * heritability of trait 2) / generation interval
Using DNA test:
- square root the genetic variation (sqrt(0.5))
- imagine a linear regression equation (r^2)
- r^2 is the correlation (the key is that its SQUARED)
- the h^2 (heritability) of a DNA test is 1
How do you calculate the generation interval?
Lf = (#1y.o females 1) + (#2y.o females2) + (#3y.o females *3), etc / total number of females
Same for males
Average the two
What units is the generation interval in?
Years
What units is the response to selection in?
trait units /year
What is the equation for effective population size?
Ne = (4sd)/(s+d)
What is the equation for the change in inbreeding coefficient per generation?
Delta F = 1/(2*Ne)
How do you calculate selection intensity?
First calculate the proportion of progeny required as replacements (separately for male and female).
Find the corresponding selection intensity from the table.
Average the selection intensities
What are the 3 methods for multiple trait selection?
—–TANDEM SELECTION:
- selecting for one trait at a time, ignoring all others
- order the traits in terms of importance (value) and make selections based on the most important trait for a number of years
- Then the next important trait becomes the basis for selection, and so on.
> Advantages
- Easy
- Can make the maximum genetic improvement in 1 trait
> Disadvantages
- Can only focus on one trait at a time
- If traits are negatively correlated, can be taking 2 steps forward and one (or more) steps backward
—–INDEPENDENT CULLING
- pretty much happens in all breeding programs to some degree
- A proportion of animals are culled based on individual traits alone
> Advantages:
- Culling can follow the life cycle of the animal; e.g. culling at weaning, yearling, etc.
> Disadvantages:
- Its difficult to estimate the response to selection and how much pressure to put on each trait
- —-INDEX SELECTION
- individuals are ranked based on numerous traits that are weighted on economic value
- selection is of those with the highest ranking
- Advantages
- captures the economic value of multiple traits, meaning animals can be selected based on their overall economic superiority
- Complicated
What is the difference between heritability and heterosis
- —-HERITABILITY
- The proportion of phenotypic superiority of the parents that is seen in the progeny
- a function of additive genetic effects
h^2 = Va/Vp
= addative variance / phenotypic variance
- —-HETEROSIS
- The increased performance of CROSSBRED progeny above the mean performance of the parents
- a function of non-additive genetic effects (i.e. dominance and epistasis)
= (crossbred mean performance - parent breed mean performance) / (parent breed mean performance) * 100
- crossbreds often don’t out-perform parents on individual traits, but often do out-perform them on total productivity; e.g. Angus x Hereford, the progeny won’t have better muscling than purebred Hereford
- Heterosis is reduced in subsequent generations (F2 = 25%:50%:25% heterosis)
Compare a single terminal cross and three-way cross
- —-SINGLE TERMINAL CROSS
- e.g. (existing) purebred Jersey cows x Angus bull (terminal sire) —-> Market F1 progeny
> Advantage: individual heterosis completely utilised
Disadvantage: no utilisation of maternal or paternal heterosis
- Best to use a female with superior material qualities and mate them to sires with superior growth and carcass qualities (complementarity)
- —-THREE-WAY CROSS
- The same as the single terminal cross, but instead of the F1 progeny going to market, they’re mated with another (the terminal) breed, then those progeny are marketed
e. g. existing purebred Jersey cow x Angus —-> F1 progeny x Wagyu bull —–> Market progeny
> Advantages:
- all individual heterosis utilised
- maternal heterosis from crossbred female utilised
- complementarity
> Disadvantages:
- having to maintain another (crossbred) herd
- purchasing female F1 replacements
- complex management
- —–FOUR-WAY CROSS
- Often occurs in the poultry industry
Purebred A x Purebred B —–> F1a
Purebred C x Purebred D —–> F1b
F1a x F1b —–> hybrid commercial production
> Pros:
- Utilises individual, maternal, and paternal heterosis
- Utilises complementarity
Cons:
- Only useful in production systems that have a high reproductive rate (short generation interval) because of the high management costs (of keeping 4 flocks for 1 lot of progeny)
What is the equation for calculating the amount of retained heterosis in a rotational cross?
(2^n - 2)/(2^n - 1)
where n = # breeds
Explain the equations:
alpha = a + d * (q - p)
and
2/A = 2pqomega^2A
alpha = the average effect of allele substitution
a = the additive effect of each allele in the trait of interest
d = the dominance effect of each allele in the trait of interest
q = frequency of minor allele
p = frequency of major allele
omega^2(sub)A = the additive genetic variance due to a major gene
If the trait has been lost or fixed in the population, i.e. q and p are close to 1, there will be little-to-no phenotypic variation in the trait of interest due to additive or dominant genetic effects. Therefore, any variation would be due to epistatic or environment interactions. However, as p and q approach 0.5, phenotypic variation increases until p=q and variation is maximised. This allelic frequency yields the greatest proportion of heterozygotes, while still displaying homozygous individuals for both alleles.
How would you calculate the accuracy of a genomic test if you were given the amount of genetic variation it accounted for?
square root it
e.g. test acc. for 49% of variation
accuracy = sqrt(0.49)
= 0.7
What traits respond well to crossbreeding?
Lowly heritable traits
e.g. fitness type traits (e.g. reproductive rate)
What is the equation for calculating the amount of retained heterosis in a composite cross?
(n-1)/n
n = number of breeds
Describe a rotational crossbreeding system and why it might be better than a single crossing system
> Breed A males are mated to Breed B females
Their progeny are mated to Breed B males
Their progeny are mated to Breed A males
Their progeny are mated to Breed B males, etc
After ~ 7 generations, the genetic composition reaches an equilibrium where the progeny of Breed A sires will be 2/3A, 1//3 B, and the progeny of Breed B sires will be 2/3B, 1/3A
The retained heterosis is 2/3 of individual and maternal
In contrast, single crossing systems utilise 100% of the individual heterosis, but none of the maternal heterosis, where a significant proportion of the potential gains lie.
ANIMAL GENETIC RESOURCES
Describe the desired genetic structure of small populations (5 features)
> Ne ≥ 50 per generation
delta F Constant Ne (avoid bottleneck)
Long generation interval (to minimize homozygosity/genetic drift
Spatially separated (but genetically linked) populations
- decr. risk of being wiped out by disease, natural disaster, etc
ANIMAL GENETIC RESOURCES
Name the 5 “The big five” most important species in animal production and give their chromosome numbers
Pigs - 38 Sheep - 54 Cattle - 60 Goats - 60 Chickens - 78
ANIMAL GENETIC RESOURCES
Explain ‘genetic bottleneck’ and ‘genetic meltdown’
- —-GENETIC BOTTLENECK
- A temporal (or temporary) severe reduction in effective population size
- Even though population sizes can recover after genetic bottlenecks, the genetic variation remains the same for 100s or 1000s of years
- —-GENETIC MELTDOWN
- The accumulation of deleterious mutations that result in the inability to reproduce and survive
ANIMAL GENETIC RESOURCES
Name 4 different ways of germplasm conservation
> in situ (where it developed)
ex situ (in a different environment)
in vivo (i.e. live animals)
in vitro (i.e. frozen gametes, embryos, etc)
ANIMAL GENETIC RESOURCES
List the 4 objectives of the global strategy for Domestic Animal Diversity (DAD)
- Document existing animal genetic resources
- Develop and improve their use in agriculture
- Maintain those not currently of interest
- Facilitate access to important resources
How do you convince someone that 3 is worth doing when they don’t benefit from it but it costs them money? Very difficult!
