Quantitative Genetics Flashcards
What is quantative genetics about?
The Genetic Basis of Complex Inheritance
Complex inheritance
traits that have a genetic component that does not follow strict Mendelian inheritance (relationship between genotype and phenotype)
Instead it’s the quantity of phenotype that differs rather than the type
What are complex traits known as?
Quantitative traits as they are often measured
Examples of complex traits/inheritance
Adult weight, height, blood pressure, insulin resistance, milk yield, meat production, weight gain
What is genetics of quantitive traits often referred to as?
Polygenic
Polygenic meaning
More than 1 gene involved
– A single genotype can have…
– A single phenotype can result from…
Many phenotypes
Many genotypes
2 contributing factors to quantative traits=
Genotype- many genes
Environmental factors
Examples of environmental effects influencing genes involved in traits
-weight is influenced by prenatal and childhood nutrition
-congenital hip dysplasia is influenced by the position of legs in utero
Formula of quantitative traits
P=G+E
Phenotype = genetics plus environment
What is continuous distribution/variation
Constantly seeing distribution of a specific trait; its showing continuously with changing quantity of it
(Graph on anki)
What are discontinuous traits
Single gene disorders where animals is either affected or not; not a measureable trait
In what ways does the action of genes and the environment result in continuous variation?
- Additive gene action
Additive gene action
-what is it
-effect is…
-what does it result in?
Each allele at a given locus (location) has a specific value that it contributes to the phenotype
-effects are additive (cumulative)
-results in each gene having a slightly different quantitive value
In additive gene action, alleles contributing to a single phenoytpe results in…
Huge variation which gives rise to continuous distribution
What is additive variation responsible for?
Resemblance between parents and offspring- heritability
What are the 3 types of gene action in high genes that can interact in complex gene disorder?
- Additive gene action
- Dominance gene action
- Epistasis gene action
What is dominance gene action
-not always…
- one allele dominant over another; homozygous dominant will have same phenotype as heterozygous
-not always inherited
What is Epistasis gene action
Presence of one gene affects another gene
Describe an example of Epistasis gene action
Coat colour of dogs controlled by 2 genes;
Gene 1-> codes for coat colour
B allele= dominant (black)
b allele= recessive (brown)
Gene 2-> codes for expression of pigment
E allele= allows pigment expression
e allele= no pigment expressed
Mixes of these will give different variations of coat colour of dog eg BbEe=black, bbEe= brown, Bbee= white
Threshold traits
Threshold traits refers to traits that you would at first assume are discontinuous, however on a graph are depicted by increasing liability of getting the disease for example hip dysplasia or diabetes. Individuals with a liability above threshold develop the trait
(Graph on anki)
Heritability
-what is it
-2 types
the portion of the phenotypic variation VP due to
genotypic variation VG (genetics)
-broad sense heritability
-narrow sense heritability
Broad sense heritability
How much of genetics is responsible for the phenotype
H^2=Vg/Vp
H=heritability
Vg=genotypic variation
Vp=phenotypic variation
Narrow sense heritability
-formula
h^2=Va/Vp
(Va=additive variation)
If a trait has a large genetic contribution, then…
If a trait has a low genetic contribution, then…
-trait can be selected for
-more beneficial to alter environmental conditions
What is the heritability value scale?
0:1
0 means no differences in trait are due to genetic factors (no genetic factors)
1 means all differences in trait are due to genetic factors
3 steps to measuring heritability
-heritability for weight in sheep
- Measure the phenotypic variation (weight) of a large flock of sheep
- The sheep are then allowed to mate and when the lambs grow up, their weights are measure
- The offspring weights (phenotypic values) are plotted against the mid parent weights (mid-parent offspring regression analysis).
What are Quantitative Trait Loci (QTLs)
Stretches of DNA that contain genes that are linked to quantitative traits
What is genetic selection used for?
Improving livestock traits;
– milk production,
– rapid growth,
– leanness,
– high meat production,
– more eggs
– higher fecundity
What can genetic overselection do?
Can compromise animal welfare eg double muscling in cattle
Formula for genotypic aviation
VG=…
VG = VD + VI + Va
