Lesson 11 Quantitative Genetics Flashcards
Discontinuous traits
Traits that fall into discrete categories
Quantitative traits
Show a continuum of phenotypic variation within a group of individuals
variance
Sum of squared deviations from the mean divided by the degrees of freedom (N-1)
Vx = sum(x -meanX)^2/N-1
What does variance help us understand?
Genetic variation in a population
Standard deviation
square root of variance. Helps up appreciate the amount of variation in a bell curve.
correlation
statistic that can compare the degree of association between two variables
Polygenic inheritance
Transmission of traits that are governed by two or more different genes
Quantitative trait loci (QTLs)
Locations on chromosomes where the genes reside that influence quantitative traits
Experiment 24A: polygenic inheritance
hypothesis
DDT resistance is a polygenic trait
Experiment 24A: polygenic inheritance
Starting materials
A DDT resistant and a DDT sensitive strain of fruit flies
Experiment 24A: polygenic inheritance
protocol
- Cross DDT-resistant strain to the sensitive strain. In each strain, chromosomes X, 2 and 3 were marked with alleles that provided easily discernible phenotypes.
- Take the F1 flies and backcross to both parental strains
- Identify the origin of the chromosomes in each fly according to their phenotypes
- Expose each fly to DDT on a filter paper for 18-24 hours
- Record number of survivors
Experiment 24A: polygenic inheritance
Interpreting the data
Some offspring obtained all three alleles from one parental strain, but most had a combination. The more from the DDT resistant strain, the more resistant. One allele alone did not offer maximal resistance.
What is used to map QTLs?
RFLPs and other molecular markers
What is the basis of QTL detection?
The association between genetically determined phenotypes and molecular markers such RFLPs
RFLP mapping strategy
Mate individuals that are very dissimilar with regard to their RFLP markers and are also different for quantitative traits. The experimenter then determines which RFLPs are segregating with the trait of interest.
Heritability
Amount of phenotypic variation within a group of individuals that is due to genetic factors
Hb^2 = Vg/Vt
Heritability squared = genetic variance/total variance
What is phenotypic variance due to?
Additive effects of genetic variance and environmental variance
True heritability in the broad sense
Hb^2 = Vg/Vt
Takes into account all genetic factors that could affect the phenotype
What do geneticists divide Vg into?
Vg = Va + Vd + Vi A = variance due to additive alleles D = variance due to alleles that follow a dominant/recessive pattern of inheritance I = variance due to genes that interact in an epistatic manner
What variance do geneticists focus on the most?
Va
Narrow-sense heritability
Heritability of a trait due to the additive effects of alleles
hN^2 = Robs/Rexp
Robs is observed phenotypic correlation bet. Related individuals and Rexp is the expected correlation based on the known genetic relationship
Experiment 24B: human fingerprints
hypothesis
Dermal ridge count has a genetic component. The goal of this experiment is to determine the contribution of genetics in the variation of dermal ridge counts.
Experiment 24B: human fingerprints
Starting materials
A group of human subjects from Great Britain
Experiment 24B: human fingerprints
protocol
- Take a person’s finger and blot it onto an inkpad
- Roll the person’s finger onto a recording surface to obtain a print
- With a low-power binocular microscope, count the number of ridges using the Bonnevie method
- Calculate the correlation coefficients between different pairs of individuals
Experiment 24B: human fingerprints
Interpreting the data
Genetically unrelated individuals have a negligible correlation for this trait. Individuals who are genetically related have a substantially higher correlation.
triradius
Triple junction in fingerprint
arch
No triradius
whorl
Two triradii
Bonnevie’s method of counting
A line is drawn from a triradius to the center of the fingerprint. The ridges that touch this line are counted. The triradius ridge and the last ridge are not counted if the latter forms the center of fingerprint.
When are heritability values relevant?
To particular groups raised in a particular environment
Selective breeding
A practice in which a breeder chooses the individuals who possess traits that are desirable- from a human perspective - as parents of the next generation.
Realized heritability
Out of an artificial selection experiment can be used to estimate heritability in the starting population hN^2 = R/S hN^2 is realized heritability R is response in the offspring S is selection differential in parents
Response in offspring
R = meanXoffspring - mean of starting population
Selection differential
S = mean of parents - mean of the starting population
heterosis or hybrid vigor
When two different inbred strains are crossed to each other, the resulting offspring are more vigorous than either of the inbred parental strains
Two hypothesis to explain heterosis
- Dominance hypothesis
2. overdominance hypothesis
Dominance hypothesis
Highly inbred strains have become homozygous fro one or more recessive genes that are somewhat deleterious (but not lethal). Because homozygosity happens as a matter of chance, two different inbred strains are likely to be homozygously recessive for different genes. When they are crossed with each other, the resulting hybrids are heterozygous and do not suffer the overdominance hypothesis.
overdominance hypothesis
Proposes that heterosis is explained by the resulting hybrids being heterozygous for one or more genes that display overdominance
