Qualitative traits are discrete and can be categorized. Quantitative traits are characters with continuously distributed phenotypes and are determined by the combined influence of the genotype at many different loci and the environment.

Describe Edward East’s experiments in quantitative genetics with Tobacco.  

He measured corolla length in tobacco plants. He started with opposite homozygous parents and then let the F1 self fertilize. Instead of a .25-.50-.25 ratio, he saw a continuum of phenotypes since many alleles are involved.

1) unless we breed and measure thousands of plants, the range of variation we will see in the F2s will not extend all the way to the original parental phenotypes.

2) With a few generations of selective breeding for short or tall corollas we should be able to recover the original parental phenotypes     

Even though the plants all had the same genotype, each plant was exposed to a different environment. Thus small phenotypic variation was incurred.

Quantitative trait loci – the loci that influence quantitative traits. QTL mapping uses marker loci to scan chromosomes and identify regions containing genes that contribute a quantitative trait.

M. lewisii is purple, has a landing pad, and is bee pollinated. M. cardinalis is red, tubular, and is hummingbird pollinated. Bradshaw wanted to know if QTLs had a strong or weak affect on phenotype.

Describe the differing viewpoints of Fisher and Orr.  Who was right? 

Fisher thought that alleles driven to fixation by natural selection were virtually all extremely subtle in their effects on phenotype. Orr thought that alleles fixed during adaptive evolution are obvious in their effect on phenotype. Not sure who was right.

Talk us through a QTL analysis of flower color in monkey flowers.  What is necessary and what do the results look like?

Begin by crossing completely homozygous parents for each species. Self cross the F1s to get an F2 and do a QTL map with marker loci for which each species was homozygous. Look for linkage disequilibrium between loci. It is necessary to be able to distinguish genotype by the phenotype.

Did Bradshaw’s results support Fisher or Orr’s theory of quantitative trait evolution?

Sometimes we know to evaluate a particular coding locus because we already know something about the function of its gene product and suspect that it may play a role in the phenotype. Other times we know to evaluate a particular coding locus because its location matches that of a QTL we have mapped using markers.

People with high novelty seeking scores are impulsive, active, and exploratory. There was reason to suspect that variation in novelty seeking is associated with allelic variation in the gene for the D4 dopamine receptor, D4DR. D4DR is a neurotransmitter receptor that participates in thoughts and emotions.

To find out whether the 3020insC allele can increase the risk of Crohn’s, they screened a large sample of disease patients and found 56 independent cases in which a patient had a parent who carried one copy of the 3020insC allele. If the allele plays no role in Crohn’s disease, then we expect the heterozygous parent to have transmitted the allele to the patient in half of the cases. However, the parent transmitted the allele in 39 cases and failed to transmit in 17.

Write a short equation that can partition phenotypic variation and modify this equation so that it represents heritability. 

Heritability = (genetic variation/phenotypic variation) = (genetic variation/ (genetic variation + environmental variation))

What is the slope of a midoffspring phenotype versus midparent phenotype regression line referred to as? 

Heritability. Narrow heritability?

No, the young’s beak size strongly resembled the biological parents. This is an important test to rule out the possibility of environment affecting phenotype.

Why are twin studies so useful for measuring heritability – but what drawbacks are there and how could you control these?  

If heritability is high, and variation among individuals is due mostly to variation in genes, then monozygotic twins will be more similar to each other than are dizygotic twins. If heritability is low and variation among individuals is due mostly to variation in environments, then monozygotic twins will be as different from each other as dizygotic twins. Data on heritability of traits is frequently interpreted.

S = the mean of the selected individuals – mean of the entire population

R = h²S

Predicted response to selection = (heritability)(selection differential)

Graphing the Midoffspring value (O – O*) vs. the Midparent value (P – P*) gives you a slope of R/S = h²

Directional selection, stabilizing selection, disruptive selection

1) Environment is always changing

2) a steady supply of new favorable mutations creates genetic variation for fitness-related traits. There is a steady supply of new deleterious alleles

3) Rare phenotypes may be selected for. Disruptive selection is more common than is generally recognized. (frequency dependent)

Describe the experiment of Lenski and Elena and describe which of the answers to question 20 their experiment supported?

6 pops of E. coli from an ancestral culture evolved for 10,000 generations. Mean fitness had increased by 50% (competition experiments) relative to the common ancestor. Most of the increased fitness occurred in the first few thousand gens.

Best explained by frequency dependent selection. Typical E. coli strain, when rare, did have a fitness higher than the source population.

Describe the famous experiments of Clausen Keck and Heisey and super impose the arguments of the “Bell Curve” for IQ on these experiments.   

Reared cuttings from Achillea plants in 2 different environments. In each environment, the microenvironments were the same. Thus, differences among them in height and maturation are due entirely to genetic variation. Both pops (Mather and Stanford) had heritability of 1, but there were still differences in height between pops.

The fact that heritability is high in each population tells us nothing about the cause of differences between the populations, because the populations were reared in different environments.

It allows us to predict whether selection on the trait will cause a population to evolve.