Chapter 11
Practice problems
11.1 Tarsus length in barn swallows is a polygenic trait. A population has an average length of
21 mm. The survivors of a severe winter storm from this population have an average tarsus
length of 25 mm. The average tarsus length in the progeny of the swallows that survived the
storm is 24 mm. What is the estimated narrow-sense heritability (HN) of tail length in this
population?
11.2 Long-term behavioral observations and cross-fostering (transfer of eggs among nests) were
used to determine whether antipredator behavior is inherited or taught by parents in alpine swifts
(Bize et al. 2012). The following two graphs show parent-offspring regressions between
antipredator behavioural score of cross-fostered offspring with their (a) biological midparents
(slope =0.326; Prob=0.0042) and (b) foster midparents (slope =−0.016; Prob=0.85).
(a) Why was a cross-fostering experiment used to answer this question?
(b) Do these results suggest antipredator behavior has a genetic basis or is it learned from
parents?
(c) Variance components were estimated using the animal model approach. The total
phenotypic variance VP for antipredator behavior score was 0.260, and the additive genetic
variance VA was 0.038. What is the estimated narrow-sense heritability (HN) for this trait?
1
11.3 The following two graphs show the relationship between parental and mean progeny
phenotypes for two different traits (body length and number of eggs produced) in pink from
Morgan Creek, Resurrection Bay, Alaska. Which trait has the smaller narrow-sense heritability
(HN) based on these relationships?
11.4 How much quantitative genetic variation would you expect to find in the Wollemi pine
(Example 4.2)?
11.5 The slope of the regression of progeny vertebrae number on maternal vertebrae number in
velvet belly sharks is 0.42 (Figure 2.6). What is the estimated narrow-sense heritability for this
trait?
11.6 We have seen that small population size will cause the loss of molecular genetic variation
in populations (Chapter 6). However, a number of studies have found that additive genetic
variation (HN) for traits associated with fitness sometimes increases following a population
bottleneck. Is this increase in variance expected to occur following all bottlenecks? What is the
genetic explanation for this observation? Will all bottlenecked populations have a greater
potential to adapt quickly than populations that have not been through a bottleneck?
2
Assignment problems
11.7 A common garden experiment was established with 498 loblolly pine tree genotypes that
had each been cloned (Queseda et al. 2010). Trees were inoculated with the fungal pathogen
causing the disease pitch canker, and trees were scored for their degree of infection.
(a) Below is a histogram showing the distribution of infection scores for the 500 clones. Is
resistance to this fungal pathogen single gene or polygenic?
(b) Each clone was genotyped for nearly 4000 SNPs for a genome-wide association study,
and ten SNPs were statistically associated with disease resistance. The broad sense
heritability of resistance HB was estimated to range from 30 to 40%. The 10 associated SNPs
together explained about 14% of the phenotypic variation, with individual SNPs explaining
0.29 to 3.83% of variation. Would more genetic gain in disease resistance be obtained
breeding loblolly pine for resistance to pitch canker if resistant phenotypes were selected as
parents, or if individuals were selected based on their SNP genotypes for one or all 10 of
these SNPs?
(c) Given the complex genetic architecture of this trait, would you expect strong selection on
phenotypes to quickly eliminate genetic variation for resistance to this pathogen in a
population? (See section 11.4.2).
3
11.8 Alan Robertson’s classic 1967 paper on the nature of quantitative genetic variation contains
the following quote from a colleague of his, “I can’t help feeling that those people who
introduced the notion of the selective advantage of a gene have completely confused the issue.
After all, natural selection acts on phenotypes.” Robertson states that this statement represents a
legitimate point of view and would make a very good subject for a student essay. Why do
geneticists insist on understanding the genetic basis for phenotypic differences associated with
differences in fitness? HINT: Section 11.2.
11.9 Conservation geneticists have long debated the merits of using selectively neutral genetic
markers versus quantitative phenotypic traits for understanding genetic variation and population
structure. What evolutionary forces do neutral markers provide key information about that
quantitative genetic approaches do not, and vice versa? Could genomic approaches using large
numbers of SNPs provide all relevant information with a single tool?
4