Chapter 5: Linkage, Recombination, and the Mapping of Genes on Chromosomes Learning Objectives

Question 1

o Explain how a testcross can provide evidence for or against linkage

Answer 1

	Testcrosses simplify detection of linkage because each phenotypic class of progeny corresponds to each gamete type produced by dihybrid parent
	Autosomal genes can also exhibit linkage

Question 2

o Establish relative gene positions using two-point cross data

Answer 2

 Two-point crosses can establish order of linked genes and distances between them through pairwise analysis of recombination frequencies
 Limitations of two-point crosses- distances measured between genes can be inaccurate

Question 3

o Explain why the value of the recombination frequency between any two genes is limited to 50%

Answer 3

 If two genes are far apart, crossover and non-crossover events will occur at equal frequency (50%)
 Genes that are closer together undergo fewer crossing over events and non-crossover gametes will exceed than the number of crossover gametes

Question 4

o Refine genetic maps based on data from three-point testcrosses

Answer 4

 Three-point testcrosses can refine map distances and reveal existence of crossover interference; more accurate

Question 5

o Discuss the relationship between the recombination frequency and the map distance separating two loci on a chromosome

Answer 5

 Recombination frequency reflects distance between two gene

 One map unit distance on a chromosome is equal to 1% recombination

Question 6

o Discuss the concept of the null hypothesis and its use in data analysis

Answer 6

 Null hypothesis: model that leads to a discrete numerical prediction
 Probability value (p) measures the likelihood that deviations from the predicted values have occurred by chance alone; the null hypothesis is rejected when p < 0.05

Question 7

o Evaluate the significance of experimental data based on the chi-square test

Answer 7

 Probability value (p) measures the likelihood that deviations from the predicted values have occurred by chance alone; the null hypothesis is rejected when p < 0.05

Question 8

o Explain how a genetic map (in map units) is related to actual physical distance (in base pairs of DNA)

Answer 8

 As two genes become further apart, the RF value increasingly underestimates the actual crossover frequency
 Recombination hotspots: small DNA regions where frequency of recombination is much higher than average

Question 9

o Define linkage with respect to gene loci and chromosomes

Answer 9

 Linked genes are usually located close together on same chromosome

Question 10

o Explain the purpose of the chi-square test

Answer 10

 Chi-square test helps determine whether two genes are linked by comparing differences between numbers of progeny of different classes observed in an experiment and numbers of progeny of these classes expected from the null hypothesis that genes are unlinked and thus assort independently

Question 11

o Describe the relationship between linkage groups and chromosomes

Answer 11

 Genes in a linkage group are by definition syntenic, with enough mapped genes, the entire chromosome becomes a single linkage group

Question 12

o Describe the role of chiasmata in chromosome segregation during meiosis

Answer 12

 Point where two homologous non-sister chromatids exchange genetic material during chromosomal crossover

Question 13

o Explain the physical process by which recombination takes place

Answer 13

 Mixing of information contained on two homologous chromosomes
 Homologous pairs of chromosomes line up side by side and exchange parts; occurs during prophase I

Question 14

o Conclude from ratios of progeny in a dihybrid cross whether two genes are linked

Answer 14

 9:3:3:1 ratio is altered when genes are linked

 Dihybrids produce more parental gametes than recombinant gametes

Question 15

o Differentiate between parental and recombinant gametes

Answer 15

 Parental: combinations of alleles present in original parental generation
 Recombination: reshuffled combination of alleles that weren’t present in parental generation; result of crossing-over during meiosis