1. Chromosome Mapping in Eukaryotes
PowerPoint® Lecture Presentation for Concepts of Genetics Ninth Edition Klug, Cummings, Spencer, Palladino
Chapter 5
Chromosome Mapping in Eukaryotes
Lectures by David Kass with contributions from John C. Osterman.
Copyright © 2009 Pearson Education, Inc.

2. Section 5.1
Genes assort independently if they are on different chromosomes but show linkage if they are on the same chromosome.

3. Section 5.1
In complete linkage, only parental (noncrossover) gametes are produced.
If crossing over between two linked genes occurs between two nonsister chromatids, both parental and recombinant (crossover) gametes are produced (Figure 5.1).

4. Figure 5.1

5. Section 5.1
The degree of crossing over between any two loci on a single chromosome is proportional to the distance between them, known as the interlocus distance.

6. Section 5.1
If complete linkage exists between two genes because of their close proximity and organisms heterozygous at both loci are mated, a unique F2 phenotypic ratio designated the linkage ratio results.

7. Section 5.1 Genes on the same chromosome are part of a linkage group.
The number of linkage groups should correspond to the haploid number of chromosomes.

8. Section 5.2
The percentage of offspring resulting from recombinant gametes depends on the distance between the two genes on the chromosome.
Alfred H. Sturtevant.
Thomas H. Morgan.

9. Section 5.2
Synapsed chromosomes in meiosis wrap around each other to create chiasmata that are points of genetic exchange.

10. Section 5.2
Two genes located relatively close to each other along a chromosome are less likely to have a chiasma form between them, and it is less likely that crossing over will occur.

11. Section 5.2
The recombination frequencies between linked genes are additive, and the frequency of exchange is an estimate of the relative distance between two genes along the chromosome.

12. Section 5.2
One map unit (mu) is defined as 1 percent recombination between two genes on a chromosome.
Map units are often called centimorgans (cM) and are relative distances, not exact ones.

13. Section 5.2
A single crossover (SCO) alters linkage between two genes only if the crossover occurs between those two genes (Figure 5.5).

14. Figure 5.5

15. Figure 5.6

16. Section 5.3
Single crossovers can be used to determine the distance between two linked genes, but double crossovers (DCOs) can be used to determine the order of three genes on the chromosome and the distance between the linked genes.

17. Section 5.3
To study double exchanges, three pairs of genes must be investigated, each heterozygous for two alleles (Figure 5.7).

18. Section 5.3
The expected frequency of double-crossover gametes is much lower than that of either single-crossover gamete class.

19. Section 5.3 In Three-Point Mapping,
Parent must be heterozygous for all 3 genes under consideration.
Cross constructed so genotypes can be determined by offspring phenotypes.
Many offspring produced to get a representative sample of crossover events.
A three-point mapping cross is shown in Figure 5.8 (Drosophila).

20. Figure 5.8

21. Section 5.3
The noncrossover F2 phenotypes occur in the greatest proportion of offspring.
The double-crossover phenotypes occur in the smallest proportion.

22. Section 5.3
Because the F2 phenotypes complement each other (i.e., one is wild type and the other is mutant for all three genes), they are called reciprocal classes of phenotypes.

23. Section 5.3
Distance between two genes in a three-point cross is equal to the percentage of all detectable exchanges occurring between them and includes all single and double crossovers.

24. Section 5.3 Determining Gene Sequence We assumed sequence is y-w-ec
But don’t know if that is correct order
Figure 5-9 The three possible sequences of the white, yellow, and echinus genes, the results of a double crossover in each case, and the resulting phenotypes produced in a testcross. For simplicity, the two noncrossover chromatids of each tetrad are omitted.

25. Section 5.3
2 methods for determining gene order from a three-point cross.
Method I
Method II

26. Section 5.3
There are only 3 possible arrangements of our 3 genes

27. Section 5.3
Method I
(1) Determine arrangement of alleles on homologs of heterozygote producing crossover gametes by identifying reciprocal noncrossover phenotypes
(2) test each of 3 possible orders to see which one gives you the observed double-crossover phenotypes – the one that does is the winner!

28. Section 5.3
Method II
(1) Determine arrangement of alleles on homologs of heterozygote producing crossover gametes by identifying reciprocal noncrossover phenotypes
(2) Look at observed double-crossover phenotypes and id single allele that has been switched
Will be one that is by itself (no longer associated w/other mutant alleles)
That is the one in the middle!

29. Section 5.3 Mapping Problem in Maize 3 Linked Genes
An example of a three-point cross and mapping of the three genes involved is shown in Figure 5.10 and Figure 5.11.
3 Linked Genes
bm = brown midrib
v = virescent seedling
pr = purple aleurone

30. Figure 5.10

31. Figure 5.11

32. Section 5.4
The expected frequency of multiple exchanges between two genes can be predicted from the distance between them.
The coefficient of coincidence (C) is the observed number of DCOs divided by the expected number of DCOs.

33. Section 5.4
Interference reduces the expected number of multiple crossovers when a crossover event in one region of the chromosome inhibits a second event nearby.
Interference is positive if fewer double-crossover events than expected occur and negative if more double-crossover events than expected occur.

34. Section 5.5
When two genes are close together, the accuracy of mapping is high.
As the distance between them increases, the accuracy of mapping decreases.

35. Section 5.5
Two exchanges between linked genes that are far apart on a chromosome can involve two, three, or all four strands.
They can result in production of different percentages of recombinant chromatids (Figure 5.12).

36. Figure 5.12

37. Figure 5-13 The relationship between the percentage of recombinant chromatids that occur and actual map distance when (a) Poisson distribution is used to predict the frequency of recombination in relation to map distance; and (b) there is a direct relationship between recombination and map distance.
Figure 5.13

38. Section 5.6- Drosophila Genes Have Been Extensively Mapped
The large numbers of mutants in Drosophila, maize, and mice have allowed extensive chromosome mapping in these organisms (Figure 5.14).

39. Figure 5.14

40. Section 5.7
Lod score analysis relies on probability calculations to demonstrate linkage between two genes in organisms in which linkage analysis relies primarily on pedigrees.

41. Section 5.7
Somatic cell hybridization involves fusion of two cells in culture to form a single hybrid cell, called a heterokaryon.

42. Section 5.7 Synkaryon – nuclei eventually fuse in cell
Upon continued culturing of the hybrid cell, chromosomes from one of the two parental species are gradually lost until only a few chromosomes of one species remain and most chromosomes are from the other species

43. Figure 5.15

44. Section 5.8
Chromosome Mapping is Now Possible Using DNA Markers and Annotated Computer Databases
DNA markers represent landmarks along the chromosome.
RFLPs – Restriction Fragment Length Polymorphisms
Microsatellites
SNPs – Single Nucleotide Polymorphisms

45. Section 5.10 Recombination Occurs between Mitotic Chromosomes
In Drosophila and certain fungi, homologs pair up during mitosis, allowing crossing over to take place.
Sister chromatid exchanges (SCEs) occur during mitosis but do not produce new allelic combinations.

46. The End