PPP_Chapter 5 b - CHAPTER 5 Chromosome Mapping in...

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CHAPTER 5 Chromosome Mapping in  Eukaryotes Dr. Sanghamitra Mohanty
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MAIN AREAS OF DISCUSSION: Linkage in genetic cross Crossing over and chromosome mapping Single and multiple crossovers Determination of gene sequence Interference Lod score analysis and Somatic cell hybridization in chromosome mapping Chromosome mapping using DNA markers Mitotic recombination Sister chromatid exchange Mapping in haploid organism Linkage and mapping
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Genes assort independently if they are on different chromosomes but show linkage if they are on the same chromosome. If complete linkage exists between two genes because of their close proximity and organisms heterozygous at both loci are mated, a unique F 2 phenotypic ratio designated the linkage ratio results. Linked Genes
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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). 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.
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Fig. 5.1
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Genes on the same chromosome are part of a linkage group. The number of linkage groups should correspond to the haploid number of chromosomes. Linkage Ratio is the unique ratio that results from crossing between two heterozygotes with linked genes.
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Fig. 5.2
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Fig. 5.3
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The percentage of offspring resulting from recombinant gametes depends on the distance between the two genes on the chromosome. Synapsed chromosomes in meiosis wrap around each other to create chiasmata that are points of genetic exchange. 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.
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The First Gene Map by Sturtevant
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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. 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.
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SAMPLE QUESTION 1: The genes dumpy (dp), clot (cl), and apterous (ap)  are linked on chromosome 2 of Drosophila. In a  series of two-point mapping crosses, the following  genetic distances were determined:  dp-ap 42  dp-cl 3  ap-cl 39  What is the sequence of the three genes?
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SAMPLE QUESTION 2: Q: With two genes on a single pair of homologous 
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PPP_Chapter 5 b - CHAPTER 5 Chromosome Mapping in...

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