Bio3400 Chapter 5 Chromosome Mapping in Eukaryotes
  1. Genes         independently if they are on different chromosomes but show          if they are on the same chromosome.

      Independent assortment of two pairs of chromosomes, each containing a heterozygous gene pair, shows no linkage patterns: 4 genetically different gametes are formed in equal proportions, each containing a different combination of alleles of the two genes.


      If no crossing over occurs between two genes on the same chromosome, only two genetically different gametes are formed. This complete linkage produces only parental or noncrossover (NCO) gametes in equal proportions. Thus genes on the same chromosome form a linkage group.


      DNA synthesis occurs during interphase before the beginning of meiosis I but does not occur again before meiosis II. In prophase I, homologous chromosomes pair up into tetrads in a process called synapsis, and crossing over occurs, where genetic information is exchanged between nonsister chromatids of the homologues. Crossing over produces recombinant chromosomes and contributes to genetic variation in sexual reproduction. metaphase I


      Complete linkage produces only parental chromosomes and phenotypes in the F[2]. All F[1] flies are heterozygous for both loci and exhibit dominant red eyes and thin veins.
    • Crossing the F[1] should yield a 1:2:1 ratio in the F[2].
    • Testcross with homozygous recessive should also yield only parental phenotypes.


      flies produces a 1:1 ratio of brown,thin and red,heavy F[2], with no recombinants. If the genes were not linked, the testcross would have produced four phenotypes.
     
     
     
     
  2. Genes on the same chromosome would produce              gametes if            between nonsister chromatids occurs during meiosis.

      Crossing over of two linked genes between nonsister chromatids generates two new allele combinations, called recombinant or crossover gametes. The chromatids not involved in the exchange produce noncrossover gametes, as in complete linkage.


      If no crossing over occurs between two genes on the same chromosome, only two genetically different gametes are formed. This complete linkage produces only parental or noncrossover (NCO) gametes in equal proportions. Thus genes on the same chromosome form a linkage group.
     
     
     
     
  3. Genes located        to each other on the same chromosome are less likely to have cross-over events occur between them.

      In experiments with Drosophila, Morgan (Nobel 1933) crossed X-linked mutant yellow-bodied (y) and white-eyed (w) females with wild-type males (gray body and red eyes).


      flies show recombinant phenotypes. The recombinants express either white eyes (with gray bodies) or yellow bodies (with red eyes).


      In another cross, Morgan crossed white-eye, miniature-wing mutants with wild-type flies.


      flies show the parental phenotypes. A full 37.2% exhibited recombination.
     
     
     
     
  4. The                frequencies between linked genes provide an estimate of the           between them (interlocus distance) and can be used to construct a chromosome      .

      A map of the yellow (y), white (w), and miniature (m) genes on the X chromosome of Drosophila melanogaster. Each number represents the percentage of recombinant offspring produced between two loci and is defined as the interlocus distance. 1% recombination between two genes on a chromosome is defined as 1 map unit (mu), or 1 centimorgan (cM). The interloci distances are additive.
     
     
     
     
  5. The percentage of          involved in an exchange between two genes is        the percentage of recombinant gametes produced.

      Since a single crossover occurs between two nonsister chromatids, the other two chromatids of the tetrad are not involved in the exchange. The theoretical limit of recombination due to crossing over is 50%. So the percentage of tetrads involved in such an exchange is twice the percentage of recombinant gametes.
     
     
     
     
  6. A         crossover (SCO) alters linkage between two genes only if the crossover occurs          those two genes.

      Single Crossover
    • The exchange does not alter the linkage arrangement between the alleles of the two genes, only parental gametes are formed, and the exchange is undetected.
    • The exchange separates the alleles, resulting in recombinant gametes, which are detectable.
     
     
     
     
  7.         crossovers (DCOs) can be used to determine the order of        genes on the chromosome.

      A double crossover between two nonsister chromatids produces two double-crossover gametes. By the product law, the probability of two crossovers occurring is equal to the product of the individual probabilities; so a double crossover (DCO) is much rarer than a single crossover (SCO).
    • What are possible SCO gametes?
    • Abc, aBC, ABc, abC.


      Assuming 2 pairs of traits are inherited independently (independent assortment), the product law predicts that the combined probability of the two phenotypes is equal to the product of their individual probabilities.
     
     
     
     
  8. The              mapping technique can be used to map three         genes in one cross by two methods.

      A three-point mapping cross involving the yellow (y) body color, white (w) eye color, and echinus (ec) eye shape genes in Drosophila. In the F[1], females are heterozygous at all three loci and males are hemizygous for the mutant alleles. Most of the F[2] show parental (NCO) phenotypes, with DCOs the least common among the recombinants.


      4


      Determining gene sequence in a three-point mapping cross. Of the 3 possible arrangements, arrangement III (with w in the middle) yields the observed DCO (least frequent) phenotypes: yellow, echinus flies and white flies. Mapping gene distances. The map distance between two genes is the sum of all SCO and DCO frequencies between them. The distance between y and w is the sum of the frequencies of offsprings (3)/(4) and (7)/(8): 1.50% + 0.06% = 1.56 mu. The distance between w and ec is the sum of offsprings (5)/(6) and (7)/(8): 4.00% + 0.06% = 4.06 mu.


      4


      A three-point mapping cross involving the brown midrib (bm), virescent seedling (v), and purple aleurone (pr) genes in maize. Following a DCO in a three-point mapping cross, the allele in the middle position will be flanked by the other two alleles. So a prvbm sequence will show pr^+vbm^+ after a DCO (least frequent) event. This is not observed.


      Determining gene sequence in a three-point mapping cross. First, determine the arrangement of alleles by locating the reciprocal NCO (most frequent) phenotypes: we conclude the pr^+ is on the other homolog from the v^+ and bm^+ alleles, as shown in (a). Now find DCO (least frequent) phenotypes among the 3 possibilities in (b), (c), and (d): pr must be in the middle. Mapping gene distances. The map distance between two genes is the sum of all SCO and DCO frequencies between them. The distance between v and pr is the sum of the frequencies of SCO (14.5%) and DCO (7.8%), or 22.3 mu. The distance between pr and bmc is the sum of the frequencies of SCO (35.6%) and DCO (7.8%), or 43.4 mu.
     
     
     
     
  9. As the distance between linked genes increases, the mapping accuracy            due to undetected multiple crossovers that can be predicted by a          distribution.

      A two-strand double exchange yields no recombinant chromatids. A three-strand double exchange yields 50% recombinant chromatids. A four-strand double exchange yields 100% recombinant chromatids. Overall, multiple events "even out" and two linked genes yield a theoretical maximum of 50% recombination.


      A four-strand double exchange yields 100% recombinant chromatids.


      The Poisson distribution is a mathematical function that assigns probabilities of observing random events in a sample. As the distance between two linked genes increases, a mapping function can help correct for the underestimate of multiple exchanges observed experimentally. For example, When about 30% recombinants are detected, the true map distance is 50 mu.
     
     
     
     
  10. The large numbers of mutants in             has allowed extensive chromosome mapping in this and other organisms such as maize and mice.
     
     
     
     
  11. Mapping in maize using              markers established that crossing over involves a           exchange of chromosome regions.

      Crossing over between the colorless (c) / colored (C) and starchy (Wx) / waxy (wx) endosperm traits in maize can be correlated with cytological markers: a knob at one end of the chromosome and a translocation at the other end. For example, the colorless, waxy recombinant chromosome (case I) has lost its knob, providing visual evidence that physical exchange of genetic material had occurred.
     
     
     
     
  12. In Drosophila, homologs can pair up during          , allowing mitotic recombination to take place.

      Female flies heterozygous for the X-linked recessive mutations yellow body (y) and singed bristles (sn) show the wild type phenotype in most tissue cells (gray body and gray, straight bristles). Mitotic recombination can produce some recombinant tissue such as yellow spot or twin spot patches.


      A SCO between the loci for yellow body (y) and the centromere can produce twin spot ((singed and yellow spots)) patches of tissue where the recombinant cells (clones from the original cell that had mitotic recombination) are homozygous for either the y or the sn allele.
     
     
     
     
  13. Agents that induce chromosome         increase the frequency of sister            exchange (SCE) during          but do not produce recombination.

      Sister chromatids can engage in genetic exchanges SCEs) during mitosis, forming harlequin chromosomes. This does not produce new allelic combinations.
     
     
     
     
  14. In Neurospora, meiosis produces eight ordered haploid             within an        , allowing ordered         analysis.

      The bread mold Neurospora spends most of it life cycle in a multicellular haploid stage. Following fertilization of conidia of opposite mating types, the zygote undergoes meiosis in an ascus, which retains the haploid tetrad (do not confuse with tetrad formed in prophase I). Each cell in the tetrad undergoes mitosis to produce 8 haploid ascospores. Because the 8 cells reflect the sequence of their formation following meiosis, the tetrad is "ordered" and can be subjected to ordered tetrad analysis.


      DNA synthesis occurs during interphase before the beginning of meiosis I but does not occur again before meiosis II. In prophase I, homologous chromosomes pair up into tetrads in a process called synapsis, and crossing over occurs, where genetic information is exchanged between nonsister chromatids of the homologues. Crossing over produces recombinant chromosomes and contributes to genetic variation in sexual reproduction. metaphase I


      Mapping the centromere. If no crossover event occurs between a gene and the centromere, the pattern of ascospores within an ascus is (aaaa++++) due to first-division segregation, since the two alleles are separated during the first meiotic division. Crossover would produce recombinant patterns due to second-division segregation. To calculate the distance between the gene and the centromere: d = ((1/2 recombinant asci) / total asci) * 100. The distance (d in mu) counts half the number of recombinant asci. since crossing over in each occurs in only two of the four chromatids during meiosis.


      Since a single crossover occurs between two nonsister chromatids, the other two chromatids of the tetrad are not involved in the exchange. The theoretical limit of recombination due to crossing over is 50%. So the percentage of tetrads involved in such an exchange is twice the percentage of recombinant gametes.
     
     
     
     
  15. Somatic cell                can be used for          testing in which a specific gene product can be correlated with a chromosome.

      Cells from different organisms can be fused in culture by somatic cell hybridization to form a hybrid cell with two nuclei called a heterokaryon; the nuclei can fuse to form a synkaryon in which chromosomes from from one of the two parental species are gradually lost. A panel of synkaryons with mostly mouse chromosomes and a few human chromosomes can be used to associate a particular gene product with a specific chromosome.
    • A synteny testing example: how can products A, B, C, D be mapped to chromosomes?
    • Product A can be assigned to chromosome 5. + Product B can be assigned to chromosome 3. + Product C is not on any of the chromosomes 1-7. + Product D can be assigned to chromosome 1.


      Representative regional gene assignments for human chromosome 1 and the X chromosome. Key: AMY Amylase (salivary and pancreatic) AT3 Antithrombin (clotting factor IV) CB Color Blindness DMD Duchene Muscular Dystrophy FHM Fumarate Hrdratase (mitochondrial) GDH Glucose Dehydrogenase G6PD Glucose-6-phosphate Dehydrogenase GEMA Hemophilia A (classic) HGPRT Hypoxanthine-Guanine-Phosphoribosyl Transferase PEPC Peptiidase C PGK Phosphoglycerate Kinase PGM Phosphoglucomutase Rh Rhesus Blood Group (erythroblastosis fetalis)