Example: Distance in M.U. % Crossing Over Why? Double crossovers

Transcription

1 Example: Distance in M.U. % Crossing Over Why? Double crossovers 232

2 .. A B. a b.

3 1. A fully heterozygous gray-bodied (b+), normal winged (vg+) female F 1 fruit fly crossed with a black-bodied (b), vestigial-winged (vg) male gave the following results: gray, normal 126; gray, vestigial 24; black, normal 26; and black, vestigial 124. a) Does this indicate linkage? b) If so, what type? c) F 1 configuration? d) What is the percentage of crossing-over?

4 1. b+ = gray bodied b = black bodied vg+ = normal winged vg = vestigial winged Gray, normal b+ vg+ F 1 b vg X Black, vestigial b vg b vg Gray, normal Black, vestigial Gray, vestigial Black, normal b+ vg+ b vg b vg b vg b+ vg b vg b vg+ b vg P P R R a) yes b) normal c) cis d) [24+26] 300x100=16.7% = 16.7 m.u.

5 2. Another fully heterozygous gray-bodied, normal winged female F 1 fruit fly crossed with a black-bodied, vestigial-winged male gave the following results: gray, normal 23; gray, vestigial 127; black, normal 124; and black, vestigial 26. a) Does this indicate linkage? b) If so, what type? c) F 1 configuration? d) What is the percentage of crossing-over?

6 2. b+ = gray bodied b = black bodied vg+ = normal winged vg = vestigial winged Gray, normal b+ vg F 1 b vg+ X Black, vestigial b vg b vg Gray, normal Gray, vestigial Black, normal Black, vestigial b+ vg+ b vg b+ vg b vg b b vg+ vg b vg b vg R P P R a) yes b) normal c) trans d) [23+26] 300x100=16.3% = 16.3 m.u.

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12 F 1 Gametes Testcross progeny C S E 786 c s e 753 Region I Region II C s e 107 S or s c S E 97 C or c E or e C S e 86 c s E 94 Extreme C s E 1 c S e 2 Total To determine order of genes (the gene in the middle): a) Determine the N.C.O.s or parental types (two opposites with the most). b) Determine the D.C.O.s (two opposites with the least). c) Determine the one gene in the D.C.O.s or N.C.O.s that differs from the parental types. d) The gene that differs goes in the middle.

13 2. F 1 genotype: Distances: C & S = 10.7 M.U. 3. % C.O. C & S = x 100 = 10.7% % C.O. S & E = x 100 = 9.5% 1926 S & E = 9.5 M.U. C & E (distance between extremes) = 20.2 M.U. 241

14 5. F 1 (CSE) (cse) x (cse) (cse) F 1 gametes T.C. progeny C S E 1000 No C.O. in male Drosophila c s e Interference - A crossover in Region reduces the probability of a crossover in Region. Interference is expressed in terms of coincidence. C = % D.C.O. observed x 100 % D.C.O. expected % D.C.O. observed = No. of D.C.O. observed = 3 =.0016 Total 1926 % D.C.O. expected = % C.O. Region x % C.O. Region = 10.7% x 9.5% =.0102 (.107) (.095) C =.0016 x 100 = 16%.0102 = 100% - C = 100% - 16% = 84% If the distance between two genes is twenty map units or less, the % D.C.O. will be zero. Complete inter: C = 0% No interference: C = 100%

15 E Gray R - Rough D - Normal e - Ebony r - Smooth d Dumpy Drosophila Number F 1 Gametes testcross progeny E R d 402 e r D 398 E r D 56 e R d 38 E r d 52 e R D 48 E R D 4 e r d 2 Total Determine order of genes 2. % C.O. E & R 3. % C.O. E & D 4. % C.O. R & D 5. C. of C. 247

16 6. C. of I. 7. Minimum distance between extremes = 8. P - phenotype genotype 9. F 1 - phenotype genotype 10. Testcross parent - phenotype genotype

17 MUTATION Mutation: Definition: Sudden, heritable changes in genetic material and the processes by which these changes occur. Most mutations are lethal and recessive. I. These changes are not explained by recombination, but by either change in chromosome number [ex., euploidy (diploid, triploid, tetraploid)], or aneuploidy (chromosome # is not an exact multiple of the monoploid; ex., trisomy 21), chromosomal aberrations [ex., the addition or loss of chromosome segments (ex., deletions, duplications, inversions, translocations)], and changes in individual genes [ex., most common uses of mutation (ex., mutons, where there are point mutations or changes in single base pairs, and, therefore, genes; example of a spontaneous mutation)]. II. May occur in any cell A) Somatic (asexual reproduction) B) Germ

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19 Philadelphia chromosome Philadelphia chromosome (Ph chromosome) is an abnormality that causes chronic myeloid leukemia (CML) in humans. The Ph chromosome is chromosome 22 that has become involved in a translocation (an exchange of material) with chromosome 9. In this mutational process, chromosome 22 becomes abnormally shortened. This translocation takes place in a single bone marrow cell, and through clonal expansion of this one mutant cell, gives rise to CML.

20 III. Random rather than directed by environment Mutations are not directed by the environment per se, but rather the environment selects for those pre-existing mutations which better enable the organism to survive in the environment. Ex., Streptomycin - resistant mutants in a population of bacteria (E. coli). IV. Types of mutations Some mutations are good; they introduce variation (alleles) and, thus, help drive natural selection. Mutations are the basic source of all genetic variation. However, too many mutations would disrupt the normal transmission of genetic material from generation to generation. A) Spontaneous mutations - mutations that are not under the control of man. Are relatively infrequent, and occur without a known cause. Are due to biochemical mistakes during DNA replication, or intrinsic errors in DNA repair (which may also be induced by environmental agents; ex., lung cancer + smoking, skin cancer + uv irradiation).

21 B) Induced mutations - mutations produced by man with a mutagen. Due to mutagens like ionizing radiation, uv light (solar radiation), chemicals that react with DNA or RNA, or viruses. V. Mistakes in DNA nucleotide (base pair) selection are corrected to some extent by DNA polymerase activity; however, a minimal level of mistakes (1 x = errors/synthesized nucleotide) may still normally go unrepaired. The level of this occurrence may be increased by environmental mutagenic agents. Mutagen or mutagenic agent - an environmental agent that is capable of inducing a mutation.

22 A) Ionizing radiation (short wavelength and high energy): i) x-rays ii) alpha ( ), beta ( ), gamma ( ) radiation ( particles = 2 protons and 2 neutrons from nuclei; particles = emission of electrons; particles = emission of high energy protons) - These include the following radioisotopes: - 32P, 3H, 35S, 14C, 35Ca - 125I, 22Na - Ra (radium) (workers licking brushes used in watch dial manufacturing - radium poisoning). iii) Cosmic rays Dosage of ionizing radiation is cumulative. Low intensity x long time (chronic dosage) = high intensity x short time (acute dosage).

23 Linear relationship between dosage and frequency of mutation. Percent sex-linked lethal mutations Roentgen units units of ionizing radiation (x or ) Efficiency of ionizing radiation to induce mutations is dependent upon: 1) Quality of DNA repair mechanisms: DNA repair mechanisms: i) Photoreactivation - "Light stimulated repair" ii) Excision repair - "Dark repair" iii) Post replication recombination - "Dark repair" iv) Point in cell cycle (ex., metaphase, interphase) at which a cell is irradiated and the sensitivity the cell at that particular point in its cycle to irradiation. v) Rate of cell division - faster dividing cells (i.e., cancer cells) are more sensitive. vi) O 2 tension and temperature of cellular environment.

24 O 2, temp. = Mutation rate. B) Non-ionizing i) Ultraviolet light C) Chemical mutagens can (1) penetrate cells and (2) alter the chemical structure of the DNA within the cells. D) Viruses i) Mustard gas ii) Formaldehyde iii) Benzene

25 Point Mutations - Mutations involving changes in single base pairs. 1. Substitutions: A one-base substitution is usually not deleterious. However, this mainly applied to degenerate codons. For example: CUU CUC CUA These may all code for (leu) so a change in the third nucleotide is not deleterious, but a change in one of the first two nucleotides in the codon may cause what is known as a mis-sense mutation, which could be detrimental.

26 2. Deletions and Insertions: A deletion or insertion can be of any size. However, deletions or insertions involving one base pair are point mutations. These alterations may radically alter protein function, and may be considered frame shift mutations. Most large DNA insertions will stop gene expression. Example: Deletion G Template strand TAC GGG TTT CCC Instead of: Now transcribes: AUG CCC AAA GGG AUG CCA AAG GGX Example: Insertion G Template strand TAC GGG TTT CCC Instead of: Now transcribes: AUG CCC AAA GGG AUG CCC CAA AGG Additionally, a one-base insertion that changes a codon to a stop codon is considered a nonsense mutation and leads to a truncated protein. This could greatly decrease the activity of a protein, and could sometimes be lethal. A suppressor mutation is an addition mutation following a deletion mutation, or a deletion mutation following an addition mutation. 259

27 DNA Repair: DNA repair occurs in three steps: excision of the defective nucleotide(s) from one strand of the duplex, replacement of the

28 DNA REPAIR

29 Xeroderma Pigmentosum, or XP A genetic disorder of DNA repair in which the body's normal ability to remove damage caused by ultraviolet (UV) light is deficient. This leads to multiple basaliomas and other skin malignancies at a young age. In severe cases, it is necessary to avoid sunlight. Damage to DNA in epidermal cells occurs during exposure to UV light. The absorption of the high energy light leads to the formation of pyrimidine dimers. The pathway, using multiple enzymes, which repairs the UVdamage in healthy cells is called nucleotide excision repair. Briefly, the damage is excised by exonucleases, then the gap is filled by DNA polymerase I and sealed by DNA ligase. The most common defect in XP is a genetic defect whereby nucleotide excision repair enzymes are mutated, leading to less or no repair of UV-lesions. Unrepaired damage can lead to mutations, altering the information of the DNA. If mutations affect important genes, like tumour suppressor genes (e.g. p53) or proto-oncogenes, then this may lead to cancer. Since in XP patients the frequencies of mutations is much elevated, these patients have a predisposition for cancer.

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