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Drosophila Chromosomes

Drosophila Chromosomes. Outline. X-Linked Alleles Human X-Linked Disorders Gene Linkage Crossing-Over Chromosome Map Changes in Chromosome Number Changes in Chromosome Structure Human Syndromes. Chromosomal Inheritance. Humans are diploid (2 chromosomes of each type)

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Drosophila Chromosomes

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  1. Chromosomal Inheritance

  2. Drosophila Chromosomes

  3. Outline • X-Linked Alleles • Human X-Linked Disorders • Gene Linkage • Crossing-Over • Chromosome Map • Changes in Chromosome Number • Changes in Chromosome Structure • Human Syndromes

  4. Chromosomal Inheritance • Humans are diploid (2 chromosomes of each type) • Humans have 23 different kinds of chromosomes • Arranged in 23 pairs of homologous chromosomes • Total of 46 chromosomes (23 pairs) per cell • One of the chromosome pairs determines the sex of an individual (The sex chromosomes) • The other 22 pairs of chromosomes are autosomes • Autosomal chromosomes are numbered from smallest (#1) to largest (#22) • The sex chromosomes are numbered as the 23rd pair

  5. Sex Determination in Humans • Sex is determined in humans by allocation of chromosomes at fertilization • Both sperm and egg carry one of each of the 22 autosomes • The egg always carries the X chromosome as number 23 • The sperm may carry either and X or Y • If the sperm donates an X in fertilization, the zygote will be female • If the sperm donates a Y in fertilization, the zygote will be male • Therefore, the sex of all humans is determined by the sperm donated by their father

  6. X-Linked Alleles • Genes carried on autosomes are said to be autosomally linked • Genes carried on the female sex chromosome (X) are said to be X-linked (or sex-linked) • X-linked genes have a different pattern of inheritance than autosomal genes have • The Y chromosome is blank for these genes • Recessive alleles on X chromosome: • Follow familiar dominant/recessive rules in females (XX) • Are always expressed in males (XY), whether dominant or recessive • Males said to be monozygous for X-linked genes

  7. Eye Color in Fruit Flies • Fruit flies (Drosophila melanogaster) are common subjects for genetics research • They normally (wild-type) have red eyes • A mutant recessive allele of a gene on the X chromosome can cause white eyes • Possible combinations of genotype and phenotype:

  8. X-Linked Inheritance

  9. Human X-Linked Disorders:Red-Green Color Blindness • Color vision In humans: • Depends three different classes of cone cells in the retina • Only one type of pigment is present in each class of cone cell • The gene for blue-sensitive is autosomal • The red-sensitive and green-sensitive genes are on the X chromosome • Mutations in X-linked genes cause RG color blindness: • All males with mutation (XbY) are colorblind • Only homozygous mutant females (XbXb) are colorblind • Heterozygous females (XBXb) are asymptomatic carriers

  10. Red-Green Colorblindness Chart

  11. X-Linked Recessive Pedigree

  12. Human X-Linked Disorders:Muscular Dystrophy • Muscle cells operate by release and rapid sequestering of calcium • Protein dystrophin required to keep calcium sequestered • Dystrophin production depends on X-linked gene • A defective allele (when unopposed) causes absence of dystrophin • Allows calcium to leak into muscle cells • Causes muscular dystrophy • All sufferers male • Defective gene always unopposed in males • Males die before fathering potentially homozygous recessive daughters

  13. Human X-Linked Disorders:Hemophilia • “Bleeder’s Disease” • Blood of affected person either refuses to clot or clots too slowly • Hemophilia A – due to lack of clotting factor IX • Hemophilia B – due to lack of clotting factor VIII • Most victims male, receiving the defective allele from carrier mother • Bleed to death from simple bruises, etc. • Factor VIII now available via biotechnology

  14. Hemophilia Pedigree

  15. Human X-Linked Disorders:Fragile X Syndrome • Due to base-triplet repeats in a gene on the X chromosome • CGG repeated many times • 6-50 repeats – asymptomatic • 230-2,000 repeats – growth distortions and mental retardation • Inheritance pattern is complex and unpredictable

  16. Gene Linkage

  17. Gene Linkage • When several genes of interest exist on the same chromosome • Such genes form a linkage group • Tend to be inherited as a block • If all genes on same chromosome: • Gametes of parent likely to have exact allele combination as gamete of either grandparent • Independent assortment does not apply • If all genes on separate chromosomes: • Allele combinations of grandparent gametes will be shuffled in parental gametes • Independent assortment working

  18. Linkage Groups

  19. Constructing a Chromosome Map • Crossing-over can disrupt a blocked allele pattern on a chromosome • Affected by distance between genetic loci • Consider three genes on one chromosome: • If one at one end, a second at the other and the third in the middle • Crossing over very likely to occur between loci • Allelic patterns of grandparents will likely to be disrupted in parental gametes with all allelic combinations possible • If the three genetic loci occur in close sequence on the chromosome • Crossing over very UNlikely to occur between loci • Allelic patterns of grandparents will likely to be preserved in parental gametes • Rate at which allelic patterns are disrupted by crossing over: • Indicates distance between loci • Can be used to develop linkage map or genetic map of chromosome

  20. Crossing Over

  21. Complete vs. Incomplete Linkage

  22. Chromosome Number:Polyploidy • Polyploidy • Occurs when eukaryotes have more than 2n chromosomes • Named according to number of complete sets of chromosomes • Major method of speciation in plants • Diploid egg of one species joins with diploid pollen of another species • Result is new tetraploid species that is self-fertile but isolated from both “parent” species • Some estimate 47% of flowering plants are polyploids • Often lethal in higher animals

  23. Chromosome Number:Aneuploidy • Monosomy (2n - 1) • Diploid individual has only one of a particular chromosome • Caused by failure of synapsed chromosomes to separate at Anaphase I (nondisjunction) • Trisomy (2n + 1) occurs when an individual has three of a particular type of chromosome • Diploid individual has three of a particular chromosome • Also caused by nondisjunction • This usually produces one monosomic daughter cell and one trisomic daughter cell in meiosis I • Down syndrome is trisomy 21

  24. Nondisjunction

  25. Trisomy 21

  26. Chromosome Number:Abnormal Sex Chromosome Number • Result of inheriting too many or too few X or Y chromosomes • Caused by nondisjunction during oogenesis or spermatogenesis • Turner Syndrome (XO) • Female with single X chromosome • Short, with broad chest and widely spaced nipples • Can be of normal intelligence and function with hormone therapy

  27. Chromosome Number:Abnormal Sex Chromosome Number • Klinefelter Syndrome (XXY) • Male with underdeveloped testes and prostate; some breast overdevelopment • Long arms and legs; large hands • Near normal intelligence unless XXXY, XXXXY, etc. • No matter how many X chromosomes, presence of Y renders individual male

  28. Turner and Klinefelter Syndromes

  29. Chromosome Number:Abnormal Sex Chromosome Number • Ploy-X females • XXX simply taller & thinner than usual • Some learning difficulties • Many menstruate regularly and are fertile • More than 3 Xs renders severe mental retardation • Jacob’s syndrome (XYY) • Tall, persistent acne, speech & reading problems

  30. Abnormal Chromosome Structure • Deletion • Missing segment of chromosome • Lost during breakage • Translocation • A segment from one chromosome moves to a non-homologous chromosome • Follows breakage of two nonhomologous chromosomes and improper re-assembly

  31. Deletion, Translocation,Duplication, and Inversion

  32. Abnormal Chromosome Structure • Duplication • A segment of a chromosome is repeated in the same chromosome • Inversion • Occurs as a result of two breaks in a chromosome • The internal segment is reversed before re-insertion • Genes occur in reverse order in inverted segment

  33. Inversion Leading toDuplication and Deletion

  34. Abnormal Chromosome Structure • Deletion Syndromes • Williams syndrome - Loss of segment of chromosome 7 • Cri du chat syndrome (cat’s cry) - Loss of segment of chromosome 5 • Translocations • Alagille syndrome • Some cancers

  35. Williams Syndrome

  36. Alagille Syndrome

  37. Review • X-Linked Alleles • Human X-Linked Disorders • Gene Linkage • Crossing-Over • Chromosome Map • Changes in Chromosome Number • Changes in Chromosome Structure • Human Syndromes

  38. Chromosomal Inheritance

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