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Chromosomes and Inheritance

Chromosomes and Inheritance. Thomas Hunt Morgan. The common fruit fly – Drosophila melanogaster. Gene Linkage. Each chromosome has hundreds or thousands of genes.

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Chromosomes and Inheritance

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  1. Chromosomes and Inheritance

  2. Thomas Hunt Morgan

  3. The common fruit fly – Drosophila melanogaster

  4. Gene Linkage • Each chromosome has hundreds or thousands of genes. • Genes located on the same chromosome, linked genes, tend to be inherited together because the chromosome is passed along as a unit. • Results of crosses with linked genes deviate from those expected according to independent assortment.

  5. EXPERIMENT P Generation (homozygous) Double mutant(black body,vestigial wings) Wild type(gray body, normal wings) b b vg vg b b vg vg

  6. EXPERIMENT P Generation (homozygous) Double mutant(black body,vestigial wings) Wild type(gray body, normal wings) b b vg vg b b vg vg Double mutant F1 dihybrid(wild type) TESTCROSS b b vg vg b b vg vg

  7. EXPERIMENT P Generation (homozygous) Double mutant(black body,vestigial wings) Wild type(gray body, normal wings) b b vg vg b b vg vg Double mutant F1 dihybrid(wild type) TESTCROSS b b vg vg b b vg vg Testcrossoffspring Eggs b vg bvg bvg b vg Gray-vestigial Wild type(gray-normal) Black-normal Black-vestigial bvg Sperm b bvg vg bbvgvg b bvgvg bbvg vg

  8. EXPERIMENT P Generation (homozygous) Double mutant(black body,vestigial wings) Wild type(gray body, normal wings) b b vg vg b b vg vg Double mutant F1 dihybrid(wild type) TESTCROSS b b vg vg b b vg vg Testcrossoffspring Eggs b vg bvg bvg b vg Wild type(gray-normal) Black-normal Black-vestigial Gray-vestigial bvg Sperm b bvg vg bbvgvg b bvgvg bbvg vg PREDICTED RATIOS : 1 If genes are located on different chromosomes: : 1 1 1 : If genes are located on the same chromosome andparental alleles are always inherited together: 1 0 1 0 : : : RESULTS : 944 185 : 965 : 206

  9. Genetic Recombination • The production of offspring with new combinations of traits inherited from two parents is genetic recombination. • Genetic recombination can result from independent assortment of genes located on nonhomologous chromosomes or from crossing over of genes located on homologous chromosomes.

  10. b vg b+ vg+ F1 dihybrid femaleand homozygousrecessive malein testcross b vg b vg b vg b+ vg+ Most offspring or b vg b vg

  11. Mendel and recombination • Mendel’s dihybrid cross experiments produced some offspring that had a combination of traits that did not match either parent in the P generation. • If the P generation consists of a yellow-round parent (YYRR) crossed with a green-wrinkled seed parent (yyrr), all F1 plants have yellow-round seeds (YyRr). • A cross between an F1 plant and a homozygous recessive plant (a test-cross) produces four phenotypes. • Half are parental types, with phenotypes that match the original P parents, either with yellow-round seeds or green-wrinkled seeds. • Half are recombinants, new combination of parental traits, with yellow-wrinkled or green-round seeds. • A 50% frequency of recombination is observed for any two genes located on different (nonhomologous) chromosomes.

  12. Gametes from yellow-rounddihybrid parent (YyRr) yr Yr yR YR Gametes from green-wrinkled homozygousrecessive parent (yyrr) yr Yyrr yyrr yyRr YyRr Recombinant offspring Parental-type offspring

  13. Black body, vestigial wings(double mutant) Gray body, normal wings(F1 dihybrid) Testcrossparents bvg bvg bvg bvg Replicationof chromosomes Replicationof chromosomes bvg bvg bvg bvg bvg bvg bvg bvg Meiosis I bvg Meiosis I and II bvg bvg bvg Meiosis II Recombinantchromosomes bvg bvg bvg bvg Eggs 206Gray-vestigial 965Wild type(gray-normal) 944Black-vestigial 185Black-normal Testcrossoffspring bvg bvg bvg bvg bvg bvg bvg bvg bvg Sperm Recombinant offspring Parental-type offspring 391 recombinants2,300 total offspring Recombinationfrequency   100  17%

  14. Gray body, normal wings(F1 dihybrid) Black body, vestigial wings(double mutant) Testcrossparents bvg bvg bvg bvg Replicationof chromosomes Replicationof chromosomes bvg bvg bvg bvg bvg bvg bvg bvg Meiosis I bvg Meiosis I and II bvg bvg bvg Meiosis II Recombinantchromosomes bvg bvg bvg bvg bvg Eggs Sperm

  15. Recombinantchromosomes bvg bvg bvg bvg Eggs 206Gray-vestigial 944Black-vestigial 185Black-normal 965Wild type(gray-normal) Testcrossoffspring bvg bvg bvg bvg bvg bvg bvg bvg bvg Sperm Parental-type offspring Recombinant offspring 391 recombinants2,300 total offspring Recombinationfrequency  100  17% 

  16. Alfred Sturtevant

  17. Linkage Maps • A linkage map is an ordered list of the genetic loci along a particular chromosome. • Sturtevant hypothesized that the frequency of recombinant offspring reflected the distances between genes on a chromosome. • The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore a higher recombination frequency. • The greater the distance between two genes, the more points between them where crossing over can occur. • Sturtevant used recombination frequencies from fruit fly crosses to map the relative position of genes along chromosomes, a linkage map.

  18. RESULTS Recombinationfrequencies 9% 9.5% Chromosome 17% b cn vg

  19. Mutant phenotypes Blackbody Cinnabareyes Vestigialwings Browneyes Shortaristae 104.5 48.5 57.5 67.0 0 Long aristae(appendageson head) Gray body Red eyes Redeyes Normalwings Wild-type phenotypes

  20. Linkage distances on fruit fly chromosome 2

  21. More on Linkage Maps • A linkage map provides an imperfect picture of a chromosome. • Map units indicate relative distance and order, not precise locations of genes. • The frequency of crossing over is not actually uniform over the length of a chromosome. • Combined with other methods like chromosomal banding, geneticists can develop cytological maps. • These indicated the positions of genes with respect to chromosomal features. • More recent techniques show the absolute distances between gene loci in DNA nucleotides.

  22. Errors and Exceptions in Chromosomal Inheritance 1. Alterations of chromosome number or structure cause some genetic disorders 2. The phenotypic effects of some mammalian genes depend on whether they are inherited from the mother or the father (imprinting) 3. Extranuclear genes exhibit a non-Mendelian pattern of inheritance

  23. More Errors and Exceptions in Chromosomal Inheritance • Sex-linked traits are not the only notable deviation from the inheritance patterns observed by Mendel. • Gene mutations are not the only kind of changes to the genome that can affect phenotype. • Major chromosomal aberrations and their consequences produce exceptions to standard chromosome theory.

  24. Nondisjunction • Nondisjunction occurs when problems with the meiotic spindle cause errors in daughter cells. • This may occur if tetrad chromosomes do not separate properly during meiosis I. • Alternatively, sister chromatids may fail to separate during meiosis II. • As a consequence of nondisjunction, some gametes receive two of the same type of chromosome and another gamete receives no copy. • Offspring resulting from fertilization of a normal gamete with one after nondisjunction will have an abnormal chromosome number or aneuploidy. • Trisomic cells have three copies of a particular chromosome type and have 2n + 1 total chromosomes. • Monosomic cells have only one copy of a particular chromosome type and have 2n - 1 chromosomes.

  25. Meiosis I Nondisjunction

  26. Meiosis I Nondisjunction Meiosis II Non-disjunction

  27. Nondisjunction of homo-logous chromosomes inmeiosis I (b) (a) Nondisjunction of sisterchromatids in meiosis II Meiosis I Nondisjunction Meiosis II Non-disjunction Gametes n 1 n n 1 n 1 n n 1 n 1 n 1 Number of chromosomes

  28. Aneuploidy • If the organism survives, aneuploidy typically leads to a distinct phenotype. • Aneuploidy can also occur during failures of the mitotic spindle. • If aneuploidy happens early in development, this condition will be passed along by mitosis to a large number of cells. • This is likely to have a substantial effect on the organism. • Several serious human disorders are due to alterations of chromosome number and structure. • Although the frequency of aneuploid zygotes may be quite high in humans, most of these alterations are so disastrous that the embryos are spontaneously aborted long before birth. • These developmental problems result from an imbalance among gene products. • Certain aneuploid conditions upset the balance less, leading to survival to birth and beyond.

  29. Down’s syndrome or Trisomy 21

  30. Aneuploidy and Sex Chromosomes • Nondisjunction of sex chromosomes produces a variety of aneuploid conditions in humans. • Unlike autosomes, this aneuploidy upsets the genetic balance less severely. • This may be because the Y chromosome contains relatively few genes. • Also, extra copies of the X chromosome become inactivated as Barr bodies in somatic cells.

  31. Aneuploidy and Sex Chromosomes • Klinefelter’s syndrome, an XXY male, occurs once in every 1000 live births. • These individuals have male sex organs, but are sterile. • There may be feminine characteristics • Their intelligence is normal. • Males with an extra Y chromosome (XYY) tend to somewhat taller than average. Occurs in about 1 in every 1000 live births. • Trisomy X (XXX), which occurs once in every 1000 live births, produces healthy females. May have learning disabilities. • Monosomy X or Turner’s syndrome (X0), which occurs once in every 2500 births, produces phenotypic, but immature females. Most have normal intelligence.

  32. Polyploidy • Organisms with more than two complete sets of chromosomes, have undergone polypoidy. • This may occur when a normal gamete fertilizes another gamete in which there has been nondisjunction of all its chromosomes. • The resulting zygote would be triploid (3n). • Alternatively, if a 2n zygote failed to divide after replicating its chromosomes, a tetraploid (4n) embryo would result from subsequent successful cycles of mitosis. • Polyploidy is common among plants and much less common among animals. • The spontaneous origin of polyploid individuals plays an important role in the evolution of plants – in fact as many as 60% of all flowering plant species may have arisen via hybridization which is frequently followed by polyploidy

  33. Hybridization followed by Polyploidy

  34. The Red Viscacha Rat from Argentina – A tetraploid mammal?

  35. Mammalian Chromosome Variation • Indian muntjac deer, 2n=6 • Red Viscacha Rat, 4n=102 • Siberian roe deer, 2n=70 + up to 14 B’s (accessory chromosomes) • Transcaucasian vole, 2n=17, both sexes XO

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