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Chapter 15 Chromosomes, Morgan & Mutations Figure 15-01 Chromosome Basis of Inheritance • Genes have specific positions on chromosomes • It is the chromosomes that undergo segregation and independent assortment LE 15-2a P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) Meiosis Fertilization Gametes All F1 plants produce yellow-round seeds (YyRr) LE 15-2b All F1 plants produce yellow-round seeds (YyRr) F1 Generation Meiosis LAW OF INDEPENDENT ASSORTMENT LAW OF SEGREGATION Two equally probable arrangements of chromosomes at metaphase I Anaphase I Metaphase II Gametes LE 15-2c F2 Generation Fertilization among the F1 plants Why use Fruit Flies in genetic experiments? • Small / vials can hold hundreds • Short Generation Time • Many offspring • Only 8 chromosomes Morgan originally an Embryologist • He was the one that came up with the term “Wild Type” for the trait that was normally found in nature • All other traits he called “Mutants” • Born in Lexington, KY - 1866 • Awarded the Nobel Prize 1933 (Physiology & Medicine) • Died in 1945 Figure 15-03 More Morgan • Sex linked traits • He was responsible for determining that genes are located on the “X” chromosome LE 15-4a P Generation F1 Generation F2 Generation LE 15-4b P Generation Ova (eggs) Sperm F1 Generation Ova (eggs) F2 Generation Sperm Linked Genes • Each chromosome has hundreds or even thousands of genes • Genes that are on the same chromosome and are close together are called linked genes • Often these are inherited together Linked Genes – Two Traits • Morgan did other experiments with fruit flies to see how linkage affects inheritance of two or three traits • Morgan crossed flies that differed in traits of body color, wing size and/or eye color. LE 15-UN278-1 Parents in testcross Most offspring or LE 15-5 P Generation (homozygous) Wild type (gray body, normal wings) Double mutant (black body, vestigial wings) b+ b+ vg+ vg+ b b vg vg F1 dihybrid (wild type (gray body, normal wings) Double mutant (black body, vestigial wings) TESTCROSS b+ b vg+ vg b b vg vg Ova 965 944 Wild type Black(gray-normal) vestigial 206 Grayvestigial 185 Blacknormal Sperm Parental-type Recombinant (nonparental-type) offspring offspring Morgan & Crossing Over • He discovered that genes can be linked, but sometimes the connection between genes on the same chromosome appears to break • Morgan realized that crossing over of homologous chromosomes – was responsible for this LE 15-6a Testcross parents Gray body, normal wings (F1 dihybrid) Black body, vestigial wings (double mutant) Replication of chromosomes Replication of chromosomes Meiosis I: Crossing over between b and vg loci produces new allele combinations. Meiosis I and II: No new allele combinations are produced. Meiosis II: Separation of chromatids produces recombinant gametes with the new allele combinations. Ova Gametes Recombinant chromosomes Sperm LE 15-6b Sperm Ova Gametes Ova Testcross offspring Sperm 965 Wild type (gray-normal) 944 Blackvestigial Parental-type offspring 206 Grayvestigial 185 Blacknormal Recombinant offspring Recombination 391 recombinants 100 = 17% = frequency 2,300 total offspring Parentals & Recombinants • Morgan noticed that sometimes traits in the offspring were different than either parent • Offspring with a phenotype matching one of the parental phenotypes are called parental types • Offspring with nonparental phenotypes (new combinations of traits) are called recombinants • A 50% frequency of recombination is observed for any two genes on different chromosomes LE 15-UN278-2 Gametes from yellow-round heterozygous parent (YyRr) Gametes from greenwrinkled homozygous recessive parent (yyrr) Parental-type offspring Recombinant offspring Linkage Map • Alfred Sturtevant: one of Morgan’s protégés, made a genetic map, a list of the location of genes “loci” on a chromosome • He predicted that the farther apart two genes are, the higher the chance that a crossover will occur, so the higher the recombination frequency Map units or CentiMorgans = cM • A linkage map is a genetic map of a chromosome based on recombination frequencies • Distances between genes can be shown as map units: – one map unit, or centimorgan = 1% recombination frequency • Map units indicate relative distance and order, not exact locations of genes LE 15-7 Recombination frequencies 9% 9.5% 17% b Chromosome cn vg Cytogenetic Maps • Unlike linkage maps that just show general position – • Cytogenetic maps show the positions of genes and the chromosomal features • “banding patterns” exact locations LE 15-8 I II Y X IV III Mutant phenotypes Short aristae 0 Long aristae (appendages on head) 48.5 Gray body Vestigial wings Cinnabar eyes Black body 57.5 67.0 Red eyes Wild-type phenotypes Brown eyes 104.5 Normal wings Red eyes Morgan’s gene representation: • For example: – b = black = mutant – b+ = normal body = WILD – vg = vestigial = mutant – vg+ = normal wings = WILD – w = white = mutant – w+ = red eyes (normal) = WILD – ♀ = female – ♂ = male Example • Wild (normal) body & Wild (normal) wings crossed with a Black body & Vestigial wings: – b+ b+ vg+ vg+ X b b vg vg – AABB x aabb (representing the same thing) • RESULT: – b+ b vg+ vg (heterozygote) – This is sometimes confusing – – AaBb = same thing – much easier If on separate chromosomes & not linked • Aa Bb X aabb • Counts will be even: – – – – 100 100 100 100 AB Ab aB ab (AaBb) (Aabb) (aaBb) (aabb) AB AaBb ab ab ab ab Ab Aabb aB aaBb ab aabb If genes located on same chromosome = LINKED … • Aa Bb X aabb • Counts will be NOT even: – 150 – 50 – 50 – 150 AB Ab aB ab (AaBb) (Aabb) (aaBb) (aabb) • To calculate the map distance: – add the recombinants (50 + 50) / total (400) 100/400 = 25 (x 100) = 25% cM Three Point Cross • AABBCC x aabbcc = AaBbCc • AaBbCc x aabbcc (test cross counts) – to determine the linkage distance: Example: – ABC – ABc – AbC – Abc 95 5 700 50 aBC aBc abC abc 50 700 5 95 A few hints … – ABC – ABc – AbC – Abc 95 5 700 50 aBC aBc abC abc 50 700 5 95 • Parentals: (largest number) AbC & aBc – Notice they are reciprocals AbC & aBc • Recombinants: all others • Double Crossovers: the smallest number & the “middle” gene (ABc & abC) – ABC 95 aBC 50 – ABc 5 aBc 700 – AbC 700 abC 5 – Abc 50 abc 95 • Compare Parentals & Double Crossovers: – ABc – aBc 5 700 abC AbC 5 700 • Notice the only gene that is different when comparing the two is the “A” gene • That tells you that “A” is in the middle It is an excellent idea to rewrite the offspring chart in correct order … – ABC – ABc – AbC – Abc 95 5 700 50 aBC aBc abC abc 50 700 5 95 • -----------------------------------– BAC – BAc – bAC – bAc 95 5 700 50 BaC Bac baC bac 50 700 5 95 Calculations – BAC – BAc – bAC – bAc 95 5 700 50 BaC Bac baC bac 50 700 5 95 Total: 1700 • 95 + 95 + 5 + 5 = 200/1700 = 11.8 (bet. B & A) • 50 + 50 + 5 + 5 = 110/1700 = 6.5 (bet. A & C) • 95 + 95 + 50 + 50 = 290/1700 = 17.1 (bet. B & C) – (all of the above totals are x 100) = 11.8 / 6.5 / 17.1 Order of genes 17.1 B 11.8 A 6.5 C Sex Determination • An organism’s sex is determined by the presence or absence of certain chromosomes • In humans and other mammals, there are two sex chromosomes, X and Y • Other animals have different chromosome arrangement to determine male / female LE 15-UN282 X Y LE 15-9a Parents Ova Sperm Zygotes (offspring) The X-Y system LE 15-9b The X-0 system Grasshoppers, Cockroaches, Crickets, Praying Mantis & some other insects LE 15-9c The Z-W system Birds, some fish and some insects LE 15-9d The haplo-diploid system Bees & Ants LE 15-9 Parents Ova Sperm Zygotes (offspring) The X-Y system The X-0 system The Z-W system The haplo-diploid system Sex Linked • The sex chromosomes can have other genes on them – not related to sex determination • A gene located on either sex chromosome is called a sex-linked gene • Sex-linked genes follow specific patterns of inheritance Sex Linked Diseases • Some disorders caused by recessive alleles on the X chromosome in humans: – Color blindness – Muscular dystrophy (Duchenne MD) – Hemophilia LE 15-10a Sperm Ova LE 15-10b Sperm Ova LE 15-10c Sperm Ova X-Inactivation in Females • In mammals, one of the two X chromosomes is randomly inactivated when “embryo” • If a female = heterozygous for a gene located on the X chromosome, she will be a mosaic for that character • Very little is understood about how this works; Alex will tell you more… very soon! LE 15-11 Two cell populations in adult cat: Active X Early embryo: Orange fur X chromosomes Cell division Inactive X and X chromosome Inactive X inactivation Black fur Allele for orange fur Allele for black fur Active X Mutations • In non-disjunction, pairs of homologous chromosomes do not separate normally during meiosis • As a result: – one gamete receives two chromosomes – the other gamete receives zero LE 15-12 Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n+1 n+1 n–1 n–1 n+1 n–1 n Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I Nondisjunction of sister chromatids in meiosis I n Abnormalities • Aneuploidy = abnormal number of a particular chromosome • Trisomy 3n = three copies of a particular chromosome • Monosomic = chromosome missing in the zygote (only one present) • Polyploidy = is a condition in which an organism has more than two complete sets of chromosomes • Tetraploidy = 4n = four sets of chromosomes Chromosome Breaking Apart • Breakage = four types of changes in chromosome structure: – Deletion removes a chromosomal segment – Duplication repeats a segment – Inversion reverses a segment within a chromosome – Translocation moves a segment from one chromosome to another LE 15-14a Deletion A deletion removes a chromosomal segment. LE 15-14b Duplication A duplication repeats a segment. LE 15-14c Inversion An inversion reverses a segment within a chromosome. LE 15-14d Reciprocal translocation A translocation moves a segment from one chromosome to another, nonhomologous one. Genetic Disorders • Down syndrome is an aneuploid condition that results from three copies of chromosome 21 (Trisomy 21) • It affects about 1 / 700 children born in the United States • The frequency of Down syndrome increases with the age of the mother (several theories here…) Down’s Syndrome Maternal Age < Risk of chromosomal abnormality Risk of Down’s Syndrome 15-24 1/500 1/1500 25-29 1/385 1/1100 35 1/178 1/350 40 1/63 1/100 45 1/18 1/25 Figure 15-15 Sex Chromosome Abnormalities • Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals • Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable (survivable) monosomy in humans Chromosomal Abnormalities • One syndrome, cri du chat (“cry of the cat”), results from a deletion in chromosome 5 • A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood • Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes LE 15-16 Normal chromosome 9 Reciprocal translocation Translocated chromosome 9 Philadelphia chromosome Normal chromosome 22 Translocated chromosome 22 Exceptions to the Rule • There are two normal exceptions to Mendelian genetics • One involves genes located: – in the nucleus – involves genes located outside the nucleus Genes located in Organelles • Extra-nuclear genes are genes found in organelles in the cytoplasm • This depends on the maternal parent because the zygote’s cytoplasm comes from the egg • The first evidence of extra-nuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant Figure 15-18 Genetic Imprinting • For a few traits in mammals - the phenotype depends on which parent the trait came from • This is called genetic imprinting • This involves the silencing of certain genes that are “stamped” with an imprint during gamete production LE 15-17a Normal lgf2 allele (expressed) Paternal chromosome Maternal chromosome Normal lgf2 allele (not expressed) Wild-type mouse (normal size) A wild-type mouse is homozygous for the normal lgf2 allele. LE 15-17b Normal lgf2 allele (expressed) Paternal Maternal Mutant lgf2 allele (not expressed) Normal size mouse Mutant lgf2 allele (expressed) Paternal Maternal Normal lgf2 allele (not expressed) Dwarf mouse When a normal lgf2 allele is inherited from the father, heterozygous mice grow to normal size. But when a mutant allele is inherited from the father, heterozygous mice have the dwarf phenotype. Human Example • Depends on whether the deletion is inherited from Mother or Father on chromosome 15q: • If deletion in Father’s 15q: (Child inherits both copies of 15q from Mother) – Prader-Willi syndrome: developmental delay, small or undescended testes, obesity (never feel satisfied), short stature, and mild retardation. • If deletion in Mother’s 15q (Child inherits both copies of 15q from Father) – Angelman syndrome: seizures, severe mental retardation, inappropriate laughter, and a characteristic face that is small with a large mouth and prominent chin. Prader-Willi Syndrome Angelman’s Syndrome Figure 15-13