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Chapter 15 Chromosomal Basis of Inheritance Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes • Mitosis and meiosis were first described in the late 1800s • The chromosome theory of inheritance states: – Mendelian genes have specific loci (positions) on chromosomes – Chromosomes undergo segregation and independent assortment © 2011 Pearson Education, Inc. Figure 15.2a P Generation Yellow-round seeds (YYRR) Y Y R R r y y r Meiosis Fertilization Gametes R Y y r Green-wrinkled seeds (yyrr) Figure 15.2b All F1 plants produce yellow-round seeds (YyRr). F1 Generation R y r R y r Y Y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. r R Y y r R Metaphase I y Y 1 1 R r r R Y y Anaphase I Y y r R 2 y Y Y R R 1/ 4 YR r 1/ 4 yr y Y Y Y y r R 2 y Y Gametes r Metaphase II r r 1/ 4 Yr y y R R 1/ 4 yR Figure 15.2c LAW OF INDEPENDENT ASSORTMENT LAW OF SEGREGATION F2 Generation 3 Fertilization recombines the R and r alleles at random. An F1 F1 cross-fertilization 9 :3 :3 :1 3 Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. Morgan’s Experimental Evidence: Scientific Inquiry • -specific gene / specific chromosome came from Thomas Hunt Morgan • Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors © 2011 Pearson Education, Inc. • Worked with fruit flies- why? – wild type, or normal, phenotypes that were common in the fly populations – mutant phenotypes -alternative to the wild type © 2011 Pearson Education, Inc. Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair • In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) – The F1 generation all had red eyes – The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes • Morgan determined that the white-eyed mutant allele must be located on the X chromosome © 2011 Pearson Education, Inc. Figure 15.4b CONCLUSION P Generation X X w X Y w w Eggs F1 Generation Sperm w w w w w Eggs F2 Generation w w w Sperm w w w w w w Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance • In humans other animals, there is a chromosomal basis of sex determination • Only ends of Y chromosome have regions that are homologous with regions of the X chromosome • The SRY gene on the Y chromosome – Sex determining Region of Y – If absent gonads develop into ovaries © 2011 Pearson Education, Inc. Figure 15.5 X Y • Females are XX, and males are XY • Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome • Other animals have different methods of sex determination © 2011 Pearson Education, Inc. • A gene that is located on either sex chromosome is called a sex-linked gene • Genes on the Y chromosome are called Y-linked genes; there are few of these • Genes on the X chromosome are called X-linked genes – Fathers- x-linked to all daughters/no sons – Mothers-x-linked to sons and daughters © 2011 Pearson Education, Inc. Recessive Sex-Linked Trait Female must be homozygous in order to express the trait • Fewer females with sexlinked disorders Males are hemizygous • a condition where only one copy of a gene is present in a diploid organism • More males than females have x-linked disorders Examples • Color blindness, hemophilia, Duchenne muscular dystrophy Sex Linkage Problem Color Blindness - recessive A man who is color blind is married to a female that carries the color blindness allele. What is the probability that a son would be color blind? Daughter? A daughter that is a carrier? Sex Linkage Problem Hemophilia- recessive A man with hemophilia marries a woman with normal clotting factors but is heterozygous. What is the probability that they will have a … daughter with hemophilia? What is the probability that they will have a… son with hemophilia? What is the probability that their son… will have hemophilia? X Inactivation in Female Mammals • one of the two X chromosomes in each cell is randomly inactivated during embryonic development • The inactive X condenses into a Barr body – Reactivated in gonad cells – Females are a “mosaic” of cells © 2011 Pearson Education, Inc. Figure 15.8 X chromosomes Allele for orange fur Early embryo: Two cell populations in adult cat: Allele for black fur Cell division and X chromosome inactivation Active X Inactive X Active X Black fur Orange fur Concept 15.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome • Each chromosome has hundreds or thousands of genes (except the Y chromosome) • Genes located on the same chromosome that tend to be inherited together are called linked genes © 2011 Pearson Education, Inc. How Linkage Affects Inheritance • Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters • Saw higher # of parental types than expected if it was a dihybrid cross- determined wing/body color inherited together – these genes do not assort independently, and reasoned that they were on the same chromosome © 2011 Pearson Education, Inc. Figure 15.9-4 EXPERIMENT 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) Double mutant TESTCROSS b b vg vg b b vg vg Testcross offspring Eggs b vg b vg b vg Wild type Black(gray-normal) vestigial b vg Blacknormal Grayvestigial b vg Sperm b b vg vg b b vg vg b b vg vg b b vg vg PREDICTED RATIOS If genes are located on different chromosomes: 1 : 1 : 1 : 1 If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 1 : 0 : 0 965 : 944 : 206 : 185 RESULTS Genetic Recombination • nonparental phenotypes produced • the production of offspring with combinations of traits differing from either parent – Recombinants – 50% recombinants expected for genes on different chromosomes- ie independent assortment © 2011 Pearson Education, Inc. Figure 15.UN02 Gametes from yellow-round dihybrid parent (YyRr) Gametes from greenwrinkled homozygous recessive parent (yyrr) YR yr Yr yR YyRr yyrr Yyrr yyRr yr Parentaltype offspring Recombinant offspring Recombination of Linked Genes: Crossing Over • Morgan discovered that genes can be linked, but the linkage was incomplete-some recombinant phenotypes were observed • He proposed: – some process must occasionally break the physical connection between genes on the same chromosome • That mechanism was the crossing over of homologous chromosomes © 2011 Pearson Education, Inc. Figure 15.10b Recombinant chromosomes Eggs Testcross offspring bvg 965 Wild type (gray-normal) b vg b vg b vg 944 Blackvestigial 206 Grayvestigial 185 Blacknormal b vg b vg b vg b vg b vg b vg b vg b vg Parental-type offspring Recombinant offspring Recombination 391 recombinants 100 17% frequency 2,300 total offspring b vg Sperm Recombinant Frequency Problem • • • • Total offspring – 1566 Parental types – 1472 Recombinants – 94 Frequency = (# of recombinants/Total) * 100 = % =(distance between the two alleles aka map units) Recombinant Frequency Problem • A wild type fruit fly (heterozygous for gray body color and red eyes) was mated with a black fruit fly with purple eyes. What is the recombinant frequency for these genes? • Wild type – 721 • Black purple – 751 • Gray purple – 49 • Black red - 45 Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry • Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome • Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency © 2011 Pearson Education, Inc. • A linkage map is a genetic map of a chromosome based on recombination frequencies • Distances between genes can be expressed as map units; one map unit, represents a 1% recombination frequency • Map units indicate relative distance and order, not precise locations of genes © 2011 Pearson Education, Inc. Errors and Exceptions Alterations of chromosome number • Increase or decrease in the number of chromosomes from the normal diploid number • Occurs during anaphase of meiosis Alterations of chromosome structure • Actual chromosome is altered Alterations of Chromosome Number Nondisjunction • Certain homologous chromosomes fail to separate during meiosis • May occur in anaphase I or anaphase II • Results in aneuploidy Figure 15.13-3 Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n1 n1 n1 n1 n1 n1 n n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II • Aneuploidy results from the fertilization of gametes in which nondisjunction occurred – Offspring have an abnormal number of a particular chromosome • monosomic zygote has only one copy of a particular chromosome • trisomic zygote has three copies of a particular chromosome © 2011 Pearson Education, Inc. • Polyploidy is a condition in which an organism has more than two complete sets of chromosomes – Triploidy (3n) is three sets of chromosomes – Tetraploidy (4n) is four sets of chromosomes – common in plants, but not animals – more normal in appearance than aneuploids – Ex. Downs Syndrome © 2011 Pearson Education, Inc. Figure 15.15 Alterations of Chromosome Structure Deletion • Chromosomes lose a fragment of DNA • Cri du chat syndrome (#5) – A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood Inversion • Fragments break off then reattach in reverse orientation Alterations of Chromosome Structure Translocation • Fragments reattach to nonhomologous chromosomes • Chronic mylegenous leukemia (22 switches with 9) Concept 15.5: Some inheritance patterns are exceptions to standard Mendelian inheritance • There are two normal exceptions to Mendelian genetics – genes located in the nucleus – genes located outside the nucleus • In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance © 2011 Pearson Education, Inc. Mother v. Father Inheritance For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits Genomic imprinting • Induces intrinsic changes in chromosomes inherited from males and females • Silencing of certain genes that are “stamped” with an imprint during gamete production • Prader-Willi Syndrome (paternal) • Angelman Syndrome (maternal) Fragile X Syndrome • consequence of maternal genomic imprinting Figure 15.17b Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father Normal-sized mouse (wild type) Dwarf mouse (mutant) Normal Igf2 allele is expressed. Mutant Igf2 allele is expressed. Mutant Igf2 allele is not expressed. Normal Igf2 allele is not expressed. (b) Heterozygotes • It appears that imprinting is the result of the methylation (addition of —CH3) of cysteine nucleotides • Genomic imprinting is thought to affect only a small fraction of mammalian genes • Most imprinted genes are critical for embryonic development © 2011 Pearson Education, Inc. Inheritance of Organelle Genes • Extranuclear genes (or cytoplasmic genes) are found in organelles in the cytoplasm • Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules • Extranuclear genes are inherited MATERNALLY because the zygote’s cytoplasm comes from the egg • The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant • defects in mitochondrial genes prevent cells from making enough ATP – Make protein complexes of electron transport and ATP synthase – diseases that affect the muscular and nervous systems © 2011 Pearson Education, Inc.