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Chapter 15 The Chromosomal Basis of Inheritance PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes • The chromosome theory of inheritance states that: – Mendelian genes have specific loci (positions) on chromosomes – It is the chromosomes that undergo segregation and independent assortment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-2 P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) Meiosis Fertilization Gametes All F1 plants produce yellow-round seeds (YyRr) F1 Generation Meiosis LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT Two equally probable arrangements of chromosomes at metaphase I Anaphase I Metaphase II Gametes F2 Generation Fertilization among the F1 plants Morgan’s Experimental Evidence: Scientific Inquiry • The first solid evidence associating a specific gene with a a specific chromosome came from Thomas Hunt Morgan, an embryologist. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s Choice of Experimental Organism • Characteristics that make fruit flies a convenient organism for genetic studies: – They breed at a high rate – A generation can be bred every two weeks – They have only four pairs of chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Morgan noted wild type, or normal, phenotypes that were common in the fly populations • Traits alternative to the wild type are called mutant phenotypes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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-eye mutant allele must be located on the X chromosome • Morgan’s finding supported the chromosome theory of inheritance Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings P Generation F1 Generation F2 Generation P Generation Ova (eggs) Sperm F1 Generation Ova (eggs) F2 Generation Sperm Concept 15.2: Linked genes tend to be inherited together because they are located near each other on the same chromosome • Genes located on the same chromosome that tend to be inherited together are called linked genes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How Linkage Affects Inheritance: Scientific Inquiry • Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters 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) b+ b vg+ vg Double mutant (black body, vestigial wings) TESTCROSS b b vg vg Ova 965 944 Wild type Black(gray-normal) vestigial 206 Grayvestigial 185 Blacknormal Sperm Parental-typeRecombinant (nonparental-type) offspring offspring Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • From the results, Morgan reasoned that body color and wing size are usually inherited together in specific combinations (parental phenotypes) because the genes are on the same chromosome • However, nonparental phenotypes were also produced Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombination of Unlinked Genes: Independent Assortment of Chromosomes • Parental types - Offspring with a phenotype matching one of the parental phenotypes. • Recombinant types or Recombinants - Offspring with nonparental phenotypes (new combinations). – A 50% frequency of recombination is observed for any two genes on different chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-UN278-2 Gametes from yellow-round heterozygous parent (YyRr) Gametes from greenwrinkled homozygous recessive parent (yyrr) Parental-type offspring Recombinant offspring Recombination of Linked Genes: Crossing Over • Crossing over of homologous chromosomes explains how recombinants can sometimes occur to otherwise linked genes. Animation: Crossing Over Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-6 Testcross parents Black body, vestigial wings (double mutant) Gray body, normal wings (F1 dihybrid) 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. Recombinant chromosomes 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 Linkage Mapping: Using Recombination Data: Scientific Inquiry • The farther apart two genes are, the higher the probability that a crossover will occur between them, and therefore the higher the recombination frequency Recombination frequencies 9% 9.5% 17% b Chromosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings cn vg • 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, or centimorgan, represents a 1% recombination frequency Recombination frequencies 9% 9.5% 17% b Chromosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings cn vg 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 Concept 15.3: Sex-linked genes exhibit unique patterns of inheritance • An organism’s sex is an inherited phenotypic character determined by the presence or absence of certain chromosomes • In humans and other mammals, there are two varieties of sex chromosomes, X and Y Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-9 Parents Ova Sperm Zygotes (offspring) The X-Y system The X-0 system The Z-W system The haplo-diploid system Inheritance of Sex-Linked Genes • The sex chromosomes have genes for many characters unrelated to sex • A gene located on either sex chromosome is called a sex-linked gene • Sex-linked genes follow specific patterns of inheritance Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-10 Sperm Ova Sperm Ova Sperm Ova • Some disorders caused by recessive alleles on the X chromosome in humans: – Color blindness – Duchenne muscular dystrophy – Hemophilia Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings X inactivation in Female Mammals • In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development • If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character Two cell populations in adult cat: Active X Early embryo: X chromosomes Orange fur Cell division Inactive X and X Inactive X chromosome inactivation Allele for orange fur Active X Allele for Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings black fur Black fur Abnormal Chromosome Number • In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 • Aneuploidy results from the fertilization of gametes in which nondisjunction occurred – Trisomic zygote has three copies of a particular chromosome – Monosomic zygote has only one copy of a particular chromosome – Polyploidy is a condition in which an organism has more than two complete sets of chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Aneuploidy • Trisomy 21 (Down’s Syndrome) affects about one out of every 700 children born in the United States. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alterations of Chromosome Structure • Breakage of a chromosome can lead to 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-14 A deletion removes a chromosomal segment. A duplication repeats a segment. An inversion reverses a segment within a chromosome. A translocation moves a segment from one chromosome to another, nonhomologous one. Deletion Duplication Inversion Reciprocal translocation Aneuploidy of Sex Chromosomes • Nondisjunction of sex chromosomes produces a variety of aneuploid conditions • 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 monosomy in humans Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Disorders Caused by Structurally Altered Chromosomes • One syndrome, cri du chat (“cry of the cat”), results from a specific 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 Chromosomal Disorders Website Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings