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Chapter 15 The Chromosomal Basis of Inheritance PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Mr. Barnes Biology 2012 Lectures by ChrisHonors Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Locating Genes on Chromosomes • A century ago the relationship between genes and chromosomes was not obvious • Today we can show that genes are located on chromosomes (locus) • The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes • Several researchers proposed in the early 1900s that genes are located on chromosomes • The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 • Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors 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 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 LE 15-4 P Generation F1 Generation F2 Generation P Generation Ova (eggs) Sperm F1 Generation Ova (eggs) F2 Generation Sperm Try a basic Punnett Square to prove this is an Xlinked trait: Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Try a basic Punnett Square to prove this is an Xlinked trait: Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.2: 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 • 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 • Morgan crossed flies that differed in traits of body color and wing size Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 • 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 • Understanding this result involves exploring genetic recombination, production of offspring with combinations of traits differing from either parent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-UN278-1 Parents in testcross Most offspring or Genetic Recombination and Linkage • The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombination of Unlinked Genes: Independent Assortment of Chromosomes • Mendel observed that combinations of traits in some offspring differ from 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 recombinant types, or recombinants • 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 • Morgan discovered that genes can be linked, but the linkage was incomplete, as evident from recombinant phenotypes • Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome • That mechanism was the crossing over of homologous chromosomes 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 Quiz time • Get your clicker • QUIZ Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Linkage Mapping: 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 • Map units indicate relative distance and order, not precise locations of genes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-7 Recombination frequencies 9% 9.5% 17% b Chromosome cn vg • Sturtevant used recombination frequencies to make linkage maps of fruit fly genes • Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes • Cytogenetic maps indicate the positions of genes with respect to chromosomal features Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 • In humans and other animals, there is a chromosomal basis of sex determination Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Chromosomal Basis of Sex • 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 • Other animals have different methods of sex determination Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-9 Parents Sperm Ova The X-Y system The X-0 system The Z-W system The haplo-diploid system Zygotes (offspring) Males have only one sex chromosome (XO). Sex is determined by if sperm contains X chromosome or not. Sex chromosome in ovum determines sex of offspring. No sex chromosomes! Females develop from fertilized ovum (Diploid). Males develop from unfertilized ova (Haploid) and have no father. 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 Transmission of sex-linked recessive traits Sperm Ova Ova Father with disorder will transmit mutant allele to all daughter, but none of his sons. Daughter will become carriers. Sperm Sperm Ova If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier and a 50% chance a son will have the disorder. If a carrier mates with a male with the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. • 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 (Barr Body) • If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character (X chromosomes from both the father and the mother) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders • Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Abnormal Chromosome Number • In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis • As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy 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 II n • Aneuploidy results from the fertilization of gametes in which nondisjunction occurred • Offspring with this condition have an abnormal number of a particular chromosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A trisomic zygote has three copies of a particular chromosome • A 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 Tetraploid mammal: Plains Viscacha Rat (Tympanoctomys barrerae) has four copies of each chromosome probably caused by some error in some past event of mitosis of meiosis. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Most of the strawberries you but today are octoploidy 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 Human Disorders Due to Chromosomal Alterations • Alterations of chromosome number and structure are associated with some serious disorders • Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond • These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Down Syndrome • Down syndrome is an aneuploid condition that results from three copies of chromosome 21(trisomy 21) • It affects about one out of every 700 children born in the United States • The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 • The most common symptom is infertility. Other symptoms may include: – Abnormal body proportions (long legs, short trunk, shoulder equal to hip size) – Enlarged breasts (gynecomastia) – Sexual problems – Less than normal amount of pubic, armpit, and facial hair – Small, firm testicles – Small penis – Tall height Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Aneuploidy of Sex Chromosomes • Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans • Possible symptoms in young infants include: • – Swollen hands and feet – Wide and webbed neck A combination of the following symptoms may be seen in older females: – Absent or incomplete development at puberty, including sparse pubic hair and small breasts – Broad, flat chest shaped like a shield – Drooping eyelids – Dry eyes – Infertility – No periods (absent menstruation) – Short height – Vaginal dryness, can lead to painful intercourse 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 • Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-16 The cancerous cells in nearly all CML patients contain an abnormally small chromosome 22 and a long chromosome 9. Normal chromosome 9 Reciprocal translocation Translocated chromosome 9 Philadelphia chromosome Normal chromosome 22 Translocated chromosome 22 Concept 15.5: Some inheritance patterns are exceptions to the standard chromosome theory • There are two normal exceptions to Mendelian genetics • One exception involves genes located in the nucleus, and the other exception involves genes located outside the nucleus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genomic Imprinting • For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits • Such variation in phenotype is called genomic imprinting • Genomic imprinting involves the silencing of certain genes that are “stamped” with an imprint during gamete production Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-17 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. Normal lgf2 allele (expressed) Igf2 – insulin like growth factor: Only paternal allele is 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. Inheritance of Organelle Genes • Extranuclear genes are genes found in organelles in the cytoplasm • The inheritance of traits controlled by extranuclear genes depends on the maternal parent 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stripped or spotted leaves are a result of mutations in pigment genes located in plastids, inherited from the maternal parent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some diseases affecting the muscular and nervous systems are caused by defects in mitochondrial genes, most of which come from the cytoplasm of the egg. • These mitochondrial genes help make up the protein complex of the ETC and ATP synthase which leads to lower production of ATP. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The nervous system is impacted greatly by the reduced amount of ATP. • Leads to weakness, intolerance to exercise and muscle deterioration. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Homework • Textbook: – Pgs. 291 – 292: 1-3, 6-10, 12, 14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings