Chromosomal Basis of Inheritance PDF

The Chromosomal Basis of Inheritance 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 ?...

The Chromosomal Basis of Inheritance 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 ? The location of a particular gene can be seen by tagging isolated chromosomes with a ? 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 Green-wrinkled seeds (YYRR) 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) Sperm F2 Generation  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 Double mutant (gray body, (black body, normal wings) vestigial wings) b+ b+ vg+ vg+ b b vg vg F1 dihybrid Double mutant (wild type (black body, (gray body, TESTCROSS vestigial wings) normal wings) b+ b vg+ vg b b vg vg Ova 965 944 206 185 Wild type Black- Gray- Black- (gray-normal) vestigial vestigial normal 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 or offspring  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 green- wrinkled homozygous recessive parent (yyrr) Parental-type Recombinant offspring offspring Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  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  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 (Cm), 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 cn vg Chromosome end  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 X IV II Y III Mutant phenotypes Short Black Cinnabar Vestigial Brown aristae body eyes wings eyes 0 48.5 57.5 67.0 104.5 Long aristae Gray Red Normal Red (appendages body eyes wings eyes on head) Wild-type phenotypes  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 Zygotes (offspring) The X-Y system The X-0 system The Z-W system The haplo-diploid system  Inheritance of Sex-Linked Genes Question: All genes located on the sex chromosomes are related to sex-related traits? Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Answer: The sex chromosomes have genes for many characters unrelated to sex A gene located on either sex chromosome is called a sex-linked gene Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 15-11 Two cell populations in adult cat: Active X Early embryo: Orange X chromosomes fur Cell division and X Inactive X chromosome Inactive X inactivation Allele for Black orange fur fur Allele for Active X black fur  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 n Number of chromosomes Nondisjunction of homologous Nondisjunction of sister chromosomes in meiosis I chromatids in meiosis II  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 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 Deletion A deletion removes a chromosomal segment. Duplication A duplication repeats a segment. An inversion reverses a segment Inversion within a chromosome. A translocation moves a segment from one chromosome to another, Reciprocal nonhomologous one. 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 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 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 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 Normal chromosome 9 Reciprocal Translocated chromosome 9 translocation Philadelphia chromosome Normal chromosome 22 Translocated chromosome 22  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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings