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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 15 The Chromosomal Basis of Inheritance What you learned already Chapter.

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Presentation on theme: "Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 15 The Chromosomal Basis of Inheritance What you learned already Chapter."— Presentation transcript:

1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 15 The Chromosomal Basis of Inheritance What you learned already Chapter 13 : Meiosis and sexual life cycles Chapter 14 : Mendel and the gene idea

2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ☞ Genes (  passengers) and chromosomes (  wagon) Overview: Locating Genes on Chromosomes Mendel’s “hereditary factors” were genes, though this wasn’t known at the time Today we can show that genes are located on chromosomes Q: How to visualize that? The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene  Chromosomes: a physical basis for the principles of heredity (=Mendel’s laws of segregation and independent assortment)  This chapter’s aim: Is to study the chromosomal basis for the transmission of genes from parents to offspring

3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 The behavior of chromosomes during meiosis can account for Mendel’s laws of segregation and independent assortment

4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 15.2 P Generation F 1 Generation Yellow-round seeds ( YYRR ) Green-wrinkled seeds ( yyrr )  Meiosis Fertilization Gametes Y Y R R Y R y y r y r All F 1 plants produce yellow-round seeds ( YyRr ). Meiosis Metaphase I Anaphase I Metaphase II R R R R R R R R R R R R rr r r rr r r r r r r YY Y Y Y Y Y Y Y Y Y Y y y y y y y y y y y y y Gametes LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation /41/4 1/41/4 1/41/4 1/41/4 YR yr Yr yR F 2 Generation 3 3 Fertilization recombines the R and r alleles at random. Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. An F 1  F 1 cross-fertilization 9 : 3 : 1 r

5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 15.2b F 1 Generation All F 1 plants produce yellow-round seeds ( YyRr ). Meiosis Metaphase I Anaphase I Metaphase II R R R R R R R R R R R R r r r r r r r r r r r r YY Y Y Y Y Y Y Y Y Y Y yy y y y y y y y y y y Gametes LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation /41/4 1/41/4 1/41/4 1/41/4 YR yr Yr yR

6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 15.2c F 2 Generation 3 Fertilization recombines the R and r alleles at random. Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. An F 1  F 1 cross-fertilization 9 : 3 : 1 LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT 3

7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Summary for the chromosomal basis of Mendel’s laws Law of segregation: Two alleles for each gene segregate during gamete formation and end up in different gametes  Each gamete gets only one of the two alleles  In terms of chromosomes, this segregation corresponds to the distribution of homologous chromosomes to different gamete (R or r allele; Y or y allele) Law of independent assortment: Alleles of genes on nonhomologous chromosomes assort independently during gamete formation  Each gamete gets one of four allele combinations (YR:Yr:yR:yr = 1:1:1:1) depending on the alternative arrangements of homologous chromosome pairs in metaphase I

8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s Experimental Evidence for chromosomal theory: Scientific Inquiry Thomas Hunt Morgan (Embryologist at Columbia Univ. early in the 20 th century) – Provided the first solid evidence associating a specific gene with a specific chromosome – Namely, provided convincing evidence that chromosomes are the location of Mendel’s heritable factors Morgan’s Choice of Experimental Organism Several characteristics 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

9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan first observed and noted : Wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type: Are called mutant phenotypes Morgan first invented a notation for symbolizing alleles in Drosophila (still widely used) – The allele for white eyes in flies is symbolized by w (indicated as a mutant allele) – A superscript + identifies the allele for the wild-type trait: w + for the allele for red eyes

10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How to Correlate 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 F 1 generation all had red eyes – The F 2 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 Morgan’s finding supported the chromosome theory of inheritance

11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s discovery that transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait – Was the first solid evidence indicating that a specific gene is associated with a specific chromosome

12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 성염색체 연관 유전자의 유전현상의 특징 Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance  We consider the role of sex chromosome in heritance  First step: reviewing the chromosomal basis of sex determination in humans and some other animals What will you examine here The Chromosomal Basis of Sex ( 성결정과 염색체 ) An organism’s sex – Is an inherited phenotypic character determined by the presence or absence of certain chromosomes (sex chromosomes) How to determine sex in an organism?

13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome Only the ends of the Y chromosome have regions that are homologous with the X chromosome The SRY gene on the Y chromosome codes for the development of testes

14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Different systems of sex determination – Are found in other organisms Figure 15.6b–d 22 + XX 22 + X 76 + ZZ 76 + ZW 16 (Haploid) 16 (Diploid) (b) The X–0 system (c) The Z–W system (d) The haplo-diploid system In grasshoppers, cockroaches, there is only one type of sex chromosome (the X) In birds, some fishes, and some insects, the sex chromosome present in the egg (not the sperm) determines the sex of offspring (Z and W) In most species of bees and ants, there are no sex chromosomes. Female develop from fertilized eggs and are thus diploid. Males develop from unfertilized eggs and haploid (no fathers) 32

15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Sex-Linked Genes Key points to keep in mind The sex chromosomes have genes for many characters unrelated to sex A gene located on either sex chromosome is called a sex-linked gene In humans, sex-linked usually refers to a gene on the larger X chromosome Female: has two X chromosomes – Homozygous recessive alleles : expression of phenotype – Heterozygous: no expression of phenotype (called carrier) Male: has only one X chromosome – You don’t use the terms homozygous and heterozygous: use “hemizygous” – Any male receiving the recessive allele from his mother will express the trait  far more males than females have sex-linked recessive disorders (The freq. of sex-linked recessive disorders in male is higher than that in female) Sex-linked trait by recessive allele on X chromosome

16 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some recessive alleles found on the X chromosome in humans cause certain types of disorders – Color blindness: – Duchenne muscular dystrophy : 1/3500 males born in US; progressive weakening of the muscles; the absence of a key muscle protein called dystrophin – Hemophilia: shows delayed blood clotting due to the absence of one or more of the proteins required for blood clotting; treated with intravenous injections of the missing protein What kinds of X-lined disorders are found?

17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 15.7a–c A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation. (a) How do sex-lined recessive traits transmit to offspring? XAXaXAXa

18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The transmission of sex-linked recessive traits Figure 15.7a–c If a carrier female mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder. (b) XaXAXaXA

19 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The transmission of sex-linked recessive traits Figure 15.7a–c If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele. (c) XaXaXaXa

20 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 The inactive X condenses into a Barr body If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character Mystery of the presence or absence of sweat glands in female skin In humans, mosaicism can be observed in a recessive X-linked mutation that prevents the development of sweat glands A woman who is heterozygous for this trait has patches of normal skin and patches of skin lacking sweat glands

21 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Location of some genes on the same chromosome is expected The genes tend to be inherited together in genetic crosses: are said to be linked genes The linked genes do not always follow Mendel’s law of independent assortment

22 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How Linkage Affects Inheritance 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

23 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings However, nonparental phenotypes were also produced The production of a small number of offspring with nonparental phenotypes indicated that some mechanism occasionally breaks the linkage between genes on the same chromosome Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent

24 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Recombination and Linkage Meiosis and random fertilization generate genetic variation among offspring of sexually reproducing organisms What you learned in the Chapter 13 What is the chromosomal basis of recombination in relation to the genetic findings of Mendel and Morgan? What will you examine next The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination

25 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

26 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, because some recombinant phenotypes were observed He proposed that some process must occasionally break the physical connection between genes on the same chromosome That mechanism was the crossing over of homologous chromosomes

27 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure Testcross parents Replication of chromosomes Gray body, normal wings (F 1 dihybrid) Black body, vestigial wings (double mutant) Replication of chromosomes Meiosis I Meiosis II Meiosis I and II Recombinant chromosomes Eggs b  vg  b vg b  vg  b  vg b vg  b vg b  vg  b  vg b vg b vg  Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Sperm Parental-type offspring Recombinant offspring Recombination frequency 391 recombinants 2,300 total offspring  100  17%  b  vg  b vg  b  vg b vg Chromosomal basis for recombination of linked genes: Linked genes exhibit RF less than 50%

28 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 15.10a Testcross parents Replication of chromosomes Gray body, normal wings (F 1 dihybrid) Black body, vestigial wings (double mutant) Replication of chromosomes Recombinant chromosomes Eggs b  vg  b vg b  vg  b  vg b vg  b vg b  vg  b  vg b vg b vg  Sperm b vg Meiosis I: Crossing over between b and vg loci produces new allele combinations. Meiosis II: Segregation of chromatids produces recombinant gametes with the new allele combinations. Meiosis I and II: Even if crossing over occurs, no new allele combinations are produced.

29 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 15.10b Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Sperm Parental-type offspringRecombinant offspring Recombination frequency 391 recombinants 2,300 total offspring  100  17%  b  vg  b vg  b  vg b vg Eggs Recombinant chromosomes b  vg  b vg b  vg b vg  RF: the percentage of recombinants in the total offspring

30 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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

31 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Example  Consider three genes: the body-color (b), wing-size (vg), and cinnabar eyes (cn) [brighter red than wt eye color]  The observed recombination frequencies between three Drosophila gene pairs : b–cn 9%, cn–vg 9.5%, and b–vg 17%  Make a linear order : cn is positioned about halfway between the other two genes: Figure How to construct a linkage map Chromosome Recombination frequencies 9% 9.5% 17% b cn vg RESULTS 1 st cross-over 2 nd CO b, black body color vg, vestigial wing cn, cinnabar eyes

32 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Because double crossovers are fairly likely to occur between b and vg in matings tracking these two genes.  A second crossover would “cancel out” the effect of the first crossover and thus reduce the observed b–vg RF. Example How to construct a linkage map (continued) Inquiry: The b–vg recombination frequency (17%) is slightly less than the sum (18.5%) of the b–cn and cn–vg frequencies. Why?

33 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Canceling out of the effect of the 1 st crossover by 2 nd cross-over

34 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genes that are far apart on the same chromosome can have a recombination frequency near 50% Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes 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

35 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders Plants tolerate such genetic changes better than animals do

36 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

37 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The consequences of nondisjunction after fertilization? Aneuploidy – Results from the fertilization of aberrant gametes (nondisjunction occurred) with a normal one – Is a condition in which offspring have an abnormal number of a particular chromosome Chromosome number of haploid (gamete): n Chromosome number of normal zygote: 2n If a zygote is trisomic (2n+1 chromosomes) – It has three copies of a particular chromosome If a zygote is monosomic (2n-1 chromosomes) – It has only one copy of a particular chromosome 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 Polyploidy is common in plants, but not animals Polyploids are more normal in appearance than aneuploids

38 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 orientation of a segment within a chromosome – Translocation moves a segment from one chromosome to another

39 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Down Syndrome (Trisomy 21) 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

40 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

41 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Disorders Caused by Structurally Altered Chromosomes The 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

42 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance 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 (Monoallelic expression) Genomic imprinting involves the silencing of certain genes that are “stamped” with an imprint during gamete production

43 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genomic imprinting Involves the silencing of certain genes that are “stamped” with an imprint during gamete production A zygote expresses only one allele of an imprinted gene (either from the mother or the father)  The imprints are transmitted to all the body cells during development A zygote expresses only one allele of an imprinted gene (either from the mother or the father)  The imprints are transmitted to all the body cells during development A gene imprinted for maternal allele expression is always imprinted for maternal allele expression, generation to generation

44 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 15.17a Genomic imprinting of the mouse Igf2 gene.

45 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 (see next slide)


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