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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.

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Presentation on theme: "Chapter 15 Chromosomal Basis of Inheritance. Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes Mitosis and meiosis."— Presentation transcript:

1 Chapter 15 Chromosomal Basis of Inheritance

2 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.

3 Figure 15.2a P Generation Yellow-round seeds ( YYRR ) Green-wrinkled seeds ( yyrr )  Meiosis Fertilization Gametes Y Y R R Y R y y r y r r

4 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. 12 21 1/41/4 1/41/4 1/41/4 1/41/4 YR yr Yr yR

5 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

6 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.

7 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.

8 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 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 © 2011 Pearson Education, Inc.

9 Figure 15.4b F 2 Generation P Generation Eggs Sperm X ww CONCLUSION X X Y ww ww ww ww ww ww ww ww ww ww w w w w w w F 1 Generation

10 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.

11 Figure 15.5 X Y

12 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.

13 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.

14 Recessive Sex-Linked Trait Female must be homozygous in order to express the trait Fewer females with sex- linked 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

15 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?

16 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?

17 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.

18 Figure 15.8 Early embryo: X chromosomes Allele for orange fur Allele for black fur Two cell populations in adult cat: Cell division and X chromosome inactivation Active X Inactive X Active X Black fur Orange fur

19 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 Concept 15.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome © 2011 Pearson Education, Inc.

20 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.

21 Figure 15.9-4 P Generation (homozygous) Wild type (gray body, normal wings) F 1 dihybrid (wild type) Testcross offspring TESTCROSS b  b  vg  vg  b  b vg  vg b b vg vg Double mutant (black body, vestigial wings) Double mutant Eggs Sperm EXPERIMENT RESULTS PREDICTED RATIOS Wild type (gray-normal) Black- vestigial Gray- vestigial Black- normal b  vg  b vg b  vgb vg  b  b vg  vg b b vg vgb  b vg vgb b vg  vg 965 944 206 185 1 1 1 1 1 0 1 0 If genes are located on different chromosomes: If genes are located on the same chromosome and parental alleles are always inherited together: : : : : : : : : : b vg

22 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.

23 Figure 15.UN02 Gametes from green- wrinkled homozygous recessive parent ( yyrr ) Gametes from yellow-round dihybrid parent ( YyRr ) Recombinant offspring Parental- type offspring YR yr Yr yR yr YyRr yyrr Yyrr yyRr

24 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.

25 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 

26 Recombinant Frequency Problem Total offspring – 1566 Parental types – 1472 Recombinants – 94 Frequency = (# of recombinants/Total) * 100 = % =(distance between the two alleles aka map units)

27 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

28 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.

29 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.

30 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

31 Alterations of Chromosome Number Nondisjunction Certain homologous chromosomes fail to separate during meiosis May occur in anaphase I or anaphase II Results in aneuploidy

32 Meiosis I Meiosis II Nondisjunction Non- disjunction Gametes Number of chromosomes Nondisjunction of homo- logous chromosomes in meiosis I (a) Nondisjunction of sister chromatids in meiosis II (b) n  1 n  1 n  1 n  1 n  1 n n Figure 15.13-3

33 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.

34 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.

35 Figure 15.15

36 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

37 Alterations of Chromosome Structure Translocation Fragments reattach to nonhomologous chromosomes Chronic mylegenous leukemia (22 switches with 9)

38 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.

39 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

40 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

41 It appears that imprinting is the result of the methylation (addition of —CH 3 ) 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.

42 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

43 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.


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