1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 15 The Chromosomal Basis of Inheritance

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings الامتحان الاول لمساق ب 101 (شعبة 3 ) يوم الاربعاء 14-12-2005l/الساعة 4,15- 5،15 المكان الكليات الهندسية وعلى النحو التالي: رقم التسلسل القاعة CH2 111 1-26 E2 011 27-50 E2 113 51-74 E2 114 75-98 E2 115 99-119 M2 008 120-166 M5 124 167-213 M5 125 214-238

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Based on Mendelian observations, biologists developed the Chromosomal Theory of Inheritance. This theory indicates that: 1-Mendelian factors or genes are located on the chromosomes. 2-It is the chromosomes that segregate and independently assort. Overview: Locating Genes on Chromosomes Genes: Are located on chromosomes – Can be visualized using certain techniques

4. 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

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The chromosome theory of inheritance states that – Mendelian genes have specific loci on chromosomes – Chromosomes undergo segregation and independent assortment

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s Experimental Evidence: Scientific Inquiry Morgan traced a gene to a specific chromosome: Morgan selected the fruit fly Drosophila melanogaster as the experimental organisms because these flies:  A re easily cultured in the laboratory.  Are prolific breeders ( سريعة التكاثر): they breed at a high rate  Have a short generation time (A new generation can be bred every two weeks).  Have only four pairs of chromosomes: three pairs of autosomes (II, III and IV) and one pair of sex chromosomes.

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan’s Choice of Experimental Organism Thomas Hunt Morgan – Provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

8. 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 – Wild type: Normal or most frequently observed phenotype. Traits alternative to the wild type – Are called mutant phenotypes – Mutant phenotypes: phenotypes that are alternative s to the wild type and which are due to mutations in the wild type gene.

9. 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 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 discovered a single male fly with white eyes instead of the wild-type (normal) red. Morgan made the following cross: – w = white eye allele. w + = red eye allele.

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Since white eyes appeared only in males, then it is sex linked. Morgan concluded that this gene is located only on the X chromosome. Morgan determined t hat the white-eye mutant allele must be located on the X chromosome

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 – Linked genes: genes that are located on the same chromosome and tend to be inherited together. –  linked genes do not assort independently because they are on the same chromosome. –  A dihybrid cross following two linked genes will not produce an F2 phenotypic ratio of 9:3:3:1.

12. 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

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings How Linkage Affects Inheritance: Scientific Inquiry Independent assortment of chromosomes and crossing over cause genetic variations. Genetic Recombination : The production of offspring with new combinations of traits differing from those found in parents. Mendel discovered that some offspring from dihybrid crosses have phenotypes unlike either parent. Morgan did other experiments with fruit flies – To see how linkage affects the inheritance of two different characters

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan crossed flies – That differed in traits of two different characters

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan determined that – Genes that are close together on the same chromosome are linked and do not assort independently – Unlinked genes are either on separate chromosomes of are far apart on the same chromosome and assort independently

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Recombination and Linkage Genetic Recombination : The production of offspring with new combinations of traits differing from those found in parents. Recombinants because these trait were not found in the parents. In this cross the seed shape and the seed color are unlinked. This is called Recombination of Unlinked Genes.( independent assortment of chromosomes). The recombination of linked genes: Crossing over If genes are totally linked, some possible phenotypic combinations should not appear. Sometimes however, the unexpected recombinants appear. -Mendel discovered that some offspring from dihybrid crosses have phenotypes unlike either parent.

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombination of Unlinked Genes: Independent Assortment of Chromosomes When Mendel followed the inheritance of two characters -He observed that some offspring have combinations of traits that do not match either parent in the P generation – The green round seeds (yyRr) and the yellow wrinkled (Yyrr) are Recombinants because these trait were not found in the parents. – In this cross the seed shape and the seed color are unlinked. This is called Recombination of Unlinked Genes.( independent assortment of chromosomes).

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombinant offspring – Are those that show new combinations of the parental traits When 50% of all offspring are recombinants – Geneticists say that there is a 50% frequency of recombination

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombination of Linked Genes: Crossing Over Morgan discovered that genes can be linked But due to the appearance of recombinant phenotypes, the linkage appeared incomplete If genes are totally linked, some possible phenotypic combinations should not appear. Sometimes however, the unexpected recombinants appear. Example: dihybrid cross of Drosophila with following traits: b = black body vg = vestigial wings b + = gray body vg + = wild-type wings (normal) b + b vg + vg x bbvgvg gray normal wings black vestigial wings

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morgan proposed that – Some process must occasionally break the physical connection between genes on the same chromosome – Crossing over of homologous chromosomes was the mechanism –The recombination of linked genes: Crossing over

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 931 recombinants Recombination frequency = --------------------------------- x 100 = 17% 2300 total offspring Conclusion : The results show that the genes are neither unlinked nor totally linked: · if wing type and body color genes were linked, they would assort independently and give a phenotypic ratio of 1:1:1:1. · If they were completely linked, expected results from the testcross would be 1:1 phenotypic ratio of parental types only. · Since the 17% were recombinants, it means that the linkage must be incomplete. Some mechanism must break the linkage between genes. · This mechanism is now known as crossing over. Recombination data is used to map a chromosomes genetic loci. (see the book for details).

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Linkage Mapping: Using Recombination Data: Scientific Inquiry A genetic map – Is an ordered list of the genetic loci along a particular chromosome – Can be developed using recombination frequencies

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A linkage map – Is the actual map of a chromosome based on recombination frequencies

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The farther apart genes are on a chromosome – The more likely they are to be separated during crossing over

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Many fruit fly genes – Were mapped initially using recombination frequencies

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.3: Sex-linked genes exhibit unique patterns of inheritance

27. 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 Sex Chromosomes: –  in most species sex is determined by the presence or absence of special chromosomes. –  As a result of meiotic segregation, each gamete has one sex chromosome to contribute to fertilization. –  Mammals, including humans, have an X-Y that determines sex at fertilization. –  Each gamete has one sex chromosome, when sperm and ovum unite at fertilization, the zygote receives one of two possible combinations: XX or XY. –  Males are XY and females are XX. –  Sex of embryo depends on Y chromosome.

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In humans and other mammals – There are two varieties of sex chromosomes, X and Y

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Different systems of sex determination – Are found in other organisms

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

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sex-linked genes – Follow specific patterns of inheritance

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sex-linked disorders in humans: In humans, the term sex-linked traits refers to X-linked traits Some recessive alleles found on the X chromosome in humans cause certain types of disorders Sex chromosomes also carry genes for other traits. The human X chromosome is much longer than the Y, therefore there are more X-linked than Y-linked traits.  Most X-linked genes have no loci on Y chromosome.  Examples of sex-linked disorders are: Color blindness (عمى ألوان), Duchenne muscular dystrophy and hemophilia.  Father pass X-linked alleles to ONLY and ALL of their daughters.  Males receive their X chromosome only from their MOTHER.  Fathers can not pass sex-linked traits to their sons.  Mothers can pass sex-linked alleles to BOTH sons and daughters.  Females receive two X chromosomes, one from each parent.  Mothers pass on one X chromosome to every daughter and son.

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings X inactivation in Female Mammals X-inactivation in Females : In mammalian females, One of the two X chromosomes in each cell is randomly inactivated during embryonic development  In female mammals, most diploid cells have only on functional X chromosome.  The other X chromosome is not active and is called: Barr body.  Most Barr body genes are not expressed.  Barr body is activated in gonadal cells ( خلايا الغدد الجنسية) that undergo meiosis to form gametes.  X chromosome inactivation is associated with DNA methylation.  Methylation of DNA means addition of methyl group (-CH 3 ) to cytosine.

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If a female is heterozygous for a particular gene located on the X chromosome – She will be a mosaic for that character

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.4: Alteration of chromosome number or structure cause some genetic disorders: Large-scale chromosomal alterations – Often lead to spontaneous abortions or cause a variety of developmental disorders I.Alteration of chromosome number: Nondisjunction: (عدم الانفصال) Chromosomes or sister chromatids do not separate during meiosis or mitosis.

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Abnormal Chromosome Number When nondisjunction occurs – Pairs of homologous chromosomes do not separate normally during meiosis – Gametes contain two copies or no copies of a particular chromosome

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Meiotic nondisjunction:  may occur during meiosis so that homologues do not separate.  May occur during Meiosis II,sister chromatids do not separate.  Results in one gamete receiving two copies of a chromosome and the other gamete receives zero copy. Mitotic nondisjunction:  also results in abnormal number of chromosomes.  If occurred in embryonic cells, division passes it to a large number of cells resulting in a big effect. Aneuploidy (Is a condition in which offspring have an abnormal number of a particular chromosome) Monosomy: cell having one copy of a chromosome. – Results from the fertilization of gametes in which nondisjunction occurred

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If a zygote is trisomic Trisomy: cell having three copies of a particular chromosome. Example: Down Syndrome (المنغولية) which results from trisomy of chromosome 21. If a zygote is monosomic Monosomy: cell having one copy of a particular chromosome.

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Polyploidy: Is a condition in which there are more than two complete sets of chromosomes in an organism. occur more commonly in plants but rare in animals. Triploidy: is a polyploid chromosome number with three haploid chromosome sets (3N). Tetraploidy: with four haploid chromosome sets (4N).

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alterations of Chromosome Structure Alteration of Chromosome Structure: Breakage of a chromosome can lead to four types of changes in chromosome structure 1-Deletion: chromosomes which lose a fragment lacking a centromere will have a deficiency.  Duplication  Fragments without centromeres may: join to homologous chromosome 3-Translocation: Join non homologous chromosome 4: Inversion: R eattach to the original chromosome in reverse order

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alterations of chromosome structure Figure 15.14a–d A B CD E FG H Deletion A B C E G H F A B CD E FG H Duplication A B C B D E C F G H A A MN OPQR B CD EFGH B CDEFGH Inversion Reciprocal translocation A BPQ R M NOCDEF G H A D CBEFH G (a) A deletion removes a chromosomal segment. (b) A duplication repeats a segment. (c) An inversion reverses a segment within a chromosome. (d) A translocation moves a segment from one chromosome to another, nonhomologous one. In a reciprocal translocation, the most common type, nonhomologous chromosomes exchange fragments. Nonreciprocal translocations also occur, in which a chromosome transfers a fragment without receiving a fragment in return.

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Human Disorders Due to Chromosomal Alterations Alterations of chromosome number and structure: Are associated with a number of serious human disorders I-alteration of chromosome numbers: Down Syndrome: Aneuploid condition: usually the result of trisomy 21.  The child has characteristic facial features, short statures, heart defects, mental retardation(تخلف عقلي).  May be related to mother’s age. Patau Syndrome: trisomy 13/ Edwards Syndrome: trisomy 18. These two disorders are rare.

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Down Syndrome Down syndrome – Is usually the result of an extra chromosome 21, trisomy 21

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Aneuploidy of Sex Chromosomes Nondisjunction of sex chromosomes – Produces a variety of aneuploid conditions – Condition of having an abnormal number of certain chromosomes – Monosomy: cell having one copy of a chromosome. – Trisomy: cell having three copies of a chromosome. – Example: Down Syndrome (المنغولية) which results from trisomy of chromosome 21.

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Klinefelter syndrome : Is the result of an extra chromosome in a male, producing XXY individuals Aneuploidy in males: Genotype: usually XXY, but may be XXYY, XXXY, XXXXY. Phenotype: male sex organs with abnormally small testis, sterile (عقيم), feminine (أنثوي) body but with normal intelligence. Extra Y: Genotype: XYY /Phenotype: normal male, usually taller than average, normal intelligence & fertile. Turner syndrome : Is the result of monosomy X, producing an X0 karyotype Genotype: XO (monosomy) Phenotype: short stature, sex organs do not mature, sterile. Triple-X syndrome: Genotype: XXX Phenotype: usually fertile with normal phenotype.

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Disorders Caused by Structurally Altered Chromosomes II-Alteration of chromosome structure: Cri du Chat: Is a disorder caused by a deletion in a chromosome # 5 Cause mental retardation, small head and a cry like a mewing cat.

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Certain cancers: Are caused by translocations of chromosomes Chronic myelogenous Leukemia (CML): caused by translocation between chr 22 & 9

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.5: Some inheritance patterns are exceptions to the standard chromosome theory Two normal exceptions to Mendelian genetics include – Genes located in the nucleus – Genes located outside the nucleus

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genomic Imprinting In mammals – The phenotypic effects of certain genes depend on which allele is inherited from the mother and which is inherited from the father

50. 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

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inheritance of Organelle Genes Extranuclear genes – Are genes found in organelles in the cytoplasm

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The inheritance of traits controlled by genes present in the chloroplasts or mitochondria – Depends solely on the maternal parent because the zygote’s cytoplasm comes from the egg

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some diseases affecting the muscular and nervous systems – Are caused by defects in mitochondrial genes that prevent cells from making enough ATP