1. 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

2. 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

3. LE 15-2 P Generation Gametes Meiosis Yellow-round seeds (YYRR) Fertilization Green-wrinkled seeds (yyrr) All F 1 plants produce yellow-round seeds (YyRr) Meiosis Two equally probable arrangements of chromosomes at metaphase I Anaphase I LAW OF INDEPENDENT ASSORTMENT Metaphase II Fertilization among the F 1 plants F 2 Generation Gametes F 1 Generation LAW OF SEGREGATION

4. 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 determined that the white-eye mutant allele must be located on the X chromosome Morgan’s finding supported the chromosome theory of inheritance

5. LE 15-4 P Generation F 1 Generation F 2 Generation P Generation F 1 Generation Ova (eggs) Sperm F 2 Generation Ova (eggs) Sperm

6. 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

7. LE 15-UN278-1 Parents in testcross Most offspring or

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Recombination and Linkage The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination

9. 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

10. LE 15-UN278-2 Gametes from green- wrinkled homozygous recessive parent (yyrr) Parental-type offspring Gametes from yellow-round heterozygous parent (YyRr) Recombinant offspring

11. 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 Animation: Crossing Over

12. LE 15-6 Testcross parents Parental-type offspring Recombinant offspring Ova Sperm 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Gray body, normal wings (F 1 dihybrid) Sperm Testcross offspring Recombination frequency 391 recombinants 2,300 total offspring  100 = 17% = Ova Gametes Replication of chromosomes Black body, vestigial wings (double mutant) Replication of chromosomes Meiosis I and II: No new allele combinations are produced. Meiosis I: Crossing over between b and vg loci produces new allele combinations. Recombinant chromosomes Meiosis II: Separation of chromatids produces recombinant gametes with the new allele combinations.

13. 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

14. 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

15. LE 15-7 Recombination frequencies 9%9.5% 17% bcn vg Chromosome

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

17. LE 15-8 Short aristae Black body Cinnabar eyes Mutant phenotypes Vestigial wings Brown eyes Wild-type phenotypes Red eyes Normal wings Red eyes Long aristae (appendages on head) Gray body 104.567.0 57.548.5 0 III I IV X Y II

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 15.3: Sex-linked genes exhibit unique patterns of inheritance In humans and other animals, there is a chromosomal basis of sex determination

19. 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

20. LE 15-9 The X-Y system The X-0 system The Z-W system The haplo-diploid system Parents Sperm Ova Zygotes (offspring)

21. 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 Sex-linked genes follow specific patterns of inheritance

22. LE 15-10 Sperm Ova Sperm Ova Sperm Ova

23. 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

24. 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

25. LE 15-11 Early embryo: X chromosomes Allele for orange fur Cell division and X chromosome inactivation Allele for black fur Active X Inactive X Black fur Orange fur Active X Two cell populations in adult cat:

26. 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 often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders

27. 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

28. LE 15-12 Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n + 1 Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I n + 1n – 1 n + 1n – 1nn Nondisjunction of sister chromatids in meiosis I

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Aneuploidy results from the fertilization of gametes in which nondisjunction occurred Offspring with this condition have an abnormal number of a particular chromosome

30. 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

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

32. 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

33. LE 15-14 Deletion Duplication Inversion Reciprocal translocation 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.

34. 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

35. 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

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

37. 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

38. 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

39. LE 15-16 Reciprocal translocation Normal chromosome 9 Normal chromosome 22 Translocated chromosome 9 Translocated chromosome 22 Philadelphia chromosome

40. 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

41. 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

42. LE 15-17 Normal lgf2 allele (expressed) Normal lgf2 allele (not expressed) Wild-type mouse (normal size) Paternal chromosome Maternal chromosome A wild-type mouse is homozygous for the normal lgf2 allele. Normal lgf2 allele (expressed) Mutant lgf2 allele (not expressed) Normal size mouse Paternal Maternal Mutant lgf2 allele (expressed) Normal lgf2 allele (not expressed) Dwarf mouse Paternal Maternal 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.

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

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

45. Some diseases affecting the muscular and nervous systems are caused by defects in mitochondrial genes that prevent cells from making enough ATP