AP Biology Day 50 Friday, January 20, 2017
Do-Now 1. Review your Gene/cs Problems with your neighbors 2. Prepare for your Ch. 14 quiz
Announcements Make sure to have a calculator Complete prac/ce quizzes! Semester 2 Grades Receive a 3+ on AP exam for +10% to your final semester 2 grade Only applies to students who completed all work
CW/HW Assignments 6. CH. 15 VCN (Linked Genes) 7. Ch. 15 Lecture Notes 8. GeneDcs WS 3 PLANNER Complete odd #s for Genetics WS 3 (separate paper) See Ms. Fleming for ALL stamps Ch. 15 Quiz Study for genetics test Ch. 13-15, with some Ch. 6-7
Quiz Protocol Clear your desks Noise level 0 Eyes on your own paper Check answers when finished Flip over when finished
EssenDal knowledge standards 3.A.4: The inheritance pasern of many traits cannot be explained by simple Mendelian gene/cs 3.C.1: Changes in genotype can result in changes in phenotype
I will be able to: FLT Explain why sex-linked diseases are more common in human males than females disdnguish between sex-linked genes and linked genes Explain how meiosis accounts for recombinant phenotypes Explain how linkage maps are constructed By comple1ng Ch. 15 Lecture Notes
Ch. 15: The Chromosomal Basis of Inheritance
Overview Mendel s hereditary factors were genes, though this wasn t known at the Dme. The locadon of a pardcular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene. 9
I. Mendelian GeneDcs and Chromosomes A. Chromosome Theory of Inheritance The chromosome theory of inheritance states: Mendelian genes have specific loci (posi/ons) on chromosomes Chromosomes undergo segrega/on and independent assortment. The behavior of chromosomes during meiosis was said to account for Mendel s laws of segregadon and independent assortment. 10
Mendel s Laws P Genera/on Y R Y R Yellow-round seeds (YYRR) y r y r Green-wrinkled seeds ( yyrr) Meiosis Gametes R Y Fer/liza/on y r All F 1 plants produce yellow-round seeds (YyRr) F 1 Genera/on R r y R r y LAW OF SEGREGATION The two alleles for each gene separate during gamete forma/on. R Y r y Y Meiosis Metaphase I Y r Y R y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete forma/on. 1 R r r R 1 Anaphase I Y y Y y R r Metaphase II r R 2 Y y Y y 2 Gametes Y R R Y r y y r r Y Y r R y R y 1 / YR 1 / yr 1 / 4 4 4 Yr 1 / 4 yr F 2 Genera/on 3 An F 1 F 1 cross-fer/liza/on 9 : 3 : 3 : 1 3
I. Mendelian GeneDcs and Chromosomes What were Mendel s hereditary factors? Where were they located?? 12
I. Mendelian GeneDcs and Chromosomes A. Morgan and Drosophila Melanogaster Thomas Hunt Morgan studied mutadon in Drosophla melanogaster (fruit flies) Morgan s experiments with fruit flies provided convincing evidence that chromosomes are the locadon of Mendel s heritable factors 13
I. Mendelian GeneDcs and Chromosomes A. Chromosome Theory of Inheritance B. Morgan and Drosophila Melanogaster Several characterisdcs make fruit flies a convenient organism for genedc studies: They breed at a high rate A generadon can be bred every two weeks They have only four pairs of chromosomes. 14
I. Mendelian GeneDcs and Chromosomes A. Morgan and Drosophila Melanogaster 1. Wild type and mutant type Different traits have been studied Commonly, eye color is looked at Ex/ Wild type = red-eyes Mutant type = white eyes 15
Normal /Wild Type Mutant /AlternaDve Phenotypes
I. Mendelian GeneDcs and Chromosomes A. Morgan and Drosophila Melanogaster In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) The F 1 generadon all had red eyes The F 2 generadon showed the 3:1 red:white eye rado, 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. 17
Recall: Sex Chromosomes vs. Autosomes 18
II. Inheritance of Sex-Linked Genes The sex chromosomes have genes for many characters unrelated to sex. Sex-linked gene = A gene located on one of the sex chromosomes In humans, sex-linked usually refers to a gene on the larger X chromosome. 19
Recall: Sex Chromosomes vs. Autosomes 20
I. Mendelian GeneDcs and Chromosomes A. Morgan and Drosophila Melanogaster P: Morgan crossed a red-eyed female with a whiteeyed male F 1 : All red-eyed offspring F 2 : 3 red-eyed: 1 white-eyed male Have we seen this padern before? 21
I. Mendelian GeneDcs and Chromosomes NotaDon for X-linked traits: X A = dominant allele, X a = recessive allele Males always exhibit X-linked traits they inherit (dominant or not) since they only have one X Examples: X R X R = homozygous dominant red-eyed female X R X r = heterozygous red-eyed female X r X r = homozygous recessive white-eyed female X R Y = red-eyed male X r Y = white-eyed male 22
I. Mendelian GeneDcs and Chromosomes Example: P generadon True-breeding red-eyed female x white-eyed male 23
I. Mendelian GeneDcs and Chromosomes Do-Now: Cross the F 1 (heterozygous female x red-eyed male) 24
II. Inheritance of Sex-Linked Genes A. Sex DeterminaDon In humans and other mammals, there are two variedes 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. 25
Systems of Sex Determina/on 44 + XY Parents 44 + XX 22 + 22 + X or Y + Sperm 22 + X Egg 44 + XX or 44 + XY (a) The X-Y system Zygotes (offspring) 22 + XX 22 + X (b) The X-0 system 76 + ZW 76 + ZZ (c) The Z-W system 32 (Diploid) 16 (Haploid) (d) The haplo-diploid system
II. Inheritance of Sex-Linked Genes B. Sex-Linked inherited traits Sex-linked genes follow specific paderns of inheritance. For a recessive sex-linked trait to be expressed A female needs two copies of the allele A male needs only one copy of the allele Because males always express X-linked genes, sexlinked recessive disorders are much more common in males than in females. 27
N = Normal is dominant n = disorder is recessive X N X N X n Y X N X n X N Y X N X n X n Y Sperm X n Y Sperm X N Y Sperm X n Y Eggs X N X N X n X N Y Eggs X N X N X N X N Y Eggs X N X N X n X N Y X N X N X n X N Y X n X n X N X n Y X n X n X n X n Y (a) (b) (c)
II. Inheritance of Sex-Linked Genes C. Hemizygous = Having only one copy of a gene (instead of two) Ex/ Males are typically hemizygous for X-chromosome genes 29
II. Inheritance of Sex-Linked Genes D. Some disorders caused by recessive alleles on the X chromosome in humans: 1. Muscular dystrophy 2. Color blindness 3. Hemophilia 30
II. Inheritance of Sex-Linked Genes E. X-InacDvaDon in Females 1. Barr bodies In mammalian females, one of the two X chromosomes in each cell is randomly inac/vated during embryonic development. The inac/ve X condenses into a Barr body. If a female is heterozygous for a pardcular gene located on the X chromosome, she will be a mosaic for that character. 31
X Inac/va/on is Random in Female Mammal Cells Early embryo: X chromosomes Allele for orange fur Allele for black fur Two cell popula/ons in adult cat: Ac/ve X Cell division and X chromosome inac/va/on Inac/ve X Ac/ve X Black fur Orange fur
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III. Chromosomal RecombinaDon A. Linked Genes 34
III. Chromosomal RecombinaDon A. Linked Genes 35
III. Chromosomal RecombinaDon A. Linked Genes Linked genes = genes on the same chromosome that tend to be inherited together due to proximity 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. 36
Morgan: Linked Genes Parents in testcross b + vg + b vg b vg b vg b + vg + b vg Most offspring b vg or b vg
Morgan: Linked Genes EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) Double mutant (black body, vestigial wings) b + b + vg + vg + b b vg vg F 1 dihybrid (wild type) TESTCROSS Double mutant b + b vg + vg b b vg vg Testcross offspring Eggs b + vg + b vg b + vg b vg + Wild type (gray-normal) Blackvestigial Grayvestigial Blacknormal b vg Sperm PREDICTED RATIOS If genes are located on different chromosomes: b + b vg + vg b b vg vg b + b vg vg b b vg + vg 1 : 1 : 1 : 1 If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 1 : 0 : 0 RESULTS 965 : 944 : 206 : 185
III. Chromosomal RecombinaDon B. GeneDc RecombinaDon Even when genes are linked, genedc recombinadon can occur This means that new phenotypes (non-parental phenotypes) are produced Gene/c recombina/on = Parents produce offspring with a mix of traits (not iden/cal to parents) The genedc findings of Mendel and Morgan relate to the chromosomal basis of recombina/on. 39
III. Chromosomal RecombinaDon C. Parental Types Parental types = phenotypes that match one of the parents 40
III. Chromosomal RecombinaDon D. Recombinants Non-parental phenotypes = recombinants 41
Parental Types and Recombinants Gametes from yellow-round heterozygous parent (YyRr) YR yr Yr yr Gametes from greenwrinkled homozygous recessive parent ( yyrr) yr YyRr yyrr Yyrr yyrr Parentaltype offspring Recombinant offspring
Crossing Over: Separates Linked Genes Recombinant Frequency Calculated Testcross parents Replication of chromosomes Gray body, normal wings (F 1 dihybrid) b + vg + b vg b + vg + b + vg + b vg Black body, vestigial wings (double mutant) b vg b vg b vg b vg b vg Replication of chromosomes Meiosis I b vg b vg b + vg + b + vg b vg + Meiosis I and II Meiosis II b vg Recombinant chromosomes Eggs b + vg + b vg b + vg b vg + Testcross offspring 965 Wild type (gray-normal) b + vg + 944 Blackvestigial b vg 206 Grayvestigial b + vg 185 Blacknormal b vg + b vg b vg b vg b vg b vg Sperm Parental-type offspring Recombinant offspring Recombination frequency = 391 recombinants 100 = 17% 2,300 total offspring
III. Chromosomal RecombinaDon E. Crossing Over 1. RecombinaDon Frequency = % of recombinant offspring produced in a genedc cross Genes on different chromosomes à 50% recombina/on frequency If genes are linked, then they may not assort independently during meiosis. Thus, LESS THAN 50% recombina/on frequency 44
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IV. GeneDc Maps & Crossing Over Data Alfred Sturtevant, one of Morgan s students, constructed a gene/c map, an ordered list of the genedc loci along a pardcular 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 recombina9on frequency. 46
IV. GeneDc Maps & Crossing Over Data A. Linkage map/map units Linkage map = mapping genes on a chromosome based on recombina/on frequencies Distances between genes can be expressed as map units; one map unit, or cendmorgan, represents a 1% recombinadon frequency. Map units indicate rela/ve distance and order, not precise loca/ons of genes. 47
Crossing Over --> Recombinants Frequency --> Distance Apart RESULTS Recombina/on frequencies Chromosome 9% 9.5% 17% b cn vg
IV. GeneDc Maps & Crossing Over Data A. Linkage map/map units Genes that are far apart on the same chromosome can have a recombinadon frequency near 50%. Such genes are physically linked, but genedcally unlinked, and behave as if found on different chromosomes. Sturtevant used recombinadon frequencies to make linkage maps of fruit fly genes. Using methods like chromosomal banding, genedcists can develop cytogenedc maps of chromosomes. CytogeneDc maps indicate the posidons of genes with respect to chromosomal features. 49
Pair-Share-Respond 1. What is meant by the term linked genes? 2. What % recombina/on frequency would be expected for linked genes? 3. What % recombina/on frequency would be expected for genes that are not linked? 4. What is a linkage map, and what does it tell us? 50
V. Chromosomal AlteraDons What kind of chromosomal alteradons can occur and what are their effects? 51
V. Chromosomal AlteraDons A. AlteraDons in Chromosome Number 1. Nondisjunc/on = when homologous chromosomes do not separate during anaphase I of meiosis This causes one gamete to receive 2 copies of a chromosome, and another gamete to have no copies 2. Aneuploidy 3. Polyploidy - 52
Fig. 15-13-3 Meiosis I Nondisjunc/on Meiosis II Nondisjunc/on Gametes n + 1 n + 1 n 1 n 1 n + 1 n 1 n n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II
V. Chromosomal AlteraDons A. AlteraDons in Chromosome Number 2. Aneuploidy = when the gametes resul/ng from nondisjunc/on are fer/lized, an abnormal chromosome number results A monosomic zygote has only one copy of a pardcular chromosome 2n - 1 A trisomic zygote has three copies of a pardcular chromosome 2n + 1 3. Polyploidy - 54
V. Chromosomal AlteraDons A. AlteraDons in Chromosome Number 3. Polyploidy Polyploidy = when 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. 55
V. Chromosomal AlteraDons B. AlteraDons of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure: 1. Dele/on = removes a chromosomal segment 2. Duplica/on = repeats a segment 3. Inversion = reverses a segment within a chromosome 4. Transloca/on = moves a segment from one chromosome to another. 5. Nonreciprocal Crossover = results in one chromosome having a dele/on and another a duplica/on 56
Breakage (a) Causes Change A B C D E F G H A B C E F G H Dele/on (b) A B C D E F G H Duplica/on A B C B C D E F G H (c) A B C D E F G H Inversion A D C B E F G H (d) A B C D E F G H Reciprocal transloca/on M N O C D E F G H M N O P Q R A B P Q R
V. Chromosomal AlteraDons C. Chromosomal AlteraDons in Human Disease 1. Aneuploid Disorders = caused by nondisjunc/on of sex chromosomes 58
V. Chromosomal AlteraDons 1. Aneuploid Disorders a. Down Syndrome = 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 correladon that has not been explained. 59
Down Syndrome 2n + 1 trisomy 21
V. Chromosomal AlteraDons 1. Aneuploid Disorders b. Klinefelter Syndrome = results from an extra chromosome in males (XXY) Effects include low IQ, taller body, decreased muscle mass, broad hips, extended legs, weaker bones, etc. 61
V. Chromosomal AlteraDons 1. Aneuploid Disorders c. Turner syndrome = Monosomy X = XO females Viable, but females are sterile 62
V. Chromosomal AlteraDons 1. Aneuploid Disorders d. Extra Y = XYY males (occurs in 1/1000 male births) Taller, possible acne issues, but mostly normal 63
V. Chromosomal AlteraDons 1. Aneuploid Disorders e. Metafemales = XXX (about 1:1500 female births) Taller, poor coordinadon, reproducdve concerns 64
V. Chromosomal AlteraDons C. Chromosomal AlteraDons in Human Disease 2. Structural Diseases a. Cri du chat = cry of the cat = dele/on of chromosome 5 Results in severe developmental delays, behavioral struggles, unusual facial features, and nervous system issues. 65
VI. ExcepDons to Standard Chromosome Theory One excepdon to chromosome theory involves genes located in the nucleus A. Genomic Imprin/ng = varia/on in phenotype that depends on which parent passed the alleles (mother vs. father) Genomic imprindng involves the silencing of certain genes that are stamped with an imprint during gamete producdon. 66
Genomic Imprinting Paternal chromosome Maternal chromosome Normal Igf2 allele is expressed Normal Igf2 allele is not expressed Wild-type mouse (normal size) (a) Homozygote Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father Normal size 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
VI. ExcepDons to Standard Chromosome Theory A. Genomic ImprinDng: It appears that mammalian genomic imprindng, gene silencing, is the result of the methyla9on of DNA (addidon of CH 3 ). Genomic imprindng is thought to affect only a small fracdon of mammalian genes. Most imprinted genes are cridcal for embryonic development. 68
VI. ExcepDons to Standard Chromosome Theory B. Organelle Genes = extranuclear DNA (such as that found in mitochondria and chloroplasts) 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. 69
REVIEW Sperm Egg P genera/on gametes D C B A E + d c b a e F f This F 1 cell has 2n = 6 chromosomes and is heterozygous for all six genes shown (AaBbCcDdEeFf ). Red = maternal; blue = paternal. Each chromosome has hundreds or thousands of genes. Four (A, B, C, F) are shown on this one. F D A B C E c b a e d f The alleles of unlinked genes are either on separate chromosomes (such as d and e) or so far apart on the same chromosome (c and f ) that they assort independently. Genes on the same chromosome whose alleles are so close together that they do not assort independently (such as a, b, and c) are said to be linked.
Recombinants Due to Crossing-Over
Assignment 1. GeneDcs set #3 (odd) 72