* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download MF011_fhs_lnt_002b_May11 - MF011 General Biology 2 (May
Saethre–Chotzen syndrome wikipedia , lookup
Non-coding DNA wikipedia , lookup
Genomic library wikipedia , lookup
Genetic drift wikipedia , lookup
Oncogenomics wikipedia , lookup
Therapeutic gene modulation wikipedia , lookup
Hybrid (biology) wikipedia , lookup
Extrachromosomal DNA wikipedia , lookup
Nutriepigenomics wikipedia , lookup
No-SCAR (Scarless Cas9 Assisted Recombineering) Genome Editing wikipedia , lookup
Cre-Lox recombination wikipedia , lookup
Ridge (biology) wikipedia , lookup
Biology and consumer behaviour wikipedia , lookup
Population genetics wikipedia , lookup
Minimal genome wikipedia , lookup
Quantitative trait locus wikipedia , lookup
Polycomb Group Proteins and Cancer wikipedia , lookup
Gene expression profiling wikipedia , lookup
Frameshift mutation wikipedia , lookup
Site-specific recombinase technology wikipedia , lookup
History of genetic engineering wikipedia , lookup
Genome evolution wikipedia , lookup
Gene expression programming wikipedia , lookup
Skewed X-inactivation wikipedia , lookup
Designer baby wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
Dominance (genetics) wikipedia , lookup
Epigenetics of human development wikipedia , lookup
Point mutation wikipedia , lookup
Y chromosome wikipedia , lookup
Genomic imprinting wikipedia , lookup
Genome (book) wikipedia , lookup
Neocentromere wikipedia , lookup
GENETICS & EVOLUTION: CHROMOSOMAL INHERITANCE & MUTATION Chapter 2.2 Overview Chromosomal Inheritance Sex-linked Genes Gene linkage and analysis Mutations Gene Mutations Chromosomal Abberations Overview: Locating Genes Along 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 The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene Fig. 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 was said to account for Mendel’s laws of segregation and independent assortment Fig. 15-2a Green-wrinkled seeds ( yyrr) Yellow-round seeds (YYRR) P Generation Y Y R R r y y r Meiosis Fertilization Gametes R Y y r All F1 plants produce yellow-round seeds (YyRr) Fig. 15-2b F1 Generation All F1 plants produce yellow-round seeds (YyRr) 0.5 mm R R y r Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. y r Y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis r R r R Y y Metaphase I Y y 1 1 r R r R Y y Anaphase I Y y r R Metaphase II R r 2 2 Gametes y Y Y R R 1 4 YR r 1 3 4 yr Y Y y r y Y y Y r r 14 Yr y y R R 14 yR 3 Fig. 15-2c F2 Generation An F1 F1 cross-fertilization 3 3 9 :3 :3 :1 Fig. 15-2 P Generation Yellow-round seeds (YYRR) Y Y R r R y Green-wrinkled seeds ( yyrr) y r Meiosis Fertilization y R Y Gametes r All F1 plants produce yellow-round seeds (YyRr) F1 Generation R R y r Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. y r LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis R r Y y r R Y y Metaphase I 1 1 R r Y y r R Y y Anaphase I R r Y y Metaphase II r R Y y 2 2 Y Y Gametes R 1/ 4 YR F2 Generation R y r Y Y y r r 1/ yr 4 r 1/ 4 Yr y R y R 1/ yR 4 An F1 F1 cross-fertilization 3 3 9 :3 :3 :1 The Chromosomal Basis of Sex 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 Fig. 15-5 X Y 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 Fig. 15-6 44 + XY 44 + XX Parents 22 + 22 + or X Y Sperm + 44 + XX or 22 + X Egg 44 + XY Zygotes (offspring) (a) The X-Y system 22 + XX 22 + X 76 + ZW 76 + ZZ 32 (Diploid) 16 (Haploid) (b) The X-0 system (c) The Z-W system (d) The haplo-diploid system 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 In humans, sex-linked usually refers to a gene on the larger X chromosomeSex-linked genes follow specific patterns 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 Sex-linked recessive disorders are much more common in males than in females Fig. 15-7 XNXN Sperm Xn XnY Sperm XN Y Eggs XN XNXn XNY XN XNXn XNY (a) XNXn XNY XNXn Sperm Xn Y XnY Y Eggs XN XNXN XNY Eggs XN XNXn XNY XnXN XnY Xn XnXn XnY Xn (b) (c) Some disorders caused by recessive alleles on the X chromosome in humans: Color blindness Duchenne muscular dystrophy Hemophilia 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 Fig. 15-8 X chromosomes Early embryo: Two cell populations in adult cat: Active X Allele for orange fur Allele for black fur Cell division and X chromosome inactivation Active X Inactive X Black fur Orange fur Linked genes tend to be inherited together Each chromosome has hundreds or thousands of genes Genes located on the same chromosome that tend to be inherited together are called linked genes 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 How Linkage Affects Inheritance Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) He noted that these genes do not assort independently, and reasoned that they were on the same chromosome However, nonparental phenotypes were also produced Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent Fig. 15-UN1 b vg b+ vg+ Parents in testcross Most offspring b vg b vg b+ vg+ b vg or b vg b vg Fig. 15-9-1 EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) b+ b+ vg+ vg+ Double mutant (black body, vestigial wings) b b vg vg Fig. 15-9-2 EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) b b vg vg b+ b+ vg+ vg+ F1 dihybrid (wild type) b+ b vg+ vg Double mutant (black body, vestigial wings) TESTCROSS Double mutant b b vg vg Fig. 15-9-3 EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) Double mutant (black body, vestigial wings) b b vg vg b+ b+ vg+ vg+ F1 dihybrid (wild type) Double mutant TESTCROSS b+ b vg+ vg Testcross offspring b b vg vg Eggs b+ vg+ Wild type (gray-normal) b vg b+ vg b vg+ Blackvestigial Grayvestigial Blacknormal b vg Sperm b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg Fig. 15-9-4 EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) Double mutant (black body, vestigial wings) b b vg vg b+ b+ vg+ vg+ F1 dihybrid (wild type) Double mutant TESTCROSS b+ b vg+ vg Testcross offspring b b vg vg Eggs b+ vg+ Wild type (gray-normal) b vg b+ vg b vg+ Blackvestigial Grayvestigial Blacknormal b vg Sperm b b vg vg b+ b vg vg b b vg+ vg b+ b vg+ vg PREDICTED RATIOS If genes are located on different chromosomes: 1 : 1 : 1 : 1 If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 1 : 0 : 0 965 : 944 : 206 : 185 RESULTS Genetic Recombination and Linkage The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination 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 Fig. 15-UN2 Gametes from yellow-round heterozygous parent (YyRr) Gametes from greenwrinkled homozygous recessive parent ( yyrr) YR yr Yr yR YyRr yyrr Yyrr yyRr yr Parentaltype offspring Recombinant offspring 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 Fig. 15-10 Testcross parents Gray body, normal wings (F1 dihybrid) Black body, vestigial wings (double mutant) b+ vg+ b vg b vg b vg Replication of chromosomes Replication of chromosomes Meiosis I b+ vg+ b vg b+ vg+ b vg b vg b vg b vg b vg b+ vg+ Meiosis I and II b+ vg b vg+ b vg Meiosis II Recombinant chromosomes Eggs Testcross offspring b vg b+ vg+ b+ vg b vg+ 965 944 206 185 Wild type (gray-normal) Blackvestigial Grayvestigial Blacknormal b+ vg+ b vg b+ vg b vg+ b vg b vg b vg b vg Parental-type offspring Recombination frequency = Recombinant offspring 391 recombinants 2,300 total offspring 100 = 17% b vg Sperm Fig. 15-10a Testcross parents Black body, vestigial wings (double mutant) Gray body, normal wings (F1 dihybrid) Replication of chromosomes Meiosis I b+ vg+ b vg b vg b vg b+ vg+ b vg b+ vg+ b vg b vg b vg b vg b vg b+ vg+ b+ Meiosis I and II vg b vg+ b vg Meiosis II Recombinant chromosomes b+ vg+ b vg Eggs b+ vg b vg+ b vg Sperm Replication of chromosomes Fig. 15-10b Recombinant chromosomes Eggs Testcross offspring b+ vg+ 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 b vg b vg Parental-type offspring Recombination frequency = Recombinant offspring 391 recombinants 2,300 total offspring 100 = 17% b vg Sperm Mapping the Distance Between Genes Using Recombination Data 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 Fig. 15-11 RESULTS Recombination frequencies 9% Chromosome 9.5% 17% b cn vg 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 Fig. 15-12 Short aristae 0 Long aristae (appendages on head) Mutant phenotypes Black body 48.5 Gray body Cinnabar eyes 57.5 Red eyes Vestigial wings 67.0 Normal wings Wild-type phenotypes Brown eyes 104.5 Red eyes Abnormal Chromosome Number Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders 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 Fig. 13-8a Prophase I Metaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Sister chromatids remain attached Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Homologous chromosomes Fragments of nuclear envelope Telophase I and Cytokinesis Anaphase I Microtubule attached to kinetochore Cleavage furrow Fig. 13-8d Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming Fig. 15-13-1 Meiosis I Nondisjunction (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II Fig. 15-13-2 Meiosis I Nondisjunction Meiosis II Nondisjunction (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II Fig. 15-13-3 Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n+1 n+1 n–1 n–1 n+1 n–1 n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II n Aneuploidy results from the fertilization of gametes in which nondisjunction occurred Offspring with this condition have an abnormal number of a particular chromosome A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies 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 Fig. 15-14 Mutagens Spontaneous mutations can occur during DNA replication, recombination, or repair Mutagens are physical or chemical agents that can cause mutations Mutations are changes in the genetic material of a cell or virus Gene mutation Chromosomal abberation Causes of Mutations Spontaneous mutation DNA can undergo a chemical change Movement of transposons from one chromosomal location to another Replication Errors 1 in 1,000,000,000 replications DNA polymerase Proofreads new strands Generally corrects errors Induced mutation: Mutagens such as radiation, organic chemicals Many mutagens are also carcinogens (cancer causing) Environmental Mutagens Ultraviolet Radiation Tobacco Smoke Effect of Mutations on Protein Activity Point Mutations Involve change in a single DNA nucleotide Changes one codon to a different codon Affects on protein vary: Nonfunctional Reduced functionality Unaffected Frameshift Mutations One or two nucleotides are either inserted or deleted from DNA Protein always rendered nonfunctional Normal : After deletion: After insertion: THE CAT ATE THE RAT THE ATA TET HER AT THE CCA TAT ETH ERA T Point mutations can affect protein structure and function Point mutations are chemical changes in just one base pair of a gene The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein Fig. 17-22 Wild-type hemoglobin DNA C T T 3 5 G A A Mutant hemoglobin DNA C A T 5 3 G T A 3 5 mRNA 5 5 3 mRNA G A A Normal hemoglobin Glu 3 5 G U A Sickle-cell hemoglobin Val 3 Types of Point Mutations Point mutations within a gene can be divided into two general categories Base-pair substitutions Base-pair insertions or deletions Fig. 17-23 Wild-type DNA template strand 3 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end A instead of G 3 5 Extra A 5 3 3 5 3 5 U instead of C 5 5 3 Extra U 3 Stop Stop Silent (no effect on amino acid sequence) Frameshift causing immediate nonsense (1 base-pair insertion) T instead of C missing 3 5 5 3 3 5 3 5 5 3 A instead of G missing 5 3 Stop Missense Frameshift causing extensive missense (1 base-pair deletion) missing A instead of T 5 3 3 5 U instead of A 5 5 3 3 5 missing 3 5 Stop Stop Nonsense (a) Base-pair substitution 3 No frameshift, but one amino acid missing (3 base-pair deletion) (b) Base-pair insertion or deletion Fig. 17-23a Wild type DNA template 3 strand 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end A instead of G 5 3 3 5 U instead of C 5 3 Stop Silent (no effect on amino acid sequence) Fig. 17-23b Wild type DNA template 3 strand 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end T instead of C 5 3 3 5 A instead of G 3 5 Stop Missense Fig. 17-23c Wild type DNA template 3 strand 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end A instead of T 3 5 5 3 U instead of A 5 3 Stop Nonsense Fig. 17-23d Wild type DNA template 3 strand 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end Extra A 5 3 3 5 Extra U 5 3 Stop Frameshift causing immediate nonsense (1 base-pair insertion) Fig. 17-23e Wild type DNA template 3 strand 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end missing 5 3 3 5 missing 5 Frameshift causing extensive missense (1 base-pair deletion) 3 Fig. 17-23f Wild type DNA template 3 strand 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end missing 5 3 3 5 missing 5 3 Stop No frameshift, but one amino acid missing (3 base-pair deletion) Substitutions A base-pair substitution replaces one nucleotide and its partner with another pair of nucleotides Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code Missense mutations still code for an amino acid, but not necessarily the right amino acid Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein Insertions and Deletions Insertions and deletions are additions or losses of nucleotide pairs in a gene These mutations have a disastrous effect on the resulting protein more often than substitutions do Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation 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 Fig. 15-15 (a) (b) (c) (d) A B C D E F G H A B C D E F G H A B C D E F G H A B C D E F G H Deletion Duplication Inversion A B C E F G H A B C B C D E A D C B E R F G H M N O C D E Reciprocal translocation M N O P Q F G H A B P Q R F G H 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 Fig. 15-16 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 Changes in Sex Chromosome Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. Turner syndrome b. Klinefelter syndrome a: Courtesy UNC Medical Illustration and Photography; b: Courtesy Stefan D. Schwarz, http://klinefeltersyndrome.org 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 Fig. 15-17 Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) 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 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 Fig. 15-18a Paternal chromosome Normal Igf2 allele is expressed Maternal chromosome Normal Igf2 allele is not expressed (a) Homozygote Wild-type mouse (normal size) Fig. 15-18b 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 Fig. 15-18 Paternal chromosome Normal Igf2 allele is expressed Maternal chromosome 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 It appears that imprinting is the result of the methylation (addition of –CH3) of DNA Genomic imprinting is thought to affect only a small fraction of mammalian genes Most imprinted genes are critical for embryonic development Fig. 15-UN3 Inheritance of Organelle Genes Extranuclear genes (or cytoplasmic genes) are genes 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 Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems For example, mitochondrial myopathy and Leber’s hereditary optic neuropathy You should now be able to: 1. 2. 3. 4. 5. Explain the chromosomal theory of inheritance and its discovery Explain why sex-linked diseases are more common in human males than females Distinguish between sex-linked genes and linked genes Explain how meiosis accounts for recombinant phenotypes Explain how linkage maps are constructed 6. 7. 8. 9. 10. 11. 12. Explain how nondisjunction can lead to aneuploidy Define trisomy, triploidy, and polyploidy Define mutation Distinguish between different gene mutations Distinguish among deletions, duplications, inversions, and translocations Explain genomic imprinting Explain why extranuclear genes are not inherited in a Mendelian fashion Morgan’s Experimental Evidence Imagine that Morgan had used a grasshopper (2N = 24 and an XX, XO sex determination system). Predict where the first mutant would have been discovered. a. b. c. d. on the O chromosome of a male on the X chromosome of a male on the X chromosome of a female on the Y chromosome of a male Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. The Chromosomal Basis of Sex Think about bees and ants, groups in which males are haploid. Which of the following are accurate statements about bee and ant males when they are compared to species in which males are XY and diploid for the autosomes? a. b. c. d. e. Bee males have half the DNA of bee females whereas human males have nearly the same amount of DNA that human females have. Considered across the genome, harmful (deleterious) recessives will negatively affect bee males more than Drosophila males. Human and Drosophila males have sons but bee males do not. Inheritance in bees is like inheritance of sex-linked characteristics in humans. none of the above Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. The Chromosomal Basis of Sex In some Drosophila species there are genes on the Y chromosome that do not occur on the X chromosome. Imagine that a mutation of one gene on the Y chromosome reduces the size by half of individuals with the mutation. Which of the following statements is accurate with regard to this situation? This mutation occurs in all offspring of a male with the mutation. This mutation occurs in all male but no female offspring of a male with the mutation. This mutation occurs in all offspring of a female with the mutation. This mutation occurs in all male but no female offspring of a female with the mutation. This mutation occurs in all offspring of both males and females with the mutation. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. The Chromosomal Basis of Sex Imagine that a deleterious recessive allele occurs on the W chromosome of a chicken (2N = 78). Where would it be most likely to appear first in a genetics experiment? a. b. c. d. e. in a male because there is no possibility of the presence of a normal, dominant allele in a male because it is haploid in a female because there is no possibility of the presence of a normal, dominant allele in a female because all alleles on the W chromosomes are dominant to those on the Z chromosome none of the above Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. Inheritance of Sex-Linked Genes In cats, a sex-linked gene affects coat color. The O allele produces an enzyme that converts eumelanin, a black or brown pigment, into phaeomelanin, an orange pigment. The o allele is recessive to O and produces a defective enzyme, one that does not convert eumelanin into phaeomelanin. Which of the following statements is/are accurate? a. b. c. d. e. The phenotype of O-Y males is orange because the functional allele O converts eumelanin into phaeomelanin. The phenotype of o-Y males is black/brown because the non-functional allele o does not convert eumelanin into phaeomelanin. The phenotype of OO and Oo males is orange because the functional allele O converts eumelanin into phaeomelanin. The phenotype of Oo males is mixed orange and black/brown because the functional allele O converts eumelanin into phaeomelanin in some cell groups (orange) and because in other cell groups the non-functional allele o does not convert eumelanin into phaeomelanin. The phenotype of O-Y males is orange because the non-functional allele O does not convert eumelanin into phaeomelanin while the phenotype of o-Y males is black/brown because the functional allele o converts eumelanin into phaeomelanin. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. Inheritance of Sex-Linked Genes In cats, a sex-linked gene affects coat color. The O allele produces an enzyme that converts eumelanin, a black or brown pigment, into phaeomelanin, an orange pigment. The o allele is recessive to O and produces a defective enzyme, one that does not convert eumelanin into phaeomelanin. Which of the following statements is/are accurate? a. b. c. d. e. The phenotype of O-Y females is orange because the functional allele O converts eumelanin into phaeomelanin. The phenotype of o-Y females is black/brown because the non-functional allele o does not convert eumelanin into phaeomelanin. The phenotype of OO and Oo females is orange because the functional allele O converts eumelanin into phaeomelanin. The phenotype of Oo females is mixed orange and black/brown because the functional allele O converts eumelanin into phaeomelanin in some cell groups (orange) and because in other cell groups the non-functional allele o does not convert eumelanin into phaeomelanin. The phenotype of O-Y females is orange because the non-functional allele O does not convert eumelanin into phaeomelanin while the phenotype of o-Y males is black/brown because the functional allele o converts eumelanin into phaeomelanin. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. X Inactivation in Female Mammals Imagine two species of mammals that differ in the timing of Barr body formation during development. Both species have genes that determine coat color, O for the dominant orange fur and o for the recessive black/brown fur, on the X chromosome. In species A, the Barr body forms during week 1 of a 6-month pregnancy whereas in species B, the Barr body forms during week 3 of a 5-month pregnancy. What would you predict about the coloration of heterozygous females (Oo) in the two species? a. b. c. Both species will have similar sized patches of orange and black/brown fur. Species A will have smaller patches of orange or black/brown fur than will species B. The females of both species will show the dominant fur color, orange. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. Mapping the Distance between Genes Imagine a species with three loci thought to be on the same chromosome. The recombination rate between locus A and locus B is 35% and the recombination rate between locus B and locus C is 33%. Predict the recombination rate between A and C. a. b. c. d. e. The recombination rate between locus A and locus C is either 2% or 68%. The recombination rate between locus A and locus C is probably 2%. The recombination rate between locus A and locus C is either 2% or 50%. The recombination rate between locus A and locus C is either 2% or 39%. The recombination rate between locus A and locus C cannot be predicted. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. Triploid species are usually sterile (unable to reproduce) whereas tetraploids are often fertile. Which of the following are likely good explanations of these facts? a. b. c. d. In mitosis, some chromosomes in triploids have no partner at synapsis, but chromosomes in tetraploids do have partners. In meiosis, some chromosomes in triploids have no partner at synapsis, but chromosomes in tetraploids do have partners. In mitosis, some chromosomes in tetraploids have no partner at synapsis, but chromosomes in triploids do have partners. In meiosis, some chromosomes in tetraploids have no partner at synapsis, but chromosomes in triploids do have partners. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. Chromosomal rearrangements can occur after chromosomes break. Which of the following statements are most accurate with respect to alterations in chromosome structure? a. b. c. d. Chromosomal rearrangements are more likely to occur in mammals than in other vertebrates. Translocations and inversions are not deleterious because no genes are lost in the organism. Chromosomal rearrangements are more likely to occur during mitosis than during meiosis. An individual that is homozygous for a deletion of a certain gene is likely to be more damaged than is one that is homozygous for a duplication of that same gene because loss of a function can be lethal. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. Imagine that you could create medical policy for a country. In this country it is known that the frequency of Down syndrome babies increases with increasing age of the mother and that the severity of characteristics varies enormously and unpredictably among affected individuals. Furthermore, financial resources are severely limited, both for testing of pregnant women and for supplemental training of Down syndrome children. The graph on the next slide shows the incidence of Down syndrome as a function of maternal age. Which of the following policies would you implement? a. No testing of pregnant women should be conducted and all the health care money should be used for training of Down syndrome children. b. The health care system should provide testing only for women over 30. c. The health care system should provide testing only for women over 40. d. The health care system should require termination of all Down syndrome fetuses. e. The health care system should provide training for the 30% most seriously affected children only. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. The lawyer for a defendant in a paternity suit asked for DNA testing of a baby girl. Which of the following set of results would demonstrate that the purported father was not actually the genetic father of the child? a. b. c. d. e. The mitochondrial DNA of the child and “father” did not match. DNA sequencing of chromosome #5 of the child and “father” did not match. The mitochondrial DNA of the child and “mother” did not match. DNA sequencing of chromosome #5 of the child and “mother” did not match. The mitochondrial DNA of the child and “father” matched but the mitochondrial DNA of the child and “mother” did not. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.