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Chapter 13-15 Meiosis, Sexual Life Cycles Mendelian Genetics Exceptions to the Rule Overview: Living Things Can Reproduce!! Heredity • Is the transmission of traits from one generation to the next Variation • Shows that offspring differ somewhat in appearance from parents and siblings Genetics • Is the scientific study of heredity and hereditary variation Asexual Reproduction In asexual reproduction • One parent produces genetically identical offspring by mitosis Parent Bud Figure 13.2 0.5 mm Sexual Reproduction In sexual reproduction • Two parents give rise to offspring that have unique combinations of genes inherited from the two parents Life Cycle Fertilization & meiosis alternate in sexual life cycles A life cycle • Is the generation-to-generation sequence of stages in the reproductive history of an organism After Replication: identical sister chromatids Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Figure 13.4 Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) The Human Life Cycle Key Haploid gametes (n = 23) Haploid (n) Ovum (n) Diploid (2n) Sperm Cell (n) MEIOSIS Ovary FERTILIZATION Testis Diploid zygote (2n = 46) Mitosis and development Figure 13.5 Multicellular diploid adults (2n = 46) Meiosis reduces chromosome from diploid to haploid Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Figure 13.7 Haploid cells with unreplicated chromosomes INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I Centrosomes (with centriole pairs) Sister chromatids Nuclear envelopeTetrad METAPHASE I Chiasmata ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Spindle Metaphase plate Homologous Microtubule chromosomes attached to Chromatin separate kinetochore Pairs of homologous Chromosomes duplicate Tertads line up Homologous chromosomes chromosomes split up (red and blue) pair and exchange segments; 2n = 6 in this example Figure 13.8 MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS PROPHASE II Cleavage furrow Figure 13.8 Two haploid cells form; chromosomes are still double METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase I Prophase Chromosome replication Duplicated chromosome (two sister chromatids) Chromosome replication Tetrad formed by synapsis of homologous chromosomes 2n = 6 Metaphase Chromosomes positioned at the metaphase plate Anaphase Telophase Sister chromatids separate during anaphase 2n Tetrads positioned at the metaphase plate Homologues separate during anaphase I; sister chromatids remain together Metaphase I Anaphase I Telophase I Haploid n=3 Daughter cells of meiosis I 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Figure 13.9 Sister chromatids separate during anaphase II Genetic Variations contributes to Evolution Reshuffling of genetic material in meiosis • Produces genetic variation due to the chromosome behaviors • Homologous pairs orient randomly • Independent assortment • Crossing over • Random fertilization • mutations Figure 14.4 Words to know Allele for purple flowers Locus for flower-color gene Pair of homologous chromosomes Allele for white flowers Mendel and Genetics Law of Segregation: • Two alleles for a trait separate during gamate formation. Law of Independent Assortment • Each pair of alleles separates independently from other pairs. Inheritance Patterns Dominance /recessive Codominance Incomplete dominance Levels: ex: Tay-Sachs Disease Frequencies of dominant allele Multiple alleles Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease © 2011 Pearson Education, Inc. EPISTASIS: BbEe Eggs 1/ 4 BE 1/ 4 bE 1/ 4 Be 1/ 4 be Sperm 1/ BE 4 1/ BbEe 4 bE 1/ 4 Be 1/ 4 be BBEE BbEE BBEe BbEe BbEE bbEE BbEe bbEe BBEe BbEe BBee Bbee BbEe bbEe Bbee bbee 9 : 3 : 4 POLYGENETIC: AaBbCc AaBbCc Sperm 1/ 1/ 8 8 1/ 1/ Eggs 8 1/ 1/ 8 8 1/ 8 1/ 1/ 8 8 8 8 1/ 8 1/ 8 1/ 1/ 8 1/ 8 1/ 8 1/ 8 Phenotypes: Number of dark-skin alleles: 1/ 64 0 6/ 64 1 15/ 64 2 20/ 64 3 15/ 64 4 6/ 64 5 1/ 64 6 Pedigrees Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The norm of reaction is the phenotypic range of a genotype influenced by the environment © 2011 Pearson Education, Inc. Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair © 2011 Pearson Education, Inc. The Chromosomal Basis of Sex • In humans and other mammals: X vs. Y • Y is tiny!! • The SRY gene on the Y chromosome • Some disorders caused by recessive alleles on the X: • Color blindness (mostly X-linked) • Duchenne muscular dystrophy • Hemophilia © 2011 Pearson Education, Inc. X Inactivation in Female Mammals Barr body Females are mosaic © 2011 Pearson Education, Inc. Figure 15.8 X chromosomes Allele for orange fur Early embryo: Two cell populations in adult cat: Allele for black fur Cell division and X chromosome inactivation Active X Inactive X Active X Black fur Orange fur Figure 15.11 Linked genes tend to be inherited together because they are located near each other on the same chromosome Recombination frequencies 9% Chromosome 9.5% 17% b cn vg Figure 15.12 Mutant phenotypes Short aristae 0 Long aristae (appendages on head) Black body Cinnabar Vestigial eyes wings 48.5 57.5 Gray body Red eyes Brown eyes 67.0 104.5 Normal wings Red eyes Wild-type phenotypes (a) Deletion A B C D E F G Mutations H A deletion removes a chromosomal segment. A B C Nondisjunct ion Aneuploidy Breakage of a chromosome : • Deletion • Duplication • Inversion © 2011 Pearson Education, Inc. E F G H (b) Duplication A B C D E F G H A duplication repeats a segment. A B C B C D E F G H (c) Inversion A B C D E F G H An inversion reverses a segment within a chromosome. A D C B E F G H (d) Translocation A B C D E F G H M N O P Q R A translocation moves a segment from one chromosome to a nonhomologous chromosome. M N O C D E F G H A B P Q R Figure 15.15 Down Syndrome (Trisomy 21) Aneuploidy of Sex Chromosomes Nondisjunctio n of sex chromosome s: • XXX • Klinefelter syndrome (XXY) • Monosomy X, called Turner syndrome, © 2011 Pearson Education, Inc. Genomic Imprinting 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 Inheritance of Organelle Genes Mitochondria, chloroplasts, and inherited maternally © 2011 Pearson Education, Inc. Ch 15: Chromosomes! Figure 15.2 P Generation Yellow-round seeds (YYRR) Y Y Green-wrinkled seeds (yyrr) ry R R r y Meiosis Fertilization y R Y Gametes r All F1 plants produce yellow-round seeds (YyRr). F1 Generation R y r Y R r Y y Meiosis LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. r R r R Y y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Metaphase I Y y 1 1 R r r R Y y Anaphase I Y y R r Y y r R Y y 2 2 Gametes R R 1/ 4 YR F2 Generation 3 y Y Y Fertilization recombines the R and r alleles at random. Metaphase II r 1/ 4 Y Y y r r r 1/ yr 4 y y R R 1/ Yr 4 yR An F1 F1 cross-fertilization 3 9 :3 :3 :1 Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. Figure 15.4b CONCLUSION P Generation X X w X Y w w Eggs F1 Generation Sperm w w w w w Eggs F2 Generation w w w Sperm w w w w w w Figure 15.5 X Y Figure 15.6 44 XY 44 XX Parents 22 22 X or Y 22 X Sperm Egg 44 XX or 44 XY (a) The X-Y system Zygotes (offspring) 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 Figure 15.8 X chromosomes Allele for orange fur Early embryo: Two cell populations in adult cat: Allele for black fur Cell division and X chromosome inactivation Active X Inactive X Active X Black fur Orange fur Figure 15.UN01 Linked Genes F1 dihybrid female and homozygous recessive male in testcross b+ vg+ b vg b vg b vg b+ vg+ b vg Most offspring or b vg b vg Genetic Recombination and Linkage 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 Figure 15.UN02 Gametes from yellow-round dihybrid parent (YyRr) Gametes from greenwrinkled homozygous recessive parent (yyrr) YR yr Yr yR YyRr yyrr Yyrr yyRr yr Parentaltype offspring Recombinant offspring Figure 15.12 Mutant phenotypes Short aristae 0 Long aristae (appendages on head) Black body Cinnabar Vestigial eyes wings 48.5 57.5 Gray body Red eyes Brown eyes 67.0 104.5 Normal wings Red eyes Wild-type phenotypes Figure 15.14 (a) Deletion A B C D E F G H A deletion removes a chromosomal segment. A B C E F G H (b) Duplication A B C D E F G H A duplication repeats a segment. A B C B C D E F G H (c) Inversion A B C D E F G H An inversion reverses a segment within a chromosome. A D C B E F G H (d) Translocation A B C D E F G H M N O P Q R A translocation moves a segment from one chromosome to a nonhomologous chromosome. M N O C D E F G H A B P Q R Figure 15.15 Others Imprinting • Silencing of a gene during gamate formation (depends on which parent) Organelle genes • Mitochondria, choloroplast • Maternally inherited