Meiosis, Sexual Life Cycles Mendelian Genetics Exceptions to the Rule 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 Figure 13.2 Parent Bud 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 Figure 13.4 Key Maternal set of chromosomes (n = 3) Unlike somatic cells Gametes, sperm and egg cells , are haploid cells containing only one set of chromosomes 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

The Human Life Cycle Key Figure 13.5 Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46) At sexual maturity The ovaries and testes produce haploid gametes by meiosis During fertilization These gametes, sperm and ovum, fuse, forming a diploid zygote The zygote Develops into an adult organism

Meiosis reduces chromosome from diploid to haploid Figure 13.7 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous separate Haploid cells with replicated chromosomes Sister chromatids Haploid cells with unreplicated chromosomes Meiosis I Meiosis II

Figure 13.8 Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Synapsis and crossing over: physical connection and exchange genetic information during the tetrad stage Separation of the homologous pairs Figure 13.8

MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND MEIOSIS II: Separates sister chromatids Cleavage furrow 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 Two haploid cells form; chromosomes are still double Figure 13.8 Sister chromatids splits

(before chromosome replication) Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n Daughter cells of mitosis 2n = 6

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

Words to know Allele for purple flowers Pair of homologous chromosomes Figure 14.4 Words to know Allele for purple flowers Pair of homologous chromosomes Locus for flower-color gene 2 alleles per trait, dominant, recessie Allele for white flowers

Mendel and Genetics Law of Segregation: Law of Independent Assortment 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 Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain At the organismal level, the allele is recessive At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant At the molecular level, the alleles are codominant

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 BbEe Sperm 1/4 1/4 1/4 1/4 Eggs 1/4 1/4 1/4 1/4 9 : 3 In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus bE BbEE bbEE BbEe bbEe 1/4 Be BBEe BbEe BBee Bbee 1/4 be BbEe bbEe Bbee bbee 9 : 3 : 4

Polygenetic: AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 Figure 14.13 A simplified model for polygenic inheritance of skin color. 1/8 Eggs 1/8 1/8 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 15/64 6/64 1/64 Number of dark-skin alleles: 1 2 3 4 5 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 For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity © 2011 Pearson Education, Inc.

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 F1 generation all had red eyes The F2 generation showed the 3:1 red:white eye ratio, 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 © 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 In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development © 2011 Pearson Education, Inc.

Cell division and X chromosome inactivation Figure 15.8 X chromosomes Allele for orange fur Early embryo: Allele for black fur Cell division and X chromosome inactivation Two cell populations in adult cat: Active X Figure 15.8 X inactivation and the tortoiseshell cat. Inactive X Active X Black fur Orange fur

Recombination frequencies Figure 15.11 Linked genes tend to be inherited together because they are located near each other on the same chromosome Recombination frequencies 9% 9.5% Chromosome Figure 15.11 Research Method: Constructing a Linkage Map 17% b cn vg

Long aristae (appendages on head) Gray body Red eyes Normal wings Figure 15.12 Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes 48.5 57.5 67.0 104.5 Figure 15.12 A partial genetic (linkage) map of a Drosophila chromosome. Long aristae (appendages on head) Gray body Red eyes Normal wings Red eyes Wild-type phenotypes

Mutations Nondisjunction  Aneuploidy Breakage of a chromosome: (a) Deletion Mutations A B C D E F G H A deletion removes a chromosomal segment. A B C E F G H Nondisjunction  Aneuploidy Breakage of a chromosome: Deletion Duplication Inversion Translocation (b) Duplication A B C D E F G H A duplication repeats a segment. A B C D E F G H , pairs of homologous chromosomes do not separate normally during meiosis Offspring with this condition have an abnormal number of a particular chromosome (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 H F E D G A B P Q R © 2011 Pearson Education, Inc.

Down Syndrome (Trisomy 21) Figure 15.15 Down Syndrome (Trisomy 21) Figure 15.15 Down syndrome. Down syndrome is an aneuploid condition that results from three copies of chromosome 21

Aneuploidy of Sex Chromosomes Nondisjunction of sex chromosomes: XXX Klinefelter syndrome (XXY) Monosomy X, called Turner syndrome, (X0) females, who are sterile © 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. Figure 15.17 Genomic imprinting of the mouse Igf2 gene. It appears that imprinting is the result of the methylation (addition of —CH3) of cysteine nucleotides Genomic imprinting is thought to affect only a small fraction of mammalian genes Most imprinted genes are critical for embryonic development 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! The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

Figure 15.2 The chromosomal basis of Mendel’s laws. P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) Y r y  Y R R r y Meiosis Fertilization R Y y r Gametes All F1 plants produce yellow-round seeds (YyRr). F1 Generation R R y y r r Y Y Meiosis LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. R r r R Metaphase I Figure 15.2 The chromosomal basis of Mendel’s laws. Y y Y y 1 1 R r r R Anaphase I Y y Y y R r r R 2 2 Y y Metaphase II Y y y Y Y y Y Y y y Gametes R R r r r r R R 1/4 YR 1/4 yr 1/4 Yr 1/4 yR F2 Generation An F1  F1 cross-fertilization 3 Fertilization recombines the R and r alleles at random. 3 Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. 9 : 3 : 3 : 1

P Generation F1 Generation F2 Generation Figure 15.4b CONCLUSION P Generation w w X X X Y w w Sperm Eggs F1 Generation w w w Figure 15.4 Inquiry: In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have? Morgan’s experiment for X-linked gene w w Sperm Eggs w w F2 Generation w w w w w w

X Y Figure 15.5 Figure 15.5 Human sex chromosomes. 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 corresponding regions of the X chromosome The SRY gene on the Y chromosome codes for a protein that directs the development of male anatomical features Y

(d) The haplo-diploid system Figure 15.6 44  XY Parents 44  XX 22  X or 22  Y 22  X Sperm Egg 44  XX or 44  XY Zygotes (offspring) (a) The X-Y system 22  XX 22  X Figure 15.6 Some chromosomal systems of sex determination. (b) The X-0 system 76  ZW 76  ZZ (c) The Z-W system 32 (Diploid) 16 (Haploid) (d) The haplo-diploid system

Cell division and X chromosome inactivation Figure 15.8 X chromosomes Allele for orange fur Early embryo: Allele for black fur Cell division and X chromosome inactivation Two cell populations in adult cat: Active X Figure 15.8 X inactivation and the tortoiseshell cat. barr bodies Inactive X Active X Black fur Orange fur

Figure 15.UN01 Linked Genes b+ vg+ b vg F1 dihybrid female and homozygous recessive male in testcross b vg b vg Inherited together! Body color and wing size are inherted together 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

Gametes from yellow-round dihybrid parent (YyRr) Figure 15.UN02 Gametes from yellow-round dihybrid parent (YyRr) YR yr Yr yR Gametes from green- wrinkled homozygous recessive parent (yyrr) Figure 15.UN02 In-text figure, p. 294 yr YyRr yyrr Yyrr yyRr Parental- type offspring Recombinant offspring

Long aristae (appendages on head) Gray body Red eyes Normal wings Figure 15.12 Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes 48.5 57.5 67.0 104.5 Figure 15.12 A partial genetic (linkage) map of a Drosophila chromosome. Long aristae (appendages on head) Gray body Red eyes Normal wings Red eyes Wild-type phenotypes

A deletion removes a chromosomal segment. 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 D E F G H Figure 15.14 Alterations of chromosome structure. (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 H F E D G A B P Q R

Figure 15.15 Figure 15.15 Down syndrome. Down syndrome is an aneuploid condition that results from three copies of chromosome 21

Others Imprinting Organelle genes Silencing of a gene during gamate formation (depends on which parent) Organelle genes Mitochondria, choloroplast Maternally inherited example, mitochondrial myopathy and Leber’s hereditary optic neuropathy