Ch09-Form Meiosis to Mendelocw.nctu.edu.tw/course/biology/modrenbiologyI_lecturenotes/ch09.pdf ·...

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Copyright 2005—Brooks/Cole—Thomson Learning

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Tobin and Dusheck: Asking About Life, 3E Chapter 9

From Meiosis toMendel

Chapter 9


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Key Questions

•How do sexually reproducingorganisms use meiosis to keepthe same number ofchromosomes from generationto generation?•How do the mechanics of

meiosis explain Mendel’s rules ofsegregation and independentassortment?

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Key Questions 2

•What is the value ofrecombination and geneticvariation?•Why are some diseases “sex-

linked”?


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Historical views of inheritance

Hippocrates: “particles”fromorgans in mother’s andfather’s body merged to formchild (460~377 B.C.)

Sperm (or eggs) contain tinyhuman (17th century)

Male and female traits blend inoffspring

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Chromosome could carryinformation?

19th Century: studies of mitosis,meiosis, and transmission of traitschanged ideas about inheritance

By 1883, scientists knew theprocesses that leads to an equaldistribution of the nuclear material totwo daughter cells.

Not until 1885, the link betweennuclear material and generation oforganisms was postulated.


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Early Theories of Inheritance

•Preformed individually in eitheregg or sperm

•Egg contributed material; spermcontributes life force

•Blending inheritance

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Rediscovery of Mendel’s work

Mendel’s work was published in 1866but was ignored

In 1870’s and 1880’s: mitosis, meiosisand chromosomes described, studied

Connected to Mendel’s work in early1900’s; repeated independently byseveral scientists, e.g., Carl Correns,Hugo de vries and Eric con tschermak.


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Why is the yellow dog yellow?

Walter Sutton, at age25, discovered thelocation of the genes incell. Genes, he said,had to be in thenucleus on thechromosomes.

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Genetics•Genetics is the study of

inheritance•Transmission genetics looks at

variation passed from generationto generation•Molecular genetics looks at the

details of how DNA works in acell


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Phenotype and Genotype

•Phenotype is the physical andbehavioral traits of an organism;“What we can see”•Genotype is the genetic makeup

of an organism, either a singlegene or its entire genome

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Gene

•A region of DNA that either:–Specifies the structure of a

single protein–Or, regulates the expression of

other genes

•Gene expression is theproduction of a protein encodedby a gene


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Phenotype•The way an organism looks or behaves is a

result of both genotype and environment

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Phenotype is affect byenvironmental factors

•Organisms can express differentphenotypes in response to differentenvironments, a phenomenon calledphenotypic plasticity.

•Exp1-Tadpoles of spadefoot toads inthe Sonoran desert.

•Exp2-Women living in groups ovulatesynchrony with one another


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Inheritance in Eukaryotes

•All use DNA as the genetic material•DNA is organized as chromosomes•Most chromosomes exist as pairs

at some time in life cycle•Pairs of chromosomes behave the

same during meiosis andfertilization

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Asexual Reproduction

•All offspring are geneticallyidentical to the parent; they areclones


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Sexual Reproduction

•Offspring inherit geneticinformation from 2 parents

•Offspring are unique in eachgeneration

•Offspring have paired chromosomes— diploid

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Homologous Chromosomes

•Pairs of matching chromosomesare homologous chromosomes

•Homologs have the same genes, inthe same order along thechromosome

•Gametes (reproductive cells) have1 set of homologs and are haploid


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Karyotype

•Homologouschromosomescan be arrangedinto a karyotype

•Humans have 23pairs ofchromosomes

•Is this a male orfemale?

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Gametes

•Most sexually reproducingorganisms produce 2 kinds ofgametes or germ cells–Egg or ovum is large–Sperm or spermatozoon is small,

mobile•Gametes are produced by cell

division called meiosis


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•The germ cells are found inspecial gamete-producingorgans, called gonads.•The gonads are called ovaries in

female and testes in males.

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Life Cycle •Germ cells fuseduring fertilizationto make a zygotewith somatic cells


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Fertilization

•Some aquaticorganisms andplants“broadcast”sperm and eggs

•Internalfertilizationrequirescopulation

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Meiosis•Specialized

type of celldivision thatproduceshaploid cellsfrom diploidparent cell


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Stages of Meiosis

•2 cell divisions after replication:–Meiosis I–Meiosis II

•Each division has 4 stages:–Prophase–Metaphase–Anaphase–Telophase

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Meiosis


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Prophase I

•Each chromosome is doubled with2 chromatids

•As in mitosis, chromosomescondense

•Homologous chromosomes pair insynapsis to form tetrads

•Cross over at chiasma

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Prophase I Pause

•Prophase I is longest phase ofmeiosis

•Production of proteins andstructures in egg

•In humans, meiosis starts beforebirth, pauses in prophase I untilpuberty, then 1 egg/monthcontinues for up to 40~50 years


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Crossing Over

•Duringprophase I,homologouschromosomesexchangepieces toproducerecombinantchromosomes

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Genetic Diversity•Independent assortment

and crossing over createnew combinations ofgenes in gametes

•2n

combinations:–n=2

4 types–n=23

8 million


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Metaphase I and Anaphase I•Homologous chromosomes

separate•Sister chromatids remain attached•Alignment of chromosomes is

random or independent•Resulting cells have mixture of

maternal and paternalchromosomes

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Meiosis I to Meiosis II•Telophase I and cytokinesis are

rapid; chromosomes may notdecondense

•Daughter cells are haploid withduplicated chromosomes

•Meiosis II separates chromatids;resulting cells are haploid withunduplicated chromosomes


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Practice Problems•Salamanders

have a diploidchromosomenumber of 30

•How manychromosomesdo they havein thesestages ofmeiosis?

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Practice Problems 2•Prophase I•Anaphase I•Prophase II•After

telophase II

•Salamandern=30•How many

tetrads inmetaphase I?


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Practice Problems 3•How many

tetrads inmetaphase II?•How many

chromatids inprophase I?

•How manychromatids inmetaphase II?•How many

chromatidsafter telophaseII?

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Nondisjunction•Chromosomes do not separate in

either anaphase I or anaphase II•Gamete is missing 1

chromosome or has 1 extrachromosome•Fertilization with a normal

gamete will produce a zygotewith 45 or 47 chromosomes


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Down’s Syndrome•Trisomy 21 results

fromnondisjunction ofchromosome 21

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Down’s Syndrome•Down syndrome

was firstdescribed by Dr.J. Langdon H.Down in 1866.“OBSERVATIONS ONAN ETHNICCLASSIFICATION OFIDIOTS”in J. ofMental Science(1867).


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Symptoms of Downsyndrome

The individuals with downsyndrome exhibited delayedskeletal system maturation (shortstocky build, short hands, andflattened facial features), weakmuscle tone, and mentalretardation.

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•“If these great racial divisions are fixed anddefinite, how comes it that disease is able tobreak down the barrier, and to stimulate soclosely the features of another division? Icannot but think that the observations which Ihave recorded are indications that thedifference in the races are not specific, butvariable. These examples of the result ofdegeneracy among the mankind appear tome to furnish some argument in favour of theunity of the human species.”- J. L. H.Down, 1867.


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Maternal Age Effects

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Why Sex?•Asexual reproduction is fast and

cheap•Sexual reproduction is expensive–Overproduce gametes; few are

fertilized–Energy of courting and mating

•Variation increases odds of someoffspring surviving


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Sex Chromosomes•In humans, X and Y chromosomes

are sex chromosomes–Women have 2 X’s–Men have 1 X and 1 Y

•Other 22 pairs are autosomes•Sex chromosomes pair during

meiosis as homologs

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The birth of the study of inheritance

Mendel chose to study peas because:

They were small, easy, and inexpensive

They had a short generation time

Unlike other species of plant, the petalsof pea flower remain closed until afterfertilization take place, called self-fertilization.


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Chromosomes and Alleles•Diploid cells have 2 copies of

each chromosome, and so 2copies of each gene•Genes can have slight

differences; they are alleles

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Alleles•Shorthand to denote different

alleles:–R1, R2–Or, R and r

•Individual with 2 identical allelesis homozygous, with 2 differentalleles is heterozygous


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FlowerColor —Snapdragons•R1 allele

red pigment•R2 allele

no pigment•R1R1: homozygous genotype, red

phenotype•R1R2: heterozygous genotype, pink

phenotype

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Breeding•Homozygotes are “true breeding”•Hybrids are the offspring of 2

genetically different true breedingparents (heterozygotes)–P1 generation — parents

–F1(first filial) generation —offspring–F2(second filial) generation —

grandchildren of P1


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Punnett Square•Genotypes of possible gametes

of parents on top and side•Inside squares have genotypes

of possible offspring

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Monohybrid Cross•Parents are both heterozygotes•Ratio of offspring genotypes:–1 R1R1

–2 R1R2

–1 R2R2

•Ratios apply tolarge numbers


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Crossing Pea Plants•Pea plants

can selffertilize•Mendel

controlledfertilization

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Mendel’s Experiments•P1 — 2 different

homozygotes•F1 generation are

heterozygotes•F2 generation has

phenotype ratio of3:1


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Dominant and Recessive

•Dominant allele controlsphenotype in heterozygotes

•Recessive allele controlsphenotype only in homozygotes

•Dominant alleles usually specifyfunctional proteins

•Recessive alleles usually specifynonfunctional proteins

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Incomplete Dominance•Snapdragon heterozygotes have

intermediate phenotype•Appears to be “blending”

inheritance


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Practice Problems•Tay-Sachs

disease isrecessive.•What ratio of

affected childrenwould 2heterozygoteshave?

•Polydactyly(6 fingers oneach hand) isdominant.

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Practice Problems 2•Cystic fibrosis

is recessive.•What ratio of

children woulda heterozygoteand ahomozygousrecessiveperson have?

•PKU is recessive.•What ratio of

children would 2affected peoplehave?

•A heterozygoteand ahomozygous isdominant?


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Principle of Segregation•Each sexually reproducing organism has 2

genes for each characteristic; these 2 genes

segregate during the production ofgametes

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Backcross

•Dominant phenotypes do notreveal their genotypes (homzygousor heterozygous?)

•Cross dominant phenotype withhomozygous recessive

•Offspring will reveal genotype ofthe dominant parent


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Backcross Problem•What would be the ratio of

offspring from a homozygousrecessive parent and ahomozygous dominant parent?•What would be the ratio of

offspring from a homozygousrecessive parent and aheterozygous parent?

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Independent Assortment•Test to see if 2 traits are inherited together•R-round peas• r-wrinkled peas•Y-yellow peas•y-green peas•P1 RRYY x rryy


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Independent Assortment 2•F1 RrYy•F2:–315 yellow/round–32 green/wrinkled–101 yellow/wrinkled–108 green/round

•New combinations in offspring

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9:3:3:1•If genes are not inherited together

(independent assortment), ratio ofoffspring in the second generationis 9:3:3:1

•All possible combinations ofgametes are formed in parents–RY, Ry, rY, ry


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2 FactorBackcross•Heterozygote

crossed withhomozygousrecessive–RrYy x rryy

•Ratio ofoffspring is1:1:1:1

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Evidence for ChromosomalTheory of Inheritance


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Drosophila melanogaster•Fruit flies are good experimental

subjects for genetics–Breed rapidly–Prolific: one female lays

hundreds of eggs–Small, easy to keep–Many observable traits–Only 4 chromosomes

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Drosophila Life Cycle


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Drosophila Chromosomes

•4 pairs ofchromosomes

•3 pairs ofautosomes

•1 pair of sexchromosomes, Xand Y–Females XX–Males XY

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X-Linked Traits•White-eye trait in

Drosophila is arecessive traiton X chromosome•Genes on the X

chromosome havedifferent patternof inheritance


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Sex-Linked Problems•Hemophilia is

an X-linkedrecessive trait.

•What are theprobabilities if acarrier(heterozygote)woman marriesa normal man?

•Color-blindnessis an X-linkedrecessive trait.

•What are theprobabilities if acarrier womanmarries a color-blind man?

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Chromosome Maps•Likelihood of crossing over

increases as genes are fartherapart on the chromosome•Genetic map is a calculation of

the distances between genes•1 map unit = 1% recombination


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Key Concepts•Genes are on chromosomes;

inheritance of both is parallel•In meiosis, homologous

chromosomes are separated ingametes

•Each gamete contributes 1 set ofchromosomes during fertilization

•Phenotype results from interactionof genotype and the environment

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