heredity – passing traits to offspring
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Heredity – passing traits to offspring. Chapters 11, 12. Kevin Bleier Milton HS, GA. How produce offspring?. Two major modes of reproduction Asexual reproduction Sexual reproduction. section 11.1. Asexual reproduction. - PowerPoint PPT PresentationTRANSCRIPT
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Heredity – Heredity – passing traits to offspringpassing traits to offspringChapters 11, 12
Kevin BleierMilton HS, GA
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How produce offspring?How produce offspring?Two major modes of reproduction
1)Asexual reproduction
2)Sexual reproduction
section 11.1
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Asexual reproductionAsexual reproductionOne parent making exact genetic
copy of itself (offspring are clone of parent)
Advantages: quick, no need for mate, both males and females can produce offspring
Disadvantages: no genetic diversity in offspring
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Asexual reproductionAsexual reproductionMany organisms can do this – plants,
fungi, protists, some animals, bacteria
What type of eukaryotic cell division creates exact copies of cells?
What type of prokaryotic cell division?
mitosis
binary fission
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Sexual reproductionSexual reproductionTwo parents both contribute half
of their genes to offspring
Which half they contribute can be different each time = genetically DIFFERENT offspring
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Sexual reproductionSexual reproductionAdvantages: genetic diversity
Why is this important?
Disadvantages: finding a mate, only females bear children, more energy expense
more to come in evolution unit next
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Sexual reproductionSexual reproductionWho reproduces sexually?
Most multicellular eukaryotes can reproduce sexually
(protists, fungi, plants, animals)
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Sexual reproductionSexual reproductionWhat type of cell division produces
sperm and egg cells needed for sex?
meiosis (focus of section 11.2)
germ line cells gametes(sperm / egg)
(beginning of meiosis) (end of meiosis)
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Sexual vs. asexualSexual vs. asexualBacteria can ONLY reproduce
asexually
Some organisms can ONLY reproduce sexually (like us humans)
MANY organisms can reproduce both ways … so when might they use one method?
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Sexual vs. asexualSexual vs. asexualSexual reproduction: generating
genetic diversity needed to overcome a challenge
(threatening, dangerous, unstable environments)
Asexual reproduction: producing many offspring when conditions are stable
(safe, stable environment with lots of resources)
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Preparation for meiosis Preparation for meiosis discussiondiscussionOur goal: how do we make cells
in preparation for sexual reproduction?
How does this process generate the genetic diversity important for sexual reproduction?
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Some vocabulary reviewSome vocabulary review
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Some vocabulary reviewSome vocabulary reviewDNA – chemical code (order of letters)
gene – small segment of DNA letters that codes for a specific protein (that leads to a specific trait)
chromosome – entire strand of DNA that is packed up for cell division (carries 100s to 1000s of genes)
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Some vocabulary reviewSome vocabulary review
R
chromosome
gene
T A C G G T
A AT G C C
DNA code
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Some new vocabularySome new vocabularyOrganisms’ body
cells (somatic cells) have chromosomes that come in pairs
Pairs called homologous chromosomes (carry same genes, may carry different versions of gene = alleles)
R R
R r r r
gene = seed shape
2 alleles:R = round seedr = wrinkled seed
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Some new vocabularySome new vocabularyCells that contain homologous
chromosome pairs = diploid
Somatic cells are diploid, as are germ-line cells that start meiosis
Gamete cells (sperm and egg) are haploid (only one of each chromosome, not pairs)
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Why haploid gametes?Why haploid gametes?
R R r r
parent 1’s diploid germ-line cell
parent 2’s diploid germ-line cell
Rparent 1’s haploid gamete (sperm)
meiosis meiosis
rparent 2’s haploid gamete (egg)
sexual intercourse
results in diploid zygote (which grows into new offspring)
so sexual reproduction overall = meiosis + sexual intercourse
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Different species = Different species = different chromosome #different chromosome #
Diploid = 2n
Haploid = n
2n for humans is 46
n for dogs is 39
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Types of chromosomesTypes of chromosomesHuman karyotype
22 of 23 pairsall sexes have samegenes (autosomes)
23rd pair determinessex (sex chromosomes)
Males = XY Females = XX
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Meiosis – creating Meiosis – creating gametesgametesTwo questions to answer while
studying meiosis:
1)How does a diploid germ-line cell eventually become haploid gametes?
2)How does the process generate genetic variety? (making every gamete different)
section 11.2
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Meiosis simulationMeiosis simulationWe will work with a diploid germ-
line cell where 2n = 8 (or 4 pairs of homologous chromosomes)
So haploid number (n = ___ )4
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Meiosis simulationMeiosis simulation
diploid germ-line cell
(2n = 8)
meiosis
haploid gamete (n = 4)
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MeiosisMeiosisCopies all DNA at the beginning
(like all cell divisions)
Two cell divisions and DNA divisions yields haploid cells
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2 divisions in meiosis2 divisions in meiosis
beginning of meiosis – diploid germ-line cell(here, 2n = 8)
first step of any cell division – copy the DNA
homologous pairs exist, just not organized or close together yet
exact copies
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First meiotic cell divisionFirst meiotic cell division
in meiosis I, put homologous pairs together
exact copies
homologous pairs
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First meiotic cell divisionFirst meiotic cell division
in meiosis I, put homologous pairs together
now, cell lines up pairs together (2 lines of Xs … DIFFERENT than mitosis!)
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First meiotic cell divisionFirst meiotic cell division
in meiosis I, put homologous pairs together
now, cell lines up pairs together (2 lines of Xs … DIFFERENT than mitosis!)any division of DNA must be symmetrical – here, meiosis I splits the homologous pairs
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Finishing meiosis IFinishing meiosis I
this cell has divided its DNA up equally, and now splits into 2 cells
but meiosis is not finished yet, as both of these cells will divide again
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Second meiotic cell divisionSecond meiotic cell division
Looks disorganized because chromosomes unpacked at end of meiosis I, then repacked for meiosis II
Line up into 1 line of Xs (just like mitosis)
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Second meiotic cell divisionSecond meiotic cell division
Meiosis II must reorganize chromosomes
Line up into 1 line of Xs (just like mitosis)
Meiosis II splits DNA evenly by splitting exact copies
Each cell splits into 2, creating 4 haploid gametes at the end
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Summary of first questionSummary of first questionHow does a diploid germ-line cell
eventually become haploid gametes?
Diploid germ-line cell copies all DNA
Then divides DNA in two separate divisions◦Meiosis I separates homologous pairs◦Meiosis II separates exact copies
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Meiosis – creating Meiosis – creating gametesgametesTwo questions to answer while
studying meiosis:
1)How does a diploid germ-line cell eventually become haploid gametes?
2)How does the process generate genetic variety? (making every gamete different)
independent assortment and crossover
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b
r
T p
Rt
P
Back to early meiosis IBack to early meiosis I
Let’s label one gene on each homologous pair
We also assume all heterozygous here –organisms can be homozygous (RR or rr)
B
PP
tt RR
ppTT
rr
bb
BB
gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele
B = yellow seedp = white allele t = short allele r = wrinkled allele b = green seed
Remember, there are actually 100s / 1000s of genes on each pair
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b
Independent assortmentIndependent assortment
When homologous chromosomes pair up, they do so randomly
B
PP
tt RR
ppTT
rr
b
B
gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele
B = yellow seedp = white allele t = short allele r = wrinkled allele b = green seed
Ultimate lineup will be different in every instance of meiosis
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b
Independent assortmentIndependent assortment
Assuming this particular lineup …
B
PP
tt
RR
pp
TT
rr
bB
gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele
B = yellow seedp = white allele t = short allele r = wrinkled allele b = green seed
We will get gametes carrying these particular alleles
gamete 1: pTbrgamete 2: PtBR
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b
Independent assortmentIndependent assortment
What if another round of meiosis had the “R” chromosomes lineup differently?
B
PP
tt
RR
pp
TT
rr
bB
gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele
B = yellow seedp = white allele t = short allele r = wrinkled allele b = green seed
We will get gametes carrying these particular allelesgamete 1: pTbr
gamete 2: PtBRgamete 3: PtBrgamete 4: pTbR
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b
Independent assortmentIndependent assortment
How many different gametes are possible if every pair can line up two different ways?
B
PP
tt
RR
pp
TT
rr
bB
2
2
2
2
x
x
x
16
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Independent assortmentIndependent assortmentHow many homologous
chromosome pairs do human germ line cells have?
So how many different gametes can every human make by lining them up differently every time?
23 pairs
= 223 ~ 8,000,000 different gametes
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Independent assortmentIndependent assortmentRecall though that sexual
reproduction requires 2 parents both making gametes
And we cannot choose which gametes fertilize – that’s also random~ 8,000,000 possible sperm x 8,000,000
possible eggs~ 64,000,000,000,000 possible zygotes for 2 parents~ 6,500,000,000 people on Earth
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b
Crossing overCrossing overBefore meiosis I lineup of homologous pairs
B
PP
tt
RR
pp
TT
rr
b B
Inner, non-sister chromatids can get so close that tips of chromosomes exchange
Whether or not this occurs in each pair is random every time meiosis occurs
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b
Results of crossover eventResults of crossover event
B
PP
tt
RR
pp
TT
rr
bB
All four gametes will be genetically different
PtBR
PTBR
ptbr pTbr
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Meiosis – creating Meiosis – creating gametesgametesTwo questions to answer while
studying meiosis:
1)How does a diploid germ-line cell eventually become haploid gametes?
2)How does the process generate genetic variety? (making every gamete different)
independent assortment and crossover
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Meiosis summaryMeiosis summaryStarts with 1 diploid germ-line cell,
ends with 4 haploid gametes
Each gamete has one of the chromosome pairs, all genetically different in alleles that they carry
Sperm and egg gametes must combine to complete sexual reproduction
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Errors in meiosis = Errors in meiosis = nondisjunctionnondisjunction
Diploid germ line cell 2n = 4
Haploidgametesshouldben = 2
Some gametes have 1 extra (offspring would have too many chromosomes)
Some gametes are missing 1 (offspring would have too few chromosomes)
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Nondisjunction effectsNondisjunction effectsExample: Down
syndrome (trisomy 21)
Extra or missing chromosome in all somatic cells has large effect on phenotype
Many trisomies / monosomies result in inviable embryo
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Overall human life cycleOverall human life cycle
section 11.3
diploid somatic cells(multicellular human)(2n = 46)
diploid germ-line cells (within sex organs)
haploid gametes(n = 23)
meiosis
sperm
egg(ovum)
diploid zygote(single-cell)(2n = 46)
fertilization
mitosis and development
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possible haploid gametes
Where we are headed … Where we are headed … Punnett squares show all the
possibilities of gametes that can be made in meiosis
P p P P
parent 1 parent 2
PP pp PP PP
possible gametes:
P, ppossible gametes: PPp
P p
PP
Pp
Pp
PPPP
P
P
or
PpP p
PP
P
original diploid germ-line cells
possible diploid zygotes formed by fertilization of specific sperm and egg
PpPP
chapter 12
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Two – trait heredity Two – trait heredity analysisanalysisWe will assume that genes are on
different chromosome pairs
PP
tt
pp
TT
Parent 1: PpTt Parent 2: PPtt
PP
t t t t
P P
possible gametes: PT, pt
or
PP pp
TTtt
, Pt, pT
possible gamete: Ptrules for making gametes:
1) half of what you started with 2) one of each chromosome pair (one of each letter)
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Two – trait Punnett squareTwo – trait Punnett squareParent 1: PpTt Parent 2:
PPttpossible gametes: PT, pt, Pt, pT
possible gamete: Pt
PpTt
PT
Pt pT
pt
PPtt
Pt
Pt
Pt
Pt
PPTt
PPTt
PPTt
PPTt
PPtt
PPtt
PPtt
PPtt
PpTt
PpTt
PpTt
PpTt
Pptt
Pptt
Pptt
Pptt
PT
Pt pT
pt
Pt PPTt PPtt PpTt Pptt
or
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Two – trait Punnett Two – trait Punnett squaressquaresPlease don’t do this
PpTtP p T t
PPtt
P
P
t
t
PP
PP
Pt
Pt
Pp
Pp
pt
pt
tt
tt
PT
PT
Tt
Tt
Pt
Pt
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Researching in mid 19th century
No idea about chromosomes / DNA / meiosis
Quantitative approach yields basic principles of heredity
Mendel’s worldMendel’s world
section 12.1
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Pea plants
1.Quick reproduction produces many offspring
2.Can control parents easily (also self and cross-pollination possibilities)
3.Many simple traits
Mendel’s modelMendel’s model
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Mendel could only see phenotypes, inferred genotype from his results
A reminderA reminder
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Some plants were true-breedingAll
Some
Some plants were hybridsSome
Mendel’s observationsMendel’s observations
+
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Crosses:
Mendel’s observationsMendel’s observations
xpurebred purebred
hybrid
purebred hybrid hybrid purebred
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1) All organisms have 2 copies of a gene
Some conclusionsSome conclusions
R R
Modern understanding: homologous pairs both carry copies of gene
section 12.2
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2) Each parent passes 1 of their copies to offspring
Some conclusionsSome conclusions
R
Modern understanding: meiosis splits homologous pairs – sends one to each gamete
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3) Complete dominance – one trait completely dominates the other when both are together
hybrid organism has same phenotype as purebred dominant organism
Some conclusionsSome conclusions
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True of many traitsTrue of many traits
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Crossing two traits at Crossing two traits at onceonce
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Independent assortment – traits can be considered independently of each other
ConclusionConclusion
R r
Modern understanding: assumes that genes are carried on separate chromosome pairs
independent assortment of homologues
Y y
rR
Y y
Possible gametes:
RY ry
Y Yyy
Ry rY
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Assumes:
1)One gene determines trait, two alleles
2)Complete dominance – one dominant allele, one recessive allele
3)Genes always on different chromosomes, always independent of each other (“unlinked”)
Mendelian model of Mendelian model of geneticsgenetics
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Testcross differentiates the two
How tell organisms apart?How tell organisms apart?
section 12.3
B = black furb = brown fur
possible genotypes:BB
Bbbb
cannot distinguish“unknowns”
known – brown fur
Determine genotype of unknown organism by crossing with known brown fur organism
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TestcrossTestcrossBbB b
BBB B
bb
b
b
b
bbb
Bb
Bb
Bb
Bb
BbBb
bbbb
How does this help you distinguish between homozygous dominant and heterozygous?
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Few human traits follow Mendel’s simple model
Rolling tongue, widow’s peak
Certain human genetic disorders (sickle cell anemia, Huntington’s, cystic fibrosis)
Mendelian traits in Mendelian traits in humanshumans
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Assumes:
1)One gene determines trait, two alleles
2)Complete dominance – one dominant allele, one recessive allele
3)Genes always on different chromosomes, always independent of each other (“unlinked”)
Mendelian model of Mendelian model of geneticsgenetics
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Many organisms have inheritance patterns that do NOT follow Mendel’s assumptions
Updating Mendel’s modelUpdating Mendel’s model
section 12.4
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Assumption: complete dominance
Exception: incomplete dominance
Updating Mendel’s modelUpdating Mendel’s model
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Assumption: complete dominance
Exception: codominance
Updating Mendel’s modelUpdating Mendel’s model
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Assumption: one gene determines one trait
Exception: polygenic inheritance
Updating Mendel’s modelUpdating Mendel’s model
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Assumption: there are only 2 alleles for a gene
Exception: multiple alleles
Updating Mendel’s modelUpdating Mendel’s model
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Assumption: genes determine a phenotype
Exception: gene / environment interactions
Updating Mendel’s modelUpdating Mendel’s model
acidic soil
neutral or basic soil
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Updating Mendel’s modelUpdating Mendel’s model
Assumption: genes are unlinked (always on different chromosome pairs)
Exception: linked genes