mendelian genetics & beyond
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Mendelian Genetics & Beyond. Chapter 10.2 & 10.3 Chapter 11.2. Today, we understand much about the inheritance of traits. But this was not always so… 150 years ago we had no knowledge of how traits were passed from parents to offspring….. - PowerPoint PPT PresentationTRANSCRIPT
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Mendelian Genetics & BeyondChapter 10.2 & 10.3
Chapter 11.2
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Today, we understand much about the inheritance of traits.
But this was not always so…
150 years ago we had no knowledge of how traits were
passed from parents to offspring…..
UNTIL….
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GREGOR MENDEL – 1822-1884 Born in Austria - monk - loved nature Interested in plants, meteorology and theories of
evolution
discovered 3 basic laws which govern the passage of traits
today he is known as the… “FATHER OF GENETICS”
(postmortem)
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GREGOR MENDEL
studied - pea plants observed - 7 traits (each with 2 forms) by tracing these 7 traits, Mendel discovered 3 basic laws which govern the passage of traits
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PEA PLANT
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before Mendel could start his experiments…..
he had to get pure (or true-breeding) plants
ALWAYS produce
offspring with the same trait
Why was this important?
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Mendel got pure plants
by self-pollinating
(or inbreeding)
plants for several
generations.
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eventually, he had 14 pure strains
(7 traits X 2 contrasting forms)
each pure strain he called a parental generation (P)
Now, he was ready to begin his experiments
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MENDEL’S EXPERIMENTS
Mendel crossed pure (P) X contrasting pure (P)
P X P = F1 (first filial generation)
RESULTSonly 1 form of the trait appeared in the
F1 generation
Mendel repeated his experiment s many times – all with the same results
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then, he crossed… F1 x F1 = F2 (second filial generation)
RESULTS both forms of the trait appeared
in a ratio of 3:1
Mendel repeated his experiment s many times – all with the same results
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MENDEL concluded that the patterns of inheritance are
governed by 3 principles
PRINCIPLE OF . . . . 1. Dominance and Recessiveness
2. Segregation 3. Independent Assortment
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Principle of Dominance and Recessiveness
Mendel concluded…each trait is controlled by a pair (2) of
factors… a dominant factor will prevent a
recessive factor from being
expressed
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PRINCIPLE OF SEGREGATION
Mendel concluded…each pair of “factors” must segregate
(separate) during the formation of gametes
So that….. only one “factor” is inherited from each
parent
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Principle of Segregation
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PRINCIPLE OF INDEPENDENT ASSORTMENT
Mendel concluded… the inheritance of 1 trait is
independent of the inheritance of another trait
* the factors for different traits are distributed independently from one another
* this principle requires the observation of 2 or more traits at the same time
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Principle of Independent Assortment
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Important Terms to know Genetics: study of how
traits are passed from parent to offspring (heredity)
Gene: unit of inheritance that usually is directly responsible for one trait or characteristic
Allele: an alternate form of a gene; formerly called factors by Mendel Represented by letters
yellow = Y purple = P
Examples of Alleles
WrinkledSmooth
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Alleles can be…Homozygous or Heterozygous When alleles from each parent are the
same, they are called homozygous (pure) Written as double letters that are the same
size Ex: PP, pp, YY, yy, BB, bb
When alleles from each parent are different, they are called heterozygous (hybrid) Written as double letters that are different
sizes Ex: Pp, Yy, Bb
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Alleles can be…Dominant or Recessive A dominant allele is expressed no matter
what the second allele is Represented by a capital letter
Ex: PP, Pp, YY, Yy, BB, Bb
A recessive allele is only expressed when the second allele is the same Represented by a lower case letter
Ex: pp, yy, bb
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Phenotype vs. Genotype Phenotype is the
physical expression or appearance of the trait Ex. Purple flower yellow seeds blond hair
Genotype refers to the alleles (genes) pairs Ex. PP, Pp YY, Yy bb
This may be changed.
(Coloring your hair)
(Plastic surgery)
This can NEVER be changed.
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Mendel’s studies Studied garden peas
because: they grew fast made lots of
offspring with short generation times
few traits that were easily seen
traits showed complete dominance
Usually self-pollinate/fertilize
7 well-defined garden pea traits
Counted offspring of each phenotype and analyzed the results mathematically – saw patterns
Trait Dominant Recessive
Flower color
Purple White
Flower position
Axial Terminal
Seed color Yellow Green
Seed texture
Smooth Wrinkled
Pod color Green Yellow
Pod texture
Inflated Constricted
Height Tall Short
You MUST KNOW THESE!!
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Fig. 10.4, Mendel’s 7 garden pea characters.
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Practice assigning alleles1. PP2. TT3. GG4. Gg5. ss6. AA7. Aa8. aa9. yy
1. Homozygous purple flower
2. Homozygous tall
3. Homozygous green pod
4. Heterozygous green pod
5. Homozygous wrinkled seed
6. Homozygous axial flower
7. Heterozygous axial flower
8. Homozygous terminal flower
9. Homozygous green seeds
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Probability
The chance that an event will occur
Probability = # of 1 kind of event total # of events
What is the probability of… A coin landing on heads? Drawing a king from a deck of cards? Having a baby boy? A die landing on the number “3”?
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Punnett Squares Dr. Reginald Punnett, early 1900s Graphical way to show probability 5 steps:
Assign P genotypes Remember: use the letter of the dominant trait
(homozygous = same size; heterozygous = different sizes)
Split alleles Perform cross Report F1 genotypes Report F1 phenotypes
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Types of Crosses Monohybrid cross = cross of two different
alleles for a single trait. Ex: crossing eye color x eye color Ex: crossing hair color x hair color
Dihybrid cross = cross of two different alleles for two traits. Ex: crossing eye & hair color x eye & hair color
4 squares
16 squares
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Monohybrid Crosses can be used with… Complete dominance of traits Incomplete dominance of traits Codominance of traits Sex-linked traits Multiple allele traits
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Let’s Practice
Step 1. P = SS x ss
Step 2. Split alleles
S S
Ss Ss
Ss Sss
s
Step 3: perform Punnett
Step 4: F1 genotype
100% Ss
Step 5: F1 phenotype
100% smooth seeds
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Some practice – Show all 5 steps1. In gerbils, brown fur is completely
dominant over white fur. Cross a heterozygous brown-furred gerbil with a white-furred gerbil.
2. Cross two heterozygous tall plants. 3. In pigs, curly tails are completely
dominant over straight tails. Cross a homozygous curly-tailed pig with a heterozygous curly-tailed pig. What are the possible phenotypes of the offspring?
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INCOMPLETE DOMINANCE when neither allele is completely dominant, both alleles influence the trait there is a BLENDING of the alleles
EXAMPLE – Four O’clock Flowers red (r) and white (w) exhibit incomplete
dominance a heterozygous individual (rw) will be pink
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CODOMINANCE when both alleles are dominant both alleles are fully expressed – NO BLENDING
EXAMPLE - Some cattle and horses exhibit codominance in their coat color.
red (R) and white (W) are codominant alleles in the heterozygous individual (RW), both be fully
expressed
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SEX-LINKED GENES….are genes that are linked to (found on) the sex chromosomes
X chromosome is LARGE + carries many genes
genes on the X chromosome are called X linked genes
discovered by Thomas Hunt Morgan when working with the fruit fly, Drosophila melanogaster
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Sex-linked Traits Males affected more often; cannot be
heterozygous Females less affected; can be
heterozygous (carriers)
Examples: Red-green colorblindnessHemophilia (blood clotting disorder)
Duchenne Muscular Dystrophy
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COLORBLINDNESS a recessive trait found on the X chromosomeresults in an inability to distinguish certain colors
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Duchenne MUSCULAR DYSTROPHY a recessive trait found on the X chromosomes results in the weakening and wasting away of muscle tissue
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MULTIPLE ALLELES occur when there are 3 or more forms of a
trait although there are 3+ alleles, only 2 are
inherited
EXAMPLE – 3 alleles (A, B and o) influence blood type Alleles A and B are codominant. Allele i is recessive.
4 possible blood types –
What is a person’s blood type if their allele pair is?
AA, Ao, BB, Bo, AB, oo
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Sometimes with multiple alleles Epistasis occurs
One allele hiding the effect of another allele
Example: coat pigmentation on animals – see p. 305
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POLYGENIC TRAITSTraits that result from the interaction of several
genes (several allele pairs).
Skin color, hair color, eye color are polygenic traits.
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Some ways to check yourself…. In monohybrid crosses
heterozygous x heterozygous crosses: Genotypic ratio ALWAYS = 1:2:1 Phenotypic ratio ALWAYS = 3:1
In dihybrid crosses heterozygous x heterozygous crosses:
Phenotypic ratio ALWAYS = 9:3:3:1
TRY SOME INTERACTIVE PUNNETT SQUARES @http://glencoe.mcgraw-hill.com/sites/dl/free/
0078695104/383934/BL_05.html
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Dihybrid Punnett Squares Follow the same steps as in monohybrids Extra step (FOIL the alleles)
Assign P genotypes Remember: use the letter of the dominant trait
(homozygous = same size; heterozygous = different sizes)
FOIL alleles Split alleles Perform cross Report F1 genotypes Report F1 phenotypes
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Practice with FOILing1. Homozygous purple & axial x white & terminal
P= PPAA x ppaaFOIL = PA PA PA PA x pa pa pa pa
2. Homozygous smooth & yellow seeds x heterozygous smooth & yellow seedsP= SS YY x Ss YyFOIL = SY SY SY SY x SY Sy sY sy
3. Heterozygous green & inflated pods x heterozygous green & inflated podsP= Gg Ii x Gg IiFOIL = GI Gi gI gi x GI Gi gI gi
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Cross #1
PpAa PpAa PpAa PpAa
PpAa PpAa PpAa PpAa
PpAa PpAa PpAa PpAa
PpAa PpAa PpAa PpAa
P= PPAA x ppaaFOIL = PA PA PA PA x pa pa pa pa
PA PA PA PA
pa
pa
pa
pa
F1 genotypes:
100% PpAa
F1 phenotypes:
100% purple & axial flowers
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Cross #2P= SS YY x Ss YyFOIL = SY SY SY SY x SY Sy sY sy
SSYY SSYY SSYY SSYY
SSYy SSYy SSYy SSYy
SsYY SsYY SsYY SsYY
SsYy SsYy SsYy SsYy
SY SY SY SY
SY
Sy
sY
sy
F1 genotypes:
25% SSYY, 25% SSYy
25% SsYY, 25% SsYy
F1 phenotypes:
100% smooth, yellow seeds
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Cross #3P= Gg Ii x Gg IiFOIL = GI Gi gI gi x GI Gi gI gi
GGII GGIi GgII GgIi
GGIi GGii GgIi Ggii
GgII GgIi ggII ggIi
GgIi Ggii ggIi ggii
GI Gi gI gi
GI
Gi
gI
gi
F1 genotypes:
1 GGII, 2 GGIi, 2 GgII, 4 GgIi, 1 GGii, 2 Ggii, 1 ggII, 2 ggIi, 1 ggii
F1 phenotypes:
9 green & inflated pods
3 green & contricted pods
3 yellow & inflated pods
1 yellow & constricted pods
(9:3:3:1)
There should always be a 9:3:3:1 phenotypic ratio in a heterozygous x heterozygous dihybrid cross.
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Polyploidy One or more extra sets of all
chromosomes in an organism Triploid organism (3n) Rarely occurs in animals Always fatal in humans Plants OFTEN exhibit polyploidy
Exhibit more vigor and size oats & wheat = 6n Sugar cane = 8n
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Genetic Recombination understoodAfter studying meiosis and
After Mendel’s studies…
Why don’t you look identical to any other human being?
*the possible number of allele combinations is 223 x 223 70 trillion (not including variation from crossing over)
Scientists now use this knowledge to artificially recombine genes to breed plants and animals with desirable traits.
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Gene Linkage Is an exception to the Law of Independent
Assortment Genes that are closer together on a
chromosome are more likely to travel together during gamete formation.
Scientists studied the Drosophila melanogaster (fruit fly) to demonstrate gene linkage
Chromosome maps are used to show this frequency 1 map unit = 1% cross over
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CHROMOSOME MAP (GENE MAP)A “map” showing the relative location of genes on a
chromosome.The “HUMAN GENOME PROJECT” was an effort to
map the human genome (the human chromosomes).
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How to map genes…Frequency between gene X and Y = 10%Frequency between gene Y and Z = 15%Frequency between gene X and Z = ?
X Y Z10 map units 15 map units