Transcript R - My CCSD

Fundamentals of
Genetics
 Mendel’s Legacy
• Mendel’s Experiments
• Mendel’s Results &
Conclusions
Genetic Crosses
• Probability
• Monohybrid Crosses
• Dihybrid Crosses
Historical Background
of Genetics
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Mendel’s Legacy
• Gregor Mendel
 Mendel’s Garden Peas
 Mendel’s Methods
• Mendel’s Experiments
• Mendel’s Results & Conclusions
 Dominance & Recessiveness
 Law of Segregation
 Law of Independent Assortment
• Chromosomes & Genes
Learning
Objectives
•
TSW …
1. Describe the steps involved in
Mendel’s experiments on garden
peas
2. Distinguish btw/ dominant &
recessive traits
3. State 2 laws of heredity that were
developed from Mendel’s work
4. Describe how Mendel’s results can
be explained by scientific knowledge
of genes & chromosomes
Gregor Mendel
mathematician
natural philosopher
priest & abbot
The Garden Pea
Pisum sativum
• Annual plant
• Heritable features easily observed
• Monoecious: flowers have both male
and female organs
 plants can self-pollinate or crosspollinate
 male stamens can easily be removed to
prevent self-pollination
 pollen easily transferred by dusting to
cross-pollinate
Genetic Terminology
• Heredity – transmission of
characters from parents to offspring
• Character – heritable feature
 flower color
 pea pod color
 stem length
• Trait – heritable variant of a
character
 purple flower -v- white flower
 green pea pod -v- yellow pea pod
 tall stems -v- dwarf (short) stem
Pea Plant Traits
• Features investigated by Mendel







flower color: purple or white
flower position: axial or terminal
seed color: yellow or green
seed shape: smooth or wrinkled
pod shape: inflated or constricted
pod color: green or yellow
stem length: tall or short
Mendel’s Methods:
Flower Structure
• Pollination – pollen grains from
male flower part are transferred
to female flower part
 Anther (of stamen): male flower
part
 Stigma (of carpal): female flower
part
Mendel’s Methods:
Pollination Techniques
• Self-pollination – pollen
transferred from anthers of a
flower to a stigma of the same
flower or a different flower on
the same plant
• Cross-pollination – pollen
transferred from anthers of a
flower on one plant to a stigma
of a flower on a different plant
Mendel’s Experiments:
Parental Generation
• P1 generation = strain of
plants pure for a trait (truebreeding)
 “Parental” plants
 Developed by self-pollinating for
generations to produce offspring
that always have the same trait as
the parents
Ex: purple-flowered parents produce
only purple-flowered offspring
Mendel’s Experiments:
Filial Generations
• F1 generation = offspring
(hybrids) of the P1 generation
 Produced by cross-pollinating 2
pure (P1) strains
Ex: purple-flowered strain crosspollinated w/ white flowered strain
• F2 generation = offspring of
the F1 generation
 Produced by self-pollinating F1s
Mendel’s Results
Results of
P1 & F1 Crosses
• 7 characters, each showing 2
possible traits, were tested
• 1,000s of crosses were made in
each case
• In every P1 cross – 1 trait
disappeared in the F1
• In every F1 cross – the trait
reappeared in the F2 generation
Mendel’s Experiments &
Conclusions
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Mendel’s Conclusions
• Based on mathematics, each
character (ex., flower color) was
controlled by factors for 2
contrasting traits (exs., purple
flower or white flower)
• One trait-factor dominated the
other in expression of the
character when both were
present in the same plant
Mendel’s Conclusions:
Dominance & Recessiveness
• Dominant factor (trait) – masks
or “dominates” the other trait in
the F1
• Recessive factor (trait) –
disappears (or “recedes”) in the
F1 generation but reappears in
the F2
– Mendel discovered this after
carrying out monohybrid crosses
for specific characters.
Mendel’s Conclusions:
Segregation
• Law of Segregation – A pair of
factors is segregated (or
separated) during the formation
of gametes
 Mendel discovered this after
carrying out P1 & F1 crosses for
specific characters.
 The two paired factors that control
the expression of a trait separate
during the formation of
reproductive cells
Mendel’s Conclusions:
Independent Assortment
• Law of Independent
Assortment – Factors for
different characters are
distributed to gametes
independently
 Mendel discovered this after
carrying out P1 & F1 crosses for 2
specific characters.
 Plants showing the dominant trait
for one character could also show
the recessive trait for a different
character
Law of Independent
Assortment
• When considering two or more
different characters (genes), the
traits (alleles) for each segregate
into different gametes independently
Flower color
of each other
 the different characters
reside on different pairs
of homologous
chromosomes
Pod color
Law of Independent
Assortment
• Mendel discovered this after
carrying out repeated dihybrid
crosses.
 he crossed a true-breeding plant w/
yellow, round seeds (dominants)
and a true-breeding plant w/ green,
wrinkled seeds (recessives)
 all the F1s had the dominant traits
of yellow, round seeds
Law of Independent
Assortment
 he then crossed the yellow, round
F1s
 in the F2 generation, plants
appeared with: yellow, round seeds
(56%); yellow, wrinkled (19%);
green, round (19%); green,
wrinkled (6%)
Factors & Chromosomes
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Chromosomes & Genes
• Chromosomes are DNA
molecules
• DNA molecules are divided into
many distinct segments called
genes
• Genes control specific
hereditary traits
• Chromosomes occur in pairs
• Therefore genes also occur in
pairs
Chromosomes & Genes
• Alleles
 Alternate versions of the same
gene
 1 allele carried on each
homologous chromosome
 Occur at the same locus
• the DNA of each allele has a slightly
different nucleotide sequence
• results in slightly different variations
of the same character
Alleles
Example:
 The gene for the character
“flower color” on one homologue
can have an allele (factor) for purple
flowers
 The gene for the character
“flower color” on the other
homologue can have an allele
(factor) for white flowers
pp
PP
Genetic Symbology
Dominant alleles are
Pp
symbolized w/ a capital letter
Recessive alleles are
symbolized w/ a the lower
case letter used for the
dominant allele
Chromosomes &
Inheritance
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Video Quiz:
Genetics & Meiosis
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• 5 Questions
Genetic Crosses
• Genotype & Phenotype
• Probability
• Predicting Results of
Monohybrid Crosses
 6 examples
• Predicting Results of Dihybrid
Crosses
 2 examples
Learning
Objectives
•
TSW …
1. Explain how probability is used to
predict the results of genetic crosses
2. Use a Punnet square to predict the
results of monohybrid & dihybrid
crosses
3. Explain how a testcross is used to
show the genotype of an individual w/
the dominant phenotype
4. Differentiate a monohybrid cross
from a dihybrid cross
Genetic Vocabulary
• Phenotype
 outward appearance of an
organism
Ex: purple flowers
• Genotype
 genetic make-up of an organism
Ex: PP, Pp (– purple flowers)
pp (– white flower)
Genetic Vocabulary
• Homozygous
 an organism having a pair of
identical alleles for a character
Ex: PP = homozygous dominant
Ex: pp = homozygous recessive
• Heterozygous
 an organism having 2 different
alleles for a character
Ex: Pp
Probability
• Probability – the likelihood that
a specific event will occur
–



Expressed as:
decimal
percentage
fraction
Probability = # times an event is expected to occur
# opportunities for an event to occur
Probability
Q. What is the probability that
the dominant trait for purple
flowers will appear in an F2
generation?
Probability =
# times an event occurs
# opportunities for an event to occur
Mendelian Experiment:
Probability =
6,022 purple plants
= 0.75
8,023 total plants produced
Genetic Predictions
Based on laws of probability
• Multiplication Rule
 The probability that 2 or more
independent events will occur together
in a specific way is determined by
multiplying the probability of 1 event by
the probability of the other event
Ex: the probability of a coin flip ending up
heads is 50% or ½. The probability that
a second coin flip will end up heads is
also 50% or ½.
Therefore, the probability that 2 coins
flipped together will both end up heads is
½ X ½ = ¼ or 25%
Genetic Predictions
Based on laws of probability
• Addition Rule
 The probability that any 1 of 2 or more
independent events will occur is
determined by adding their individual
probabilities
Ex: the probability that flipped coin A will
come up heads while flipped coin B will
come up tails is 25% or ¼. The
probability that flipped coin A will come
up tails while flipped coin B will come up
heads is also 25% or ¼.
Therefore, the probability that both events
will occur is ¼ + ¼ = ½ or 50%
Predicting Results of
Monohybrid Crosses
• Monohybrid cross – a cross
btw/ individuals that involves 1
pair of contrasting traits
• Punnett square – a diagram
used as an aid in predicting the
probability that certain traits
will be inherited by offspring
Monohybrid crosses
investigate
inheritance patterns
for a single character
Mendel’s Crosses &
Punnett Squares
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Monohybrid Cross:
Procedure
• Step 1: determine the genotypes for
each mate in the cross. The
genotype shows the allelic
combination.
Ex: Tt X Tt
• Step 2: determine all the possible
kinds of gametes that each mate can
produce. Each gamete will have 1
allele from the genotype.
Ex: T & t for one parent; T & t for the
other parent
Monohybrid Cross:
Procedure
• Step 3: construct a Punnett square.
Place the gametes for one parent
across the top & the gametes for the
other parent along the left side.
Ex: 4 boxes for a monohybrid cross. T
& t across top; T & t along side
• Step 4: combine the possible gametes
of each parent in the 4 boxes
Ex: TT in top left box; Tt in top right &
bottom left boxes; tt in bottom right box
Example 1:
Homozygous X Homozygous
•
•
A plant that is true-breeding for
purple flower color (dominant) is
crossed with a plant that is true
breeding for white flower color
(recessive).
State the probability that any
offspring will have the following
flower color:
a) Purple
b) White
♂
♀
PP
X
purple
p
p
pp
Purple: 100%
white
P
P
Pp
Pp
purple
purple
Pp
Pp
purple
purple
Example 2:
Homozygous X Heterozygous
•
•
A plant that is true-breeding for
purple flower color is crossed
with a purple flowered plant that
carries the allele for white flower
color.
State the probability that any
offspring will have the following
flower color:
a) Purple
b) White
♂
♀
PP
X
purple
P
p
Pp
Purple: 100%
purple
P
P
PP
PP
purple
purple
Pp
Pp
purple
purple
Example 3:
Heterozygous X Heterozygous
• Two purple flowered plants
that carry the allele for white
flower color are crossed.
• State the probability that any
offspring will have the
following flower color:
a) Purple
b) White
♂
♀
Pp
X
purple
P
p
Purple: 75%
Pp
White: 25%
purple
P
p
PP
Pp
purple
purple
Pp
pp
purple
white
Phenotypic ratio
3:1
Genotypic ratio
1:2:1
Genetic
Ratios
1 PP : 2 Pp : 1 pp
• Phenotypic ratio
 ratio of offspring w/ dominant
appearance to offspring w/ recessive
appearance
Ex: for offspring of 2 heterozygotes = 3:1
for monohybrid crosses
• purple-flowered to white flowered individuals
after a cross of Pp X Pp
• Genotypic ratio
 ratio of homozygous dominants to
heterozygotes to homozygous recessives
Ex: for offspring of 2 heterozygotes = 1:2:1
for monohybrid crosses
Testcross
If an organism has the recessive phenotype,
we know its genotype because there is
only one way that the recessive form of the
character can occur.
Ex: White-flowered plants always have the pp
?
genotype
Question: If an organism has the dominant
phenotype, how can we determine which of
the 2 possible genotypes it has?
Ex: Purple-flowered plants can have either
the PP or the Pp genotypes
Testcross
• If you cross a homozygous
recessive, pp, with an organism of
the dominant phenotype, P?, there
are only two possible results.
 If 100% of the offspring have the
dominant phenotype, the dominant
parent MUST BE a homozygous
dominant, PP
 If any of the offspring have the
recessive phenotype, the dominant
parent MUST BE a heterozygote, Pp
• Determine the genotype of a
plant with the dominant
phenotype for flower color
(red-flowered).
• Note the probability that any
offspring will have the
following flower color:
a) Red
b) White
♂
♀
R?
red
r
X
Red: 100%
rr
White: 0%
white
R
R
?
Rr
?r
R
red
red
Phenotypic ratio
4:0
Genotypic ratio
4:0
r
Rr
red
?r
R
red
Dominant
color plant
is homozygous
♂
♀
R?
red
r
X
Red: 50%
rr
White: 50%
white
R
?r
Rr
?
rr
red
white
Phenotypic ratio
1:1
Genotypic ratio
1:1
r
Rr
red
?
rr
white
Dominant
color plant
is heterozygous
Example 5:
Incomplete Dominance
• Hybrids have phenotypes that
are a blend of the
characteristics of the two
parental varieties
 P generation: Red flower X
White flowers
 F1 generation: All Pink flowers
 F2 generation: ¼ Red, ¼ White,
& ½ Pink flowers
Example 5a:
Incomplete Dominance
•
•
Determine the genotypic &
phenotypic ratios for a cross
between two true-breeding plants
showing incomplete dominance
for flower color.
Note the probability that any
offspring will have the following
flower color:
a) Red
b) White
♀
♂
Red: 0%
RR
rr
White: 0%
white
Pink: 100%
X
red
r
r
R
R
Rr
Rr
pink
pink
Rr
Rr
pink
pink
Phenotypic ratio
4:0
Genotypic ratio
4:0
Example 5b:
Incomplete Dominance
•
•
Determine the genotypic &
phenotypic ratios for a cross
between two hybrid plants
showing incomplete dominance
for flower color.
Note the probability that any
offspring will have the following
flower color:
a) Red
b) White
♀
♂
Red: 25%
Rr
Rr
Pink: 50%
pink
White: 25%
X
pink
R
r
R
r
RR
Rr
red
pink
Rr
rr
pink
white
Phenotypic ratio
1:2:1
Genotypic ratio
1:2:1
Codominance
Phenotype
Genotype
• Hybrids show the
phenotypes of both parental
varieties in separate,
distinguishable ways
 P generation: Type A RBC
antigen X Type B RBC antigen
 F1: All Type AB RBC antigen
Example 6a:
Codominance
• A woman who is homozygous
for type A blood marries a
man who is homozygous for
type B blood.
• State the probability that any
child they produce will have
the following blood types:
a) A
b) B
c) AB
♂
A
A
B
B
I I X I I
♀
IB
B
I
100% probability
for AB blood type
IA
A
I
A
B
I I
A
B
I I
Type AB
Type AB
A
B
I I
A
B
I I
Type AB
Type AB
Example 6b:
Codominance
• A woman who is heterozygous
for blood type (AB) marries a
man who is heterozygous for
blood type (AB) .
• State the probability that any
child they produce will have the
following blood types:
a) A
b) B
c) AB
♂
A
B
A
B
I I X I I
♀
IA
B
I
50% probability for AB
blood type
25% probability each for
blood types A & B
IA
B
I
A
A
I I
A
B
I I
Type A
Type AB
A
B
I I
Type AB
B
B
I I
Type B
Mendel’s Experiments
• Mendel deduced the law
independent assortment from
repeated observations of P &
F1 dihybrid crosses.
 the appearance of dominant and
recessive traits according to
principles of probability led to
this conclusion
Mendel’s Experiments
• Experimental Model
 With “dependent” assortment,
traits will be inherited together
F1 gametes will be either YR or yr
Phenotypic ratio will be 3:1
 Hypothesis for independent
assortment, offspring will show 4
different combinations of traits
F1 gametes will be YR, Yr, yR, & yr
Phenotypic ratio will be 9:3:3:1
Mendel’s Experiments
• The Dihybrid Cross
 Mendel crossed a true-breeding
Dihybrid
crosses
plant
w/ yellow,
round seeds (both
investigate
dominant
traits) and a truebreeding
plant w/
green, wrinkled
inheritance
patterns
seeds
(bothcharacters
recessive traits)
for two
 all the F1s had the dominant traits:
yellow, round seeds
Mendel’s Experiments
 he then crossed the yellow, round
F1s
 in the F2 generation, plants
appeared with: yellow, round seeds
(56%); yellow, wrinkled (19%);
green, round (19%); green,
wrinkled (6%)
Dihybrid Crosses:
Procedure
• Step 1: determine the genotype for
each mate in the cross. The
genotype shows the allelic
combination.
Ex: TtAa X TtAa
• Step 2: determine all the possible
kinds of gametes that each mate can
produce. Each gamete will have 1
allele from each character.
Ex: for both parents – TA, Ta, tA, ta
Dihybrid Crosses:
Procedure
• Step 3: construct a Punnett square.
Place the gametes for one parent
across the top & the gametes for the
other parent along the left side.
Ex: 16 boxes for a dihybrid cross. TA,
Ta, At, ta across top & along side
• Step 4: combine the possible gametes
of each parent in the 16 boxes
Ex: 1 TTAA; 2 TTAa; 2 TtAA; 1 TTaa;
2 Ttaa; 4 TtAa; 1 ttAA; 2 ttAa; 1 ttaa
Punnett’s Contribution:
Cross Predictions
Dihybrid
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Example
1:
100% Round & Yellow
Homozygous
X Homozygous
100% heterozygous
for both characters
• A plant that is true-breeding for
round, yellow seeds is crossed
w/ a plant that is true-breeding
for wrinkled, green seeds.
• State the probability that any
offspring will have the following
seed phenotypes:
a) Round & yellow c) Round & green
b) Wrinkled & green d) Wrinkled &
yellow
Note: 9 : 3 : 3 : 12:
Example
phenotypic ratio
Heterozygous X Heterozygous
•
Two plants w/ round, yellow seeds
that are carrying alleles for wrinkled
& green are crossed.
• State the probability that any
offspring will have the following
flower color:
a) Round & yellow
c) Round & green
b) Wrinkled & green d) Wrinkled &
yellow
Common Genetic Ratios
• Phenotypic ratios
 Monohybrid heterozygote crosses:
3:1
 Dihybrid heterozygote crosses:
9:3:3:1
• Genotypic ratios
 Monohybrid heterozygote crosses:
1:2:1
 Dihybrid heterozygote crosses:
not commonly done
1:2:2:1:4:1:2:2:1
Dominance/Recessiveness
Relationships
• Range
 complete dominance – degrees of
incomplete dominance – codominance
• Reflection of mechanisms by which
specific alleles are expressed in
phenotype & do not involve the
ability of one allele to subdue
another at the level of DNA
• Do not determine or correlate with
the relative abundance of alleles in a
population
Effect of Environment
on Phenotype
hydrangea in
acidic soil
hydrangea in
alkaline soil
Effect of Environment
on Phenotype
Introduction to DNA
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Chromosomes, Genes, &
DNA
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