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Mendel’s workplace
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Prior history
To appreciate Mendels work, one must keep in mind the
prevailing theories of inheritance
Preformationism: the idea that gamete contains an intact
organism, was first proposed in the late 1600s.
A preformed human infant or homunculus contained within a
sperm (Male centric view of the world)
Blending inheritance: essences of both sperm and egg mixed to
form offspring intermediate between the parents.
Strains of plants and animals generated by blending
Mendel ignored development, focused solely on transmission of traits
2
Darwin and Heredity
Darwin: Among individuals of any species, there are
differences (variations)
Evolution cannot occur unless there are differences
among individuals
If individuals are identical and remain so generation
after generation, there can be no evolution.
Variation is important
Variation must be transmitted from parent to
offspring
Mechanism of inheritance is important for
understanding evolution (1860s)
Mechanism of inheritance not understood
Certain traits suddenly appear in an individual
These traits are then transmitted to their progeny
Porcupine man- protuberances on body
Not due to the environment- only the man and his
offspring have these traits
3
Pangenesis
Pangenesis: (1860s)
Whole organism reproduces itself
Gemmules determine characteristics (traits) of
organism
Germ cells contain gemmules of all sorts and these are
transmitted to the next generation.
Fertilization- gemmules unite and produce new cells of
the types from which they are produced.
4
Cytology
Cytology:
1600s Organisms are composed of cells
1830s The most distinct structure in a cell is the
nucleus. ALL cells have a nucleus. Role of nucleus was
controversial because it disappears during cell
division.
1840s Cells are formed by the division of preexisting cells (Mitosis)
1850s Sperm and Ovum are cells.
1873 Mitosis described in detail- nuclear and
chromosome dynamics described
1883 Fertilization in sea urchin showed that sperm
and ovum fuse. This links parents to offspring
Problem of ploid:. Nuclei of parents and progeny are
diploids. Are nuclei of germ cells haploid?
1885 Meiosis described- Reductional division of
chromosomes keeps number of chromosomes constant.
No similar phenomenon seen in any other cellular
organelle.
5
Cytology’s contribution to Mendel
Heredity is a consequence of genetic continuity
Germ cells are the vehicle of transmission from one
generation to the next.
Germ cells contain half the number of chromosomes
found in body cells.
Fertilization involves union of sperm and eggs.
Fertilization involves union of nuclei.
Nucleus is crucial. During division it resolves into long
chromosomes that split lengthwise.
Chromosomes do not lose their individuality. They are
inherited intact.
Diploid embryo descends from maternal/paternal fusion
of haploid chromosomes
6
The origin of genetics:
The study of genetics begins when Gregor Mendel, in 1865,
addressed the question :
"How are characters passed on from one generation to the
next?”
Mendel was the first to make a serious attempt of
experimentally answering the question of heredity and not only
were his answers correct, they were a complete and compelling
proof.
Mendel published in 1866 but little attention was paid to his
work until 1900, when it was simultaneously rediscovered by
three scientists, one in Holland, one in Austria, and one in
Germany.
There are often impressions that Mendel was removed from the
scientific community, or that his papers were not well
circulated. This was not true. Over 200 copies of Mendels
papers have been discovered in different libraries.
All three of Mendel's rediscovers had read Mendel's work prior
to publishing their own work.
7
Gregor Mendel was born on either 20th or 22nd July, 1822 in
Heizendorf (today Hynice in the Czech Republic).
From 1851 to 1853, Gregor Mendel studied zoology, botany,
chemistry, and physics at the University of Vienna.
He studied botany under Prof. Unger where he learned genetic
crosses
He studied physics under Prof. Doppler where he learnt
statistics
Mendel returned to Brno and began his experiments with the
hybrid cultivation of pea plants in 1856.
After spending eight years carrying out experimental work in
the monastery garden, he reported on the results of his
observations at the meetings of the Association for Natural
Research in Brno on the evenings of February 8th and March
8th, 1865
Why Peas?
8
Model organisms
The genomes of many organisms have been sequenced.
www.genomenewsnetwork.org
A model organism is a species that has been widely studied, usually
because it is easy to maintain and breed in a lab and/or has particular
experimental advantages and are used to obtain information about species
that are more difficult to study directly.
Remember: processes are conserved!!!!!
Genetic model organisms
Amenable to genetic analysis
Breed in large numbers
Short generation time (large-scale crosses over several generations.)
Many different mutants available
Detailed genetic maps
Baker's yeast (Saccharomyces cerevisiae), the fruit fly (Drosophila
melanogaster) and the nematode (Caenorhabditis elegans)
Experimental model organisms
Long generation intervals and poor genetic maps
Produce robust embryos that can be studied and manipulated with
ease.
Used in developmental biology.
Chicken, Zebrafish and Xenopus laevis
Genomic model organisms
Occupy a pivotal position in the evolutionary tree
Special quality of their genome.
An example is the puffer fish (Fugu rubripes) which has a similar
gene repertoire to humans but a much smaller genome (400 million
base pairs instead of 3000 million
9
Model organisms
Human model organisms
Relevance of model organisms to humans.
Over 60 per cent of the human disease genes have counterparts in the fly
and worm. About 1500 gene families are conserved in all animals.
Genes affecting immune system etc, are less likely to have counterparts in
simple animals. For these systems, we require closer models such as the
mouse . Mice have been used to establish disease models by mimicking the
gene defects seen in humans, and these models can be used to test the
efficacy of new drugs.
E.coli
Yeast
Worm
Fly
Zebra fish
Mouse
Arabidopsis
Modern genetics:
Location in evolutionary tree
Genome size,
Ease of cloning a gene
Ability to insert DNA back into genome
Ability to monitor development
Xenopus, dog, tetrahymena
10
The Pea
Mendel chose the common garden pea to study patterns of
inheritance. This was a excellent choice as a model system
for the following reasons:
1
Many varieties
2
It can self pollinate and cross pollinate
3
Cheap
4
Short generation time
He identified over 20 traits and studied 7 (why 7?)
Seed shape: round versus wrinkled
Seed color: yellow versus green
Flower color: red versus white
Pod shape: inflated versus pinched
Pod color: yellow versus green
Flower position: axial versus terminal
Stem length: long versus short
11
True breeding
The first two years of Mendel's work were devoted to
selecting lines that breed true (pure lines) for a particular
character or trait.
He identified over 20 traits that bred true and studied 7
Breeding True:
smooth seed
|
Self cross
|
smooth seed
------------------wrinkled seed
|
Self cross
|
wrinkled seed
He identified plants that produced only smooth seeds and
plants that produced only wrinkled seeds.
He identified plants that produced only purple flowers and
plants that produced only white flowers.
12
Mendel’s first cross
P
purple X
male anther
white ----> All purple
female stigma
F1
purple
purple
X
-----> purple:white
705 purple:224 white
------------P
purple X
female
white
male
----> purple -->
Blending may not be correct
Reciprocal cross also gave the same result therefore
preformationism also not correct
13
Mendel’s first cross
P
yellow X
green
----> All yellow
Mendel crossed pure breeding yellow pea plants to pure
breeding green pea plants. All of the progeny were yellow pea
plants. Next he selfed these yellow plants allowing the pollen
to fall on its own stigma.
F1
yellow X
yellow=6022
yellow
----> yellow:green
green=2001
14
Keys to success
He did these experiments with all seven traits!!!!
The ratios obtained were between 2.82:1 and 3.15:1
This cross involving only one character, seed color, is called a
monohybrid cross.
Keys to success:
1
Generated a pure breeding line before starting the
cross
2
Analyzed individual character
3
Counted progeny
15
Conclusions
Although others were doing crosses at the time, Mendel work
was unique
Results and Conclusions:
1
In F1 all progeny showed only one of the two possible
traits for all crosses
2
Males and females not important
3
Character missing in F1, reappeared in F2
Traits did not blend in the offspring but were transmitted in
a discrete fashion and remained unchanged.
Reciprocal crosses produced the same results, this indicated
that each parent makes an equal contribution to genetic
makeup of the offspring. The sperm does not contain a
homunculus.
16
Terms:
Phenotype and genotype:
The parental yellow pea plants are a pure line- they only produce
yellow pea plants when selfed.
However the F1 yellow pea plants produce some green pea plants
when selfed.
P
F1
YY
X
(yellow)
yy
(green)
Yy
(yellow)
17
The Yellow of the P generation is different from the yellow
from the F1 generation
P
YY
x
(yellow)
YY
(yellow)
F1
YY (All yellow)
F1
Yy
(yellow)
F2
Yy (yellow)
X
Yy (yellow)
1YY:2Yy:1yy
1 Yellow:2Yellow:1Green
Therefore it is necessary to make the distinction between the
appearance of an organism and its genetic make-up.
Phenotype refers to the appearance of an organism
Genotype refers its genetic makeup
Would say the parental yellow pea plants and F1 yellow pea
plants have the same phenotype but a different genotype.18
Terms:
Dominant and recessive: All F1 seeds were yellow but when
selfed the F2 produced some green seeds.
Mendel termed the trait that is expressed in the F1 as
dominant
The trait that is hidden but re-expressed in the F2 as
recessive.
The F1 plants must contain factors for green and yellow since
both reappear in the F2.
From the fact the reciprocal crosses produce the same result,
Mendel concluded that male and females contribute equally
With these assumptions the simplest model is that the F1
contains two hereditary factors
One for green and another for yellow
We will use the Uppercase Y to represent the dominant yellow
factor and the lower case y to represent the recessive green.
19
The Principle of Segregation:
Mendel reasoned that without a mechanism to halve the number
of factors in each generation, that factors would multiple with
each generation and become unmanageable.
Mendel reasoned that during gamete formation the paired
factors separate and each gamete receives one of the two
factors.
Parent
YY
Gametes
Y
F1
yy
Y
y
y
Y y
Sperm and egg randomly combine to produce F2 progeny
Notice that while the parents have two factors, they produce
gametes containing only a single factor and the progeny again
have two factors
20
YY
Yellow
yy
Green
Grows into
A plant
Grows into
A plant
Generates
Gametes
Generates
Gametes
Y
Or
Y
y
Or
y
Gametes combine at Random
Yy
Yellow
21
Mendel's assumption of two factors and segregation makes
a strong prediction concerning the genetic make-up of the of
F2 yellow pea plants
F1
Yy
Y
X
y
Y
y
Yy
Y
y
Y
y
YY
Yy
F2
yy
yY
Mendel's prediction:
Phenotype ratio: 3:1 (yellow:green)
Genotype ratio: 1:2:1
1/4YY
Yellow
1/2Yy
yellow
3/4
1/4yy
green
1/4
genotype
Of the yellow F2 plants, 1/3 should be YY and 2/3 Yy
22
How would you test this prediction?
Selfing
Mendel selfed each of the green F2 plants
Green
yy
F3
X
X
Green
yy
yy
green
120 green F2 were selfed: 120 green progeny seen
23
Selfing
Mendel selfed each of the yellow F2 plants
Yellow
YY
F3
X
Yellow
Yellow
YY
Yy
YY
yellow
Yellow
X
Yy
YY:Yy:yy
3 yellow:1green
1/3 of all yellow plants
2/3 of all yellow plants
519 yellow F2 were selfed: 166 gave all yellow and 363
gave yellow/green
Therefore:
Phenotype ratio: 3:1 yellow:green
Genotype ratio: 1:2:1
1/4YY
Yellow
1/2Yy
yellow
3/4
1/4yy
green
1/4
genotype
Of the yellow F2 plants, 1/3 should be YY and 2/3 Yy
That is what Mendel observed!
Is there another way to test this prediction?
24
Test cross
If instead of selfing the F2 plants, they are crossed to
pure breeding green plants, what are the expected outcomes:
F2
YY
x
yy
Yy
x
yy
yy
x
yy
------>all yellow (Yy)
------>1:1 yellow (Yy):green (yy)
------>all green (yy)
25
Probability
Lets cross a Yy with a Yy pea plant
What is the probability of obtaining a homozygous YY plant
Chance of a Y sperm uniting with a Y egg
Chance of sperm with Y allele
1/2
Chance of egg with Y allele
Chance of Y and Y uniting
1/2
½x½=¼
-------------What is the probability of obtaining a heterozygous Yy plant
Chance of sperm with Y allele and egg with y allele
Chance of sperm with y allele and egg wit Y allele
Chance of Yy = (1/2x1/2) + (1/2x1/2)= 1/2
1/4
1/4
26
Probability
Cross Yy xYy pea plants.
Chance of Y sperm uniting with a Y egg
(1/2) chance of sperm with Y allele
(1/2) chance of egg with Y allele
Chance of Y and Y uniting = 0.5 x 0.5 = (1/4)
Chance of Yy offspring
1/4 chance of sperm with y allele and egg with Y
allele
1/4 chance of sperm with Y allele and egg with y
allele
Chance of Yy = (0.5x0.5) + (0.5x0.5) = 0.5
27
More terms:
Mendel's factors are now known as genes
Alternative forms of a gene that determine different traits are
known as---_alleles__
Individuals with two identical alleles are said to be ------_homozygous_
Individuals with two different forms of alleles are said to be—
_heterozygous__
28
The dihybrid cross and the principle of independent assortment:
In the second set of experiments Mendel investigated the
pattern of inheritance for two sets of characters simultaneously.
A cross involving two sets of characters is called a dihybrid cross.
Pea shape: smooth, wrinkled (Smooth is dominant to wrinkled)
Cotyledons color: yellow, green (Yellow is dominant to green)
P
F1
Smooth yellow
x
wrinkled green
smooth yellow
selfed
F2
315 smooth yellow
101 wrinkled yellow
108 smooth green
32 wrinkled green
9
3
3
1
29
9:3:3:1 ----> 3:1
If we examine seed shape only (smooth, wrinkled) and ignore
cotyledon color (yellow, green), in the F2, we expect to find:
3/4 smooth and 1/4 wrinkled:
Take all the smooth peas from the four classes and add them up
# Smooth = 315 + 108 = 423
# wrinkled = 101 + 32 = 133
423:133 is close to the 3:1 expected
Now, if we only examine cotyledon color and ignore seed shape
we expect 3/4 Yellow to 1/4 green.
# Yellow = 315+101
# green = 108+32
416:140 is also close to the 3:1 expected ratio
The wrinkled and green phenotypes INDIVIDUALLY behave as a
standard recessives in a monohybrid cross.
30
The 9:3:3:1 ratio.
The 9:3:3:1 ratio appears a lot more complex than the 3:1
ratios of the monohybrid cross.
Mendel's insight was to realize the 9:3:3:1 ratio is nothing
more than two 3:1 ratios combined at random. That is if one
examined the traits individually they formed a 3:1 ratio.
To determine the mode of inheritance of the two genes in this
dihybrid cross Mendel examined each of the traits separately:
31
Each trait behaves as a standard recessive found in a
monohybrid cross.
They do not affect one another
Genes segregate independently!!!!
GeneS in a gamete does not affect the segregation of geneY in
that gamete
Parent
Gamete
ss yy
SS YY
sy
SY
Ss Yy
F1
Self cross
Ss Yy
x
Ss Yy
32
In a heterozygous individual (Self cross)
SsYy
x
SsYy
Genes line up in two ways during gamete formation
S
Y
s
y
or
SY
sy
Gamete
sy
y
s
Y
Sy
sY
or
SY
S
Sy
sY
33
In a heterozygous individual with two traits
SsYy
x
SsYy
Genes line up in two ways during gamete formation giving rise
to 4 different gametes
SsYy
SsYy
SY
sy
Gamete
Sy
sY
or
SY
sy
Sy
sY
25%
25%
25%
25%
Mendel concluded that SsYy individuals produced
gametes in a 1:1:1:1 ratio ---SY Sy sY sy
34
Independent assortment
If independent assortment is occurring, four different kinds of
gametes will be produced in equal frequencies.
The only rule is that S and s segregate to separate gametes and
Y and y segregate to separate gametes
(that is one does not get an Ss gamete or a Yy gamete)
SsYy males and SsYy females can produce four types of gametes
in equal frequencies:
SY, Sy, sY, sy
The male and female gametes randomly combine to restore diploidy
35
Independent assortment of gene pairs
YYSS x
yyss
YySs
x
YySs
9Y-S-:3Y-ss:3yyS-:1yyss
Different gene pairs assort independently during gamete
formation.
The presence of a Y in a gamete does not influence the
probability of a S or s being in that gamete.
36
Calculation
A heterozygous Red eyed curly winged female fly is
crossed to a heterozygous Red eyed curly winged male
fly.
What is the expected ratio for a Red eyed flat
winged male fly?
--------------
A heterozygous smooth yellow pea is crossed to a
heterozygous smooth yellow pea.
What is the expected ratio for a smooth green pea?
37
Punnet diagram of a dihybrid cross
male
Female
1/4
SY
1/4
Sy
1/4
sY
1/4
sy
1/4
SY
1/4
Sy
1/4
sY
1/4
sy
1/16
SY/SY
Smooth
yellow
1/16
SY/Sy
Smooth
yellow
1/16
SY/sY
Smooth
yellow
1/16
SY/sy
Smooth
yellow
1/16
Sy/sY
Smooth
yellow
1/16
Sy/sy
Smooth
green
1/16
Sy/SY
Smooth
yellow
1/16
sY/SY
Smooth
yellow
1/16
sy/SY
Smooth
yellow
1/16
Sy/Sy
Smooth
green
1/16
sY/Sy
smooth
yellow
1/16
sy/Sy
Smooth
green
1/16
sY/sY
wrinkled
yellow
1/16
sy/sY
wrinkled
yellow
1/16
sy/sY
wrinkled
yellow
1/16
sy/sy
wrinkled
green
9 smooth yellow: 3 smooth green: 3 wrinkled yellow: 1 wrinkled green
Therefore the 9:3:3:1 ratio is a natural outcome of applying
Mendel's two laws
38
Monohybrid phenotype--->dihybrid phenotype
If you combine the monohybrid ratios for two PHENOTYPES
you get:
3/4 Yellow
3/4 Smooth
1/4 Green
3/4 Yellow
3/4x3/4=
9/16
3/4x1/4= 3/16
1/4x3/4= 3/16
1/4 Wrinkled
1/4 Green
1/4x1/4= 1/16
39
Probability
Yellow is dominant to green
Smooth is dominant to wrinkled
Heterozygous yellow
Heterozygous smooth
X
Heterozygous yellow
Homozygous wrinkled
Probability of yellow wrinkled?
1/2 smooth
3/4 yellow
1/2 wrinkled
1/4 green
Heterozygous yellow
Homozygous smooth
X
Heterozygous yellow
Homozygous wrinkled
Probability of yellow wrinkled?
1 smooth
3/4 yellow
0 wrinkled
40
1/4 green
More probabilities
Yellow is dominant to green
Smooth is dominant to wrinkled
Tall is dominant to short
Heterozygous yellow
Heterozygous smooth
Heterozygous tall
X
Heterozygous yellow
Homozygous wrinkled
Heterozygous tall
Probability of green wrinkled short?
smooth
green
tall
wrinkled
short
41
More probabilities
Yellow is dominant to green
Smooth is dominant to wrinkled
Tall is dominant to short
Heterozygous yellow
Heterozygous smooth
Heterozygous tall
X
Heterozygous yellow
Homozygous wrinkled
Heterozygous tall
Probability of green wrinkled short?
1/2 smooth
1/4 green
3/4 tall
1/2 wrinkled
1/4 short
42
43
Rules of Probability
Independent events: The probability of two events
occurring together
What is the probability that both X and Y will
occur together?
Answer: First determine the probability of each event
Then multiply them together.
Mutually exclusive events: The probability of one or another
event occurring.
What is the probability of X or Y occurring?
Answer: First determine the probability of each event
Then add them together.
Probability and Mendel’s Results
Cross Yy xYy pea plants.
What is the chance of Y sperm uniting with a Y egg
½ chance of sperm with Y allele
½ chance of egg with Y allele
Therefore chance of Y and Y uniting = ½ x ½ = 1/4
----------------------------------------What is the chance of Yy offspring
½ chance of sperm with y allele and egg with Y allele
or
½ chance of sperm with Y allele and egg with y allele
Therefore chance of Yy – (½ x ½) + (½ x ½) = 2/4, or
1/2
Multiple genes
P
F1
RRYYTTSS  rryyttss
RrYyTtSs  RrYyTtSs
What is the probability of obtaining
the genotype RrYyTtss in the F2?
Multiple genes
P
gametes
RYTS
F1
gametes
F2
RRYYTTSS X rryyttss
RrYyTtSs
ryts
X RrYyTtSs
RYTS RYTs RYtS RYts
RYTS RYTs RYtS RYts
RyTS RyTs RytS Ryts
RyTS RyTs RytS Ryts
rYTS rYTs rYts rYTS
rYTS rYTs rYts rYTS
ryTs rYtS
ryTs rYtS
rYts
ryts
rYts
ryts
What is the ratio of different genotypes
and phenotypes?
Punnet Square method for multiple genes
-Four loci
Each loci is heterozygous i.e. 2 types of
gametes
- 24 = 16 possible gamete combinations for
each parent
Thus, a 16  16 Punnet Square giving rise to
256 genotypes
Big Punnet Square
Alternatively –
Loci Assort Independently
Look at each locus separately.
Branched diagram
P
RRYYTTSS  rryyttss
F1
RrYyTtSs  RrYyTtSs
What is the probability of obtaining the genotype
RrYyTtss in the F2?
Rr  Rr
Yy X Yy
Tt  Tt
Ss  Ss
1RR:2Rr:1rr 1YY:2Yy:1yy 1TT:2Tt:1tt 1SS:2Ss:1ss
2/4 Rr
2/4 Yy
2/4 Tt
1/4 ss
Probability of obtaining individual with Rr and Yy
and Tt and ss.
2/4  2/4  2/4  1/4 = 8/256 (or 1/32)
Branched diagram
P
RRYYTTSS  rryyttss
F1
RrYyTtSs  RrYyTtSs
What is the probability of obtaining a completely homozygous
genotype in the F2?
Genotype could be RRYYTTSS or rryyttss
Rr  Rr
Yy  Yy
Tt  Tt
Ss  Ss
1RR:2Rr:1rr 1YY:2Yy:1yy
1TT:2Tt:1tt
1SS:2Ss:1ss
1/4 RR
1/4 rr
1/4 TT
1/4 tt
1/4 SS
1/4 ss
1/4 YY
1/4 yy
(1/4  1/4  1/4  1/4) + (1/4  1/4  1/4  1/4)
= 2/256
51
Significance of Ratios
What is the biological significance of the 9:3:3:1 ratio? This
ratio is only produced if TWO DIFFERENT GENE PAIRS assort
independently of each other during gamete formation.
The presence of one gene in a gamete does not influence
the probability of another gene being found in that gamete
Principle of segregation: for one gene, each individual has two
copies. These two copies segregate from one another during
gamete formation.
Independent assortment: Segregation of one gene pair is
independent of the segregation of any other pair of gene.
52
Mendel tested seven different traits and all assorted independently!
The pea plant has seven chromosomes
(The seven traits are not on different chromosomes)
Mendel had generated 20 true breeding lines
What about the other 13 traits?
Did he do experiments with these and find that they do not assort
independently?
Why did he so thoroughly study only the seven traits that assorted
independently.
He probably only used data he understood and it is possible that the
other data did not fit any statistical understanding and he therefore
ignored/ suppressed this data.
Fire destroyed his original notes, so we will never know!
53
Mendels laws
1.
The principle of segregation: Each individual carries two
copies of a given gene and these segregate from one another
during gamete formation.
2. The principle of independent assortment: The segregation of
one pair of genes is independent of the segregation of any
other pair of genes during gamete formation
(as we will find their are important exceptions to this rule)!!!!!
By applying these rules Mendel concluded that SsYy individuals
produced the following gametes in a 1:1:1:1 ratio
SY Sy sY sy
As described above, he inferred these gamete ratios by selfing
SsYy individuals and looking at phenotypes of progeny.
He could also have inferred these gamete ratios by crossing
SsYy individuals to ssyy individuals.
Crossing to the homozygous recessive individuals is known as a
test cross
54
A test cross is easier that a self cross for the F2
SsYy
x
Gamete:
1/4 SY
1/4 Sy
1/4 sY
1/4 sy
Test cross
ssyy
1 sy
What are the expected genotypic and phenotypic ratios of
the progeny produced from this cross?
1
sy
1/4
SY
1/4
sy
1/4
SY/sy
Smooth
Yellow
1/4
sy/sy
Wrinkled
Green
1/4
sY
1/4
sY/sy
Wrinkled
Yellow
1/4
Sy
1/4
Sy/sy
Smooth
green
What are the expected
genotypic and phenotypic
ratios of the
progeny produced from this
cross?
To answer this, first describe
the gamete classes and their
frequencies produced from
the SsYy and ssyy individuals.
Use these to construct a
Punnett square or branched
diagram
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Gene number
Power of Mendel phenotype ratios:
The ratio tells you the number of genes involved in determining a
phenotype
3;1 ratio (selfing) = 1 gene
9:3:3:1 ratio (selfing) = 2 genes
27:9:9:9:3:3:3:1 ratio (selfing) = 3 genes
Say seed color is controlled by two genes- GeneS and GeneT
Green seeds are sstt
All others are yellow (if either S or T are present, the seed is yellow)
True breeding yellow
SSTT
x
F1
true breeding green
sstt
SsTt
(yellow)
SsTt
Self
x
SsTt
How many green will you get out of this cross:
9 S-T3 S-tt
3 ssT1 sstt
yellow
yellow
yellow
green
These conclusions about gene number become very important when
applied to human traits like size, behavior, temperament etc. 56
Take two individuals
They both have blue eyes
They mate and produce 16 progeny
12 have blue eyes and 4 have black eyes
There is ________ gene/s for eye color
If instead you get 15 with blue eyes and 1 with black eyes
There are _______ gene/s for eye color
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Take two individuals
They both have blue eyes
They mate and produce 16 progeny
12 have blue eyes and 4 have black eyes
There is ___1___ gene/s for eye color
If instead you get 15 with blue eyes and 1 with black eyes
There are ___2___ gene/s for eye color
Both individuals must be heterozygous! Why? Because black eye
appears. If only one was heterozygous and the other homozygous
blue then no black should appear.
1st scenario: Bb x Bb-----> 3B- (blue) : 1bb (black)
2nd scenario BbCc x BbCc ----->
9 B-C-(blue): 3B-cc(Blue): 3bbC- (blue):1bbcc (black)
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Dog genome
The Dog Genome Project is an example of how basic Mendelian
principles are being used to identify genes that control
morphology and behavior.
Dr. Jasper Rine at U.C. Berkeley crossed
Newfoundlands to Border Collies.
These dogs differ extensively in size and behavior.
Newfoundlands are vigorous swimmers and weight about 140
pounds
Border Collies are herders and weigh about 50 pounds
By performing a series of Mendelian crosses one can begin to
determine how many and what kinds of genes are involved in
determining their behavior!!!!
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Dogs have been breed for specific traits for 10,000 years
About 150 breeds have been generated through selective breeding
Diversity in
Physical makeup:
Coat color
height
mass
muscle
Behavior:
herding
tracking
retrieval
Guarding
Intelligence:
The individual breeds can mate with one another and produce viable
fertile offspring
They can also mate with Cayotes and wolves
Border collies do not like water, Newfoundlands love water.
In the cross swimming is dominant!
This complex trait is likely mediated by a small number of genes (~2)
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Coat hair phenotypes
Coat Variation in the Domestic Dog Is Governed by
Variants in Three Genes
2 OCTOBER 2009 SCIENCE VOL 326 p150
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Specific breeds of dogs are also associated with specific diseases
Dobermans: narcolepsy
Scotties: Haemophilia
Terriers: copper metabolism (menke disease)
Labrador: hip dysplasia
Beagle: seizure risk
The ratio from a doberman cross suggests that narcolepsy is most
likely mediated by a single gene! (3:1 ratio)
Why are Mutts healthier than true breeds?
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Hybrid Vigor
Hybrid vigor:
The first cross between two purebred lines is often healthier than
either parent
Breed1
a-B-C-d-E
a-B-C-d-E
F1
Breed2
A-b-c-d-E
A-b-c-d-E
a-B-C-d-E
A-b-c-d-E
If GeneA causes narcolepsy and geneC causes haemophilia
F1 will be normal.
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Corn
Commercial corn is a F1 hybrid because of hybrid vigor
Two inbred lines are mated to generate a F1 that is sold
AAbb
F1
x
aaBB
AaBb
hybrid vigor
1930’s increase in corn output because of these hybrid varieties
Line1 -disease resistant- rust and mold resistance but low yield
Line2- large crop but susceptible to disease
Hybrid- disease resistant and large yield----Sold to farmer
The F1 hybrids do not breed true!
The farmer cannot plant the seeds he gets from the F1 hybrid in the
next season. They will not be disease resistant with high yields!
Gametes AB
Ab
aB
ab
The seeds will generate plants with
A-Bhigh yield, disease resistant
A-bb
high yield, susceptible to disease
aaBlow yield, disease resistant
aabb
low yield, susceptible to disease
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