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Genetics - 1
The problem of Inheritance (Darwin’s dilemma)
Blending or Particulate Inheritance
Gregor Mendel
Segregation and Dominance
Phenotype vs. Genotype
Independent Assortment
Darwin’s Dilemma
How does inheritance work?
If the traits of parents were
blended in their offspring,
evolutionary novelties might be
blended away before natural
selection could increase their
frequency in the population:
Gregor Mendel (1822-1884).
Priest and later Abbot at Augustinian Monastery of
St. Thomas in Brünn (now Brno) in AustriaHungary (now Czech Republic).
Peas usually selffertilize; but artificial
cross-breeding is
easy:
carpel
stamen
Mendel kept meticulous records of his crosses,
focusing on 7 contrasting traits.
P generation
(parental)
F1 generation
(first filial)
filius = son
(Latin)
Result: F1 flowers all purple.
Conclusion? “Purple is ‘prepotent’ over white”
[today we say “dominant”]
X
F1 plants
(all purple
flowers)
F2 plants
705
purple
224 white
Simplest Possible Explanation?
A
A
a
Each plant contains 2 “characters”
(genes) for flower color
a
And these characters are separated
when gametes are formed
So that a new organism (containing
two genes) is formed by the fusion of
two gametes
A
male gametes
So, the F1 purple flowered
plants must contain one white
gene and one purple gene.
female gametes
A
a
a
A technique of predicting the
outcomes of a genetic cross is
known as the Punnett Square
Write all possible gamete
genotypes on both sides
of square:
A
a
A
AA
Aa
a
Aa
aa
Reginald
Punnett
Offspring genotypes are determined by combining the
genotypes of the gametes
A
a
Aa
A
a
female gametes
male gametes
A
a
A
a
A
A
a
aa
1.
a
Aa
aa
2.
Aa
aa
A
a
A
AA
Aa
a
Aa
aa
3.
4.
And there are four possible ways in which the
gametes formed by the two F1 plants can combine to
form F2 plants.
Trait Studied
Dominant
Form
Recessive
Form
F2 Dominant-toRecessive Ratio
SEED SHAPE
5,474 round
1,850 wrinkled
2.96:1
SEED COLOR
6,022 yellow
2,001 green
3.01:1
POD SHAPE
882 inflated
299 wrinkled
2.95:1
POD COLOR
428 green
152 yellow
2.82:1
FLOWER COLOR
705 purple
224 white
3.15:1
651 long stem
207 at tip
3.14:1
787 tall
277 dwarf
2.84:1
FLOWER POSITION
STEM LENGTH
www.mendelweb.org
1) The Gene:
Heredity is controlled by units called genes. Mendel called
them "Merkmal" which means trait or character in German.
2) A Mechanism of Inheritance:
Genes are passed from parent to offspring. Therefore,
each organism has at least 2 genes for each trait: In the
F1 generation, each new plant received a gene for White
flowers (a) and a gene for Purple flowers (A), like this:
Organisms with 2
genes for each trait are
said to be diploid.
A
a
And the purple
gene is dominant
over white.
Phenotype: measurable trait or
appearance
Genotype: genetic composition
Phenotype:
Genotype?
Purple
AA or Aa
AA = homozygous (dominant)
Aa = heterozygous
aa = homozygous (recessive)
A
a
The two genes for a
trait are segregated
from each other
during gamete
formation
A “Test Cross” can be
used to determine the
genotype of a plant with
the dominant phenotype.
Test crosses are usually
done using a plant with
the recessive phenotype.
From your text (p. 283): The key to a successful test
cross is to arrange it so that two very different results
would result from different genotypes.
Next question: What about two or more traits? Are they
inherited together or independently?
TRUEBREEDING
PARENTS:
purple
flowers,
tall
white
flowers,
dwarf
x
AABB
GAMETES
:
AB
aabb
AB
ab
ab
AaBb
F1 HYBRID
OFFSPRING:
Strategy: Cross doubledominant plant with double
recessive. F1 hybrids show
the dominant phenotype for
both traits.
AABB
purpleflowered
tall parent
(homozygous
dominant)
AB
aabb
whiteflowered
dwarf parent
(homozygous
recessive)
ab
X
F1 OUTCOME: All F1 plants purple-flowered, tall
(AaBb heterozygotes)
AaBb
AaBb
AaBb
gamete formation
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
Test for “linkage” by
crossing F1
heterozygotes with
double recessive plants
Gamete
formation
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/16
1/16
1/16
1/16
AABB AABb AaBB AaBb
1/16
1/16
1/16
1/16
AABb AAbb AaBb Aabb
1/16
1/16
1/16
1/16
AaBB AaBb aaBB aaBb
1/16
AaBb
1/16
1/16
AaBb Aabb
aaBb
1/16
aabb
Possible outcomes of cross-fertilization
315 purple tall
108 white tall
101 purple dwarf
32 white dwarf
AB
AB
ab
ab
AABB AaBb
AaBb
aabb
Is the 9:3:3:1 ratio different from the 3:1 ratio?
AB
Ab
aB
ab
AB
Ab
aB
ab
1/16
1/16
1/16
1/16
AABB AABb AaBB AaBb
1/16
1/16
1/16
1/16
AABb AAbb AaBb Aabb
1/16
1/16
1/16
1/16
315 purple tall
108 white tall
101 purple dwarf
32 white dwarf
AaBB AaBb aaBB aaBb
1/16
1/16
1/16
1/16
AaBb Aabb aaBb aabb
Nope.
Purple : White = 416:140 = 2.97: 1
Tall : Dwarf = 423:133 = 3.13:1
A 3-factor (tri-hybrid) cross?
Punnett square with 64 genotypes!
Phenotypic ratios: 27:9:9:9:3:3:3:1
SsYyAa x SsYyAa
Using Genetics to solve problems:
Example: two normally-pigmented parents give birth to
an albino child. What are the chances that their next
child will be albino? [fact: albinism is a recessive trait]
Reasoning: Both parents must be genotype Pp
Therefore, the gametes they can produce are either P or p.
P
p
P
PP
Pp
p
pP
pp
Conclusion: The chances of a second child being albino
are 1 in 4 (25%).
Using Genetics to solve problems:
• Assuming independent assortment for all gene pairs,
what is the probability that the cross
AABbCc × AaBbCc will produce an AaBbCc offspring?
Possible gametes
from AABbCc :
Possible gametes
from AaBbCc :
ABC
AbC
ABc
Abc
ABC
AbC
ABc
Abc
aBC
abC
aBc
abc
First Technique: Punnett Square
ABC
AbC
ABc
Abc
aBC
abC
aBc
ABC
X
AbC
X
ABc
Abc
abc
X
X
We predict that 4 out of 32 offspring will be AaBbCc
= 1/8 or 12.5 %
Second Technique: Probability
Possible gametes
from AABbCc :
ABC
AbC
ABc
Abc
Possible gametes
from AaBbCc :
ABC
AbC
ABc
Abc
aBC
abC
aBc
abc
ABC = 1/4 of gametes
abc = 1/8 of gametes
1/4 x 1/8 = 1/32
AbC / aBc
1/4 x 1/8 = 1/32
ABc / abC
1/4 x 1/8 = 1/32
Abc / aBC
1/4 x 1/8 = 1/32
Total = 4/32 = 1/8
In a particular beetle, body color is
controlled by two independently
assorting genes. Whenever a
dominant allele is present for either
or both genes, red color is produced.
• A cross of AaBb × AaBb would be
expected to yield how many red
offspring out of 16?
• All beetles are red unless they are genotype aabb.
• AaBb produces 1/4 gametes of genotype ab
• Probability of non-red is 1/4 x 1/4 = 1/16
• Answer: 1 offspring out of 16
A plant with orange-spotted flowers is
grown from a seed gathered in the wild. It
is allowed to self-pollenate, and produces
the following offspring:
88 orange with spots
34 yellow with spots
32 orange with no spots
8 yellow with no spots
• Determine the genotype of the original plant
• This is roughly a 9:3:3:1 ratio (dihybrid cross!)
• Choose symbols: O = orange o = yellow
S = spotted s = unspotted
OS
Check with Punnett Square:
Os
oS
os
OS
OOSS OOSs OsSS OoSs
Os
OOSs OOss OoSs Ooss
oS
OoSS OoSs
ooSS
ooSo
os
OoSs Ooss
ooSs
ooss
Mendel's Principles:
1) Traits are inherited as genes, invisible,
indivisible, characters passed from one
generation to the next.
2) Some genes are dominant over others.
3) During gamete formation, the two copies of a
gene are segregated from each other
4) The segregation of genes for different
characters is independent for each character.
Remember Darwin’s
Dilemma?
If the traits of parents were blended in
their offspring, evolutionary novelties
might be blended away before natural
selection could increase their frequency in
the population.
Mendel had solved it!
By showing that genes (the units of
inheritance) were particulate, he had
shown that a beneficial trait would not
be diluted by repeated crosses.
http://www.mendel-museum.org/