Transcript Mendelian Genetics

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…..
UNTIL….
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)
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
PEA PLANT
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?
Mendel got
pure plants
by
self-pollinating
(or inbreeding)
plants for
several
generations.
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
MENDEL’S EXPERIMENTS
Mendel crossed pure (P) X contrasting
pure (P)
P X P = F1 (first filial generation)
RESULTS
only 1 form of the trait appeared in the
F1 generation
Mendel repeated his experiment s many
times – all with the same results
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
MENDEL concluded that the
patterns of inheritance are
governed by 3 principles
PRINCIPLE OF . . . .
1. Dominance and Recessiveness
2. Segregation
3. Independent Assortment
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
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
Principle of Segregation
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
Principle of Independent Assortment
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
Smooth
Wrinkled
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
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
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.
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 selfpollinate/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!!
Fig. 10.4, Mendel’s 7 garden pea characters.
Practice assigning alleles
1.
2.
3.
4.
5.
6.
7.
8.
9.
PP
TT
GG
Gg
ss
AA
Aa
aa
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
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”?
Punnett Squares
Dr. Reginald Punnett, early 1900s
 Graphical way to show probability
 5 steps:


Assign P genotypes

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

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Remember: use the letter of the dominant trait
(homozygous = same size; heterozygous = different
sizes)
Split alleles
Perform cross
Report F1 genotypes
Report F1 phenotypes
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
4 squares
Dihybrid cross = cross of two different
alleles for two traits.

Ex: crossing eye & hair color x eye & hair color
16 squares
Monohybrid Crosses can be used with…
Complete dominance of traits
 Incomplete dominance of traits
 Codominance of traits
 Sex-linked traits
 Multiple allele traits

Let’s Practice
Step 1. P = SS x ss
Step 2. Split alleles
s
s
S
S
Ss
Ss
Ss
Ss
Step 3: perform Punnett
Step 4: F1 genotype
100% Ss
Step 5: F1 phenotype
100% smooth seeds
Some practice – Show all 5 steps
1. 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?
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
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
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
Sex-linked Traits
Males affected more often; cannot be
heterozygous
 Females less affected; can be
heterozygous (carriers)

Examples: Red-green colorblindness
Hemophilia (blood clotting disorder)
Duchenne Muscular Dystrophy
COLORBLINDNESS
a recessive trait found on the X chromosome
results in an inability to distinguish certain colors
Duchenne MUSCULAR DYSTROPHY
a recessive trait found on the X chromosomes
results in the weakening and wasting away of muscle tissue
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
Sometimes with multiple alleles

Epistasis occurs

One allele hiding the effect of another allele
Example: coat pigmentation on animals –
see p. 305
POLYGENIC TRAITS
Traits that result from the interaction of several
genes (several allele pairs).
Skin color, hair color, eye color are polygenic
traits.
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.mcgrawhill.com/sites/dl/free/0078695104/383934/BL_05.html
Dihybrid Punnett Squares
Follow the same steps as in monohybrids
 Extra step (FOIL the alleles)

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Assign P genotypes

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

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
Practice with FOILing
1. Homozygous purple & axial x white & terminal
P= PPAA x ppaa
FOIL = PA PA PA PA x pa pa pa pa
2. Homozygous smooth & yellow seeds x
heterozygous smooth & yellow seeds
P= SS YY x Ss Yy
FOIL = SY SY SY SY x SY Sy sY sy
3. Heterozygous green & inflated pods x
heterozygous green & inflated pods
P= Gg Ii x Gg Ii
FOIL = GI Gi gI gi x GI Gi gI gi
P= PPAA x ppaa
FOIL = PA PA PA PA x pa pa pa pa
Cross #1
PA
PA
PA
PA
PpAa
PpAa
PpAa
PpAa
PpAa
PpAa
PpAa
PpAa
PpAa
PpAa
PpAa
PpAa
PpAa
PpAa
PpAa
PpAa
pa
pa
pa
pa
F1 genotypes:
100% PpAa
F1 phenotypes:
100% purple &
axial flowers
P= SS YY x Ss Yy
FOIL = SY SY SY SY x SY Sy sY sy
Cross #2
SY
SY
Sy
sY
sy
SY
SY
SY
SSYY
SSYY
SSYY
SSYY
SSYy
SSYy
SSYy
SSYy
SsYY
SsYY
SsYY
SsYY
SsYy
SsYy
SsYy
SsYy
F1 genotypes:
F1 phenotypes:
25% SSYY, 25% SSYy
100% smooth,
yellow seeds
25% SsYY, 25% SsYy
P= Gg Ii x Gg Ii
FOIL = GI Gi gI gi x GI Gi gI gi
Cross #3
GI
Gi
gI
gi
GGII GGIi GgII GgIi
F1 genotypes:
Gi
GGIi GGii
GgIi
Ggii
1 GGII, 2 GGIi, 2 GgII,
4 GgIi, 1 GGii, 2 Ggii,
1 ggII, 2 ggIi, 1 ggii
gI
GgII GgIi
ggII
ggIi
GI
gi
GgIi
Ggii
ggIi
ggii
F1 phenotypes:
9 green & inflated pods
3 green & contricted pods
3 yellow & inflated pods
There should always be a
9:3:3:1 phenotypic ratio in a
heterozygous x heterozygous
dihybrid cross.
1 yellow & constricted pods
(9:3:3:1)
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



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Exhibit more vigor and size
oats & wheat = 6n
Sugar cane = 8n
Genetic Recombination understood
After 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.
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
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).
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
10 map units
Y
15 map units
Z