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Chapter 6:
Genetics
18 Things You Should Know about Genetics
When told to, put the PTC paper on your tongue
PTC, or phenylthiourea, is an organic
compound having the unusual property of
either tasting very bitter, or being virtually
tasteless, depending on the genetic makeup
of the taster.
• The ability to taste PTC is a dominant genetic
trait.
T = Taster t = non-taster
– If you can taste, you are either TT or Tt.
– Those who can not taste are tt
• About 70% of people can taste PTC, varying
from a low of 58% for Aborginal people of
Australia to 98% for Native Americans
• Ever wonder why people resemble their
parents &/or siblings?
• How do farmers select the best plants
or animals for breeding purposes?
• How’d you like that PTC paper?
• Why did some people in your
class/family taste it and others didn’t?
How do we inherit our traits?
Traits are distinguishing characteristics
which we inherit from our parents and
can pass on to our children.
What are you for these traits?
– PTC tasting ability
– Tongue rolling ability
– Hand preference
– Eye color
– Hair color
– Dimples
– Freckles
______________
______________
_______________
_______________
_______________
_______________
_______________
Widows peak
Dimples
Mid – digit hair
Free earlobe
Hitchhikers thumb
Chin Dimple
2nd toe larger than
1st
Straight pinkie finger
Genetics
Studies the
transmission
of traits or
characteristics
from 1
generation to
the next
Gregor Mendel
(1822 – 1884)
The Father of Genetics
• Central European monk (Now the
Czech Republic) discovered the basic
underlying principles of heredity.
• Mathematician who studied statistic
• Work completed “Experiments on Plant
Hybridization” in 1865 but his work was
not discovered until 1900.
Bill Nye
• He used pea plants and controlled how
they were bred over many generations
• Saw that certain traits show up in
offspring plants without any blending
of parent characteristics.
P
Why all purple?
Why not pink?
Mendel's work
1.
Removed the male parts of the flower to prevent selfpollination
2. Used the pollen from a Purple flower producing plant and
pollinated only flowers from other purple flower
producers
3. Observed generations for 2 years to be
certain of Purebreds (a genetically
uniform line – pedigrees)
Purple X Purple
White X White
Got all Purples
Got all whites
4. Crossed plants with contrasting traits
Purple X white
white X Purple
Mendel expected
to get something
between purple
and white but
instead got all
purples!!
Did the white trait
disappear
forever?
Next, he took purple flowered plants from
the First Generation (F1) offspring and
crossed them.
Would the white trait reappear?
To Mendel’s surprise,
the white trait
reappeared in the
Second Generation (F2)
but only 25% of the
flowers were white.
75% of the flowers were
purple.
Why?
Mendel didn’t just use the trait for flower
color, he looked at other traits in pea
plants and found a similar pattern.
Every time he crossed on purebred with
the contrasting purebred, only one trait
ever showed.
Then when he crossed the F1 offspring,
the hidden trait reappeared.
Mendel’s conclusions
1. Traits (genes) are located on
chromosomes and sexually reproducing
organisms get two copies of a trait, one
from each parent
Gene from mom
Gene from dad
2. Each parent only donates one gene per
trait in their gametes.
Genes and chromosomes
Eye color
Hair color
Skin color
Nose size
Nose shape
Genes
Eye shape
hair texture
Ear size
widow’s peak
Ear lobes
Chromosome
Law of Segregation
3. During gamete formation (Meiosis), the two genes are separated.
50% of the gametes will contain one trait while the other 50% will
contain the other trait
In the example above, you can see that there are two parent
chromosomes. In the same location on both chromosomes, one
chromosome has a T gene while the other has a t gene for say being
tail or being short.
When the gametes are produced during Meiosis, the two genes will be
separated, thus his Law of Segregation
More about how Mendel figured
out more than he realized
Mendel never said the term genes, but he
did call them factors.
These factors or genes, we now know are
on the DNA and code for a particular
protein.
Each gene has a specific location or
locus on a chromosome
Mendel said that there are different forms of
a particular factor such as eye color.
There are 3 different factors that
determine our eye color
– Blue
– Green
– Brown
For hand preference, there are two factors
for the hand preference trait
– Left
– Right
Alleles are the different forms of a
trait/gene.
There can be two such as hand
preference or many such a hair color,
blood type and eye color
Allele for
Allele for
Allele for
Next Principle is his Law of Dominance: –
One factor “masks” the appearance of
another factor. It prevents it from showing
P = Purple flower color – Dominant
allele
P = white flower color – recessive
allele
• Dominant – trait that must show if
present
• Recessive – trait that will only
show if in the pure form
More genetic vocabulary to learn
• Genotype: Shows the actual genetic
makeup. (Use symbols for genes)
– PP or Pure dominant or homozygous (same
genes) dominant
– Pp or Hybrid or heterozygous (mixed genes_
– pp or Pure recessive or homozygous
recessive
Homozygous vs Heterozygous: Pure vs
Hybrid (mixed)
• Phenotype:
– Tells appearance (describes the trait)
Purple flower (if Pp or Pp)
white flower (if pp)
– NO HYBRIDS HERE!!
– Describe what they look like
– For Eye color:
– Blue
– Green
– Brown
– For Hair color; Red, Blond or Brown
Vocabulary Review
F
1. Genetics ___
a)
G
2. Allele ___
b)
E
3. Purebred ___
c)
d)
I
4. Law of Segregation _
5. Law of dominance __J e)
f)
C
6. Genotype ___
g)
h)
7. Phenotype ___
D
B
i)
8. Dominant ___
H
9. Recessive ___
j)
A
10.Genes ___
k)
K
11.Heterozygous ___
Inheritable distinguishing
characteristics – traits
When present, this allele will show
BB, Bb, or bb – Gene makeup
Brown eyes, left handed… Physical appearance
Homozygous – both genes for a
trait are the same
Study of how traits are passed on
Alternate form of a gene
Allele which only shows when it is
alone
Two genes for a trait separate
during meiosis
One factor masks the expression of
the other
Hybrid – having two different
alleles for a trait
Predict the likelihood of two individuals who are
hybrids for having allergies (Having allergies is a
dominant trait so we will use an “A” for the gene).
A = Allergies
Phenotype ratio
What is the likelihood of these two
a = No allergies
parents having children with
Aa x ___
Aa allergies to not having
Parent Cross: ___
allergies?
A
a
3:1 or 75%:25%
A
AA
a Aa
Aa
aa
Genotype ratio
What is the likelihood of having a
child who is a purebred for
allergies? 25%
Hybrid for allergies? 50%
Purebred for not having allergies?
25%
1:2:1
Punnett Square
Is a grid system for predicting all possible
genotypes resulting from a genetic cross
The axes will have the alleles found in the
two parents which result from segregation
of the alleles during meiosis
Trait: Hand Preference in humans. Being right handed
is dominant to left hand preference.
R = Right Hand preference
r = Left Hand preference
Cross two individuals with one being pure or homozygous
dominant and the other being a hybrid or heterozygous for
being right handed
Parent Cross: RR
___ x Rr
___ Possible Phenotypes
R
R
RR
R
RR
50%
____
Right handed
50%
____ Left handed
Possible Genotypes
50%
r
Rr
Rr
___ RR
50%
___ Rr
0%
___rr
For the same trait, cross a person who is pure
for being right handed with a lefty.
Parent Cross: RR
___ x rr___
Possible Phenotypes
100%
____
R
r
Rr
R
RR
Right handed
0%
____ Left handed
Possible Genotypes
0%
r
Rr
Rr
___ RR
100%
___ Rr
0%
___rr
Trait: Earlobe position: In humans, having free
earlobes is dominant to having an attached earlobe
F – Free earlobes
f – Attached earlobes
Cross a two people who both are hybrids x for
having free earlobes.
Parent Cross: Ff
___ x Ff
___ Possible Phenotypes
75%
___ Right handed
F
f
25%
____
F
FF
Ff
Possible Genotypes
25%
f
Ff
ff
Left handed
___ RR
50%
___ Rr
25%
___rr
Back to Mendel’s work
In pea plants, being a tall plant is dominant
over being short plants.
___ = Tall allele
___ = Short allele
Cross a Hybird tall with a homozgyous short
Parent cross: ___ x ___
Possible Phenotypes
___ Tall
___ Short
Possible Genotypes
___ TT
___ Tt
___ tt
1. In humans, having a chin dimple is dominant to not having dimples.
a.Show the cross of two parents who are both hybrid for having dimples.
b.Give the expected Phenotype ratio and genotype ratio for this cross.
____ = Dimple gene
____ = Non dimple gene
Parent cross ______ x ______
Phenotype ratio :
Genotype ratio :
2. For the same trait, cross a hybrid with an individual who does not have dimples
Parent cross ______ x ______
Phenotype ratio :
Genotype ratio :
Probability
What is the probability of flipping a head?
What is the probability of flipping a tail?
50:50 chance of getting heads or tail if you toss one
coin
Coin Toss Activity:
Flip one coin 10x. Keep track of the # of Heads
and Tails you get.
____ Heads
____ Tails
**Need large numbers to get accurate predictions**
What happens when you toss two coins?
• If you toss 2 coins 100 times, you should get:
•
25 Heads/Heads
•
50 Heads/Tails
•
25 Tails/Tails
• As the # of trials increases, the ratios predicted
by the laws of probability get closer
• Actual outcomes get closer to calculated
predictions
Now flip two coins 100 times and keep track
of the number of combo you get.
H/H
1
2
3
1
4
5
6
7
8
9
10
Total/1000
Actual
Genotype Ratio
Actual
Phenotype Ratio
Predicted
Genotype Ratio
Predicted
Phenotype ratio
H/t
t/t
H/H
1
2
3
3
4
5
6
7
8
9
10
Total/1000
Actual
Genotype Ratio
Actual
Phenotype Ratio
Predicted
Genotype Ratio
Predicted
Phenotype ratio
H/t
t/t
How could you tell if you were a hybrid
right handed person or pure for the trait?
You would need to mate with someone
who is a lefty.
• If any of your children are lefty, then
you know you had to be a hybrid
• Test Cross– A cross between an organism with an
unknown dominant genotype with an
individual with the recessive phenotype
Is your black guinea pig pure for its coat
color?
In Guinea pigs: B = Black coat
b = white coat
Cross the (BB) guinea pig with a pure recessive (bb)
Cross the (Bb) guinea pig with a pure recessive (bb)
If: BB x bb
If: Bb x
bb
If any of the offspring are white coated, then we
knew the black guinea pig was Bb
What happens when a red carnation is
crossed with a white carnation?
We would expect to get Red
carnations.
•
WRONG!!!
• We would end up getting all Pink
carnations.
• Neither the red allele nor the
white allele are dominant to
each other. Instead, their
expression blends and we
get hybrid pink carnations
How did this happen?
P1:
Red carnations (rr)
•
F1:
F1:
Pink carnation (rw)
x
White carnations (ww)
Pink carnations (rw)
F2: 25% Red (rr)
x
Pink carnation (rw)
50% Pink (rw)
25% White (ww)
1:2:1 phenotype ratio!!
1:2:1 genotype ratio!!
Incomplete Dominance
Heterozygous individuals will show a
phenotype of something in between the
other two phenotypes
A green betta (gg) crossed with a royal
blue betta (bb) gives a steel blue betta
(gb)
Incomplete Dominance
Hair color
Eye color
Height
Face shape
Wavy hair
Pitch of male’s voice (tt – tenor, tb baritones, bb – low bass)
Tay-Sachs Disease •
Inability to produce the enzyme hexosaminidase A.
•
•
•
Causes fluid pressure on brain then breakdown of
brain. Starts a ~ 6 months w/ death by 2 – 3 years.
Most common among the descendents of Eastern
European Jews (Ashkenazi Jews).
Tt individuals produce 40-60% of enzyme
Crosses:
1. Cross a Round
faced person with
an square faced
person
2. Cross a Black with
a white horse
3. Cross a Wavy
haired person with a
curly haired person
Codominance
Condition in which both alleles of a gene are
expressed or active
• Roan horses: Red is codominant with white.
Roans
Blood types in humans
A is codominant with B resulting in AB blood
A type blood has the A proteins on their RBCs
B type has the B proteins on its RBC
Both A and B are dominant to O
A=B>o
As you can see, AB blood is a mixture of both A and
B proteins.
Animation
Population
A
B
AB
O
US Whites
39.7
10.6
3.4
46.3
African Americans
26.5
20.1
4.3
49
African
25
19.7
3.7
51.7
Navaho Native Am
30.6
0.2
0
69.1
Ecuadorian Am
4
1.5
0.1
94.4
Japanese
38.4
21.9
9.7
30.1
Russians
34.6
24.2
7.2
34
French
45.6
8.3
3.3
42.7
Problems
1.
Figure out what blood type
two parents must be in
order to have 4 children,
each with a different blood
type.
2. Who could be the father? A
woman with O type blood
has a child with B type
blood
Man #1 is A type blood
Man #2 is AB blood
3. What could be your blood
type?
Polygenic Inheritance
• Some human traits are controlled by
more than one set of genes that
determine the expression of the traits.
• Ie. Eye color , hair color, height, body
weight, skin color, etc…
Light blue
0 dominate alleles
Blue
1 dominate allele
Blue-green
2 dominate alleles
hazel
3 dominate alleles
Light brown
4 dominate alleles
Brown
5 dominate alleles
Dark brown / black
6 dominate alleles
How is sex determined?
Below is a Karyotype (a picture of all the
chromsomes in a cell) of the two different
individuals.
Can you see a difference between the two.
Sex Chromosomes
Both have 22 pairs of Autosomes (Body Chromosomes)
• Sex Chromosomes are the big X & little Y
chromosome
• The female on the left has two X (XX) chromosomes
while the male on the right has an X and a Y (XY)
Abnormal Karyotypes
Extra X Chromosome – 47XXY
Only has an X – 45X0
Extra 21 – Trisomy 21
Extra 18 – Trisomy 18
Does the difference between the size of the X and Y
chromosomes make a difference in the inheritance of
some traits?
It makes a difference if the trait is
inherited only on the larger X
chromosome.
• Since the Y is so much smaller,
some traits are only inherited on
the X chromosome
• We call these traits “Sex-linked
traits”
• If a mother gives her son the trait
on the X chromosome, the
father’s Y can’t save him from
having it since the Y doesn’t have
the normal gene
Unmatched area – no
homologous genes
present on the Y
chromosome
Normal female Carrier female Female w/ trait Normal male Male w/trait
Colorblindness
– Recessive gene carried on the X chromosome
– Individuals can’t see certain colors
– If you are looking at a traffic light, where is the
red light?
In humans, colorblindness is inherited on the X chromosome and is
recessive. The gene for normal color vision is dominant.
Cross a female who is a carrier of the gene (XNXn)
with a normal male (XNY)
N = Normal color vision gene
n = Colorblind gene
• What is the probability of them having a son who
has the colorblind gene (n) on his X chromosome
50%
and is therefore colorblind? ______%
• What is the probability of them having a colorblind
0%
daughter? _______%
What if the mother is a carrier for colorblindness and
the father is colorblind? What about the children?
N = Normal color vision gene
n = Colorblind gene
Parent Cross: XNXn
x XnY
• What is the probability of them having a son who
has the colorblind gene (n) on his X chromosome
50%
and is therefore colorblind? ______%
• What is the probability of them having a colorblind
50%
daughter? _______%
How could we follow the colorblind trait through a family?
Pedigree
Is a chart that can trace the phenotypes
and genotypes in a family through
generations
Squares are males
Circles are females
If colored in, they have the trait
If half colored, they are carriers.
Pedigree for Albinism
Albinism is a recessive autosomal gene so it
isn’t sex linked
Dwarfism pedigree
Dwarfism is dominant and autosomal