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Chapter 9
Fundamentals of
Genetics
Why do you have your “father’s” eyes or
your “mother’s” hair color?
• Genetics
– field of biology
dealing with how
characteristics are
transmitted from one
generation to the
next
• Heredity
– transmission of
characteristics from
parent to offspring
Gregor Mendel
•
•
•
•
Austrian monk - 1866 – Father of Genetics
Worked with garden pea, Pisum sativum
Studied statistics
Observed 7 pea characteristics
–
–
–
–
–
–
–
White or purple flowers
Axial or terminal flower position
Long (tall) or short stems
Round or wrinkled seeds
Yellow or green seeds
Inflated or constricted pods
Yellow or green pods
Mendel’s work
• Controlled the process of
pollination
– pollen grains from the
anther of the stamen land
on the stigma of the pistil
(female repro organ)
• Peas self-pollinate
– transfer of pollen to
stigma of the same
flower
• Instead, Mendel crosspollinated them
– Taking pollen from one
plant to pollinate the
stigma of another flower
1. Removed the stamen (male reproductive
part) to prevent self-pollination
2. Used the stamen from a tall plant and
pollinated only flowers from other tall
plants
3. Observed generations for 2 years to be
certain of purebreds
4. Tall x Tall
Short x Short
5. Crossed plants with contrasting traits
6. Tall x Short
Short x Tall
•P
= Parental generation
• F1
= 1st Filial generation (Offspring)
• F2
= 2nd Filial generation (next set of
offspring from an F1 generation cross)
•P1: Pure Tall x Pure Short
•
TT
tt
•F1:
•P2: F1 Tall
•
Tt
Tall
Tt
X
T = Tall
t = Short
Only one trait showed
F1 Tall
Tt
•F2:
3 Tall
1 Short
•
The “lost” trait reappears!
•
TT Tt Tt
tt
• To get the results for the F2, do
FOIL
(Firsts, outers, inners, lasts)
******Always get:
– 75% and 25% short or 3:1 whenever this
type of cross occurred
So what did he conclude?
1. Concept of Unit Characters:
The inheritance of each trait is
determined by “units” or “factors” that
are passed on.
- We know these as “genes”
- These units occur in pairs. One from
each parent
2. Law of Dominance:
One factor “masks” the appearance of
another factor and prevents it from
showing.
– Dominant – trait that must show if present
(TT or Tt)
– Recessive – trait that will only show if in
the pure form (tt)
– Hybrid – contains a dominant and a
recessive trait (Tt)
– Allele – The alternate genes for a given
trait (Tall or short) – genes occur in pairs
3. Law of Segregation:
- Paired factors separate during
meiosis. - - - Each gamete
receives only 1 factor of each
pair.
- A pair of factors is separated
during the formation of the
gametes.
4. Law of Independent
Assortment:
- Different pairs of alleles are
passed to offspring
independent of each other as
long as the factors are not
located on the same
chromosome.
- You inherited your eye color
independently from your hair
color.
Let’s get Crossing now!
Genotype
– shows actual genetic makeup (Use symbols for
genes)
TT or Pure Dominant or homozygous dominant
Tt or Hybrid or heterozygous
tt or Pure Recessive or homozygous recessive
Phenotype
– Tells appearance (describes the trait)
Tall (if TT or Tt)
Short (if tt)
NO HYBRIDS HERE!
Homozygous – Pure for the trait (TT or tt)
Heterozygous – Hybrid or mixed for a trait (Tt)
Probability
• The likelihood of a specific event occurring.
– Expressed as a decimal, fraction, percentage or
a ratio.
• Probability =
number of times an event is expected to happen
Number of opportunities for an event to happen
What is the likelihood of being left handled in our class?
Probability = # of left handed students
X 100
Total number of students in classroom
What is the probability of pulling out an
Ace from a deck of cards?
• Probability
= 4 (# of Aces in a deck) x 100
52 (# of cards in a deck)
1:13 or 7.6% of the time
• What is the probability of getting heads
or tails from a coin flip?
• Probability
= 1:1
50%
Punnett Squares
Shows possible gene pairing and
probability of each pairing
There are two kinds of sperm &/or 2
kinds of eggs depending on the genes
that they carry.
Genes of 1 parent
T
Genes of 1 parent
T
t
Tt
Tt
t
Tt
Tt
Phenotype ratio:
100% Tall
Genotype ratio:
100% Tt (Hybrid)
Possible crosses
1. Do the Punnett squares & determine the:
– Phenotype ratios
– Genotype ratios
Trait: Purple flower is dominant to white
flower
P = Purple flower
p = white flower
1.
PP x PP
2.
PP x Pp
3.
PP x pp
4.
Pp x Pp
5.
Pp x pp
6.
pp x pp
If you are right handed, how do you know if
you are homozygous right handed or hybrid?
Test Cross:
Method used to find out if something
(someone) is pure dominant or hybrid
for a given trait.
– Cross them with a pure recessive for that
trait.
– If the results come up with a recessive
individual, then the parent was a hybrid
In Guinea pigs:
B = Black coat b = White coat
– Cross a BB guinea pig with a pure
recessive (bb)
– Cross a Bb guinea pig with a pure
recessive (bb)
BB x bb
Bb x bb
How are two traits, not linked on the
same chromosome inherited (according
to the Law of Independent Assortment)
inherited?
• Dihybrid crosses:
– A cross between individuals that involve
two pairs of contrasting traits.
– This is where your FOILing talents come
in!
• Foil this!
AaBb
In pea plants Round seeds are dominant
over wrinkled seeds and Yellow seed coats
are dominant over green seed coats
R = Round
Y = Yellow
r = wrinkled
y = green
P1: Cross two pea plants, homozygous
for contrasting traits, what will
result?
RRYY x rryy
RY
RY
RY
RY
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
Phenotype ratio = 100% Round/Yellow
Genotype ratio = 100% RrYy
F1 Cross:
Cross 2 plants from the F1 generation
RrYy x RrYy
Now FOIL RrYy to figure out the possible
gametes for each parent plant
RrYy
RY
Ry
rY
ry
RY
Ry
rY
ry
RY
RRYY RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Phenotype ratio:
Round/Yellow
Round/Green
Wrinkled/ Yellow
Wrinkled/Green
=
=
=
=
9/16
3/16
3/16
1/16
Always a 9:3:3:1 ratio
when you cross 2
dihybrids!!
3:1 ratio with a
monohybrid cross!!!
Is it always so black and white? What happens
when you cross someone with blue eyes and
brown eyes? Do you always get brown eyes?
Incomplete dominance
– Two or more alleles influence the
phenotype, resulting in an intermediate
phenotype.
– Traits are blended
When you cross a red and a white 4:00
flower, you get pink 4:00’s
Do the Punnett square now to figure out the
phenotype and genotype ratios
R = Red
r = white
P1: Red x white
F1 Cross: Pink x Pink
Genotype ratio:
Phenotype ratio:
A round face crossed with a square
face results in an oval shaped face.
1. Show the cross
which would result in
an oval shaped face.
2. Cross two individuals
who both have oval
shaped faces
Codominance
Occurs when both alleles for a gene are
expressed in a heterozygous offspring.
Roans
R = Red
R’ = White or R = Red
B = Black B’= White
R
R
R’
RR’
RR’
R’
RR’
RR’
W = White
Phenotype ratio = 100% Roan
Genotype ratio = 100% RR’
In Humans, Blood types and Sickle cell
Anemia exhibit codominance
1. Blood Types: Exhibit Dominance, Codominance
and Multiple Alleles
Allele for A proteins on RBC = Allele for B
proteins on RBC. (Codominance)
Alleles for A and B protein are Dominant to O
There are three alleles to determine blood type
A, B and O (Multiple alleles)
A = B > o or IA = IB > i
Blood
Type
Type
Receive
Donate to
from
from
A
A, AB
A, O
B
B, AB
B, O
AB
AB
A, B, AB,
O
O
A, B, AB,
O
O
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
2.
Sickle Cell Anemia
– Due to a gene mutation, normal red blood cells are not
formed, instead, sickled shaped cells are produced
HNHN = All Normal RBC
HNHS = Carrier for SCA – ½ the
RBC are sickle shaped.
Show some of the symptoms of SCA
HSHS = SCA – All RBC will be sickle shaped.
Distribution of
Malaria
Distribution of
SCA
Advantage to being a Carrier
• People who are carriers for SCA, have a
partial resistance to Malaria.
• People who are homozygous for SCA
have a total resistance to Malaria
though they usually die in childhood.
– 100,000/year
– 1.5 million/year die from Malaria