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Mendelian Genetics
Gregor Mendel
Introduction
• Genetics is the study
of heredity
• Gregor Mendel used
mathematics to
study the inheritance
of traits
Phenotypes
• These are features
you exhibit physically
( your looks)
Example: Eye color green
Alleles
• The different versions
of a characteristic
• Example: blue, green,
and brown eyes
Inheritance
• Occurs when traits
are passed down
from parent to child.
• Gametes (sex cells
like sperm or ovaries)
carry traits to
offspring.
Genes
• Bits of information
passed down from
parent to child.
• Made of chemicals
called DNA.
Heredity
• The chromosome theory of heredity states that
the inheritance of traits is controlled by
genes, located on chromosomes
Key Terms
 Genotype - gene combination for a
trait (e.g. RR, Rr, rr)
 Phenotype - the physical feature
resulting from a genotype (e.g. red,
white)
The
Average
American
Phenotype
Alleles
• T – is considered TT – is homozygous
a dominant
dominant
allele
Tt or tT – is
heterozygous
• t – is considered dominant
a recessive
tt - is Homozygous
allele
recessive recessive
Genotype & Phenotype in Flowers
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Rr
rr
Phenotypes
RED
RED
YELLOW
The Law of Dominance
• Law states that the dominant allele, if
present, will be expressed.
 Dominant - stronger of two genes
expressed in the hybrid; represented by a
capital letter (R)
 Recessive - gene that shows up less
often in a cross; represented by a
lowercase letter (r)
Predicting traits
• Probability is used to predict traits in offspring
• Punnett squares are diagrams used to predict
outcomes from a cross
Types of Genetic Crosses
 Monohybrid cross - cross
involving a single trait
e.g. flower color
 Dihybrid cross - cross involving
two traits
e.g. flower color & plant height
How to do a monohybrid cross
Law of Segregation
• During the formation of
gametes (eggs or sperm)
during meiosis, the two alleles
responsible for a trait
separate.
• Alleles for a trait are then
"recombined" at fertilization.
Applying the Law of
Segregation
Make a Cross for the following:
• Imagine you are crossing two vampire bats.
– The dominant trait for skin color is black (S).
– The recessive trait for skin color is white (s).
• It you have one parent with the genotype Ss and one with the
genotype ss, what kind of offspring would they have?
S
s
s
s
Answer
S
s
s Ss
ss
s Ss
ss
Group Assignment
• Let’s imagine we found a new species of dog called a
Gir.
– We are going to practice crossing this new species to get a
better knowledge of how Punnett Squares work.
A pure breed
GREEN Gir has
the genotype GG
A pure breed
RED Gir has
the genotype gg
A Gir with the genotype Gg will have a GREEN phenotype
because green is dominant over red.
Breed the P1 generation
• Green (GG) x Red (gg)
g
G
G
g
Solution:
Green (GG) x Red (gg) dogs
g
g
G
Gg
Gg
produces the
F1 generation
G
Gg
Gg
All Gg = Green
(heterozygous Green)
Breed the F1 generation
• Green (Gg) x Green (Gg)
G
G
g
g
Solution:
Green (Gg) x Green (Gg)
G
g
G
GG
Gg
g
Gg
gg
produces the
F2 generation
1/4 (25%) = GG
1/2 (50%) = Gg
1/4 (25%) = gg
1:2:1 genotype
3:1 phenotype
Punnett Squares can also be used to
determine the sex.
X
X
XX
X
XX
50% possibility of female XX
Y
XY
50% possibility of male XY
XY
Dihybrid Cross
• A breeding experiment that tracks the
inheritance of two traits.
• Follows Mendel’s “Law of Independent
Assortment”
– Each pair of alleles segregates
independently during gamete formation
Dihybrid Cross
• Traits: Seed shape & Seed color
• Alleles: R round
shape
r wrinkled
Y yellow
color
y green
•
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
Law of Independent Assortment
• Alleles for different traits (like eye color and
hair color) are distributed to sex cells (&
offspring) independently of one another.
Dihybrid Cross
RY
RY
Ry
rY
ry
Ry
rY
ry
Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
Pedigrees
• Pedigrees use symbols
to track a trait.
• Trait is then tracked
through generations.
Example
Generation 1
Generation 2
Generation 3
Pedigree
• Pedigree: is a diagram of family relationships
that uses symbols to represent people and lines
to represent genetic relationships.
• These diagrams make it easier to visualize
genetic relationships within families.
• Pedigrees are often used to determine the
mode of inheritance (dominant, recessive, etc.)
of genetic diseases.
Incomplete Dominance
and
Codominance
Incomplete Dominance
• F1 hybrids have an appearance somewhat
in between the phenotypes of the two
parental varieties.
• Example: snapdragons (flower)
– red (RR) x white (rr)
• RR = red flower
• rr = white flower
r
R
R
r
Incomplete
Dominance
r
r
R
Rr
Rr
R
Rr
Rr
produces the
F1 generation
All Rr = pink
(heterozygous pink)
Incomplete Dominance
Codominance
• Multiple alleles are expressed in
heterozygous individuals.
• Example: blood type
•
•
•
•
1.
2.
3.
4.
type A = IAIA or IAi
B
B
B
type B = I I or I i
type AB = IAIB
type O = ii
A and B are
types of
antigens.
O type blood
does not have
these antigens.
Codominance Problem
• Example:
Type B
B
B
(I I )
IA
IB
IB
x Type A
A
(I i)
i
What blood types are
possible in offspring?
Codominance Problem
• Example:
B
B
A
Type B (I I ) x Type A (I i)
IB
IB
IA
i
IAIB IBi
A
B
I I
B
I i
1/2 = IAIB
B
1/2 = I i
Another Codominance Problem
male Type O (ii)
x
A
B
female type AB (I I )
• Example:
IA
IB
i
i
What are the possible
blood types?
Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
A
B
I
I
i
A
I i
B
I i
i
A
I i
B
I i
1/2 =
1/2 =
A
I i
B
I i
Sex-linked Traits
• Traits (genes) located on the sex
chromosomes
• Sex chromosomes are X and Y
• XX genotype for females
• XY genotype for males
• Many sex-linked traits carried on X
chromosome ONLY.
– Color blindness, Hemophilia (can’t clot
blood), Kabuki syndrome, baldness
• This means males get genes on X gene
from MOTHER.
Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
Sex-linked Trait Problem
• Example: Eye color in fruit flies
•
(red-eyed male) x (white-eyed female)
XRY
x
XrXr
• Remember: the Y chromosome in males
does not carry traits.
r
r
X
X
• RR = red eyed
• Rr = red eyed
XR
• rr = white eyed
• XY = male
Y
• XX = female
Sex-linked Trait
Solution:
r
X
r
X
R
X
XR Xr XR Xr
Y
r
X
Y
r
X
Y
50% red eyed
female
50% white eyed
male
Alien Lab
• You will spend 2/3 class periods creating
this booklet.
• DO NOT LOSE YOUR BOOKLET.
– TURN IT IN TO ME after each class period so
you won’t lose it.
– There are NO EXTRA COPIES.
You will create a booklet of an Alien you create
and its FAMILY HISTORY in terms of genetics.
EVERYONE will have a DIFFERENT alien.
• Things to complete (look at rubric on page 11)
I will discuss each
– 1. Cover page (Day 1)
thing the DAY
– 2. Alien Trait Key (Day 1)
we will do them.
I will only cover items
– 3. Alien Personal Data (Day 1)
1-3 today.
– 4. Alien Personal Ad (Day 2)
– 5. Alien Mate Data Sheet (Day 2).
– 6 & 7 Alien Matchmaking Traits (Day 2)
– 8. Alien Offspring Trait Key (Day2/3)
– 9. Alien Offspring Drawings (Day 3)
– 10. Discussion Questions (Day 3)
Alien Trait Key
• We will do this AS A CLASS.
• We will discuss what we want our ALIEN
SPECIES TO LOOK LIKE.
#
Trait
Homozygous
Dominant
Heterozygous
Dominant
Homozygous
Recessive
Example


HH
(heads/heads)
Hh
(heads/tails)
hh
(tails/tails)
Blue
Blue
Yellow
Female
Male
Circle one
1
Hair Color
2
Eye Color
3
# Heads
4
Tall? Short?
5
# Fingers
6
# Toes
7
Skin Color
8
9
10
11
12
Sex
Alien Personal Data
• Using a penny (Heads = H) (Tails = h) flip a
coin to find out your personal genotype.
• Fill in column 1 with genotypes flipped.
• Using the information from the “alien trait key”
fill in column 2 with your personal phenotype.
• Draw and color YOUR COVER PAGE based
off this information on the cover of your Alien
Lab Booklet.
#
1
2
3
4
5
6
7
8
9
10
11
YOUR Alien’s
Traits
(Genotype)
hh
YOUR Alien’s
Traits
(Phenotype)
yellow
Mate
Information
(Genotype)
Mate Information
(Phenotype)
Cover Page
• Have the students put their parent drawing
here. They will get this information from
the “alien personal data” sheet to be filled
out on page 3.