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Chapter 8 Part Two
By:
Brianna Shields
Mendel’s Study of Traits
Punnett
Squares
1. Diagram that predicts the
outcome of a genetic cross by
considering all possible
combinations of gametes in the
cross
Mendel’s Study of Traits
Punnett
Squares
2. 4 boxes in a large square
3. One parent’s gametes written
across top, other down left side
4. Fill boxes by combining alleles
from top and left sides (creates
possible genotypes)
Mendel’s Study of Traits
Punnett
Squares
5. Steps:
A. Set up boxes
B. Create dominant and recessive key
C. Write parental gametes across top and
down left side
D. Perform monohybrid cross
E. Record genotype percentages
F. Record phenotype percentages
Punnett Example:
TT x Tt
T
T
Genotypes:
T
TT
TT
50% TT Homozygous
Dominant
t
Tt
Tt
50% Tt Heterozygous
Phenotypes
100% Tall
T= Tall
t=Short
Complete the Cross
Brown eyes is dominant. Blue
eyes is recessive. Cross a
homozygous dominant and a
homozygous recessive.
What percentages of blue-eyed
and brown eyed offspring will you
get?
Punnett Example:
BB x bb
B
B
b
Bb
Bb
b
Bb
Bb
Genotypes:
100 % Heterozygous
Phenotypes
100% Brown Eyed
B= Brown b=Blue
Perform the cross
A couple is hoping their
child will have the tongue
rolling ability when it is
born.
Tongue rolling is a
dominant trait. Non tongue
rolling is recessive.
If the mother is
heterozygous and the
father is heterozygouswhat are their chances of
having a child with the
tongue rolling ability?
Punnett Example:
Rr x Rr
R
r
R
RR
Rr
r
Rr
rr
Genotypes:
25% Homozygous Dominant
50% Heterozygous
25% Homozygous Recessive
Phenotypes
75% Tongue Rolling
R= Tongue Rolling
r= Non-tongue Rolling
25% Non-tongue Rolling
Mendel’s Study of Traits
Punnett
Squares
6. Used by horticulturists and
animal breeders to predict the
crosses that will most likely produce
offspring with desirable phenotypes
Mendel’s Study of Traits
Determining
homo and
heterozygosi
ty
Test Cross- individual with
dominant phenotype but unknown
genotype is crossed with a
homozygous individual
Ex: Yellow seeded (Y?) crossed with green
seeded (yy)
If all offspring yellow, their genotype must by
Yy and unknown parent must be YY
If half are yellow, half are green, unknown
parent must’ve been Yy
Mendel’s Study of Traits
• Probability
• Likelihood that a specific event
will occur (used to predict
genetic crosses)
• Number of one kind of possible
outcome divided by total
number of all possible outcomes
Mendel’s Study of Traits
Probability
Ex: Probability of flipping a coin
and getting heads is 1/2
Ex: Probability of a eertain seed
color when there are 2 possible
alleles for seed color is 1/2
Mendel’s Study of Traits
Probability
Probability of the outcome of a
cross (getting an allele from one
parent is separate from getting an
allele from the other)
1/2 x 1/2 equals 1/4 (2
independent events occurring
should be multiplied)
Mendel’s Study of Traits
Pedigree
Family history that shows how a
trait is inherited over generations
Useful in tracking genetic disorders to see if
an individual is a carrier or may pass it the
disorder to their offspring
Click here to watch a tutorial about pedigrees
Mendel’s Study of Traits
• Carrier
• Heterozygous for an
inherited disorder but does
not show symptoms of the
disorder
Mendel’s Study of Traits
Autosomal
Traits
Occur on chromosomes not
related to gender
Appear in both sexes equally
Mendel’s Study of Traits
Sex-linked
Traits
Trait whose allele is located on x
chromosome
Most are recessive
Males mainly affected because they only
have one x chromosome
Females usually just carriers (presence
of dominant trait to mask recessive one)
Females would have to be homozygous recessive
to show trait (less likely to inherit)
Mendel’s Study of Traits
Autosomal
Dominant
Condition
Every individual with trait has a
parent with the trait
Mendel’s Study of Traits
Autosomal
Recessive
Condition
Individual can have one, two or no
parents with the condition because
trait is recessive
Diagram: Who are the
carriers? Who is infected?
Assessment Three
Predict the expected phenotypic and genotypic ratios
among the offspring of two individuals who are
heterozygous for freckles (Ff) by using a punnett square
Summarize how a test cross can reveal the genotype of a
pea plant with round seeds
Calculate the probability that an individual heterozygous
for a cleft chin (Cc) and an individual homozygous for a
cleft chin (cc) will produce offspring that are homozygous
for a cleft chin
When analyzing a pedigree, how can you determine if an
individual is a carrier (heterozygous) for a trait being
studied?
Complex Patterns of
Heredity
Polygenic
Trait
When several genes influence a
trait (all on one chromosome or on
different)
Ex: Eye color, height, weight, hair and skin color
Have degrees of intermediate conditions between
extremes
Can be complex due to independent assortment
and crossing over during meiosis
Complex Patterns of Heredity
• Intermediat
e Traits
• Incomplete dominance- an
individual displays a trait
that is intermediate
between the two parents
– Ex: white snapdragon x red
snapdragon equals pink
snapdragon
– Ex: curly hair x straight hair (both
homoz dom) equals wavy hair
Complex Patterns of
Heredity
Multiple
Alleles
Genes with three or more alleles
EX: ABO blood groups
A and B refer to carbohydrates on
surface of red blood cells, O has
none
A and B dominant over O, but not
over each other (codominant)
Can only have 2 of the possibilities
for the gene
Possible Blood Type
Possibilities
Possible Blood Type
Possibilities
Complex Patterns of
Heredity
Codominanc
e
2 dominant alleles are expressed
at the same time and both forms
of the trait are displayed
Ex: AB blood group (has both A
and B carbohydrates on the
surface of red blood cells)
Complex Patterns of
Heredity
Traits
influenced
by
environment
EXAMPLE 1: Hydrangea flowers
Blue (acidic soil) to pink (neutral to basic soil)
Complex Patterns of
Heredity
Traits
influenced
by
environment
EXAMPLE 2: Arctic Fox
During summer, fox produces enzymes that make
red brown pigments
In cold, pigment producing genes don’t function
and coat remains white
Fox blends in with snowy white background
Complex Patterns of
Heredity
Traits
influenced
by
environment
EXAMPLE 3: Siamese Cats
Genotype results in darker fur color in cooler
areas of the body (ears, nose, paws, tail darker
than rest of body)
Complex Patterns of
Heredity
Traits
influenced
by
environment
EXAMPLE 4: Human Height
Nutrition and internal environmental conditions
Complex Patterns of
Heredity
Traits
influenced
by
environment
EXAMPLE 5: Human Skin Color
Exposure to sun
Complex Patterns of
Heredity
Traits
influenced
by
environment
EXAMPLE 6: Human Personality
Aggression influenced by environment and genes
Complex Patterns of
Heredity
Traits
influenced
by
environment
Twins used to study environmental
influences because their genes
are identical, any differences
between them are due to the
environment
Genetic Disorders
Genetic
Disorders
Harmful effects produced by
inherited mutations
Damaged or incorrectly copied
genes can result in the production
of faulty proteins
Mutations are rare, due to efficient
correction systems in cells
Often carried by recessive alleles
in heterozygous individuals
Genetic Disorders
Genetic Disorders
Sickle Cell
Anemia
Caused by mutated allele that produces
defective form of protein hemoglobin
In rbc’s, Hb binds to and transports
oxygen
Causes sickle shaped red blood cells
that rupture easily, clog blood vessels
and can’t transport oxygen well
Carriers exposed to malaria can prevent
infection when they have sickle cell. It
kills malaria protozoans and healthy
rbc’s can still transport enough oxygen
Genetic Disorders
Cystic Fibrosis
Fatal, recessive trait
Most common inherited disorder in
Caucasians
1/25 babies are carriers
1/2500 babies have disease
No known cure
Have defective copy of gene needed to
pump Cl in and out of cell
Lung airways clog with mucus, liver and
pancreas ducts get blocked
Genetic Disorders
Hemophelia
Sex linked trait
Impairs blood’s ability to clot
Mutation on one of a dozen blood
clotting genes on x chromosome is
hemophelia A
If male receives defect on x
chromosome from mother, y
chromosome can’t compensate develops disease
Genetic Disorders
Treating
genetic
disorders
Most can’t be cured, but can be
treated
Families with histories of genetic
disorders can receive genetic
counseling before having kids to
assess the risk
Genetic Disorders
Treating
genetic
disorders
Some genetic disorders can be treated if
diagnosed early on
Ex: PKU (Phenylketoneuria)- lack enzyme
for converting amino acid phenylalanine to
tyrosine
Can cause severe mental retardation
If found immediately right after birth, baby
can be given a diet low in phenylalanine to
avoid the symptoms
Many states require testing newborns
for PKU (inexpensive)
Genetic Disorders
Treating
genetic
disorders
Gene Therapy
Replace defective gene with healthy one
Isolate copy of the gene and deliver to
infected cell by attaching it to a virus
Virus with healthy gene enters cell, starts
producing healthy protein
“Cures” the genetic disorder
Genetic Disorders
Gene Therapy
Tried for cystic fibrosis by using
cold virus
BUT, most individuals are immune to cold
virus and the lung cells rejected it
Currently trying with an AAV virus that
produces almost no immune response
Assessment Four
Differentiate between incomplete
dominance and codominance
Identify two examples of traits that are
influenced by environmental conditions
Summarize how a genetic disorder can
result from a mutation
Describe how males inherit hemophelia
A nurse states that a person cannot have
the blood type ABO. Do you agree or
disagree? Explain
Genetics Websites
OLogy: The Gene Scene