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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