Download Genetics notes - Lincoln Park High School

3/19
BR: Dihybrid Cross
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
Do a Punnett Square for the following dihybrid cross:
plants heterozygous for seed shape and color (RrYy)
Write the genotype & phenotype ratios in the offspring
Dihybrid Cross
KEY
 Genotype ratio = 1 RRYY :
2 RRYy : 2 RrYY : 4 RrYy :
1 RRyy : 2 Rryy : 1 rrYY :
2 rrYy : 1 rryy
 Phenotype ratio =
9 round & yellow :
3 round & green:
3 wrinkled & yellow :
1 wrinkled & green
The allele for colorblindness is recessive and on
the X chromosome
Are YOU colorblind????
Are YOU colorblind????
Are YOU colorblind????
The ABO blood group is a case of multiple alleles AND
codominance (FYI, +/- blood type is a different group)
3/20: BR
1.
Oh no! Some babies were separated from their parents in
the hospital nursery. Help them out. Match the parents
with the children in the problem below.



2.
Couple 1: blood types A & O
Couple 2: blood type B for both
Couple 3: blood types A & B
Baby 1: Type AB
Baby 2: Type A
Baby 3: Type O
Why do you think there so many different shades of skin
in the human population?
Date: 3/20
Other forms of inheritance


Pleiotropy: When 1 gene has multiple effects (p.170)
 Ex. Sickle-cell disease – blood can’t carry oxygen
well, but makes a person resistant to malaria
Polygenic traits: traits controlled by >1 gene (p.172)
 ex. human skin tone & height have a continuous
range of phenotypes
Other forms of inheritance


Epistasis: 1 gene influences the expression of another
 ex. in mice, black (B) is dominant over brown (b)
 But another gene codes for the ability to produce ANY
pigment (C = pigment, c = no pigment / albino)
 Bbcc = white (albino)
 BbCc = black
 bbcc = brown
Environmental influence: phenotype can be influenced by
environment
 ex. the color of the arctic fox changes from brown during the
summer time to white during the winter
(3/23) BR: Pedigrees
1.
The chart to the right
is called a pedigree.
What do pedigrees
show? How are they
used to study
genetics?
Pedigrees
= female
= female carrier
= male
= male carrier
= female with trait
= male with trait
Pedigree Analysis #1
A. Draw the pedigree to
the left
B. Is this trait dominant
or recessive?
Pedigree analysis…
It is RECESSIVE,
because if it was
dominant the parents
would have had the
trait as well
Pedigree analysis #2
A. Draw the pedigree.
B. Is this trait dominant or
recessive?
C. Write the genotypes
next to each individual.
DOMINANT.
Explanation:
Tt
tt
Tt
TT or Tt
#3: Is this trait Dominant or Recessive?
CAN’T TELL.


Could be either dominant or recessive.
Just because you are seeing a trait, doesn’t mean it is
dominant.
Pedigree analysis #4
A. Draw the pedigree.
B. Is this trait dominant or
recessive?
C. Write the genotypes
next to each individual
RECESSIVE
Tt
TT or Tt
Tt
tt
Pedigree Analysis
#5
What type of
inheritance is this?
This is most likely a
SEX-LINKED trait
Is this a sex-linked trait?
NO (at least not sex-linked recessive).
Explanation: If the mother were XhXh, then each of her
sons would have to be XhY and show the trait. They
do not, and so it cannot be sex-linked.
3/24
BR: Genetic Disorders
1.
2.
3.
You have studied chromosomal disorders. How do
you think a genetic disorder is different?
What is a gene?
Why are genetic disorders usually caused by
recessive alleles?
3/24
BR: Answers
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
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Chromosomal disorders are the result of extra, missing, or
damaged chromosomes & are usually not inherited. Genetic
disorders are due to mutations in genes that are ON
chromosomes. They can be passed from parents to offspring.
A sequence of DNA that codes for a specific PROTEIN. Those
proteins determine our traits. Genetic disorders result because
a mutated gene codes for a dysfunctional protein.
Because if a disorder decreases the chances of survival or
reproduction, it will get passed on to offspring less frequently
and eventually be eliminated from the population. But recessive
alleles can remain in the population in heterozygotes who show
a “normal” phenotype (they don’t have the disorder).
3/24
Genetic Disorders (p.164)

Genes code for proteins w/ a specific fxn
 Many genes have only 1 allele
 Any ∆ in that allele is a mutation that can be passed
 Usually harmful
Genetic Disorders

Disorders may be caused by recessive, dominant, or
codominant mutations
 Recessive…why are most genetic disorders caused by recessive
alleles?

Cystic fibrosis: gene mutation causes improper folding
of membrane protein CFTR  absence of CFTR in cell’s
membrane
 chloride ions (Cl ) unable to cross cell membranes 
mucus in lungs, digestive tract
 Common in people of northern European descent
Recessive


Phenylketonuria (PKU): mutation in gene coding for an
enzyme that breaks down the AA phenylalanine 
phenyl. accumulation in tissues may cause mental
retardation  early detection can allow for survival w/
dietary restrictions
Tay-Sachs disease: mutation in gene that codes for
enzyme that breaks down ganglioside lipids in neurons
 results in nervous system break down & death
 Common in Ashkenazi Jews
Genetic Disorders

Dominant
 Huntington disease: loss of muscle control &
mental function  starts in 30s or older
 Achondroplasia: form of dwarfism
Genetic Disorders

Codominant
 Sickle-cell disease –mutation ∆’s 1 AA in hemoglobin
 must be homozygous to have disease
 heterozygotes are resistant to malaria
 abnormal hemoglobin protein causes sickle-shaped
RBCs  get caught in capillaries & block blood flow,
starving tissues of oxygen and damaging organs like
the brain, liver, &spleen
 Common in people living in tropical areas like
western Africa where malaria kills millions (very
common in African-Americans)
Recessive Disorders

Disorders are often distributed among certain ethnicities much
more frequently than others. Why do you think that is?
 Genetic disorders are inherited and due to mutations in
genes. Mutations happen naturally and frequently. The
average person has 60 in his/her DNA! But they are usually
recessive if we survive with them. The smaller a population
is, the more likely two people who breed will share the same
random mutation in their DNA (this is why incest is risky!)
Races/ethnicities exist because different populations of
people were isolated from others for many generations, and
any changes in their genes stayed within each population.
This is how evolution works!
3/25
BR: Construct a pedigree

Construct a pedigree examining hemophilia
inheritance and list the genotypes of each person:


A woman is a carrier for hemophilia and marries a man
who does not have hemophilia. This couple has 2 sons
and 1 daughter. One son has hemophilia, the other does
not. The daughter marries a man with hemophilia. They
have 2 sons and 2 daughters. One of their sons has
hemophilia, the other does not. One daughter has
hemophilia, the other does not.
List the genotypes of everyone in the pedigree.
KEY
XHY
XHXh
XHXh
XhY
XhY
XH Y
XhXh
XHXh
XHY
XhY
3/26
BR: Dihybrid Cross
About 70% of Americans perceive a bitter taste from the chemical
phenylthiocarbamide (PTC). The ability to taste this chemical results from a
dominant allele (T) and not being able to taste PTC is the result of having two
recessive alleles (t). Albinism is also a single locus trait (i.e., controlled by 2
alleles) with normal pigment being dominant (A) and the lack of pigment being
recessive (a). Gloria is a normally-pigmented woman who cannot taste PTC.
Her father is an albino who can taste PTC. Gloria marries Jay, who is a
homozygous, normally-pigmented man who can taste PTC but whose mother
cannot taste PTC.
a.
Identify the genotypes of Gloria and Jay and the possible allele
combinations in their gametes (to use in a Punnett Square).
b.
What are the genotype and phenotype ratios of Jay and Gloria’s
potential children?
c.
What can you definitively conclude about Jay and Gloria’s parents?
KEY
a.
b.
Gloria: Aatt, Jay: AATt and the
•
possible allele combinations in their gametes:
•
Gloria: At, at
•
Jay: AT, At
Offspring genotype ratio =
1 AATt : 1 AaTt : 1 AAtt : 1 Aatt

Phenotype ratio=
1 normally-pigmented who can taste PTC :
1 normally-pigmented who cannot taste PTC
KEY
c.
Gloria’s mom is A__t__ (normal pigment)
Gloria’s dad is aaTt (albino & can taste PTC)
Jay’s mom is aatt (albino & cannot taste PTC)
Jay’s dad is aaT__ (albino & can taste PTC)