Transcript 11.1 notes


Answer the question:
› What do you think is the difference between
heredity and genetics?

The delivery of characteristics from
parents to offspring is called heredity.

The scientific study of heredity is called
genetics.

Gregor Mendel was
born in 1822 in what
is now the Czech
Republic. He
worked as a priest in
a monastery and
also taught high
school and ran the
monastery garden.

He worked with
ordinary pea plants
because they were
small, easy to grow,
produce hundreds of
offspring, and were
convenient to study.
They are now known
as the model system.

Gregor Mendel worked on fertilizing
different plants and observing traits that
the offspring held. A trait is a specific
characteristic (seed color, plant height,
etc.) of an individual.

Mendel knew that one ordinary pea plant
contained both female and male parts
which means that it could self-pollinate.
These types of plants are known as true
breeding plants because they would
always produce offspring identical to
themselves.

Mendel wanted to
know how traits were
passed on so he
wanted to cross the
traits of two true
breeding plants
through a process
known as cross
pollination.

Mendel always started with true
breeding flowers. He called them his
parental generation or the “P”
generation. The parental generation he
used was always homozygous. This
means they carried two of the same
alleles (ex: TT or tt never Tt)
The factors that are passed on from
parent to offspring are called genes.
 The different forms of these factors are
called alleles.

Example:
height = gene
tall (T) = allele
Capital Letters = dominant trait
 Lowercase Letters = recessive trait
 Alleles come in pairs. If there is even one
dominant allele it will mask the other.
 Showing the dominant trait TT or Tt
 Showing the recessive trait
tt

Homozygous: TT or tt
 Heterozygous: Tt
 Genotype: The allele code (TT, Tt, or tt)
 Phenotype: Visually what you see
occurring (tall or short)

Using punnett squares we can figure out
what possible types of offspring a future
child will be.
 Example: parent plants are TT
(homozygous tall) and tt (homozygous
short)
 T = tall
t = short

tt
TT
T
T
t
P
t
gametes
t
T
T
t
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?

The first generation
of offspring were
known as the F1
generation and they
are no longer
purebred plants.
They are
heterozygous since
they carry two
different alleles.


Ratio TT:Tt:tt
0: 4: 0
Ratio Tall : short
4 : 0
Tt
Tt
Tt
Tt
T
T
t
t
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?


By allowing members
of the first generation
to reproduce the
second generation of
offspring are formed
and they are known as
the F2 generation.
Mendel studied 7
different
characteristics to see
which would be
dominant or recessive.


Ratio TT:Tt:tt
1: 2: 1
Ratio Tall : short
3 : 1
TT
Tt
Tt
tt
The Principle of Dominance
 The Law of Segregation
 The Law of Independent Assortment


On your table are 7
different traits that
Mendel studied.
Whatever
disappeared in the F1
generation but
reappeared later he
called a recessive
trait.

Mendel crosspollinated two true
breeding
homozygous plants
together and he was
surprised that after
the first round of
reproduction that
certain traits were
lost.

To find out why this occurred he allowed
his F1 generation to self-pollinate. In the
F2 generation the lost traits reappeared.
¼ of the plants in the F2 generation
showed the lost trait. Mendel figured out
that this must have been a recessive
trait.
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?

Phenotype Ratio

Genotype Ratio
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?
Genotype:
Phenotype:
Homo. Or Hetero?

Phenotype Ratio

Genotype Ratio

Dominant allele: Represented by capital
letters. Will show through in the plant as
long as one is present.
› Example: T, A, B, …

Recessive allele: Represented by
lowercase letters. Will only show through
in the plant if there are two present.
› Example: t, a, b, …

Genotype: The coded letters that
represents an outcome.
› Example: TT, Tt, tt

Phenotype: The physical / visible trait
that occurs in the plant.
› Example: tall or short

Homozygous: the genotype is both
dominant or recessive.
› Example: TT or tt

Heterozygous: the genotype is a mixture
of dominant and recessive.
› Example: Tt or tT
Probability is the likelihood that an event
will occur. We represent these with
fractions or percents.
 Complete examples 1 and 2 and leave
your answers in fractions.
 Ex1: ½
 Ex2: ½ x ½ x ½ = 1/8
 The previous outcome does NOT change
the outcome of the next flip.


The way in which alleles segregate
during gamete formation is independent
just like in example 2.
½
½x½
¼
½
½
½
½x½
¼
½x½
¼
½x½
¼

T
T
F1 possible offspring
t
Geno:
Pheno:
Geno:
Pheno:
Ratios:
Tall : Short
Probability
t
Geno:
Pheno:
Geno:
Pheno:
TT:Tt:tt
TT
Tt
tt

T
T
F1 possible offspring
t
Geno: Tt
Pheno: Tall
Geno: Tt
Pheno: Tall
Ratios:
Probability
t
Geno: Tt
Pheno: Tall
Geno: Tt
Pheno: tall
Tall : Short 4:0
TT
Tt
tt
TT:Tt:tt 0:4:0
0/4
4/4
0/4

T
t
F2 possible offspring
T
Geno:
Pheno:
Geno:
Pheno:
Ratios:
Tall : Short
Probability
t
Geno:
Pheno:
Geno:
Pheno:
TT:Tt:tt
TT
Tt
tt

T
t
F2 possible offspring
Probability
T
t
Geno: TT
Geno: Tt
Pheno: Tall Pheno: Tall
Geno: Tt
Geno: tt
Pheno: Tall Pheno: short
Ratios:
Tall : Short 3:1
TT
Tt
tt
TT:Tt:tt 1:2:1
¼
½
¼
BP
BP
bp
bp
BP
BP
bp
bp
BbPp
BbPp
BbPp
BbPp
BP
Bp
bP
bp
BP
Bp
bP
bp
BP
Geno:BBPP
Geno:
Pheno: black Pheno:
and petals
Geno:
Pheno:
Geno:
Pheno:
Bp
Geno:
Pheno:
Geno:
Pheno:
Geno:
Pheno:
Geno:
Pheno:
bP
Geno:
Pheno:
Geno:
Pheno:
Geno:
Pheno:
Geno:
Pheno:
bp
Geno:
Pheno:
Geno:
Pheno:
Geno:
Pheno:
Geno:
Pheno:
BP
Bp
bP
bp
BP
Geno:BBPP
Geno: BBPp
Pheno: black Pheno: black
and petals
and petals
Geno:BbPP
Pheno: black
and petals
Geno: BbPp
Pheno: black
and petals
Bp
Geno: BBPp Geno: BBpp
Pheno: black Pheno: black
and petals
and no petals
Geno: BbPp
Pheno: black
and petals
Geno: Bbpp
Pheno: black
and no petals
bP
Geno: BbPP Geno: BbPp
Pheno: black Pheno: black
and petals
and petals
Geno: bbPP
Pheno: white
and petals
Geno: bbPp
Pheno: white
and petals
bp
Geno: BbPp Geno: Bbpp
Pheno: black Pheno: black
and petals
and no petals
Geno: bbPp
Pheno: white
and petals
Geno: bbpp
Pheno: white
and no petals
Dom-Dom : Dom-Rec : Rec-Dom : Rec-Rec
Black-Petal : Black-no petal : white-petal : white-no petal
9:3:3:1
While Mendel only had the resources of his
monastery garden to use for studying
genetics, other scientists made use of other
resources to check his work.
 While his rules of dominance and recessive
for a single gene work there are always
exceptions to the rule.
 And as technology increases so does our
understanding of how wide is the range of
exceptions.

What do you think the word incomplete
means?
 Answer: not complete; lacking some
part

What Mendel dealt with was known as
complete dominance. In his situation
the presence of one dominant allele
always showed the dominant trait.
 An exception to this is known as
incomplete dominance where the
heterozygous form will exhibit an
incomplete / blended / intermediate
phenotype because some alleles are
neither dominant or recessive.


Ex: Snapdragons or Four O’Clock Plants
What do you
notice about
t
the F1
generation?
R
R
W
Geno:
Pheno:
Geno:
Pheno:
W
Geno:
Pheno:
Geno:
Pheno:
Probability of
RR
RW
WW
R
R
W
Geno: RW
Pheno: pink
Geno: RW
Pheno: pink
W
Geno: RW
Pheno: pink
Geno: RW
Pheno: pink
Probability of
RR 0/4
RW 4/4
WW 0/4
R
W
R
Geno:
Pheno:
Geno:
Pheno:
W
Geno:
Pheno:
Geno:
Pheno:
Probability of
RR
RW
WW
R
W
R
Geno: RR
Pheno: red
Geno: RW
Pheno: pink
W
Geno: RW
Pheno: pink
Geno: WW
Pheno: white
Probability of
RR 1/4
RW 1/2
WW 1/4

Four O’clock plant
Explain to your neighbor what it means
for a given trait to be incompletely
dominant.
 NOW on a scrap piece of paper
complete the following cross:
BB = Black WW = White BW = Grey
Find the possible offspring when a black
parent crosses with a grey parent.

What do you think is meant by the word
co in co-dominance?
 Answer: Shared


When multiple alleles are equally
dominant then both of those traits will
show through in the heterozygous. There
is no blending in this exception. It looks
as if the figure is patterned because it will
display both traits.

Erminette Chicken

Roan Cow

Erminette Chicken



Certain varieties of
chickens
BB = black
WW=white
However when you
get a BW you get a
black/white
checkered that is
otherwise known as
an erminette
chicken.
B
B
W
Geno:
Pheno:
Geno:
Pheno:
W
Geno:
Pheno:
Geno:
Pheno:


Parents are BB and
WW
Probability of
BB
BW
WW
W
W
B
B
Geno: BW
Pheno:
erminette
Geno: BW
Pheno:
erminette
Geno: BW
Pheno:
erminette
Geno: BW
Pheno:
erminette


Parents are BB and
WW
Probability of
BB 0/4
BW 4/4
WW 0/4
B
W
B
Geno:
Pheno:
Geno:
Pheno:
W
Geno:
Pheno:
Geno:
Pheno:


Parents are BW and
BW
Probability of
BB
BW
WW
B
W
B
W
Geno: BB
Pheno:
black
Geno: BW
Pheno:
erminette
Geno: BW
Pheno:
erminette
Geno:
WW
Pheno:
white


Parents are BW and
BW
Probability of
BB 1/4
BW 1/2
WW 1/4

Another example of
co-dominance is
Sickle Cell Anemia.
This is an issue where
the red blood cells
cannot carry
oxygen.



Trait (or carrier) is the
heterozygous form.
Have both normal
and sickled blood
cells
The normal blood
cells help the person
to live a near normal
life.


Disease is
homozygous for the
sickle cell and the
majority of their
blood cells are
sickled.
They show the
symptons of sickle
cell.
Family 1
A = Normal RBC ( red
blood cell)
S = Sickle RBC
Look at the hands of
the person to show
their genotype.
Family 1
Looking at family 1:
Parents AS and AA
What is the probability
of children with:
Sickle Cell Trait:
Sickle Cell Disease:
Usual/normal:
Family 2
A = Normal RBC ( red
blood cell)
S = Sickle RBC
Look at the hands of
the person to show
their genotype.
Family 2
Looking at family 1:
Parents AS and AS
What is the probability
of children with:
Sickle Cell Trait:
Sickle Cell Disease:
Usual/normal:

Why do more members of the African
American community have sickle cell
anemia than some other cultures?


Green is areas
of high malaria
Orange stripe is
areas of sickle
cell allele
carriers.

There is a connection between sickle-cell
anemia and malaria. Malaria is caused
by a parasite that can live inside
mosquitoes found in tropical regions of
the world and is often deadly.

The parasite enters the body and travels
directly to infect the RBC.

Those who are heterozygous for sicklecell will not contract malaria.

WHY you might ask? The sickled cells
puncture/kill the malaria.

In fact in countries like Africa where
malaria is an issue 1/10 will be
heterozygous for the trait as an
evolutionary measure against malaria.
Reminder:
Gene = height
allele = T tall
So the gene is on the chromosome but the
expression of the gene is the specific
allele.

It is possible for ONE gene to have 2+
alleles (or versions of a trait).
 Example: pigeons have 3 alleles for
feather color which means they can be
born either red, chocolate, or blue.
 Example: Rabbits have 4 color alleles
and therefore can be born with multiple
varieties of colors.

Blood Type
 There are three different alleles resulting
in the four blood types
 Four Blood types: A, B, AB, O

The letters A and B represent antigens on
the allele. If they are present they will be
present in the name of the blood type.
 We write the alleles as IA , IB, and I
otherwise known as A, B, and O
 The I stands for immunoglobulin protein.

Phenotype
Type A
Type B
Type AB
Type O
Genotype
Phenotype
Genotype
Type A
Type B
Type AB
Type O
IAIA
or
IAI
=
AA
or
AO
Phenotype
Genotype
Type A
Type B
Type AB
Type O
IAIA or
IBIB or
IAI
IBI
=
=
AA or
AO
BB or
BO
Phenotype
Genotype
Type A
Type B
Type AB
Type O
IAIA or IAI
IBIB or IBI
IAIB = AB
=
=
AA or
AO
BB or
BO
Phenotype
Genotype
Type A
Type B
Type AB
Type O
IAIA or
IBIB or
IAIB or
II
IAI
IBI
IAIB
= AA or
AO
= BB or
BO
= AB or
AB
=
OO
Remember that each of the alleles (A and B) are codominant so as long as they are present they are
represented.
Blood transfusions
 You can only donate to those that also
have your dominant allele. If you only
have recessive you can donate to
anyone.
 If you do not receive the correct
transfusion your body will reject it and
start to clot.

Who can donate to who?
Donor
Viable Recipient

O
O
A
A
B
B
AB
AB
What blood type is considered to be the
universal donor?
 Type O

What blood type of considered to be
the universal acceptor?
 Type AB

Just a little side note (will not be tested
on this)
 But blood types are positive and
negative so where does that come
from?
 Blood types are either + or – based on
the presence of the Rh factor on the
surface of the RBC. If it is present they
will be + but if it is absent they will be -.

On a piece of scrap paper answer the
following:
 What are all the possible genotypes of
offspring if a Type A parent breeds with a
Type B parent?


Answer: what are all the possible type
A’s (AA or AO) and type B’s (BB or BO)
cross all the possible combinations.
B
B
A
AB
AB
A
AB
AB
B
O
A
AB
AO
A
AB
AO
B
O
A
AB
AO
O
BO
OO
B
B
A
AB
AB
O
BO
BO
Answer: Without information about the
parents parents we must try all options.
 AB, AO, BO, and OO are the possible
genotypes

On the same scrap of paper answer the
following question:
 Can a child with blood type B be the
child of parents with blood types A and
type O? Explain your answer using
punnett squares.

O
O
A
AO
AO
A
AO
AO
O
O
A
AO
AO
O
OO
OO

Poly means many while genic can be
taken as gene so polygenic means
many genes. Two or more genes have
to be affected in order for this to occur
so this is NOT the same thing as multiple
allele (one gene) or co-dominance
(speckled) which both only deal with
ONE gene.

Examples: Eye Color, hair color, skin
color, and height.

Skin Color

For skin color the more capital letters one
has the more melanin they have and
therefore the darker their skin color is.

When it comes to trait expression both
nature and nurture influence
phenotypes.

Nature means your genes / what
“mother nature” gave you.

Nurture means the environment.

For instance, while nature / your genes
may have given you pale skin color. The
environment / how you nurture your skin
with regards to sun exposure can alter its
phenotype.
Environmental influences can include
things like light, nutrition, temperature,
etc.
 For example, Siamese cats and
Himalayan rabbits have dark-colored
extremities. The paws, tails, ears, and
nose are colder in temperature
compared to the core of the body and
thus are darker.


If the hair in a dark region is shaved and
the extremities are kept at warmer
temperatures, the new hair that grows in
will be white. This is because there is an
enzyme that produces the dark pigment.
It is only active at cooler temperatures.

The visual image of the complete diploid
set of chromosomes grouped together in
pairs, and arranged in order of
decreasing size, is otherwise known as a
karyotype.
Notation is given as follows:
# of chromosomes, sex chromosomes,
issues
 The normal female is 46,XX
 The normal male is 46, XY

What is the notation for the karyotype to
the right?
 47, XX, +18
they have 47
chromosomes, two x chromosomes, and
the issue is they have an extra 18th
chromosome.


Scientists get the pictures of the
chromosomes during mitosis when the
chromosomes are clearly condensed
and easy to view. They will then cut
them out and put them in order.
Complete the worksheet. Due
Wednesday upon return from winter
break.
 Quiz answer key is by my computer if you
need to check out any answers.

Females are XX and males are XY.
 What is the ratio of the sex of the possible
offspring that a male and female could
have. Show in a punnett square.

X
X
X
Y

You have a 50/50 chance of having a
boy or a girl. Some families are very
prone to having certain sexes of children
but this is a genetic anomaly.
X
Y
X
XX
XY
X
XX
XY

While there are 44 chromosomes that do
not code for sex (also known as
autosomal chromosomes) there are two
that do code for the sex of the offspring
and we call these sex chromosomes
denoted by the letters X and/or Y.

More than 1200 genes are found on the
long X chromosome while only about 140
genes are located on the Y
chromosome. Most of these genes on
the Y chromosome have to deal with
male sex determination and sperm
development.
Example on the X chromosome are
Duchenne muscular dystrophy,
hemophilia A, and color blindness.
 These are called sex-linked traits

Simple dominance
 Co-dominance and multiple alleles
 Sex-linked traits

› Almost always are issues with the X chromosome
but there are some sex-linked traits connected
to the Y for males.


Red-green
colorblindess is a
recessive X-linked
trait.
To the right is the
differences in what
you would see if you
were red-green
colorblind.

Normal female with
no colorblindness
› XNXN

Carrier female for
colorblindness (sees
normally)
› XNXn

Colorblind female
› XnXn

Normal male
› XNY

Colorblind male
› XnY
Colorblindness: What are the possible
offspring that a normal male and a
carrier female could have?
 Answer:

XN
XN
Y
XNXN
XNY
Normal male
Normal female
Xn
XNXn
Carrier female
XnY
Colorblind male
Colorblindness: What are the possible
offspring that a colorblind male and a
carrier female could have?
 Answer:

XN
Xn
Y
XNXn
XNY
Normal male
Carrier female
Xn
XnXn
Colorblind female
XnY
Colorblind male
Colorblindness: Who does the son get
the colorblind allele from his mother or
his father? Why?
 Answer: The father passes on a Y to a son
whereas the son will get the X from the
mother so she is passing on the
colorblind allele.

Also called the “bleeders disease”
 Caused by a recessive x-linked allele.
 The blood of people with this disorder
does not clot.
 This condition was passed on throughout
the royal families of Europe due to the
common practice of marrying other
royals.


It began with the son of Queen Victoria
of England and affected Spanish and
Russian royalty as well. Basically, think of
it as an incest situation.
The muscles of people with this disorder
weaken and this leads to loss of
coordination.
 This is a recessive disorder.
 Which gender is most affected by xlinked disorders? Why?
 Males because they only have one x
chromosome

Males have one x chromosome while
females have 2 and sometimes it is just one
too many. The body will adjust to this extra
chromosome by randomly switching one X
chromosome off when it is not in need.
 These switched off X chromosomes will form
a dense region in the nucleus known as a
Barr Body. These are generally only found in
females since males need the one X
chromosome they have.



Calico cats are a
good example of
this. The gene that
controls the color of
the fur is located on
the X chromosome.
One X will code for
orange color while
the other codes for
black color.


Different cells may
make different colors
inactive and therefore
the females will have
spots of different colors
showing in different
parts of their bodies.
Males will have spots
of 1 color since they
don’t have two X’s
and they don’t switch
it off.
You can use a chart that shows the
relationships in a family to track the
pattern of inheritance of a certain trait.
This chart is called a pedigree.
 A pedigree allows for you to track
multiple generations of family to see if a
trait is either
 Sex-linked dominant or recessive OR
 Autosomal dominant or recessive


What does a circle
mean?

› Girl

What does a square
mean?
› Married

› Boy

What does a colored
in square or circle
mean?
› They show the trait
What does a horizontal line
connecting two people
mean?
What does an unshaded
square or circle mean?
› Doesn’t show trait

A vertical line off of a
married couple has a set of
circles and squares
attached to it. These are
what relationship to the
married couple?
› Children.



To check to see if a
pedigree is for a
dominant trait use
the following
information:
Trait shows if even
one dominant allele
is present
We will assign all
members of a family
genotypes.
Shows
AA or Aa

Doesn’t show
aa

Anyone shaded
must have at least
one dominant allele.
AA or Aa
 Anyone not shaded
must have no
dominant alleles
aa
 Lets fill in the aa first

aa
aa
aa
aa
aa
aa
aa
aa
aa
aa
A__
aa
aa
aa
aa
A__
aa
aa
aa
aa
aa
A__
aa
A__
aa
aa
aa
aa
A__
aa
aa
aa
aa
aa
A__
aa
Aa
aa
aa
aa
aa
A__
aa
aa
aa
aa
aa
A__
aa
Anyone shaded
must have only
recessive alleles.
aa
 Anyone not shaded
must have at least
one dominant allele
AA or Aa
 Lets fill in the aa first

aa
aa
aa
aa
A_
A_
A_
A__
aa
A_
A_
A_
A_
A_
aa
A_
aa
Aa
A_
Aa
A__
aa
A_
Aa
A_
A_
A_
aa
A_
aa
Aa
A_
Aa
Aa
aa
A_
Aa
Aa
aa
A_
A_
A_
aa
Aa
A_
Aa
Aa
aa
A_
Aa
Aa
aa
A_
A_
A_
Aa
aa
Aa
aa aa aa
Aa
Aa
aa
aa Aa aa Aa aa
aa aa
aa
aa Aa
Some pedigrees represent carriers
(heterozygous form) with ½ the circle or
square colored in or a dot
 See in cystic fibrosis example.


Most genetic disorders are caused by
recessive alleles.

Common amongst white Americans and
causes mucus build up in the lungs and
digestive tract.

Build-up of lipids in
the brain




No treatment you
can just help to
make the patient
comfortable.









Deafness
Decreased eye contact,
blindness
Decreased muscle tone (loss
of muscle strength)
Delayed mental and social
skills
Dementia
Increased startle reaction
Irritability
Listlessness
Loss of motor skills
Paralysis or loss of muscle
function
Seizures
Slow growth
Phenylalanine is one of the 20 amino
acids. People with PKU cannot break
down this amino acid so it accumulates
in the central nervous system (CNS).
 Babies are tested at birth because milk
contains lots of phenylalanine. If a baby
drinks milk and has PKU disorder, mental
retardation occurs.

What are the safest foods for an
individual with PKU to consume?
 Low protein breads, fruits, juices,
vegetables


A recessive disorder in which a person
has a lack of pigment (melanin)
produced in the eyes, skin and hair.
Albus means white in latin.

One type of dwarfism. Sometimes
considered a dominant disorder but
what does this pedigree show?

The pedigree shows it is recessive
because there is no way for a couple
who is not showing to pass on dominant
alleles to their offspring.
Having extra toes or finger is actually a
dominant trait. Therefore, dominant does
not mean more common in a population.
 Sometimes they are functional sometimes
they are not. Depends on their location.
 If P = poly and p = not poly then what is the
genotype of those who do not have
polydactyl digits?
 pp


Causes the breakdown of areas of the
brain and no treatment exists but
genetic screening is recommended for
families who have the dominant gene. It
is passed on to offspring because
affected individuals do not show
symptoms until about age 30 to 50.
Widows peak
 Free hanging ear lobes
 Hitchhikers thumb

A man by the name of T.H.
Morgan worked with fruit flies called
Drosophila melangaster.
 The first word is the genus and is always
capital and the second word is the
species and is always lowercase.
 Also, they must be underlined or typed in
italics.

The normal eye color for Drosophila is
red.
 Morgan noticed a mutation occurred to
produce a fly with white eyes.
 The trait for eye color in fruit flies is
located on the X chromosome and is
therefore called a sex-linked trait or an Xlinked trait.

When it comes to doing sex-linked
problems it is important to note the
gender of the individuals.
 For example, in flies red eyes are
dominant (R) and white are recessive (r).
 A homozygous red-eyed female would
be written as XRXR.
 White-eyed female XrXr.
 Red-eyed male XRY.
 White-eyed male XrY.

The reason why the trait is only written on
the X chromosome is because the Y
chromosome carries few traits.
 Sample: Cross a red-eyed heterozygous
female with a white-eyed male. Write
the genotype and phenotype ratios.

XR
Xr
XR Xr
Y
XR Y
Xr
XrXr
XrY
Phenotypic Ratio
R. fem. : W. fem : R male : W. male
1
: 1
: 1
:
1
Genotypic Ratio
XRXR : XRXr : XrXr : XRY : XrY
0 : 1 : 1 : 1 : 1
Is it possible for a male to be a carrier of
a sex-linked trait?
 No they only have one x chromosome so
nothing to hide it.
 Can a dad pass on an x-linked trait to his
biological son?
 No because the dad passes on the Y
while the mom passes on the X.
