What is Genetics?
Defined broadly, it’s the branch of biology that deals with heredity and variation.
What is Genetics?
A key question in genetics is what is the relationship between genotype
and phenotype?
The question is simple, the answer is complex.
Unit 3 Genetics
• Every living thing is made up of different traits
that make it unique.
– i.e. Labrador retrievers. This type of dog and its traits
occur because of selective breeding.
• Traits can be passed on from generation to
generation. This transmission of traits is called
heredity and the traits that are passed on are said
to be inherited
• Genetics is the branch of biology that deals with
the principle of variation and inheritance.
Gregor Mendel
•
•
•
•
•
Born 1822
Austrian monk
Attended the university of Vienna
Studied botany and mathematics
Conducted experiments on pea plants over a 8
year period from 1853-1861
• Mendel’s questions were not new – but his
approach to understanding heredity was.
• Mendel’s experimental innovations
included:
– choice of model organism
– study of discrete traits
– use of pure-breeding lines
– painstaking control of matings
– rigorous mathematical analysis of large
numbers of offspring
Mendelian Inheritance
The Garden Pea Pisum sativum
Why did he choose pea plants?
• Commonly available in Europe
• Were easy to grow and matured quickly
• Sexual organs of the plant were entirely
enclosed in the flower which allowed him
to control reproduction whether through
cross pollination or self pollination
• Different varieties had different traits that
could be observed from generation to
generation. Each trait had two variations
Mendel's Experiments
• How did he conduct his experiments?
• His first step was to obtain purebred plants for the trait
that he wanted to study
• A purebred organism is descended from ancestors of a
distinct type or breed. This trait results from previous
generations through long term selective breeding
• He chose plants that were tall and bred these together.
From these he obtained some plants that were tall and
some that were short.
• He then proceed to breed only the tall plants together
again, he did this until he got only tall offspring
• He did the same for the short plants as well as 6 other
traits
Mendel's First experiment
Monohybrid Cross
• Once he obtained purebred or true bred plants
he named them the parental generation or P
generation.
• He crossed pure tall with pure short the
offspring from this cross he designated as the
first filial generation or the F1
• The offspring from this cross were hybrids
indicating that they were the result of a cross
between two different purebreds
• This is called a monohybrid cross because it
involves only one trait; plant height
• All offspring from this cross were tall ( there were no short plants)
• What happened according to the theory of blended inheritance
he should get all medium plants, instead all plants were tall. This
lead mendel to conclude that the trait for tall must be dominant
and the trait for short must be recessive.
• A dominant trait is one that is always expressed .
• A recessive trait is a characteristic that is present but inactive. It
is only expressed if it is the only trait present
• He conducted the experiment many times with the same results
and therefore concluded that when plants with two contrasting
plants are crossed one is always dominant over the other. This
lead to his principle of dominance
Mendel Began with
Monohybrid
Crosses Between
Parents Who Bred
True for
Antagonistic Pairs of
Traits
White
Cross Pollination –
One of Mendel’s
Experimental
Methods
Remove stamens
from purple flower.
Parental
generation
Stamens
Transfer pollen from stamens
of white flower to the stigma of
a purple flower.
Purple
Cross-pollinated flower
forms seeds.
Plant seeds
from pod.
Firstgeneration
plants
One of Mendel’s Monohybrid Crosses
Regardless of the trait, the pattern
remained the same.
Mendel Observed Consistent Patterns of Inheritance
Second experiment
• He bred the F1 generation through self
pollination
• These produced the second filial
generation or the F2 generation
• When crossed this way the F2 offspring
75% of the time resembled the dominant
parent or P generation and 25% of the
time resembled the recessive parent. This
ratio of 3:1 is known as the mendelain
ratio
Why did Mendel get these results from the F2?
• He concluded that each parent in the F1
generation starts with 2 hereditary factors. One
is dominant and the other is recessive
• The factors separate in the parent, only one
factor from each parent is contributed to the
offspring.
• Each offspring inherits one factor from each
parent. If the dominant factor is present it will
be expressed even if the recessive factor is also
present.
• The recessive factor will be expressed if only
Law of segregation
• The results from Mendel F2 generation gave rise
to the law of segregation
• A pair of factors for a trait separate ( segregate )
during the formation of gametes(sperm and egg)
and then recombine during fertilization
• We know today that these factors that Mendel
referred to are genes, the part of the chromosome
that governs the expression of a particular trait.
• Gene has two forms called alleles.
• these factors are sometimes referred to as unit
characters.
• When using letters to designate traits or
unit characters a purebred tall plant will
have 2 uppercase letter i.e., TT this means
the plant is homozygous for tall.
• The pure breed short plant has 2 alleles
for short designated in lowercase letters
as tt.
• The product of the cross between a tall and
short plant would have the alleles T and t. It
would have a dominant and a recessive
trait is said to be heterozygous for tall.
Principle of Segregation
… and these gametes unite at random in fertilization to create a new individual.
Principle of Segregation
Principle of Segregation: Genes come in pairs that segregate (separate) in the formation
of gametes…
The Principle of
Segregation Coupled with
the Postulate of Dominant
and Recessive Alleles
Explains the Outcome of
Monohybrid Crosses
Test Cross
• Test Cross” a cross in which an individual of unknown
genotype is crossed with a homozygous recessive
individual.
• This is done to determine the genotype of the unknown
individual
• Two crosses are performed
• Cross a homozygous dominant individual with the
homozygous recessive individual
• Cross a heterozygous dominant individual with a
homozygous recessive individual.
• Examine the results to tell the genotype of the unknown
individual
• Test cross A - assume the unknown genotype is
homozygous dominant
– We will use letters TT for homozygous
dominant and tt for homozygous recessive
• Test cross B- assume unknown genotype is
heterozygous dominant
– We will use letters Tt for heterozygous dominant and tt
for homozygous recessive
A Test Cross Reveals the Genotype of a Dominant Individual
Test crosses are the most important form of progeny testing.
Mendel used test crosses to buttress his proposal of segregation.
More on Mendel
•
•
•
•
•
Punnett Squares
Probability
Product Rule
Test Cross
Dihybrid and Trihybrid
Crosses
• Incomplete
Dominance—Neither
gene is completely
dominant over the
other. Snapdragons.
• Codominance—Both
alleles are expressed
simultaneously.
• Human ABO blood type
Single Factor/ Monohybrid Cross
• Single factor cross: A cross that is done for one trait.
• Example: Mary is homozygous dominant for straight hair(SS).
Steve is heterozygous for straight hair (Ss) what are the
possible ratios of offspring for these two individuals
• Solution:
• Ratios:
• Genotypic: 1:1 -- 50% homozygous straight-50% heterozygous
straight
• Phenotypic :100% straight hair
• Monohybrid cross is where the parents are
heterozygous or hybrid for the trait in
question
• Ex. Crossing two plants that are
heterozygous for tallness (Tt)
Results of monohybrid cross
• Phenotypic ratio: 3:1
• Tall : short
• Genotypic Ratio: 1:2:1 , 1 homozygous
dominant, 2 heterozygous dominant: 1
homozygous recessive.
• NOTE: In a monohybrid cross the ratios are
always as above
Product Rule
• A rule that uses the principles of probability to
determine the possible outcomes of a genetic cross.
• Product Rule: The probability or chance that two or
more independent events will occur together is the
product of their individual probabilities of occurring
alone
• Example: tossing coins, toss a single coin: chances of
getting heads = ½ chances of getting tails= ½
• toss two coins together
• Chance of getting two heads= ½ x ½ = ¼
• Chance of getting two tails= ½ x ½ = ¼
• Chance of getting heads and tails; ½ x ½ + ½ x ½ = ½
• The same
• Monohybrid Cross Example #1:
• In a particular type of fish, a "G" allele produces green
scales, which is dominant to the "g" allele which
produces blue scales. What genotype and phenotype
ratios do we get if we cross two Gg fish?
• To solve this problem,
we must first determine the possible gametes each fish
can produce. Recall that during meiosis the
chromosome number is reduced by half. Each gamete
will have only HALF the number of chromosomes as
the individual.
• Two green fish, each with a genotype of Gg, can
produce gametes that are either G or g.
Then we set up a Punnett square like this
sperm
G
g
G
GG
Gg
g
Gg
gg
eggs
Then we combine the gametes to fill in the rest of the chart:
Results:
genotype ratio: 1 GG : 2 Gg : 1 gg
phenotype ratio: 3 green fish : 1 blue fish
Be sure to write the genotype and phenotype ratios like this!
Genotype ratios must have numbers, letters (for the alleles), and colons between
them to show it is a ratio. (It is also OK to use percentages for the numbers: 25% GG
: 50% Gg : 25% gg)
Phenotype ratios must have numbers, words (that describe the trait), and colons
between them to show it is a ratio. (It is also OK to use percentages for the
numbers: 75% green fish : 25% blue fish)
Let's try another example
• Let's cross a male heterozygous green fish with
a female homozygous recessive blue fish. What
genotype and phenotype ratios do we get if we
cross these two fish?
• To solve this problem, we must first determine
the possible gametes each fish can produce. A
male heterozygous green fish has a genotype of
Gg, and can produce gametes that are either G
or g. A female homozygous recessive blue fish
has a genotype of gg, and can only produce eggs
that are g. Then we set up a Punnett square:
Mendel’s Results and
Conclusions
• Mendel concluded that:
• 1. Each trait is controlled by 2 genes or alleles.
• 2. Some alleles are dominant, while other alleles are
recessive.
• Law of Segregation—During meiosis, each allele
separates in an orderly manner from its partner, the
two always go to different gametes.
• 4. Law of Independent Assortment—During
Metaphase and Anaphase I of meiosis, the alleles for
different traits segregate independently. This mixes
chromosomes from your father and mother
• www.siskiyous.edu/class/bio1/genetics/mono
hybrid_v2.html
Mendels Crosses
• www.sumanasinc.com/webcontent/animation
s/content/mendel/mendel.html
Practice
• Recessive male (wrinkled) and Heterozygous
Female (round)
• P. 533 #’s 1,2, & 3
• P.531 #’s 1 &2
• Assignment 1
Does the Segregation of Alleles of One Gene Occur
Independently of the Segregation of Alleles of a
Second Gene?
A dihybrid cross
This was Mendel’s next question, and he addressed it using dihybrid crosses.
Dihybrid Cross
• Mendel wanted to know if the inheritance of one trait
influenced the inheritance of another trait
• He approached this problem in the same manner as he
approached the others
• First he produced plants that were purebred for the traits
he wanted to examine. He selectively bred plants until the
offspring were round and yellow. These plants were
homozygous dominant for both seed shape and colour
• He breed them with plants that were homozygous
recessive for seed colour and shape.(wrinkled and green)
• He performed a dihybrid cross. All offspring were
heterozygous for both traits. RrYy making them round and
yellow.
• He allowed the F1`s of his dihybrids to self
pollinate. Of the 551 plants in the F2
– 320 round yellow
– 104 round green
– 101 wrinkled yellow
– 26 wrinkled green
• these results show a phenotypic ratio of
9:3:3:1.
Alternative Hypotheses About the Inheritance of Two Traits
A Dihybrid Cross
Between Smooth,
Yellow and Wrinkled,
Green Strains
The result
The Dihybrid Cross and Its
Explanation
Regardless of the particular
traits, the pattern remained
the same.
These results led Mendel to
postulate the Principle of
Independent Assortment.
Example
• In pea plants tall (T) is dominant over short
plants (t). Green pods (G) is dominant over
yellow pods(g). Mendel crossed a plant
heterozygous for both tall and green with a
plant that was heterozygous for both tall and
green.
• What results did he obtain?
Mendel's Hypothesis
• He realized that the ratio 9:3:3:1 could be
explained if the alleles from one trait were
inherited independently of the alleles for
another trait. This he called the Law of
Independent assortment.
• According to this law different pairs of alleles
are passed to the offspring independently of
each other
The Principle of Independent Assortment
Segregation of pairs of alleles of one gene occurs independently of
segregation of pairs of alleles of another gene.
The Principle of Independent Assortment
• http://www.dnaftb.org/5/problem.html
Instructions: How to complete a Dihybrid cross
1.Determine what letters you will use for each trait.
Capital letters are used for dominant alleles, or traits, lower case letters are used
for recessive alleles.
If green is dominant, the green allele would be G, the yellow allele would be g. If
rough pods are dominant, the rough allele would be R, the smooth allele would
be r.
2. Heterozygous crosses have one dominant and one recessive allele for each trait.
The genotype for each parent would be GgRr. Each plant has two alleles
because it receives one copy from its parents (one from mom and one from
dad).
3. Draw a square. Divide the square into 16 boxes. Each square gets one parent at
the top (two letter in each box) and one parent on the left side (again two
letter in each box). Each possibility is given a square.
4. Multiply the letters together and place four letters in each box, capital letters
first. The offspring would have a genotype of one GGRR, two GGRr, one GGrr,
two GgRR, four GgRr, two Ggrr, one ggRR, two ggRr, and one ggrr.
• In plants round seeds are dominant to
wrinkled seeds and green pods are
dominant to yellow pods. A plant
heterozygous for round and yellow is
crossed with a plant that is heterozygous
for both traits.
• Step one
– Assign letter
– R round, r wrinkled, G green and, g yellow
• Step 2
– P generation
– RrGg x RrGg
• Step 3
– Assortment of gametes
– RG, Rg, rG, rg for both parents
• Step 4
– Draw 16 punnett square
Eggs
RG
RG
Rg
sperm
rG
rg
Rg
rG
rg
• Step 5
– Complete table as if multiplying
• Step 6
– Calculate the genotypic and phenotypic ratios
of the F1 offspring
– Once table is complete should reveal for 2
heterozygous parents a ratio of 9:3:3:1 for the
phenotype
Textbook practise: p. 540 #1 and p. 544 #’s1 and 2
Assignment 2
Beyond Mendel’s laws
• some organisms show different patterns of
inheritance where the dominant or recessive
allele are not expressed as expected.
• These include
• Incomplete dominance
• Co-dominance
• Multiple alleles
Incomplete Dominance
• This occurs when neither of the two alleles expressed are dominant
for a trait
• Examples include four o’clock flowers and Snap dragon flowers
• Some examples would include
– Snap dragon flowers RR red, RR’ would be pink, and
R’R’ would be white.
– The heterozygous condition presents with a colour
that is in between the dominant and recessive colors
– To show the red colour you would need both RR to be
present. Those with just one R are unable to make
enough red pigment to produce red flowers and
therefore they appear pink. Those with the R’R’ have
no red pigment and are therefore white.
Snap Dragon Flowers
• The gene for white petal is incompletely
dominant while the gene for red petal is also
incompletely dominant. When you cross a white
flower with a red flower you get all pink flowers.
How is this possible?
• Lets make the allele for red petal color (R). Lets
make the allele for white petal colour (R’).
• Because both of these genes are incompletely
dominant, they both have a capital letter. We
use R’ because both are dominant, but not one
over the other.
• Cross a white snap dragon with a red snapdragon.
What are the results of such a cross.
• This cross is setup exactly like the monohybrid cross we
have already studied.
–
–
–
–
–
Obtain genotypes of parents
Separate the alleles
Place them in correct places in punnett square
Complete the cross
Calculate the ratios
Red snap dragon RR
Pink snap Dragon RR’
White snap dragon R’R’
sperm
egg
R’
R’
R
RR’
RR’
R
RR’
RR’
Here are the results:
Genotypic ratio= 4:0 for heterozygous Pink flowers (RR’)
Phenotypic ratio = 4:0 all pink flowers
What would be the results if we crossed two
pink snapdragon flowers?
• Pink snap dragons (RR’)
sperm
egg
R
R’
R
RR
RR’
R’
RR’
R’R’
Genotypic ratio= 1:2:1 for homozygous red; heterozygous pink; homozygous white
Phenotypic ratio = 1:2;1 for red: white: pink
• In Japanese four o’clock plants incomplete
dominance occurs. Red is incompletely
dominant to white. Red and white crossed will
produce pink.
• Cross a red with a pink. Show the punnett
square and the genotypic and phenotypic
ratios
Co-Dominance
• Situation where both alleles for a trait may be
dominant. The alleles are said to be codominant. Both alleles are expressed in the
heterozygous individual
• example of co-dominance feather colour in
chickens, cattle
• What happens when a black Rooster is
crossed with a white hen?
• Black –B
• White – W
• Black BB and white WW
B
B
W
BW
BW
W
BW
BW
• All offspring will be speckled black and
white
• If the colours blended you would expect the
offspring to have grey plumage. If black was
dominant, then they young would be black
• However you ended with offspring that are
checkered black and white
Multiple Alleles
•
•
•
•
•
•
•
Many genes have more than two alleles
One of these genes is human blood types
In this case 3 allele are involved A, B, O
Each person has two of the three alleles
These are expressed as IA, IB ,i
IA and IB are dominant over i.
IA and IB are co-dominant and expressed
equally
• Multiple allele inheritance is often
influenced by environmental factors, such
as diet and climate
Phenotype (Blood Type)
Genotypes
A
IAIA or IA i
B
IBIB or I Bi
AB
I AI B
O
ii
Blood Types
• AB is the universal recipient (it can accept all
blood types)
• O is the universal Donor ( can donate to all
blood types because it is recessive)
Practise problems
• If a woman has blood type AB and a man has
blood type A what are the possible blood
types of their offspring
• Find the genotype of each parent first
• Mother is IA IB the father is IA I A or I A i
• Complete each of the crosses as if you were
completing a monohybrid cross
• Pg. 542 # 1 and 544 #5
Mother
Father
IA
IB
IA
I AI A
I AI B
i
IA i
IB i
Mother
Father
IA
IB
IA
I AI A
I AI B
IA
I AI A
I AI B
Chromosome Theory of Inheritance
• Proposed by: Walter Sutton and Thoeodor Boveri
(1902)
• They studied chromosomes during the various
phases of meiosis.
• They made 3 key observations
– Chromosomes occur in pairs
– Chromosomes pairs separate (segregate)
during anaphase 1. This backs up Mendel's
claims of the law of segregation
– Chromosomes align themselves independently
along the equator of a cell. (this backs up
Mendel's ideas of independent assortment
Chromosome Theory
• Mendel's factors or genes are carried on
chromosomes
• The segregation and independent
assortment of chromosomes during meiosis
is what accounts for inheritance.
• Note: the work of Sutton and Boveri
confirmed Mendel's ideas about
genes(Factors) and how they are
inherited(segregation and independent
assortment)
Thomas Morgan
• Was an American who studied fruit flies in 1910.
• He performed crosses involving eye colour
• Crossed 2 red eyed parents and produced a white
eyed male
• He crossed the red eyed female from the white
eyed male, with a normal red eyed male, all
females had red eyes, while in the males half
were red and half were white
• Found that chromosomes in Fruit flies are the
same except for one pair
• He called the dissimilar pair sex chromosomes
because he believed they determined the sex of
the fly.
• Found that certain traits such as eye colour
in fruit flies are found on the X gene. This is
what he called “sex linkage’. Today we call
this Sex Linked
• Sex linked trait; A trait that is carried on
one of the sex chromosomes (X or Y)
Linked Genes
• Morgan also found certain genes do not follow
the law of independent assortment, but
instead tend to be inherited together. He
reasoned that genes located on the same
chromosome would likely not segregate.
These is called Linked genes.
• He did find that some of these genes do
sometimes segregate. How is this possible?
Gene Chromosome Theory
• Genes exit at specific sites arranged in a
linear fashion along chromosomes.
• Morgan found that the closer the genes are
located together on the chromosome they
will almost always be inherited together,
while genes located further apart are more
likely to be separated by a crossing over
event.
• This would account for some differences in
ratios of crosses. Instead of getting 9:3:3:1
expected ratio in a Dihybrid cross, the ratio
would be different.
Law of Independent Assortment in
Modern Terms
• Mendel’s law of independent assortment
based on Morgan's work should be restated
as:
– If crossing over does not take place, genes that
are located on different chromosomes will assort
independently, while genes that are located on
the same chromosome will be inherited together.
Sex- Linked Traits
• Some traits are passed from one generation to the next depend on
the sex of the parent carrying the trait. This is because it is located
on the sex chromosomes
• Sex Linked traits are traits that are carried on the sex
chromosomes(X,Y).
• Most often, the traits are carried on the X chromosome and
referred to as X-Linked
• Some are carried on the Y and are called Y –linked. These are rare
do to the small size of the Y chromosome
• Most of these traits are recessive and often lethal (deadly)
• Sex linked traits affect males more often than females
• Examples of sex linked traits
– Red-green colour blindness
– Male pattern baldness
– Hemophilia
Sex linked traits
• Things to keep in mind about sex-linked traits
• Sex-linked traits are recessive(small letters)
• Sex-linked traits are carried on the X chromosome
normally
• If a person has a big allele (N) and a small allele
(n) they are a carrier of the trait
• When using punnett squares to predict outcomes
involving genes that are sex-linked assume the
trait is located on the X chromosome unless
stated otherwise.
• When you write the genotypes and phenotypes
you have to include the sex within the ratios.
• Sex-linked traits are traits that are carried
on the sex chromosomes, either X or Y.
• Hemophilia is a recessive, sex-linked
disorder on the X-chromosome. Therefore,
a male with a recessive allele on the Xchromosome will always express it
phenotypically.
• In order for a female to express a recessive
trait, it must be in a homozygous state.
Morgan and His White-Eye Mutant
Wild type and white-eye mutant Drosophila
T. H. Morgan
Morgan Knew Male and Female Drosophila
Had Distinct Karyotypes
(Note: sex in Drosophila is determined by
the ratio of X chromosomes to autosomes;
the Y chromosome is dispensable.)
Morgan’s Curious Results
Morgan’s Curious Results
(female is
from the F1
generation
of original
cross)
Morgan’s Explanation – the Eye
Coloration Gene is Physically
Linked to the X Chromosome
P
F1
F2
Morgan’s Explanation – the Eye
Coloration Gene is Physically
Linked to the X Chromosome
There are Many Sex
Determination Systems
44 +
XY
44 +
XX
The X–Y system in mammals
6+
X
6+
XX
The X–O system in certain insects
16 +
ZZ
16 +
ZW
The Z–W system in birds
16
haploid
32
diploid
The haplo-diploid system in bees
Examples
• Mate a carrier female (a female with the
mutant gene, but who is unafflicted
because she has the normal gene on her
other X-chromosome) with a normal male.
• Step 1: Define the symbols:
– XH - normal
– X h - hemophilia
– Y - does not have the allele
• Step 2: Determine the genotypes of the parents:
– X H X h x X HY
• Step 3: Draw your Punnett square.
• F1 Generation
• Genotype Phenotype
• X HXH
Normal female
• XH X h
Carrier female
• XH Y
Normal male
• X hY
Hemophiliac male
• Due to the male total dependence on their X chromosome,
males have a much greater chance of getting hemophilia than
females.
• For a female to get hemophilia, she has to have either a
hemophiliac father, carrier mother or bad luck, or hemophiliac
mother and hemophiliac father.
• Males and females produce the same
amounts of proteins coded by genes
located on the X chromosomes
• Females have 2 copies of the X
chromosomes and males have one.
• This does not mean that females have more
in fact one of the X chromosomes is
inactive. This inactive X chromosome is
called a Barr body. Which one is inactive is
random.
• What is the probability that two parents
with normal color vision will have color
blind sons and daughters if the mother’s
father is color blind?
• A recessive sex-linked gene (h) located on
the X chromosome increases the blood
clotting. This causes the disease
hemophilia.
– Explain how a hemophiliac can be born to two
normal parents?
– Can any of the female offspring develop
hemophilia?
Textbook p.544 # 8 Assignment # 4
Polygenic inheritance
• This is the idea that traits are affected by more
than one gene
• Results: A range of phenotypes ie. not just short
and tall, but some in the middle etc.) This is
known as continuous variation
• Continuous Variation: Variation among
individuals in a population where there is a
gradient of phenotypes for one trait.
• Ear length in corn
– Shortest (homozygous recessive for both alleles)
– short, medium, long, ( these 3 are the least popular because
the are obtained by a combination of alleles
– longest ( both alleles are homozygous dominant).
– Medium is the most popular because it can be obtained
through a different combination of alleles
• Note: either of these phenotypes may occur
• This type of inheritance is used by plant and animal breeders
to improve livestock and crops, as well as human
characteristics such as susceptibility to cardiovascular disease
and athletic ability
• In humans height and skin color are examples of polygenic
inheritance
• Polygenic inheritance is the same as multiple gene inheritance
Crosses involving autosomal traits and Sex-linked traits
• Many traits are inherited together some as a result of
gender, others not related to gender.
• Example
• In humans, normal color vision (R) is dominant to color
blindness (r). This is a sex-linked trait (X-linked). Brown
eye color (B) is dominant to blue (b) eye color. If a color
blind, heterozygous brown-eyed female mated with a
normal, blue-eyed male, what percentage of the
offspring would be color blind, blue eyed males?
Results
Parents:
Female (BbXrXr)
• Male (bbXrY)
bXR
bXR
bY
bY
BXr
BbXRXr
BbXRXr
BbXrY
BbXrY
BXr
BbXRXr
BbXRXr
BbXrY
BbXrY
bXr
bbXRXr
bbXRXr
bbXrY
bbXrY
bXr
bbXRXr
bbXRXr
bbXrY
bbXrY
• Genotypic Ratio 1:1:1:1
BbXRXr
bbXRXr
bbXrY
BbXrY
• Phenotypic ratio 1:1:1:1
• Brown and normal; female 1
• Blue and normal female 1
• Blue and colorblind male 1
• Brown and colorblind male 1
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Fruit Flies
Traits
Wing shape
Eye colour
Dominant
Normal (N)
Red (XR)
Recessive
vestigial (n)
white (Xr)
• Cross a red eyed male hybrid for normal
wings with a white-eyed female with
heterozygous for normal wings
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