Download Classical Genetics - Morinville Community High School

Biology 30
Morinville Community
High School
Unit 5b: Classical Genetics
Name: ______________
Classical Genetics Unit Outline
Chapter 1 7 tex t p. 5 84 -6 21
Key Concept A: Theory of Inheritance: Gregor Mendel’s
Pea Experiments & Genetics Vocabulary
A1. Dominant and Recessive Genes
Key Concept B: Monohybrid Crosses
B1. Monohybrid Crosses & Punnett Squares &
Mendel’s 1st law: Law of Segregation
B2. Test Crosses
B3. Multiple Alleles
B4. Incomplete and Co-Dominance
B5. Sex-Linked Traits
Key Concept C: Dihybrid Crosses
C1. Dihybrid crosses and Mendel’s 2nd law:
Law of Independent Assortment
Key Concept D: Beyond Mendel
D1. Chromosome Mapping
D2. Polygenic Inheritance
Key Concept E: Human Genetic Analysis
E1. Constructing a Pedigree
E2. Using a pedigree to analyze problems
E3. Modes of Inheritance
E4. Genetic Counseling
Key Concept A: Theory of Inheritance
A1. Dominant and Recessive Genes
Gene: a heritable factor that controls a specific
characteristic.
 'Heritable' means passed on from parent to offspring
 'Characteristic' refers to genetic traits such as your hair
color of blood type.
Allele: one specific form or version of a gene
The alleles are represented by letters.
Locus: the particular location of a gene on homologous
chromosomes
Homozygous: two identical alleles for the gene
Heterozygous: two different alleles for the gene
Phenotype: the physical form of the trait of an organism
Genotype: the symbolic representation of a pair of alleles of
a trait of an organism, typically represented by two letters
Dominant allele: an allele that has the same effect on the
phenotype whether it is paired with same allele or a different
one. Dominant alleles are always expressed in the
phenotype.
Recessive allele: an allele that has an effect on the
phenotype only when present in the homozygous state.
2
Progeny: offspring
Example: Flowers of pea plants
Filial (F): First and subsequent generations of
offspring (F1, F2, F3 etc)
Autosomal: Non-sex chromosomes (in humans
these are chromosomes 1-22)
True-Breeding: Both gene copies are the same allele
(homozygous).
3
Human Traits
Introduction:
Many of you are already familiar with many of the genetic traits
of humans such as baldness, eye color, color blindness and
blood types. However, other characters such as size and
position of eyes, number and shape of fingers, total body size
and body proportion may also be genetically determined
(although such characters as body size may be profoundly
influenced by environment). The tremendous number of genetic
traits makes humans extremely variable. With the exception of
identical twins, it is highly improbable that any two persons will
have the same (or even similar) combinations of genetically
determined traits.
In this exercise, you will inventory yourself for the series of
genetic traits listed in the table, some of which are described
below. Most of these are known to be single-gene traits,
expressions of two alleles at one gene locus.
Instructions: Get with a partner and determine your phenotype
for each of the traits. Identify as much of your genotype as you
accurately can. Record both phenotype and genotype in the
table. At the end of the exercise, we will tabulate class results.
3. Blue eyes (B, b) Are your eyes blue or some other color? Blueeyed persons are homozygous recessive and lack pigment in the iris of
the eye; heterozygous or homozygous dominant individuals have iris
pigment; the actual pigmentation is a result of the interaction of several
genes (polygenic trait).
Brown eyes are the result of a brown pigment layer in the front of
the iris. Blue eyes result when a blue layer in the back of the iris can
show through in the absence of brown. Hazel or green eye color is the
result of an unrelated gene that produces a yellow pigment. Hazel eyes
have brown iris pigment while green eyes have a blue iris.
My Phenotype: ________________ My Genotype: __________
4. Tongue rolling (R, r) Persons with a dominant allele in
heterozygous or homozygous condition can roll their tongues into a tubelike shape; homozygous recessives are non- rollers and can never learn
to roll their tongues. The ability to curl the tongue is probably the result of
several genes(polygenic trait), though in genetics labs is usually treated
as a one-gene trait.
My Phenotype: ________________ My Genotype: __________
5. Widow's Peak ( W, w) Dominant allele in heterozygous or
homozygous individuals results in a V-shaped front hairline; homozygous
recessives have straight hairlines.
My Phenotype: ________________ My Genotype: __________
Trait (alleles) Expression
1. Bent pinky (P, p) Dominant allele causes the distal segment
of the fifth finger to bend distinctly inward toward the fourth (ring)
finger. Lay both hands flat on your desk and relax the muscles.
Does your fingertip bend inward?
My Phenotype: ________________ My Genotype: __________
2. PTC tasters (T, t) Phenylthiocarbamide (PTC) tastes
extremely bitter to heterozygous or homozygous dominant
individuals, but is tasteless to homozygous recessives. Put a
small piece of paper that has been impregnated with PTC on the
tip of your tongue.
My Phenotype: ________________ My Genotype: __________
6. Interlock fingers/ Thumb crossing (C, c) The way that you
interlock your fingers is genetic. Clasp your hands together quickly,
interlocking the fingers. Which thumb is on top? Most people will
interlock their fingers so that the left thumb is on top of the right, the
dominant allele (present in either heterozygous or homozygous
individuals). Those who are homozygous recessive place their right
thumb over the left. (Try interlocking your fingers the opposite way from
the way you naturally do this to compare.)
My Phenotype: ________________ My Genotype: __________
4
7. Arm Crossing (A,a) Most people will cross their right arm
over the left., the dominant allele (present in either heterozygous
or homozygous individuals). Those who are homozygous
recessive place their left arm over the right. (Try crossing your
arms the opposite way from the way you naturally do this to
compare.)
My Phenotype: ________________ My Genotype: __________
8. Ear lobes ( E, e) Ear lobes may be either adherent or free
and pendulous. Homozygous recessives have attached ear
lobes ; heterozygous or homozygous dominant individuals have
detached (free) ear lobes.
My Phenotype: ________________ My Genotype: __________
9. Hitchhiker's thumb (H , h) Homozygous recessives can
bend the distal joint of the thumb backward to a nearly 90º
angle; heterozygous or homozygous dominant condition yields
thumbs that cannot bend backward more than approximately
30º. The ability to bend the thumb backward is a caused by a
dominant allele. The proper term for this is distal
hyperextensibility. People with dominant alleles have more
flexible ligaments and thus looser joints.
My Phenotype: ________________ My Genotype: __________
10. Freckles (F, f)
The presence of freckles is dominant over the absence of
freckles.
My Phenotype: ________________ My Genotype: __________
11. Shorter Big Toe (B,b)
The dominant allele for toe length results in a big toe shorter
than the second toe. If your big toe is longer than your second
toe you have recessive alleles for this trait.
My Phenotype: ________________ My Genotype: __________
12. Short Palmar Muscle (M,m)
Examining the tendons that run over the inside of the wrist can tell you if
you have the recessive
long palmar muscle or the dominant, shorter palmar muscle. Those who
are dominant have two tendons. Those recessive have three tendons.
To determine this, you must clench your fist and flex your hand. Look and
feel for three tendons. A middle tendon indicates the presence of the
recessive long palmar muscle.
My Phenotype: ________________ My Genotype: __________
Questions:
1. Which traits, the dominant or the recessive, are you able to
determine the genotype for? Why?
2. Which traits, the dominant or recessive, are you not able to
determine the genotype for? Why?
3. In a sample population (i.e. the students in this class), does the
dominant trait always represent the highest frequency and the
recessive trait the lowest frequency? Support your answer with
data from the class.
5
Key Concept B: Monohybrid Crosses
Examples
st
B1. Monohybrid Crosses & Mendel’s 1 Law of
Segregation
• In his first set of experiments, Mendel investigated the
pattern of inheritance of one set of traits. His one-trait
experiments have been categorized as monohybrid
crosses.
• The observed results of Mendel’s monohybrid crosses with
pea plants resulted in Mendel’s First Law: the Law of
Segregation
Mendel’s Law of Segregation:
Every individual has 2 copies of each gene (1 on each
homologous chromosome)
When any individual produces gametes, the copies of the gene
separate (segregate) so that each gamete only receives one
copy.
If an individual has 2 different alleles, the gamete will receive
one or the other.
Tips: When completing any type of classical genetics cross, use the
following problem solving steps:
* Create a key or legend for your traits (what letters you’re
going to use, what is dominant, recessive etc)
* List the genotype of the parents & show the cross
* Draw a punnet square
* Check your work
1. In peas, green pod color is dominant to yellow pod color. Show a cross between truebreeding (homozygous) green and true-breeding yellow plants. This first cross is your
parental (P1) generation. Carry out the cross through the F1 and F2 generations. Show
phenotypes, genotypes, and the phenotypic and genotypic ratios for all generations.
2. In poultry, rose comb is dominant to single comb.
a. Cross a true-breeding rose-combed rooster with a single-combed hen.
Then cross the F1's to produce an F2 generation. Show all genotypic and
phenotypic ratios.
b. Explain how it is possible for a rose-combed rooster and a rose-combed hen
to produce single-combed offspring. What percentage of the offspring are
expected to have single combs in this cross?
3. Consider blue eyes in humans to be recessive to brown eyes. Show the expected
children of a blue-eyed woman with a brown eyed man whose mother had blue eyes.
Show the predicted genotypic and phenotypic ratios of their children. State what you
know (and don't know) about the brown-eyed man's father based on the information given
here. What can you predict about the brown eyed man's brothers and sisters?
6
4. In domestic swine there is a dominant allele which produces a white belt
around the body. The recessive allele produces uniform body color. One
farmer wants to produce only belted hogs, and another wants only uniformly
colored animals. Which farmer would have an easier time producing a truebreeding herd? Explain why. Tell how each farmer would proceed. (Hint: how
could you tell if a herd were true-breeding?)
7. In Holstein cattle, spotting of the coat is caused by a recessive allele and solid coat
color by a dominant allele. What types of offspring can occur in a cross between two
spotted animals?
8. A woman has a rare eyelid abnormality called ptosis, which makes it impossible for
her to open her eyes all the way. The condition is caused by a single dominant allele. The
woman's father had ptosis but her mother was normal. Her father's mother also had
normal eyelids.
5. In cattle, the polled (hornless) condition is caused by a dominant allele,
while the recessive allele causes horns to grow. A polled cow and a polled bull
produce a calf which grows horns as it matures. Show the genotypes of all
three. What is the probability that the pair's next calf will also grow horns.
a. What are the genotypes of each of the people mentioned above?
b. What proportion of her children would be expected to have ptosis if
she marries a man with normal eyelids?
6. Albinism in corn is caused by a recessive allele, and is normally lethal
because the plant cannot manufacture food without chlorophyll, so dies after it
has exhausted the food in the seed. It is possible experimentally to keep albino
plants alive with special feeding techniques in which sugar is supplied to the
plant through the leaves. Show the expected offspring from a cross between an
albino plant and a normal plant that is heterozygous for albinism.
9. In garden of pea plants, a purple-flowered true breeding plant was cross-pollinated with
a white-flowered plant (purple is dominant to white).
a) What are the phenotypes and genotypes of the F1 progeny?
b) If the monohybrid flowers are crossed, what are the phenotypes and genotypes of
the F2 progeny?
7
12. In pigeons, the checkered pattern is dominant to plain pattern.
10. Black fur on guinea pigs is dominant over white fur. A black, pure
breeding guinea pig was crossed with a white guinea pig.
a. Using a punnett square, show the cross between two heterozygous parents.
a. Determine the phenotypic and genotypic percentages when the F1
individuals are crossed.
b. If the F1 generation was crossed, what are the phenotypic and
genotypic ratios of the F2 progeny?
11. Tay-Sachs is an autosomal recessive genetic disorder. Carriers of TaySachs are always heterozygotes and do not show any symptoms, however are
capable of passing on the recessive allele to their offspring. Tay-Sachs carriers
are found most frequently among the Jews of East European origin (Ashkenazi
Jews) with a rate 100 times higher than the general population. Tay-Sachs is a
fatal condition that causes deposition of unmetabolized intermediates in the
nervous system. Affected infants are normal at birth, but by the age of 6
months they become listless and weak and show difficulty in feeding. Sudden
noises provoke an exaggerated startle reaction. Around 12 months of age there
is a rapid decrease in mental and motor functions as well as on onset of
blindness, deafness, rigidity and brain enlargement. Death usually occurs
within the first 3 years of age from pneumonia or a lung infection.
b. A checkered pattern female pigeon mates several times with the same male
pigeon. It was noted that the female had four checkered pattern chicks and four
plain pattern chicks. What is the phenotype of the male?
13. Huntington Disease (HD) is inherited as an autosomal dominant trait. HD is
characterized by rapid, uncontrollable muscle movements such as tics or muscle jerks.
This disorder causes a loss of coordination and personality changes. As the disease
progresses, the ability to speak is impaired, memory may fade, and involuntary jerky
muscle movements become more severe. The entire nervous system progressively
degenerates, ultimately leading to death 10 or more years later. HD is located on
chromosome 4 and most of the symptoms do not show up in the early reproductive years,
therefore the gene for HD may be transmitted unknowingly to the affected individual’s
children. Scientists have identified a ‘chromosome marker’ which makes genetic testing
possible. After genetic testing, Charlie is informed that he is affected with Huntington
Disease and his wife, Lorna, is unaffected. They wish to have children. Charlie’s father
has HD and is currently bedridden. His mother is unaffected with HD. What is the
probability of Charlie and Lorna having a child who is unaffected with HD?
a. If two carriers of Tay-Sachs decided to have children, calculate the
probability of having a child with Tay-Sachs.
b. What is the probability that their adult children would not be carriers?
8
14. Neurofibromatosis-1 (NF-1) is an inherited disorder, characterized by
tumors involving nerve tissue. Problems with sensation and movement result.
NF-1 is an autosomal dominant disorder in that the allele for those affected
with the disorder, F, is dominant to the allele, f, for those unaffected with the
disorder. If a woman affected with NF-1, who carries one dominant allele and
one recessive allele, and a man unaffected with NF-1 decide to have a child,
what is the percentage probability that their child will be affected with this
disorder? Support your answer with a Punnett square.
B2. Test Cross
Test Cross: testing an unknown plant or animal by crossing it with a
known homozygous recessive (ex. aa)
Examples
1)
In mice, a condition called waltzer is recessive. A waltzer mouse has a defect in the
region of the inner ear that interferes with its balance. Consequently, waltzers run in
circles. A mouse that runs normally might be homozygous dominant for this gene,
or it might be heterozygous. If geneticists had several mice that walk normally, and
wanted to know if any of these mice were heterozygous for the waltzer mutation,
they would to a test cross. Draw two Punnett squares to show probabilities of the
test cross that would help the geneticists determine the genotype of their normal
mice.
2)
Black wool is very brittle and is difficult to dye. A rancher can avoid getting black
sheep in his flock by only breeding homozygous dominant white sheep (WW). The
rancher first has to determine if his white sheep are actually homozygous. Show
how he could determine the unknown genotype of his white sheep by using a
testcross with a black sheep (black sheep are homozygous recessive ww).
15. In rabbits, the allele for long hair, L, is dominant to the allele for short hair,
l. A cross between a short-haired female and a long-haired male produced a
litter of seven long-haired bunnies and one short-haired bunny.
a. What are the genotypes of the parent rabbits? Draw a punnett square to
illustrate this cross.
b. What phenotypic ratio was expected in the offspring generation?
c. Considering the number of offspring, explain why a typical ratio was
not produced in this instance.
16. In fruit flies, long wing, L, is dominant to dumpy wing, l. Two long
winged flies mate and produce 49 dumpy-winged and 148 long-winged
offspring. What are the genotypes of the parent flies?
9
B3. Multiple Alleles
Multiple Alleles: A gene with more than two alleles
d) A chinchilla rabbit with the genotype cchch is crossed with a himilayan rabbit with
genotype ch c. What is the expected ratio of phenotypes among the offspring of
this cross?
As there are more than two alleles, upper and lower case letters
are not used to signify dominance and recessiveness.
Examples
1)
Fly eye colour is determined by multiple alleles. There are four different
alleles that each code for a different eye colour (E1 = Red with is
dominant to E2 = Apricot, which is dominant to E3 = Honey which is
dominant to E4 = white).
a. Two flies mate and have 74 apricot colour eyed offspring and 30
honey colour eyed offspring. What are genotypes of both parent flies?
b. You are in the process of performing genetic experiments on flies in
the lab when all of your flies escape. You manage to trap a couple of
flies but you no longer know what genotype they are. One of the flies
has red eyes, and the other has white eyes. When you mate the two
flies your results show approximately half the progeny with apricot
eyes, and the other half with red eyes. What are the genotypes of the
two flies you were able to capture?
2)
e) Some of the offspring of a chinchilla rabbit and a Himilayan rabbit are albino.
What must be the genotypes of the parent rabbits?
B4. Incomplete and Co-Dominance
Incomplete Dominance: Describes a condition where there is partial
expression of both alleles: neither of two alleles for the same gene can
completely conceal the presence of the other.
Three phenotypes exist for incomplete dominant traits with heterozygous
individuals being an intermediate condition of the two alleles.
Because there is no completely dominant allele, superscript notation is
used in place of upper and lower case letters.
The gene that controls chinchilla coat colour has 4 alleles: Agouti C >
chincilla cch > Himalayan ch > albino c. Agouti coat colour must have at
least one C.
a) What are the possible genotypes for agouti colouration?
b) What are the possible genotypes for chinchilla colouration?
c) What are the possible genotypes for Himilayan colouration?
10
Examples
Examples
1. The Four O'clock plant has only two alleles for flower color, but has three
different phenotypes: red flowered plants, white flowered plants, and pink
flowered plants.
a. Show the expected offspring of a cross between two pink-flowered plants.
Include genotypes, phenotypes and ratios.
1. In shorthorn cattle the CR allele, when homozygous, produces animals with red hair
and the CW allele, when homozygous, produces cattle with white hair. An animal with a
heterozygous genotype is roan in color, meaning its coat contains both red hairs and
white hairs.
a. Describe the expected offspring when a breeder mates cows and bulls of the
following phenotypes:
red x red
b. Show the expected offspring of a cross between a red-flowered Four O'clock
plant and a pink-flowered plant.
white x white
red x roan
white x roan
red x white
c. Show the expected offspring of a cross between a pink-flowered Four
O'clock plant and a white flowered plant.
roan x roan
b. Which kind of true-breeding herd would be easier to establish: red or roan?
Explain.
Co-Dominance: Describes a condition in which both alleles are
fully expressed.
c. What is the probability of a pair of roan cattle producing (1) a red calf? (2) a
roan calf? (3) a white calf?
2. A blue roan horse is a heterozygote in which one allele is expressed in the white hairs
and the other allele is expressed in the black hairs. When both these colours are
expressed, the horses coat color sometimes looks blue. If two blue roan horses are bred
together, what is the chance that the colt will be white?
11
Incomplete and Co-Dominant Examples
1. Suppose that when true-breeding long radishes are crossed with truebreeding round radishes, the F1 generations are all oval-shaped. Is this an
example of codominance or incomplete dominance?
2. A green-leafed Coleus plant was crossed-pollinated with a purple leafed
Coleus plant. The offspring all had spotted green and purple leaves. Is this an
example of codominance or incomplete dominance?
3. The symbols IA, IB, and i represent the three alleles that control the human
ABO blood group. The alleles IA and IB are codominant, and are both
dominant to the i genotype. The phenotypes are stated as blood types A, B, AB,
and O.
5. The disorder familial hypercholesterolemia (FH) results when a person possesses two
copies of a particular nonfunctioning allele. FH is a condition that prevents the tissues
from removing low-density lipoproteins (LDL, commonly known as “bad cholesterol”)
from the blood. People who are homozygous for the trait have six times the normal
amount of cholesterol in their blood and may have a heart attack at the age of 2.
Heterozygotes, who possess only one nonfunctioning copy of the gene, have about twice
as much cholesterol in their blood an may have a heart attack by the age of 35.
a) Using Ch to represent the functional (normal) allele and CH to represent the
nonfunctioning (mutant) allele, identify the genotype for each of the following
phenotypes:
i)
Healthy. No increased risk for heart attacks:
ii)
Heart attacks in middle age (35-45):
iii) Heart attacks in childhood:
Two newborn babies were accidentally mixed up in the hospital. Use the
information given to answer the questions that follow.
b) Is the nonfunctioning allele a dominant or recessive allele?
Baby 1: Type O
Baby 2: Type A
c) How would you describe the alleles involved in FH: co-dominant, completely
dominant, or incompletely dominant? Explain your answer.
Mrs. Brown: Type B
Mr. Brown: Type AB
Mrs. Smith: Type B
Mr. Smith: Type B
a. Which child belongs to the Browns? Which child belongs to the
Smiths?
b. Indicate the genotype of everybody above if you haven’t already
done so.
4. A man with blood type B is being sued for paternity by a woman with blood
type A. The child the woman claims he fathered has blood type O.
a. If this man is the father of the child, what are the genotypes of the parents?
d) Sophie and Stephan Grosfeuille are the parents of two young girls, Isabelle and
Claire. Sadly, 14 year old Isabelle recently died from a heart attack. Sophie
and Stephan knew that many members of each of their families had suffered
from heart attacks but they never thought it would happen to someone so
young.
i)
What was the genotype of Isabelle?
ii)
What are the genotypes of Sophie and Stephan?
iii) If Sophie and Stephan have another child, what is the percentage
probability of that child having the same genotype as Isabelle?
b. If this man's blood type were AB, could he be the child's father? Explain.
12
B5. Sex-Linked Traits
Thomas Morgan (1866-1945) is considered to be one of the
modern father of genetics and was made famous for his
extensive fruit fly experiments.
4. Red-green color blindness in humans is X-linked (same inheritance pattern as
hemophilia).
a. A woman has normal vision, but her father is color blind. Is she necessarily a
carrier (heterozygous)? Explain.
He concluded that sex-linked traits are expressed in different
ratios by male and female offspring, because they are governed
by the segregation of X and Y chromosomes.
There are much fewer genes located on the Y chromosome than
on the X chromosome.
b. Diagram all genotypes and phenotypes of parents and expected children in a
family where the woman has normal vision and no family history of color
blindness, and the man is color blind.
Example
1. Human females are XX and males are XY.
a. Does a male inherit the X from his mother or father?
2. Most people’s blood clots normally. Hemophilia is a blood disorder which
causes blood not to clot properly. The allele for hemophilia is recessive to the
allele for normal blood clotting. These alleles are located on the human X
chromosome. The Y chromosome is a genetic "blank" for this trait; it contains
no allele for the blood-clotting gene.
a. A number of cases of hemophilia have occurred among male
descendants in the family of England's Queen Victoria. She,
however, did not have hemophilia, nor did her husband. Show the
genotype of Queen Victoria and her husband that would produce this
result.
b. Show the possible genotypes of parents who give birth to a
hemophiliac daughter.
c. Explain what's wrong with the statement "That hemophiliac man's
father was also a hemophiliac, so he must have inherited the disease
from his father."
c. In a large family, all nine sons were color blind and all four daughters had
normal vision. Give the genotypes and phenotypes of all concerned.
5. Red-Green colourblindness is a recessive trait that is linked to the X chromosome. The
symbol XB is used to signify normal vision and the symbol Xb is used to signify
colourblindness. The normal-visioned daughter of a father with colourblindness marries
a man who is unaffected by colourblindness. What is the percentage of probability of
having a son who is colourblind?
6. Hemophilia is a recessive, X-linked condition. Hemophiliacs bleed excessively when
injured because they are missing a certain protein required for blood clotting. The most
seriously afflicted individuals bleed to death after relatively minor bruises or cuts. The
ancient Hebrews must have had some understanding of the hereditary pattern of
hemophilia, because sons born to women having a family history of hemophilia were
exempted from circumcision. A hemophiliac man is married to a woman who is
unaffected by hemophilia. Some of their children have hemophilia. What are the
parents’ genotypes?
13
7. Pseduohypertrophic muscular dystrophy is a sex-linked disorder that causes
gradual deterioration of the muscles. It is seen only in boys born to apparently
normal parents and usually results in death in the early teens.
a) Is pseduohypertrophic muscular dystrophy caused by a dominant or
recessive allele?
b) Provide the genotypes of normal parents producing an affected son.
c) Why is this disorder always seen in boys and never in girls?
8. Brown spotting of the teeth is caused by a dominant allele found on the X
chromosome. How would the sons and daughters be affected if one parent is
unaffected by brown spotting and the other
Key Concept C: Dihybrid Crosses
B1. Dihybrid Crosses and Mendel’s 2nd Law of
Independent Assortment
• In another set of experiments, Mendel investigated the pattern of
inheritance of two sets of traits. His two trait experiments have
been categorized as a dihybrid crosses.
• The observed results of Mendel’s dihyrid crosses with pea plants
resulted in Mendel’s second law: the Law of Independent
Assortment
Mendel’s Law of Independent Assortment:
d) parent is a man affected by brown spotting?
The two alleles for one gene segregate (separate) independently of the
alleles for other genes during gamete formation.
e) parent is a woman affected by brown spotting?
14
Examples
1. A tall pea plant with terminal flowers (flowers on the ends of the stems) is
crossed with a short plant that has axial flowers. All 72 offspring are tall with
axial flowers. This is a dihybrid cross with the height and flower position traits
showing independent assortment.
a. Name the dominant and recessive alleles.
3. About 70% of Canadians get a bitter taste from the substance called
phenylthiocarbamide (PTC). It is tasteless to the rest. The "taster" allele is dominant to
non-taster. Also, normal skin pigmentation is dominant to albino. A normally pigmented
woman who is taste-blind for PTC has an albino-taster father. She marries an albino man
who is a taster, though the man's mother is a non-taster. Show the expected offspring of
this couple.
b. Give the genotypes of the parents and offspring in this cross.
c. Predict the F2 offspring when the tall-axial F1's are allowed to self
pollinate.
4. In pigeons the checkered pattern is caused by a dominant allele. A plain (noncheckered) pattern is recessive. Red color is also caused by a dominant allele and brown
color by a recessive allele.
2. Suppose a white, straight haired guinea pig mates with a brown, curly-haired
animal. All five babies in their first litter have brown fur, but three are curly
and two have straight hair. The second litter consists of six more brown
offspring, where two are curly and four are straight haired.
a. Show the expected offspring of a cross between a homozygous checkered red
bird and a plain brown one. Carry out this cross through the F2 generation.
a. Can you tell which alleles are dominant?
b. Assuming curly is dominant to straight, what are the genotypes of
the parents and the offspring?
c. What is the probability of getting two female guinea pigs with
straight hair in a row?
b. Carry out to the F2 generation a cross between a homozygous plain red bird
and its homozygous checkered brown mate.
c. A plain brown female pigeon laid five eggs. The young turned out to be: 2
plain red, 2 checkered red, and 1 checkered brown. Describe the father pigeon.
Give the genotypes of all birds in this cross. Could any other types of offspring
have been produced by this pair?
15
5. In garden pea plants, the gene for plant height has two alleles: the tall allele
is dominant over the short allele. The gene for pod colour has two alleles:
green pod is dominant over yellow pod.
a) If Mendel crossed true breeding tall plants that had green pods with
true breeding short plants that had yellow pods, what is the outcome
of the F1 generation?
b) If the F1 progeny were then self-crossed, what are the phenotypic and
genotypic ratios of the F2 progeny?
8. Suppose that normal leg size in cattle is produced by the homozygous genotype DD.
Short-legged cattle possess the heterozygous genotype Dd. The homozygous genotype
dd, is lethal, producing grossly deformed dead calves. Also, suppose that the presence of
horns in cattle is governed by the recessive allel of another gene locus, p, whereas the
polled condition (absence of horns) is produced by the dominant allele P. In matings
between horned, short-legged cattle, what percent of each kind of phenotype are expected
in the adult offspring?
9. Suppose a colourblind male with type AB blood has children with a woman who has
normal vision and no family history of colour blindness and is also type O blood. What is
the probability they will have a colourblind daughter with type A blood?
6. In rabbits, the allele for brown coat colour is dominant over white coat
colour. The allele for short fur is dominant over long fur. A true-breeding
brown, short-haired rabbit is mated with a white, long-haired rabbit. The F1
progeny were then self-crossed. Determine the phenotypic ratio and genotypic
ratios of the F2 progeny.
7. In guinea pigs, rough coat R is dominant to smooth coat r. Black coat
colour B is dominant to albino b. A heterozygous black, smooth coat male is
bred to a heterozygous black, heterozygous rough coat female. What are the
probable genotypic and phenotypic probabilities of the offspring?
16
Key Concept D: Beyond Mendel
Examples
D1. Chromosome Mapping
1)
• Mendel’s Law of Independent Assortment was based on
onbservations of genes located on different chromosomes.
What this means is that the segregation of an allele for a
gene on chromsome #1 of the pea plant had no bearing on
the segregation of alleles for a gene on chromosome #3
for example.
For a series of experiments, a linkage group composed of genes Q, R, S and T was
found to show the following gene recombinant frequencies:
Genes
Q
R
S
T
Q
3%
7%
5%
R
3%
10%
8%
S
7%
10%
2%
T
5%
8%
2%
-
Construct a gene map. Show relative positions of each gene on the chromosome and
indicate distances in map units.
• Unfortunately, the law of independent assortment only
applies when genes are located on different
chromosomes.
• When genes are located on the same chromosome, they
are said to be “linked” and are more likely to get passed on
together.
• When two genes are linked, the closer they are together,
the less likely it is for crossing over to occur between
them. The reverse is also true. The further away to genes
are on the same chromosome, the more likely it is that
crossing over will occur between them.
• The rate at which crossing over occurs between linked
genes is referred to as “crossover frequency” and this
number is used as a relative distance between genes.
2)
The genes for body colour and wing shape are found on the same chromosome in
fruit flies
Two Genes Found on the Same Chromosome in Drosophila
S
W
body colour (S = normal / s = sable)
wing shape (W = normal / w = miniature)
In a cross between a heterozygous normal fly and a sable-bodied, miniature-winged
fly the results were:
99 normal flies
99 with a sable body and miniature wings
11 with a normal body and miniature wings
11 with a sable body and normal wings
How many map units separate the genes for body colour and wing shape?
17
D2. Polygenic Inheritance
a) Write the genotype of a light blue-eyed individual.
If this child's parents both have two dominant and two recessive alleles, then what is the
genotype and phenotype of the parents?
• Polygenic inheritance concerns the inheritance of a
characteristic which is controlled by more than one gene.
Many conditions are thought to be polygenic, including
human skin colour, eye colour, and height. Polygenic
inheritance can lead to continuous variation (ex. skin
colour in humans)
• Often, variation can also be caused by environmental
factors. Human skin colour is affected by the amount of
light (tanning); tallness is controlled by polygenes for
skeleton height, but their effect may be retarded by
malnutrition, injury, and disease.
b) Which of the phenotypes would be possible if one parent has grey/blue eyes and the
other green eyes?
Example #1
Eye color in humans is in varying shades of ebony brown to crystal
blue, to Kelly green, and all points in between. Eye color is possibly governed
by 6 or more genes. There are generally 8 different described eye colors.
Basically, dark is dominant at each of the 6 genes. The more dominant alleles
that you have the darker your eyes are.
c) If one parent has light brown eyes and the other has dark brown eyes, what would be
the probability of an offspring with grey/blue eyes? (Express your answer as a whole
number percentage)
Assume there are two gene pairs to determine the color of an
offspring’s eyes, one that codes for depositing pigment in the front of the iris
and one that codes for depositing in the back of the iris.
AABB - black/brown
AABb - dark brown
AAbb - brown
AaBB - brown/green flecks
AaBb - light brown
Aabb - grey/blue
aaBB - green
aaBb - dark blue
aabb - light blue
18
Example #2
Human skin color involves the interaction of at least three
independent genes. If we assume that A, B and C will each represent alleles
for dark skin, then a, b and c will represent alleles for light skin. Each person
will have two alleles per gene, so six alleles in total (or more if more than three
genes are involved.
We can symbolize the alleles for darker skin by o and the alleles for
light skin by o . The phenotype of darkest skin would then be caused by
. The phenotype of the lightest skin would be caused by
. If these two produced offspring, they would have the
genotype
with an __________________ skin colour.
If both parents have intermediate-coloured skin then the possibilities
of skin colour in the offspring produced would be as shown in the diagram to
the right.
a) On the diagram to the right, indicate the genotypes for the following
phenotypes:

very light

very dark

intermediate
b) What are two possible genotypes for the proportion of the population that
are 15/64 and have darker skin?
c) What are two possible genotypes for the proportion of the population that are
15/64 and have lighter skin?
19
Use the following simplified genotypes for skin color to answer the next
questions .
white
(albino)
aabb
light
Aabb
aaBb
medium
AAbb
AaBb
aaBB
dark
AaBB
AABb
black
AABB
Question:
State the number or dominant and recessive alleles that must be present/absent
for the following phenotypes. The first one is already done for you.
dark: 3 dominant alleles and 1 recessive allele
black:
medium:
light:
albino:
Question: Complete the Punnett square for the following cross:
light-skinned parent x black-skinned parent. Report the phenotypic
probabilities as whole number percentages.
20
Key Concept E: Human Genetic
Analysis
Use the following pedigree to answer questions 1 to 4.
I
E1. Constructing a Pedigree
1
• A pedigree is a graphical representation of the genetic
history of an individual or family. The following symbols
are used in constructing a pedigree:
2
II
2
1
3
note: Affected individuals have the attached earlobe trait.
Carriers (heterozygotes) have not been identified.
1.
2.
3.
4.
What is the sex of the oldest child?
What is the sex of the youngest child?
In what order are the children arranged, from left to right?
What must be the genotype of person I-2?
Using a different pedigree of the same family at a later time shows three generations.
Use it to answer questions 5 through 10.
I
1
2
II
E2. Using a pedigree to solve problems
1
Questions 1 through 23 pertain to the inheritance of the earlobe trait in humans
that is thought to be controlled by a single gene. There are two general earlobe
shapes, free lobes and attached lobes. The allele responsible for free lobes, E,
is dominant to the allele for attached lobes, e.
2
3
4
III
1
note: Affected individuals have the attached earlobe trait.
Carriers (heterozygotes) have not been identified.
5.
6.
7.
8.
9.
10.
Which person is the son-in-law?
To whom is the son-in-law married?
What is the gender of their child?
What must be the genotype of person II-1?
What is the genotype of the maternal grandmother of III-1?
If the paternal grandmother of III-1 had the EE genotype, what is the genotype
of the paternal grandfather if he had free ear lobes?
21
Use the following pedigree to answer questions 11 through 17.
17.
18.
19.
I
1
ee
2
E __
Examine the following data obtained about the families of a newly married man and
woman.
II
1
E __
2
ee
3
E __
Man!s Family
4
E __
III
1
E __
note: Affected individuals have the attached earlobe trait.
Carriers (heterozygotes) have not been identified.
11.
12.
13.
14.
15.
16.
How many generations are shown?
How many persons have free ear lobes? ____ Attached ear lobes? ____
How many children did the first generation have?
Can an Ee parent and an ee parent have the results shown in generation II?
Predict the genotype of person II-3.
Predict the genotype of person II-4.
Could child II-3 be EE? Explain your answer.
Predict the genotype of person III-1.
At some time in the future, II-1 and II-2 have ten more children and
all have free earlobes. What is most likely genotype of the father?
Woman!s Family
man = 24 years old; has free earlobes
woman = 25 years old; has free earlobes
Relation
sister
mother
father
paternal
grandmother
paternal
grandfather
Relation
brother
brother
mother
father
Age
26
47
48
Phenotype
attached
free
free
70
free
77
attached
Age
23
16
45
45
Phenotype
free
attached
free
free
20.
Construct a pedigree from the information provided in the data table. Use all
pertinent pedigree symbols and conventions for labelling as described in the data booklet.
Place the man’s family on the left side of the chart, and the woman’s pedigree on the right
side.
Examine the pedigree below and predict (insofar as possible) the genotypes of
each family member. Then answer questions 17 to 19.
I
1
2
II
1
2
21.
22.
III
1
2
3
4
IV
1
note: Carriers have not been identified.
2
Identify the genotypes of all individuals, insofar as possible.
Considering what is already known about the genotype of the man, what is
the probability that he is
a)
homozygous recessive
b)
homozygous dominant
c)
heterozygous
23.
Considering all the genotype possibilities for the man and woman, what is the
greatest probability that an offspring will have attached earlobes?
22
E3. Modes of Inheritance
Summary
1. Autosomal Recessive Traits
1. both males and females are equally affected
2. affected parents produce only affected offspring
3. the trait can skip a generation and then reappear in later
generations
4. unaffected parents can produce affected offspring
Examples include albinism, hair & eye pigmentation,
Tay-Sachs
2. Autosomal Dominant Traits
1. both males and females are equally affected
2. affected parents can produce unaffected offspring
3. affected offspring must have one affected parent
4. the trait does not skip a generation and reappear in later
generations
5. unaffected parents can produce affected offspring
Examples include achondroplasia, huntington’s disease
3. Sex-Linked Inheritance
X-Linked Recessive
1. it is more common in males than females
2. affected fathers transmit the gene to all daughters but
not to any sons
3. all the sons of an affected mother are affected with the
disorder
4. an affected daughter must have an affected father
Examples include colourblindness, hemophilia
4. Mitochondrial Inheritance
1. both males and females can be affected
2. only females can transmit to offspring
5. Incomplete dominance
1. both alleles are partially expressed: neither of the two alleles of
the same gene can completely conceal the presence of the other
2. a third, intermediate phenotype occurs
Examples include sickle cell anemia, and flower colour in some
plants
6. Codominance
• both alleles are fully expressed
•
expression of both phenotypes at the same time
Examples include roan cattle & horses, blood types
7. Multiple alleles:
• a gene with more than two alleles
Examples include human blood types
8. Polygenic Inheritance
• two or more genes influences the expression of one trait
• the result is an array of possible phenotypes called continuous
variation
Examples include skin colour, eye colour, height
X - Linked Dominant
• affected males produce all affected daughters and no
affected sons
23
D.
Questions:
1. Numerical Response: Identify the pedigrees that best fit one of the
following modes of inheritance:
Autosomal recessive
Autosomal dominant
Mitochondrial
X-Linked Recessive
A.
B.
2. Make adjustments on pedigree A and pedigree D to correctly represent carriers.
3. Why are the heterozygotes in pedigree B not indicated as half-shaded?
C.
24