Download Genetics Problems Notes

Name_____________________________Per____
Genetics Problems Notes PreAP Biology
Genetics and Predictions for Autosomal Traits
Monohybrid Crosses: Predictions for one trait – in all probability
Probability – the likelihood of an event
- can be expressed as __fractions_ (1/2), ___percents__ (50%), or ratios_ (1:1)
- geneticists use a _Punnett square_ to predict the possible outcomes of a cross (give the
probability of a genotype or phenotype)
- studying one trait at a time is called a __monohybrid_ cross
- crosses are represented as:
genotype x genotype ex) Yy x Yy
or (less often)
phenotype x phenotype ex) heterozygous yellow x heterozygous yellow
Sample monohybrid cross problem.
Use a punnett square to predict the results of a cross between two heterozygous yellow pea plants.
Give the parent genotypes, the punnett square, the genotypic ratio, and the phenotypic ratio.
Parent genotypes: ___Yy________x_____Yy________
Punnett square:
Possible
gametes from
the other
parent
Numbers in the
ratio must add up
to the number of
boxes in the
Punnett square
Y
y
Y
YY
Yy
y
Yy
yy
Genoptypic ratio: __1:2:1__
Phenotypic ratio: __3:1____
Remember, these are
diploid organisms
Possible
gametes from
one parent
Each box in the Punnett square
represents a possible zygote
Always written homozygous dominant: heterozygous:
homozygous recessive
Always written dominant: recessive in simple Mendelian problems
If you are asked for the percentage, or a fraction, or a ratio be sure you express your answer in that
form. For example, if the problem asked “What percentage of the offspring would be green” your answer
would be 25% (if you said ¼ or 0.25, or 3:1, your answer would be wrong because that is not what you were
asked).
If you are asked for a fraction, be sure you give a fraction, not a decimal, or a ratio, or a percent.
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Practice Problems
Monohybrid Crosses
a.
b.
c.
d.
State the P generation
Set up the punnett square
Genotype and ratio
Phenotype and ratio
Y= yellow
y= green
1. Cross a homozygous yellow plant with a green plant.
a.
_______ x _______
b.
c.
d.
2. Take two F1 generations from #1 and cross them.
a.
b.
______ x ______
c.
d.
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Name________________________ Per____
Genetics Problems II
(Homework due ______________)
A. List the correct alleles for the traits for each.
Given: Black fur (B) is dominant over white fur (b) in rabbits.
Normal skin (N) is dominant over albinism (n) in humans.
1. _____ heterozygous black fur
2. _____ homozygous normal skin
3. _____ heterozygous normal skin
4. _____ homozygous white fur
5. _____ homozygous albinism
6. _____ homozygous black fur
B. Monohybrid Crosses: Use the symbols above to complete the following problems.
7. Cross a heterozygous normal skin man with a homozygous albino woman.
What are the genotypes of the mom and dad? _____ x _____
Show your work with a Punnett square:
What are the genotypes of the offspring? _____________
What’s the ratio or percentage of each?____________
What are the phenotypes of the offspring? _____________
What’s the ratio or percentage of each? ____________
8. Cross a homozygous black rabbit with a homozygous white rabbit.
What are the genotypes of the mom and dad rabbits? _____ x _____
Show your work with a Punnett square:
What are the genotypes of the offspring? _____________
What’s the ratio or percentage of each?____________
What are the phenotypes of the offspring? _____________
What’s the ratio or percentage of each? ____________
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(Homework continued)
9. Cross a heterozygous normal skin man with a heterozygous normal skin female.
What are the genotypes of the mom and dad ? _____ x _____
Show your work with a Punnett square:
What are the genotypes of the offspring? _____________
What’s the ratio or percentage of each?____________
What are the phenotypes of the offspring? _____________
What’s the ratio or percentage of each? ____________
C. Use your own symbols for the following crosses.
10. Cross a heterozygous red flower with a homozygous recessive white flower.
The letter for red is _____. The letter for white is _____.
The genotypes of the parent flowers are _____ x _____
Show your work in a Punnett square:
What are the genotypes of the offspring? _____________
What’s the ratio or percentage of each? ______________
What are the phenotypes of the offspring? _____________
What’s the ratio or percentage of each? ______________
11. Cross a homozygous wire hair (dominant) dog with a homozygous recessive smooth hair
texture dog.
The letter for wire hair is _____. The letter for smooth is _____.
The genotypes of the parent dogs are _____ x _____
Show your work in a Punnett square:
What are the genotypes of the offspring? _____________
What’s the ratio or percentage of each? ______________
What are the phenotypes of the offspring? _____________
What’s the ratio or percentage of each? ______________
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Test cross – seeing the invisible
Scientists wanted to know how to tell if an organism showing the dominant phenotype was homozygous
or heterozygous so they developed the test cross.
A test cross involves breeding (crossing) an organism whose genotype is recessive with a phenotypically
dominant organism. Perform the following test crosses:
a)
YY x yy
Genotypic ratio: ______________
Phenotypic ratio: _____________
b)
Yy x yy
Genotypic ratio: ______________
Phenotypic ratio: _____________
What is the difference in the results of the two test crosses? __________________________
Difficult Predictions—Intermediate Inheritance, Multiple Alleles, and Pleiotropy
 Intermediate inheritance – genes in between
Mendel was lucky in the traits that he chose. One was always completely dominant and one completely
recessive. In nature, this is rare. In intermediate inheritance the heterozygous offspring have a trait that
is not exactly like the trait of either purebred parent. There are three forms of intermediate inheritance:
incomplete dominance, codominance and polygenic inheritance.
1. Incomplete dominance - heterozygous offspring show a phenotype that is in between the phenotypes of
the two homozygous parents. Neither allele is fully expressed*.
Example: red snapdragons x white snapdragons produce pink snap dragons
Pink snapdragons x pink snapdragons produce red, white and pink snapdragons
Alleles can be indicated in several ways—just stick to one way for each problem.
Capital and lowercase: Ex. RR = red, Rr = pink, rr = white
Two different letters: Ex. RR = red, RW = pink, WW = white
Capital letter and Capital letter with prime: Ex. RR = red, RR’ = pink, R’R’ = white
Example:
Palomino horses have blonde hair with white manes and tails; result from a cross between a
chestnut horse and a white horse
Example: hypercholesterolemia – normal genotype is HH and cholesterol levels are normal
- Hh – cholesterol levels are twice as high
- hh – cholesterol levels are 5x’s as high
* in incomplete dominance, there is only one allele coding for the protein; therefore, less functional
protein is produced, but enough that the phenotype is neither dominant nor recessive
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Incomplete dominance:
RR = Red
RR’ = pink
R’R’ = white
a.= P generation b.=punnett square
c.=genotypes & ratios
d.=phenotypes & ratios
Cross a pink Japanese four o’clock flower with a pink Japanese four o’clock.
a.
_____ x _____
c.
b.
d.
2. Codominance – both alleles express themselves __fully____. An example is human
__blood ____ __types_____. Human blood types also show more than one allele, called __multiple_____
__alleles. The three alleles are _IA__, __IB__, and __i__. So in addition to codominance, human blood type
also involves multiple alleles.
Blood Type
(phenotype)
A
B
AB
O
Genotype (s)
Can Donate to
Can Receive from
Cross someone with type AB blood with someone who is heterozygous for type A.
a.
b.
_____ x _____
c.
d.
Another example includes roan horses. Roan horses have red hairs and white hairs and are born from a red
horse crossed with a white horse. Most of the time, two different letters are used to represent the alleles.
Ex. RR = red, RW = roan, WW = white
3. Polygenic Inheritance
A trait controlled by two or more genes is called polygenic. Eye color and skin color are both
examples in humans.
 Multiple alleles and pleiotropy – many alleles and traits
Multiple alleles - (see human blood types)
When a single gene affects more than one trait, the pattern of inheritance is called pleiotropy. An example is
sickle cell anemia. People with two copies of the sickle cell gene have multiple effects (called pleiotropy) such
as weakness, anemia, brain damage, spleen damage, and heart damage due to damage to the circulatory
system resulting from the shape of the sickled red blood cells.
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Intermediate Inheritance Crosses WS
Incomplete Dominance:
Name________________________ Per____
(Homework due _____________)
 Definition:____________________________________________________
Use the following for problems 1 and 2: RR’ = pink (RR=red and R’R’=white)
1.
Cross a red and a white flower.
What are the genotypes of the parent
flowers? _____ x _____
Show your work with a Punnett square:
Offspring genotypes? _____________
Ratio or percent of each?____________
Offspring phenotypes? _____________
Ratio or percentage of each? ____________
2. Cross two pink flowers.
What are the genotypes of the parent
flowers? _____ x _____
Show your work with a Punnett square:
Offspring genotypes? _____________
Ratio or percent of each?____________
Offspring phenotypes? _____________
Ratio or percentage of each? ____________
Codominance:
 Definition: ___________________________________________________
For codominance genotypes, it is appropriate to use any of the following ways to represent your alleles:
Ex. Roan horses=both red hairs & white hairs (Red parent & white parent) RW = roan (RR=red and
WW=white). Ex. Sickle Cell Anemia: abnormally shaped blood cells Use: AA=normal;
SS = sickle shaped, so AS = normal and sickle for heterozygotes.
3. Cross 2 Roan horses.
What are the genotypes of the parent
horses? _____ x _____
Show your work with a Punnett square:
Offspring genotypes? _____________
Ratio or percent of each?____________
Offspring phenotypes? _____________
Ratio or percentage of each? ____________
4. Cross a person with normal red blood cells with a sickle celled person.
What are the genotypes of the parents?
_____ x _____
Offspring genotypes? _____________
Show your work with a Punnett square:
Ratio or percent of each?____________
Offspring phenotypes? _____________
Ratio or percentage of each? ____________
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(Homework continued)
5. Cross two people who are heterozygous for normal & sickle blood cells.
What are the genotypes of the parents?
_____ x _____
Offspring genotypes? _____________
Show your work with a Punnett square:
Ratio or percent of each?____________
Offspring phenotypes? _____________
Ratio or percentage of each? ____________
Codominance & Multiple Alleles
 Definition (multiple alleles) ________________________________________
_____________________________________________________________
Human blood type is an example of a trait that is determined by multiple alleles and also illustrates
codominance. Remember that there are 4 phenotypes for blood type: A, B, AB, and O. There is one
genotype for type AB, it is IAIB. There is one genotype for type O, it is ii. There are two each for types
A and B. Homozygous type A is IAIA; heterozygous type A is IAi. Homozygous B = IBIB; Heterozygous B =
IBi.
6. Cross a person who is heterozygous for B and a person with type O .
What are the genotypes of the parents?
_____ x _____
Offspring genotypes? _____________
Show your work with a Punnett square:
Ratio or percent of each?____________
Offspring phenotypes? _____________
Ratio or percentage of each? ____________
7. Cross a person who is type AB with a person who is homozygous for type A.
What are the genotypes of the parents?
_____ x _____
Show your work with a Punnett square:
Offspring genotypes? _____________
Ratio or percent of each?____________
Offspring phenotypes? _____________
Ratio or percentage of each? ____________
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Name___________________________ Per____
Blood Type Baby Mystery--“Using Genetics to Help Solve Mysteries”
Adapted from a worksheet by Merrill Publishing Co., 1991 (Homework due _____________)
Geneticists are often called upon to solve mysteries using some of the tools you have become familiar with in this
chapter. Using genetic clues, give a possible solution for each problem below.
Problem: Four newborn babies in the delivery room of the hospital at the same time were mixed up by the
person who typed the wristbands. The blood types of the four babies were known to be AB, O, A, and B. How
did the doctors eventually find out which baby belongs to which set of parents?
Given:
Parents #1 had blood types O and AB
Parents #2 had blood types AB and B
Parents #3 both had blood type O
Parents #4 had blood types O and A
Use Punnett squares to determine the possible genotypes of the offspring.
Parents # 1
Parents # 2 Homo B
Parents # 2 Hetero B
Parents # 3
Parents # 4 Homo A
Parents # 4 Hetero A
Possible Blood Types for Babies
1. Parents # 1 ______________________________________
2. Parents # 2 ______________________________________
3. Parents # 3 ______________________________________
4. Parents # 4 ______________________________________
Conclusion:
A. Baby with type AB blood belongs to Parents # _____.
B. Baby with type B blood belongs to Parents # _____.
C. Baby with type A blood belongs to Parents # _____.
D. Baby with type O blood belongs to Parents # _____.
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Dihybrid cross – cross involving two traits
- the same theory and procedure is involved, but the number of possible gametes and the
number of possible zygotes has increased
- Example:
Cross two plants heterozygous for round, yellow peas. Give the parent genotype, the
Punnett square, and the phenotypic ratio
Each parent has two copies of each
gene and gives one copy of each gene
to a gamete (diploid to haploid)
Parent genotype: RrYy x RrYy
Punnett square:
Gametes from
the second
parent
RY
RY
*RRYY
Ry
*RRYy
RRyy
*RrYy
Rryy
*RrYY
*RrYy
*RrYy
Rryy
rrYY
@
rrYy
@
rrYy
@
rryy
&
rY
ry
Ry
*RRYy
rY
ry
*RrYY
*RrYy
Note that the ratio is always dominant dominant: dominant recessive:
Phenotypic ratio: 9:3:3:1
recessive dominant: recessive recessive
*:__:@:&
Note that 1/16 of the offspring in a dihybrid cross are homozygous recessive, while 9/16
show the dominant phenotypes.
Cross a heterozygous tall red plant with a homozygous tall, heterozygous red plant.
T = tall
R = red
t = short
r = white
a.
b.
__TtRr__ x ___TTRr___
c. not necessary to do
d.
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Cross a heterozygous free large ear lobed boy with a heterozygous free large ear lobed girl.
E = free hanging ear lobed
L = large ear lobe
e = attached ear lobe
l = small ear lobe
a.
b.
__________ x __________
c. not necessary to do
d.
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Dihybrid Crosses WS
Name________________________ Per____
(Homework due ____________)
Complete each question by answering the following information:
A. the P1 generation genotypes
C. the genotype and ratio (don’t do)
B. the punnett square
D. the phenotype & ratio
In mice, the ability to run normally is a dominant trait. Mice with this trait are called
running mice (R). The recessive trait causes mice to run in circles only. Mice with this trait
are called waltzing mice (r). Hair color is also inherited in mice. Black hair (B) is dominant
over brown hair (b).
1. Cross a heterozygous running, heterozygous black mouse with a homozygous running,
homozygous black mouse.
A. _______ x ________
B.
C. not necessary to do
D.
2. Cross a heterozygous running, heterozygous black mouse with a heterozygous running,
heterozygous black mouse.
A. _______ x ________
B.
C. not necessary to do
D.
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(Homework continued)
Complete each question by answering the following information:
A. the P1 generation genotypes, B. the punnett square, C & D answer the questions.
Remember that there are 4 phenotypes for blood type: A, B, AB, and O. There is one genotype for type
AB, it is AB. There is one genotype for type O, it is OO. There are two each for types A and B.
Homozygous for Type A is AA, while the heterozygous is AO. For Type B, the homozygous is BB, and the
heterozygous is BO. Blood types often include what is called the Rh Factor. Use P for positive and p for
negative because, for these purposes, positive is dominant over negative
3. Cross a man with blood type AB+ (assume he is heterozygous for the Rh Factor) with
a female with blood type type O+ (also heterozygous for the Rh Factor)
A. _PpAB _ x _PpOO__
C. What are the possible blood types of
B.
the offspring? (ie B+, B-, A+, A-, O+, O-,
AB+, AB-)
D. What are the chances that an
offspring of theirs will have a negative
blood type? (express in ____ out of16).
What are the chances that an offspring
will be A+ (express in ____ out of 16?
4. Cross a female with blood type A- (heterozygous A) with a male who is homozygous
blood type B+ (homozygous B and heterozygous +)
A. ________ x _________
B.
C. How many offspring will be blood type
AB? (express in ____ out of16).
D. What are the chances that an
offspring of theirs will have a positive
blood type? (express in ___ out of16).
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Sex-linked traits
Traits that are sex-linked are transmitted by genes located on the X or Y chromosome. In this class, we will
just be studying traits that are sex-linked on the X chromosome, sometimes called X-linked.
Carriers—individuals who are heterozygous for an inherited disorder but do not show symptoms. Carriers can
pass the allele to their offspring.
Examples include: Hemophilia (H=normal, h=affected); colorblindness (C=normal; c=affected); Duchenne
muscular dystrophy (D=normal, d=affected)
Ex. Hemophilia: H=normal, h=affected along with
XX or XY
Female Genotypes
Male Genotypes
Female Phenotypes
Male Phenotypes
XHXH
NORMAL
XHY
NORMAL
XHXh
CARRIER (BUT
NORMAL)
AFFECTED
XhY
AFFECTED
XhXh
Why are more males than females affected by sex-linked disorders?
__________________________________________________________________________
Can a male be a carrier for a sex-linked disorder? _______
Cross a female carrier with a male hemophiliac.
a.= P generation
b.=punnett square
c.=genotypes & ratios d.=phenotypes & ratios
a.
b.
_____ x _____
c.
d.
Who does a female child have to inherit hemophilia from?
Who does a male child have to inherit hemophilia from?
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Name________________________ Per___
Sex-linked Disorder Homework Problems (HW due_______________)
In fruit flies, white eyes (r) is a sex-linked trait occurring only on the X chromosome (Red (R) is dominant).
1. If a white-eyed female(XrXr) is crossed with a red-eyed (wild type) male (XRY), predict the following:
A. ____________ X ___________
(parental genotypes)
B. Punnett square
C.
Possible genotypes:
________________________
Genotypic ratio:
_______________________
D. Possible phenotypes:
______________________
Phenotypic ratio:
______________________
2.
Cross a white-eyed male with a homozygous wild-type female.
A. ____________ X ___________
(parental genotypes)
B. Punnett square
C.
Possible genotypes:
________________________
Genotypic ratio:
_______________________
D. Possible phenotypes:
______________________
Phenotypic ratio:
______________________
3.
Lesch-Nyhan syndrome is an X-linked recessive disorder in which an enzyme deficiency results in a buildup of uric acid in the body. Work the following: A husband and wife, neither of whom have LeschNyhan syndrome, have a boy with the syndrome. What was the probability of having a child with
Lesch-Nyhan syndrome for this couple? ______________ Show your work in the space below.
A. ____________ X ___________
(parental genotypes)
B. Punnett square
C.
Possible genotypes:
________________________
Genotypic ratio:
_______________________
D. Possible phenotypes:
______________________
Phenotypic ratio:
______________________
4. Their daughter does not have the syndrome. What is the probability that their daughter is a carrier
(careful!)? ____________________
5. If the couple has a 3rd child, what is the probability that it will NOT have Lesch-Nyhan syndrome?
_________________
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Predictions and People
Pedigree studies – all in the family
People reproduce slowly and have few offspring. Therefore, geneticists use a pedigree a chart that shows how
a trait and the genes that control it are inherited within a family over several generations, much like a family
tree for specific traits. Pedigrees show phenotypes, but can often determine genotypes as well. Pedigrees
can be used for other organisms, too. In a pedigree, an individual whose genes have a recessive trait but
expresses the dominant trait is called a carrier.
Benefits of Pedigrees: Help predict if a family member is a carrier of a disorder or trait. If a disorder is
likely to be passed down, a genetic counselor can be consulted. A genetic counselor can assess the risk of
passing on disease alleles.
Genetic counseling
Involves seeking advice from a doctor about the probability of passing a genetic disorder on to your children.
A pedigree is developed, showing the probability of one or both parents carrying certain alleles. The
probability that the disorder will occur in the offspring can then be determined. The couple can then evaluate
the options
Molecular tests on the mother, such as for Downs syndrome prior to birth, or phenylketonuria (PKU) after
the birth of the baby. PKU is the inability to break down phenylalanine because of a missing enzyme, which
results in the accumulation of this amino acid in the body and damage to nerve cells PKU can be treated with a
special diet.
Traits can be Autosomal or Sex-linked.
 Autosomal- Trait on the non-sex chromosome, will appear in both sexes equally
 Sex-Linked- Trait whose allele is located on the X chromosome. Mostly seen in Males, if it is recessive,
and most are recessive. (If, recessive, must be on both X’s for a female to have it, but on the only X in
a male to have it. Females that inherit the allele from one parent will be a “carrier”, so can pass the
trait to offspring, but will not show that they have the trait.)
Traits can be Dominant or Recessive: If the trait is autosomal dominant, every individual with the trait will
have a parent with a trait. Recessive- An individual with the trait can have one, two or neither parent exhibit
the trait. (Ex. Both parents are heterozygous or carriers)
Heterozygous or Homozygous: Two people who are heterozygous carriers of a recessive allele will not show the
trait but can produce children with the trait. Carriers—individuals who are heterozygous for an inherited
disorder but do not show symptoms. Carriers can pass the allele to their offspring.
Recessive traits – it takes two
Many traits can be carried with no ill effects on the individual heterozygote, but individuals receiving two
copies of the recessive allele are affected. If two heterozygotes cross, there is a 25% chance that any one
offspring will have the recessive trait. Three human genetic disorders that result when an individual receives
two copies of a recessive allele are:
1) Tay Sachs - which results in death before the age of five due to the inability to metabolize some
lipids
2) Cystic fibrosis (cf) - which results in an excessive secretion of thick mucus that accumulates in
the respiratory and digestive tracts; treatment is prolonging the life-span of these individuals
3) albinism - which is a complete absence of the pigment melanin; these people have very pale skin,
white hair, and pink or blue eyes; vision impairment is common in people with albinism
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Dominant traits – some alleles are more expressive than others
Many traits are the result of the inheritance of a dominant trait, including freckles, farsightedness, broad
lips, widows peak, and polydactyly - extra fingers and toes.
If a dominant disorder is fatal, it will disappear from the population. The exception to this rule is fatal
disorders that do not affect the individual until after they have had offspring. Huntington’s is a fatal
genetic disorder that results in the deterioration of the nervous system, but symptoms are not developed until
the individual is in their 40s to 50s (middle age). By that time they may have already passed the allele on to
their children.
Interpreting a Pedigree:
Circle- Female
Square- Male
Shaded in Circle or Square-- Has trait/disorder
Clear Circle or Square-- Does not show trait/disorder
Carriers-- Sometimes designated Half shaded, Half Clear; or will be clear
Horizontal Line between a Circle & Square indicates marriage
Vertical Line extended from a parent line leads to the offspring
When looking at a pedigree, you should be able to determine:
 # of generations, marriages, children, sex of individuals
 Birth order of children (1st born – last born, left to right)
 Phenotype of each person
 Sometimes, genotypes of each person for autosomal traits (if not, write a question mark for
unknown allele ex. A? could be either AA or Aa for an autosomal trait); Sex linked genotypes
are almost always able to be determined.
 Whether the trait is autosomal or sexlinked. Autosomal traits will typically be seen equally in
males and females. Sex linked traits (on the X chromosome) will be seen more in males and the
male will inherit it from the mother since he must get his Y chromosome from his Dad.
 Whether the trait is dominant or recessive. Dominant traits will be seen in each generation
and one or both parents must show the trait if they have children who do. Recessive traits
may or may not appear in each generation and one, both, or neither parent may show the trait
if child(ren) do
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Pedigree Practice Classwork
Autosomal Pedigrees
The pedigree shows the inheritance of attached earlobes.
How many generations are shown? ______ How many married couples are shown? _____
Couple P1 had ___________ children and in what order? ______________________________
Is the trait for attached earlobes, versus free earlobes, dominant or recessive? ________________
How do you know? ______________________________________________________________
Fill in the genotypes on the pedigree.
The pedigree shows the inheritance of tongue rolling.
Is the trait dominant or recessive? ________________
Explain. ______________________________________________________________
____________________________________________________________________
Fill in the genotypes on the pedigree.
Sex-linked Pedigrees
This pedigree shows the inheritance of colorblindness, a sex-linked trait.
How many males have the disorder? ________ How many females? _________
In order to have a child with colorblindness does a parent have to be colorblind? ______
Is the trait dominant or recessive? ________________ Is the mother of the colorblind girl in the F3
generation colorblind, a carrier, or a person with normal color vision? ______________
Explain. ______________________________________________________________
Fill in the genotypes on the pedigree.
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Name_______________________ Per____
(Homework due _____________)
PEDIGREE WORKSHEET
Autosomal Pedigree
Remember, when a gene is autosomal, it can appear in BOTH sexes.
This is an autosomal pedigree for dimples.
P
F1
F2
Is having dimples a dominant or recessive characteristic? ___________________________.
Explain why. ____________________________________________________________.
______________________________________________________________________.
How many generations are there? ___________.
How many children does the P generation have? _________.
What is the birth order of P’s children in the F1 Generation? ________________________________.
How many men have dimples? ___________. How many females?_________.
Critical Thinking:
(Hint: Use a Punnett Square to help answer the following questions).
In the P1 generation, is the mom heterozygous (Dd) or homozygous (DD/dd) for dimples?
_________________________. Explain._______________________
_______________________________________________________________.
In the P1 generation, is the dad heterozygous (Dd) or homozygous (DD/dd) for dimples?
________________________. Explain. ________________________
______________________________________________________________.
In the F2 generation where all the circles and squares are shaded in, is the dad heterozygous or homozygous
for dimples?____________________________.
Explain._______________________________________________________
______________________________________________________________.
What about the mother (heterozygous or homozygous)?__________________
Explain________________________________________________________
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Sex-linked Pedigree
Remember, in sex-linked pedigrees, for a female to express the sex-linked condition she
must carry the recessive allele on BOTH X’s which means she inherited the condition from
both her father AND her mother.
This is a sex-linked pedigree for colorblindness.
With a pencil show which individuals are carriers of colorblindness by shading in the
circle/box half way. (Hint: Use Punnett Squares to help you if needed).
Label the generations P, F1, F2
Is colorblindness a dominant or recessive condition?____________________.
Explain why.____________________________________________________
How many males are colorblind? __________ How many females?_________.
Do you have to have a parent that is colorblind to be colorblind?___________.
Is the mother of the colorblind female in the F2 generation colorblind, have normal vision,
or is she a carrier?____________________________.
Explain why_______________________________________________________
_________________________________________________________________
(Homework continued)
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Name________________________________ Per____
Applied Genetics - © Prentice-Hall (Homework continued)
A. Duchenne muscular dystrophy is a deadly disorder in which the muscles grow progressively
weaker. The disease is caused by a recessive gene on the X chromosome. The pedigree chart
below illustrates the inheritance of this gene. Use the chart to answer the questions that follow.
HINT: Write the genotypes for each individual in the pedigree before you start answering the
questions.
Key:
Normal female
Normal male
A
C
G
Carrier female
Female with disorder
B
D
F
E
H
Male with disorder
I
J
K
1. Is Duchenne muscular dystrophy more likely to occur in males or in females? Explain your answer.
_____________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
2. Individual H is a female with this disorder. Explain how she inherited the disease.
_______________________________________________________________________________________________________
_______________________________________________________________________________________________________
3. Individual K has this disorder, yet his father did not. Explain how this is genetically possible.
_____________________________________________________________________________________________________
_____________________________________________________________________________________________________
______________________________________________________________________________________________________
4. Individual G does not have the disease, yet his mother was a carrier and his father had the disease.
Explain how this is possible. _______________________________________________________________________
___________________________________________________________________________________________________
5. Why is the genotype of the father unimportant when investigating sex-linked traits inherited by male
offspring? ______________________________________________________
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(Homework continued)
B. Huntington disease, a disease of the nervous system, is caused by an autosomal
dominant gene. The pedigree chart below illustrates a family with individuals who have
Huntington disease, the pedigree is incomplete, though, since all of the genotypes have
not be filled in. Use the chart to answer the questions that follow. HINT: Write the
genotypes for each individual in the pedigree before you start answering the questions.
A
B
hh
Hh
C
D
E
hh
J
F
G
H
I
hh
K
L
Hh
Hh
M
N
O
HH
Key:
H
Female
Male
Huntington Disease
h
Normal Gene
6. What is the probable genotype of individual D? Explain your answer.
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_______________________________________________________________________________________________________
7. What are the probable genotypes of individuals H and I? Explain your answer.
_______________________________________________________________________________________________________
_______________________________________________________________________________________________________
8. What is the probability that N will not have Huntington disease? ________________
9. Which individuals can be determined to have Huntington disease? ___________________________________
10. Identify the individuals whose genotypes cannot be determined without more information.
______________________________________________________________________
23