CHAPTER 15. It’s all in the Genes: Understanding Basic Mendelian Genetics
Student Learning Outcomes
At the completion of this exercise, the student will be able to:
1. Discuss the role of genetics in biology and in society:
2. Define the following terms: chromosome, gene, allele, homozygous,
heterozygous, genotype, phenotype, dominant trait, and recessive trait.
3. Discuss expected results and observed results based on the inheritance of one
trait.
4. Explain the inheritance of some common genetic traits in humans.
5. Discuss basic principles of population genetics.
6. Construct a simple pedigree.
OVERVIEW
“She has her father’s eyes.” “Do you think I’ll be bald like dad?” ”Can two blue-eyed
parents have a brown-eyed child?” “If my black-and-white cat mates wit my neighbor’s
yellow cat, will we have any calico kittens?” “How can I increase the chances of my next
litter of puppies being champions?” “What are the odds of two carrier parents having a
child with cystic fibrosis?” The questions are endless when people begin discussing
inheritance. People are naturally curios about how traits are inherited from one generation
to the next. That’s genetics!
As we approach the dawn of the age of genetics with its vast potential of
understanding the human genome and engineering the gene themselves, it is hard to
believe that the science of genetics had its humble origins in an obscure monastery
garden in Austria in the mid-1800’s. Our fundamental knowledge of genetics is a result
primarily of the experiments conducted by an Austrian monk, Gregor Mendel (Fig. 15.1).
His investigation into the inheritance patterns of certain characteristics of pea plants is the
classical origin of modern genetics. Ironically, the significance of Mendel’s work was not
appreciated until the early 1900s.
Each chromosome consists of thousands of structural and functional units called
genes (units of heredity), which are segments of DNA. Today, we are beginning to
understand how genes work and where they are located on the chromosomes. As
examples, the gene for Alzheimer’s disease was found on chromosome 21 and the gene
for cystic fibrosis has been identified on chromosome 7. In a typical human (that’s you,
hopefully!) nearly 20,000 genes are responsible for producing the traits that make you
human, as well as the characteristics that make you a unique member of the human race.
A trait may be controlled by one pair of genes, or it may be controlled by more than one
pair of genes.
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Figure 15.1 Gregor Mendel (1822 – 1884) has been called “The Father of Genetics.”
Because genes are contained on homologous chromosomes, two members of a
gene pair can be alike or different. The possible form that a gene may take is called an
allele. If an individual possesses two identical alleles, they are said to be homozygous. If
an individual possesses two different alleles, they are said to be heterozygous. An
individual‘s genetic make-up, or genotype, in turn influences one’s physical
characteristics, the phenotype. In many cases, one allele may take over or prevent or
mask the expression of another allele. This allele is called the dominant allele, and the
allele that is not expressed is called the recessive allele. Other types of inheritance, such
as incomplete dominance and codominance, as well as other variables that influence an
individual’s phenotype will be discussed in lecture.
For simplicity, the following lab has been designed to consider traits that are
controlled by only one pair of genes. Keep in mind that the majority of human traits are
controlled by more than one pair of genes. In this laboratory experience, students’
knowledge of the fundamental mechanisms of genetics will be reinforced by developing a
simple model of inheritance and conducting a human genetic traits survey.
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___________________________________________
Last Name, First Name [lab partner N0. 1]
____________________________________________
Last Name, First Name [lab partner N0. 2]
_______________________________
_______________________________
Last Name, First Name [lab
partner N0. 3]
___________________________
Last Name, First Name [lab
_______________
Section
group #
partner N0. 4]
____________________
Date
STUDENT ACTIVITY – UNDERSTANDING HEREDITY
Warming Up: Understanding Heredity
If the genotypes of both parents are known, the expected genotypes and phenotypes of
their offspring can be calculated, either by mathematical methods or by using a Punnett
square. This handy genetic device is named in honor of the British geneticist Reginald C.
Punnett (1875-1967).
Punnett squares make predictions about expected results based upon laws of
probability. In nature, however, the expected results may not agree with the observed
results. Observed results are those appear in the offspring as a result of random
combinations of the genes. This exercise has been developed to help students understand
the concepts of expected results and observed results.
Students will work in teams of two. Assume that I pennies, heads are dominant to
tails, assigning heads as (H) and tails as (h).
Materials
1. Paper
2. Pennies
Procedure 15.1 Understanding Heredity
1. Complete the following Punnett square based upon two heterozygous parents.
Record the genotype s and phenotypes of the results below.
1/2
1/2
H
1/2
h
H
1/2
h
2. Determine the expected genotypes and phenotypes for the Punnett square.
Genotypes:
__
Phenotypes:
3. Using the expected genotypes and phenotypes, predict the genotype and
phenotype combinations for 100 tosses.
Genotypes: _____________________________________________________
Phenotypes: ____________________________________________________
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4. Place two pennies in your hand, and then toss them onto the tabletop. Tally the
letter combinations below, and record your group’s result, as well as class totals,
on the charts (tables) provided.
I. Group Data:
Expected and Observed Genotypes
Expected Genotypes
100 tosses
Observed Genotypes
100 tosses
HH
Hh, hH
hh
Expected and observed Phenotypes
Expected phenotypes
100 tosses
Observed phenotypes
100 tosses
Heads
Tails
II. Class Data:
Expected and Observed Genotypes
Expected Genotypes
100 tosses
Observed Genotypes
100 tosses
HH
Hh, hH
hh
Expected and observed Phenotypes
Expected phenotypes
100 tosses
Observed phenotypes
100 tosses
Heads
Tails
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1. What did the pennies represent in the exercise? Were they an accurate representation?
Why or why not?
2. Why were two coins used?
3. What is the difference between expected and observed results?
4. Compare the observed genotypes and phenotypes, from your group and the class, to th
the expected genotypes and phenotypes.
5. Did you notice any relationship between the number of tosses and the expected and
observed results?
6. In nature, what variables may affect the expected and observed results in a population?
7. What is the advantage of using larger populations in studies?
My scientific studies have afforded me great gratification; and I am convinced that it will
not be long before the whole world acknowledges the results of my work.
- Gregor Mendel (1822-1884)
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___________________________________________
Last Name, First Name [lab partner N0. 1]
____________________________________________
Last Name, First Name [lab partner N0. 2]
_______________________________
_______________________________
Last Name, First Name [lab
partner N0. 3]
___________________________
Section
Last Name, First Name [lab
_______________
group #
partner N0. 4]
____________________
Date
It’s all in the Genes: Understanding Basic Mendelian Genetics
In order to understand how alleles are passed on from parent to offspring, a
Punnet square is often used. A Punnett square – shows the possible combination of
alleles (genotypes and phenotypes) of offspring that can result. The following Punnet
square is a monohybrid cross since it is examining only one characteristic.
In order to complete a Punnett square, the alleles from one parent are listed on the top of
the Punnett square and the alleles from the other parent are listed along the side. It does
not matter which parent is along the top and which is listed along the side.
The above Punnett square is a monohybrid cross since it is examining only one characteristic.
Exercise 1: Let’s begin by randomly assign the letter “D” height. Being tall is dominant
(D) and being short is recessive (d). Complete the following Punnett square based upon
two heterozygous parents; record the genotypes, phenotypes, percent, and ratio
(genotypic, phenotypic)
What is the genotype of each parent? _________________________________________
What is the phenotype of each parent? ________________________________________
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Genotype
Phenotype
%
Genotypic
Ratio
Phenotypic
Ratio
What are the possible genotypes? ____________________________________________
What are the possible phenotypes? ___________________________________________
What is the phenotypic ratio? _______________________________________________
Now let’s try a dihybrid cross which looks at two characteristics instead of one.
There are several ways to determine these combinations. One way is the (foil
method), give information a try.
Exercise 2: Let’s assign the letter “A” for eye color. Let’s assume that in eye
color brown eyes are dominant and blue eyes re recessive. Let’s assign the
letter “B” for hair color. Let’s assume that in hair color brown hair is dominant
and blonde hair is recessive.
* Let’s say that the father is heterozygous for eye color (brown eye) and
heterozygous for hair color (brown hair) his genotype would be: “AaBb”.
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To figure out the 4 possible combination of alleles the dad could donate used the
“foil method”
(1)
(4)
(2)
(3)
List four possible combinations of alleles the father can give:
_______________
_______________ _______________ _______________
* If the mother is heterozygous dominant for eye color (brown eyes) and
homozygous recessive for hair color (blonde hair),
What is the mother genotype? _________________________________________
Figure out the four possible combinations of alleles the mother could give
_______________
_______________ _______________ _______________
Using the possible combination of alleles from each parent for eye color and
hair to fill in the Punnett square below.
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Genotype
Phenotype
%
Genotypic
Ratio
Phenotypic
Ratio
Human Genetics
In this exercise you and your partner will find out the genotype and phenotype of an
individual. The alleles necessary to make this individual will be determined by a random
toss of dice so, individuals from each group will probably look very different.
For the purpose of this experiment, the heritance of many characteristics has been
simplified. For example, eye color, hair color, and texture involve much more complex
process than portrayed by this exercise. See your textbook for a more detailed
explanation of how these traits are inherited. Since most of the characteristics that will
make up your individual occur with the domination of one allele from one parent and the
other allele from the other parent, you will only have one chance to determine the
genotype for each characteristic.
1) Sex: First look at expected outcome for determining se. Complete the Punnett
square below using “XX” (female) as one parent and “XY” (male) as the other.
Male
Female
½X
½Y
½ X
½ X
Genotype
Phenotype
%
Genotypic
Ratio
Phentypic
Ratio
What percentage of the offspring will be female? ________________________________
What percentage of offspring will be male? ____________________________________
At the end of today’s lab, you will be able to compare the expected outcome with the
random outcome generated by the class.
Continue working in pairs; each partner will toss a dice in the air to determine the X and Y
chromosome. If the sum of your two dice is an even number, then the sex of your individual is
female (XX); if the sum of your two dice is an odd number, then the sex of your individual is
male (XY), write the genotype in the space below.
Genotype: _____________________
Phenotype: __________________________
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For the remaining characteristics, you will be looking at homozygous and heterozygous traits.
Each trait will be determined to be homozygous recessive, homozygous dominant, or
homozygous. Letters representing the alleles will be given to you: the uppercase letter is always
dominant and the lowercase letter is always recessive. If both dice rolled are even numbers, it
indicates Homozygous Dominant alleles. If both dice are odd, it indicates homozygous
recessive alleles; if one is odd and the other even, it indicates heterozygous alleles.
2) Eyebrows: Use the alleles “A” and “a” for this trait. Separate brows are dominant,
whereas unibrow (o one continuous brow) is recessive.
Genotype: ______________________
Phenotype: _______________________
3) Eyebrow rising: Use the alleles “B” and “b” for this trait. The ability to raise the
eyebrow is dominant, if not it is recessive.
Genotype: ______________________
Phenotype: _______________________
4) Widows peak: Use the alleles “C” and “c” for this trait. Having a V- shaped hairline in
the middle of the forehead is dominant, while having a straight forehead hairline is
recessive.
Genotype: ______________________
Phenotype: _______________________
5) Tongue roll: Using the alleles “D” and “d” for this trait. The ability to roll you
tongue into a U-shaped is dominant; the inability to roll the tongue is recessive.
Genotype: ______________________
Phenotype: _______________________
6) Earlobe: Using the alleles “E” and “e” for this trait. Detached or free earlobes are
dominant, while attached earlobe is recessive.
Genotype: ______________________
Phenotype: _______________________
7) Ear wiggling: Using the alleles “F” and “f” for this trait. The ability to wiggle
your ears is dominant; the inability to wiggle your ear is recessive.
Genotype: ______________________
Phenotype: _______________________
8) Face shape: Using the alleles “G” and “g” for this trait. Homozygous dominant is
square, homozygous recessive is oval, and heterozygous is round.
Genotype: ______________________
Phenotype: _______________________
9) Chin shape: Using the alleles “H” and “h” for this trait. Round chin is homozygous
dominant, pointing chin is homozygous recessive, and square chin is heterozygous.
Genotype: ______________________
Phenotype: _______________________
10) Dimpled chin: Using the alleles “J” and “j” for this trait. The presence of
dimpled chin is dominant; the absence of dimpled chin is recessive.
Genotype: ______________________
Phenotype: _______________________
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11) Freckles: Using the alleles “K” and “k” for this trait. The presence of freckles is
dominant; the absence of freckles is recessive.
Genotype: ______________________
Phenotype: _______________________
12) Dimpled cheeks: Using the alleles “L” and “l” for this trait. The presence of dimples in
one or both cheeks is dominant; the absence of dimples cheek is recessive.
Genotype: ______________________
Phenotype: _______________________
13) Eye shape: Using the alleles “M” and “m” for this trait. Round eyes are
dominant, while almond shaped eyes are recessive.
Genotype: ______________________
Phenotype: _______________________
14) Eye color: Using the alleles “N” and “n” for this trait. Brown eyes are homozygous
dominant, blue eyes are homozygous recessive, green eyes are heterozygous.
Genotype: ______________________
Phenotype: _______________________
15) Hair color: Using the alleles “O” and “o” for this trait. Brown hair is homozygous
dominant, blonde hair is homozygous recessive, and red hair is heterozygous.
Genotype: ______________________
Phenotype: _______________________
16) Hair texture: Using the alleles “Q” and “q” for this trait. Straight hair is homozygous
dominant, curly hair is homozygous recessive, and wavy hair is heterozygous.
Genotype: ______________________
Phenotype: _______________________
17) Bent little finger: Using the alleles “P” and “p” for this trait. If the little finger bends
towards the ring finger it is caused by a dominant allele. If the little finger is straight it is
recessive.
Genotype: ______________________
Phenotype: _______________________
18) Hitchhiker thumb: Using the alleles “R” and “r” for this trait. If the thumb is straight
it is a dominant allele, the ability to bend the thumb back at a 60-degree angle it is
recessive.
Genotype: ______________________
Phenotype: _______________________
19) Finger hair: Using the allele “S” and “s” for this trait. Having hair in the middle
segment of your fingers is dominant; having no hair in the middle segment of your
fingers is recessive.
Genotype: ______________________
Phenotype: _______________________
20) Long toe: Using the allele “T” and “t” for this trait. If the second toe is longer than the
big toe it is dominant, if the second toe is shorter than the big toe it is recessive.
Genotype: ______________________
Phenotype: _______________________
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Complete table 1 using the genotype and phenotype obtained through rolling dice.
Characteristics
Phenotype
Genotype
Sex (Female or Male)
Eyebrow (Unibrow or separate)
Eyebrow rising (yes or no)
Widow’s peak (yes or no)
Tongue roll (yes or no)
Earlobe (Attached or Detached)
Ear wiggling (yes or no)
Face shape (square, round, oval)
Chin shape (square, round, pointed)
Dimpled Chin (yes or no)
Freckles (yes or no)
Dimpled cheeks (yes or no)
Eye shape (Round or Almond)
Eye color (brown,Blue,Green)
Hair color (brown, blonde, red)
Hair texture (straight, curly, wavy)
Bent little finger (yes or no)
Hitchhiker thumb (yes or no)
Finger hair (yes or no)
Long toe (long or Short)
Table 1: Genotype and phenotype description of individuals created through rolling dice.
Determining the frequencies for each characteristics
Frequency can be determined by calculating the number of times each allele
combination was obtained divided by the total number of times the dice was thrown.
For instance, if the class results for sex determine were 7 XX combinations and 3 XY
combinations, the frequency would be calculated like so:
Freq. XX = 7/10 = 0.70
Freq. XY = 3/10 = 0.30
Notice that the frequencies always add up to 1. Determine the frequencies for each
characteristic. Use the highest frequency for each characteristic to determine the
characteristic created by the entire class. Record your data in Table 2.
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#
1
Genotypes
Characteristics Homozygous Homozygous Heterozygous
Dominant
recessive
Sex
Phenotype
Male
Female
2
Eyebrows
Unibrow
3
Eyebrow rising
Yes
No
4
Widow’s peak
Yes
No
5
Tongue roll
Yes
no
6
Earlobe
Attached
Detached
7
Ear wiggling
Yes
No
8
Face shape
Square
Round
9
Chin shape
Square
Round Pointed
10
Dimpled chin
Yes
No
11
Freckles
Yes
No
Yes
No
Round
Almond
12 Dimpled cheeks
Separate
Oval
13
Eye shape
14
Eye color
Brown
Blue
Green
15
Hair color
Brown
Blonde
red
16
Hair texture
Straight
Curly
Wavy
17
Yes
No
Yes
No
19
Bent little
finger
Hitchhiker
thumb
Finger hair
Yes
no
20
Long toe
Long
Short
18
Table 2: Average Percent Genotypic and Phenotypic description on individuals is
created through rolling of dice.
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Characteristics
Highest
Frequency
Genotype
Phenotype
Sex (Female or Male)
Eyebrow (unibrow or separated)
Eyebrow rising (yes or no)
Widow’s peak (yes or no)
Tongue roll (yes or no)
Earlobe (attached or detached)
Ear wiggling (yes or no)
Face shape (square, Round, Oval)
Chin shape (Square, Roud, Oval)
Dimpled chin (yes or no)
Freckles (yes or no)
Dimpled cheeks (yes or no)
Eye shape (Round or Almond)
Eye color (Brown, blue, green)
Hair color (Brown, blonde, red)
Hair texture (Straight, curly, wavy)
Bent little finger (yes or no)
Hitchhiker thumb (yes or no)
Finger hair (yes or no)
Long toe (Long or short)
Table 3: Genotypic and Phenotypic description and frequencies of allele
combinations of an individual created through rolling of dice class results.
Look at the results of your first Punnett square for sex determination. You should
have come up with 50% females and 50% males (or a frequency of .50 for each sex).
How does that compare to the frequency of males and females generated by the class?
____________________________________________________________________
____________________________________________________________________
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STUDENT ACTIVITY-DEVELOPING PEDIGREES
Developing Pedigrees
For many people, the term pedigree brings about images of championship dogs
and throughout horses. Essentially everything from fruit flies to watermelons has a
pedigree. To geneticists, however, a pedigree is a valuable tool resembling a family
tree that can be used to display family relationships and to track traits through a
family. In medical genetics, pedigrees are helpful in understanding how disorders
appear in families.
Pedigrees are particularly valuable in understanding the inheritance of unifactorial
(single-gene) traits such as albinism, cystic fibrosis, and hemophilia. A pedigree can
be used to visually represent Mendelian inheritance of a trait. A typical pedigree
consists of universally accepted symbols connected by ether horizontal or vertical
lines. Generations are represented by Roman numerals, and Arabic numerals
represent individuals. Filled shapes represent individuals who express a trait, and half
shaded shapes represent carriers. A few common symbols appear below.
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Using a pedigree, autosomal recessive traits such as cystic fibrosis are easy to
follow through the generations.
1. With your knowledge of pedigrees explain the inheritance of cystic fibrosis (CF) in two
children of the third generation in the pedigree below.
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________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Explanation: The inheritance of autosomal dominant traits also can be explored
through pedigree analysis. Polydactylism, having extra digits, results from a
dominant gene.
2. Using the following pedigree, explain the appearance of polydactyl in children 1, 2,
and 4 of generation 3.
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________________________________________________________________________
________________________________________________________________________
____________________________________________________________________________________________________________
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Explanation: X-linked traits are carried exclusively on the X chromosome.
Because a male possesses only one X chromosomes, if he receives an X chromosome that
carries an X-linked trait, he will express that trait. For a female to express an X-linked
trait, she will have to inherit two copies of the gene, one on each X chromosome. Several
conditions, such as hemophilia A and color-blindness, are X-linked recessive traits.
Knowing the following; construct a pedigree for color blindness in three
generations of a family below. Answer the provided questions. In the first generation,
neither parent was color-blind. The second generation had four children. In the birth
order, one was a normal female, one was an affected male, another was an unaffected
female, and one was a normal male. The first unaffected female had one unaffected
daughter and one affected son. How did this happen, and what is the genotype of her
husband? The affected male had one affected son and one affected daughter. How did
this occur? The second unaffected female had three unaffected daughters and four
unaffected sons.
1. Explain the inheritance pattern. The normal male had three unaffected sons. How did
this happened?
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________________________________________________________________________
________________________________________________________________________
2. Construct your own pedigree on the next page. Using the traits discussed in prior
Exercises create a pedigree for your family, going as far back as possible. If you do
not know the phenotype or genotype of your ancestors construct a hypothetical
family tree for one of the traits performed in the exercise.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
I'm one of those people you hate because of genetics. It's the truth.
- Brad Pitt (1963- present)
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Pedigree for color blindness in three generations of a family.
Pedigree of your family.
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___________________________________________
Last Name, First Name [lab partner N0. 1]
____________________________________________
Last Name, First Name [lab partner N0. 2]
_______________________________
_______________________________
Last Name, First Name [lab
partner N0. 3]
___________________________
Section
Last Name, First Name [lab
_______________
partner N0. 4]
____________________
group #
Date
REVIEW QUESTIONS
1. Why is genetics considered to be one of the most important disciplines of biology?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
2. What is the difference between genotype and phenotype?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
3. What is the Hardy-Weinberg equation, and when is it used?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
4. Why is inbreeding dangerous?
________________________________________________________________________
________________________________________________________________________
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5. What are the chances of two parents who carry the gene for albinism
(an autosomal recessive disorder) having a child without albinism?
________________________________________________________________________
6. Before considering starting a family, do you think it is reasonable to perform
genetic testing on you and your spouse for common inherited disorder?
Why or why not?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
7. In Manx cats, the allele “TT” yield a cat with a normal long tail, the alleles “Tt”
yield a cat with a short or absent tail, and the alleles “tt” are lethal to the embryo.
Predict the genotypes and phenotypes of potential kittens resulting from the
mating of two short tailed cats.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
8. American cat breeders are trying to establish a new breed of cat with unusual
rounded, curled-back ears to be known as the “curl cat”. Suppose you found a
curl cat and wanted to secretly start your population. How would you determine
whether the curl allele is dominant or recessive? How would you establish and
maintain a true-breeding population based on whether the allele is dominant or
recessive?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
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9. Achondroplasia, the most common type of human dwarfism, affects 1 in 15,000 to
1 in 40,000 individuals. Many cases of achondronplasia are the result of a
spontaneous mutation. Once the mutation occurs, however, it becomes an
autosomal dominant trait. On a TV talk show, the audience was shocked to learn
that two individuals with achondroplasia had a son who was of normal height and
two other sons and a daughter with achodronplasia. Draw a pedigree of the
family and explain how the parents can have a child with normal height.
________________________________________________________________________
________________________________________________________________________
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10. The following pedigree illustrates the inheritance of Tay-Sachs disease in four
generations of a family. Interpret the pedigree and determine whether the traits
is dominant or recessive. What happened in the third generation? What are the
symptoms and incidence of Tay-Sachs disease?
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________________________________________________________________________
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