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10
PATTERNS
OF
INHERITANCE
CHAPTER REVIEW
In 1860, Gregor Mendel formulated two laws of heredity
that serve as the foundation for understanding the results
of most genetic crosses from peas to human beings. Therefore, you should study Mendel’s laws carefully because most
of the crosses you will be asked to do are explained by
them.
Exceptions to Mendel’s laws abound and include incomplete dominance, inheritance by multiple alleles, polygenic inheritance, and pleiotropy. Sex-linked inheritance
is another exception of Mendel’s laws, which has historical significance. The usual manner in which alleles on
the X chromosome are inherited allowed investigators to
determine that the genes are on the chromosomes.
The alleles that occur on a single chromosome are
said to form a linkage group because they tend to be inherited together. Before modern methods were available
to map the chromosomes, crossing-over data was used to
construct chromosome maps.
CHAPTER KEY TERMS
After studying the key terms of this chapter, match the phrases below with the alphabetized list of terms.
allele
linkage group
carrier
multifactorial trait
chromosome map
phenotype
codominance
pleiotropy
dihybrid
Punnett square
gene locus
testcross
genotype
wild-type
heterozygous
X-linked
homozygous
a. one gene affects many characteristics _______________________
b. allele that is located on an X chromosome _______________________
c. having a dominant and recessive allele for a trait _______________________
d. several alleles located on same chromosome; inherited together _______________________
e. specific location of a particular gene _______________________
f. the results indicate whether the individual is homozygous dominant or heterozygous _______________________
g. having two identical alleles for a trait _______________________
h. alternative form of a gene _______________________
i. grid used to calculate the expected results of genetic cross _______________________
j. both alleles of a gene are equally expressed in the heterozygote _______________________
k. having several phenotypes whose frequency results in a bell-shaped curve _______________________
l. alleles of an individual _______________________
m. individual who appears normal but can pass on an allele for a recessive disorder _______________________
n. outward appearance; due to genotype and environmental influences _______________________
o. heterozygous for two traits _______________________
p. sequence that shows the relative distance between gene loci on a chromosome _______________________
q. allele or phenotype that is the most common for a certain gene in a population _______________________
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S T U DY E X E R C I S E S
Study the text section by section as you answer the questions that follow.
10.1
MENDEL’ S
LAWS (PP. 142–148)
• Gregor Mendel discovered certain laws of heredity after doing experiments with garden peas during the mid1800s.
• When Mendel did one-trait crosses, he found that each organism contains two factors for each trait and that
the factors separate during formation of gametes.
• A testcross can be used to determine the genotype of an individual with the dominant phenotype.
• When Mendel did two-trait crosses, he found that every possible combination of factors is present in the
gametes.
• A testcross can also be used to determine the genotype of an individual that is dominant in two traits.
1. The following characteristics made the garden pea a good subject for Mendel’s experiments. Match the
characteristics with the statements:
1. Easy to cultivate and have a short generations time.
2. Can be cross-pollinated.
3. When self-pollinated, they breed true.
4. Traits are easily observable.
a.
The results of a cross are due to the experiment and not due to an inherent instability.
b.
This characteristic allows the experimenter to decide the parents of a cross.
c.
The offspring of each type can easily be counted.
d.
This characteristic will give a large number of offspring within a short time.
2. By using the letters T (tall) and t (short), illustrate the Law of Segregation:
a.
Each individual has two factors for each trait. What are the possible combinations of these two
letters?
b.
The factors separate during the formation of the gametes. What are the possible gametes?
c.
Fertilization restores the full number of factors. If the gametes T and t fuse, what two factors do the
offspring have?
3. a. A capital letter assigned to a factor, now called an allele, means it is the ______________________________.
b. A lowercase letter assigned to an allele means it is the _______________________________.
4. Complete the following table using the letters T (tall) and t (short), to show the difference between genotype
and phenotype:
Genotype (in words)
Genotype (in letters)
Phenotype
Homozygous dominant
_________________________
_____________________________
Homozygous recessive
_________________________
____________________________
Heterozygous
_________________________
_____________________________
5. Using this key, E = unattached, e = attached, complete Punnett squares for cross 1 and cross 2:
Cross 1
Cross 2
heterozygous × homozygous recessive
heterozygous × heterozygous
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a. What is the phenotypic ratio for cross 1? ______________________
b. What are the chances of the recessive phenotype for cross 1? ___________
c. What is the phenotypic ratio for cross 2? ______________________
d. What are the chances of the recessive phenotype for cross 2? ___________
e. Which of these crosses could be the result of a testcross? ___________
6. A man with a straight hairline reproduces with a woman with a widow’s peak whose father has a straight
hairline and whose mother had a widow’s peak. The couple produce a child with a straight hairline. What are
the genotypes of these individuals involved?
Use the alleles W (widow’s peak) and w (straight hairline):
a. man with a straight hairline _____ _____
b. woman with a widow’s peak _____ _____
c. child _____ _____
d. woman’s father _____ _____
7. By using the letters T (tall), t (short) and G (green), g (yellow), illustrate the Law of Independent Assortment:
a.
Each pair of factors segregates independency of the other pair. Considering the pair Tt, what could
be in the gametes?
b.
Considering the pair Gg, what could be in the gametes?
c.
All possible combinations of factors can occur in the gametes. What are the possible gametes?
8. The cross WwSs × WwSs usually results in a phenotypic ratio close to 9:3:3:1. If W = widow’s peak,
w = straight hairline, S = short fingers, and s = long fingers, then out of 16 individuals:
9 individuals are expected to have the phenotype a. ____________________________________________________.
3 individuals are expected to have the phenotype b. ____________________________________________________.
3 individuals are expected to have the phenotype c. ____________________________________________________.
1 individual is expected to have the phenotype d. ______________________________________________________.
9. A parent with a widow’s peak and short fingers has a child with a straight hairline and long fingers. What is
the genotype of the parent? a. ____________ If the parent were homozygous dominant for both traits, what
would be the phenotype of the child? b. ____________
10. a. Complete a Punnett square for the cross in horses BbTt × Bbtt, where:
B = black
b = brown
T = trotter
t = pacer
b. What is the phenotypic ratio among offspring? _________________: _________________;
_________________: _________________
11. Mendel was a mathematician who understood the laws of probability. When we use a a. ______________ square
to calculate the results of a cross, we are using the laws of probability.
12. Today, we know that Mendel’s laws hold because of the manner in which the process a. ______________ occurs.
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10.2 BEYOND MENDEL’S LAWS (PP. 149–151)
• The genotype must be considered an integration of all the genes as a whole, because genes often work together
to control the phenotype.
• Certain forms of inheritance involve degreees of dominance, multiple alleles, and polygenes.
• Environmental conditions can influence gene expression.
• Sometimes one mutation affects many parts of the body, causing a syndrome.
13. In the following diagram of four o’clocks, complete the genotype for a and c and give the genotype for b using
this key, C R = red, C W = white.
a. CR
b.
c. CW
d. Why is this incomplete dominance? _______________________________
14. The alleles for ABO blood groups are as follows:
I A = A antigen on red blood cells
I B = B antigen on red blood cells
i = neither A nor B antigen on red blood cells
a. Why is this pattern of inheritance called multiple alleles? ______________________________
b. How many of these alleles does each person have? ______________________________
c. Considering that some individuals have the genotype I A I B , blood type AB, why is this pattern of inheritance
also an example of codominance? ______________________________
d. A man with blood type A reproduces with a woman who has blood type B. Their child has blood type O.
Give the genotype of all persons involved: man ______________, woman ______________, and child _________.
e. If a child has AB blood and the father has type B blood, what could the genotype of the mother be? _______
15. Consider a model in which there are three gene pairs of alleles; a dominant allele in any pair adds pigment to
the skin. Use the letters A, B, C to indicate pigment formation and a, b, c to indicate lack of pigment
formation:
a. What is the genotype for the darkest individual? ______________
b. What is the genotype for the lightest individual? ______________
c. What is the genotype for the offspring from a cross of the individuals from a and b? ______________
d. How does the skin color of this person compare to either of the parents? ______________
16. A person with Marfan syndrome has many deformities, all due to connective tissue defects. A person with
sickle cell disease has many physiological problems due to red blood cells that are sickle-shaped. How do you
define pleiotropy? ______________________________
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10.3 SEX-LINKED INHERITANCE (PP. 152–153)
• Certain traits, unrelated to gender, are controlled by genes located on the X chromosome.
• Alleles on the X chromosome have a pattern of inheritance that differs from that of alleles on the autosomes.
17. Study the adjacent Punnett square:
a. From these results, how can you infer that the
×
alleles for red/white eye color are on the X
chromosome? ______________________________
R
X RX r
X Y
___________________________________________
eggs
___________________________________________
b. From which parent do males get an X-linked
XR
Xr
X RX R
X RX r
X RY
X rY
allele? ______________________________
XR
sperm
____________________________________
____________________________________
c. Why don’t any of the females have white eyes?
Y
______________________________
d. Why is a fly with the genotype X R X r called a
carrier? ______________________________
_____________________________________
_____________________________________
offspring
Phenotypic Ratio
females: all red-eyed
males: 1 red-eyed
1 white-eyed
Key:
X R = red eyes
X r = white eyes
18. If XB = normal vision and Xb = color blindness, state the sex and the phenotype of each of these genotypes:
XBXB
a. _________________________
XBXb
b. _________________________
XbXb
c. _________________________
XBY
d. _________________________
XbY
e. _________________________
19. Under what circumstances can a couple have a color-blind daughter?
__________________________
20. Use the Punnett square at the right to show the expected outcome if a color-blind woman reproduces with a
man who has normal vision:
a. What are the chances of a color-blind daughter? ______________
b. What are the chances of a color-blind son? ______________
21. A son is color blind, but his mother and father are not color blind. Give the genotype
of all persons involved:
son ____________________
mother _________________
father __________________
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10.4 INHERITANCE
OF
LINKED GENES (PP. 154–155)
• Despite linkage, crossing-over can bring about recombinant gametes, and recombinant phenotypes do occur.
• The percentage of recombinant phenotypes when alleles are linked can be used to map that chromosome.
22. All the genes on one chromosome form a linkage group and tend to be inherited together. Mendel’s law of
independent assortment ______________ (does/does not) hold for linked genes.
In questions 23–24, consider that, in humans, arched eyebrow (E) is dominant over curved eyebrow (e) and hitchhiker
thumb (T) is dominant over normal thumb (t). Imagine that these two genes are linked and that two dihybrids having
these gametes reproduce.
23. From the diagram, decide the phenotype of the offspring:
a. __________________________________________
b. __________________________________________
c. __________________________________________
d. __________________________________________
24. What is the phenotypic ratio among the offspring? a. _____________________________ What would the ratio
have been if these genes were not linked? b. ______________
25. The percentage of crossing-over indicates the distance between two gene loci. For example, A and B
recombine with a and b in 10% of the offspring, A and C recombine with a and c in 20% of the offspring, and
B and C recombine with a and c in 10% of the offspring.
How many map units are between the following?
a. A and C ______________
b. A and B ______________
c. B and C ______________
d. Draw a line to represent the chromosome and map the positions of A, B, and C here:
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D E F I N I T I O N S WO R D S E A R C H
Review key terms by completing this wordsearch and filling in the blanks below, using the following alphabetized list of terms:
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f.
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______________________
______________________
______________________
______________________
______________________
______________________
______________________
allele
genotype
locus
multiple
phenotype
Punnett
X-linked
Alternative form of a gene on a chromosome.
Appearance due to expression of a gene.
Particular site where a gene is found on a chromosome.
Genetic makeup of an individual.
Type of square used to determine genetic outcome.
Carried on the X chromosome.
More than two alleles for one trait are present in the population. (_______ allele)
CHAPTER TEST
OBJECTIVE QUESTIONS
Do not refer to the text when taking this test.
1. When Gregor Mendel began crossing plants,
most breeders of organisms believed that
a. dominance was complete.
b. many genes affected one trait.
c. red and white flowers produced pink offspring.
d. the genetic material was always stable.
2. The phenotypic ratio from a genetic cross is
1:1:1:1. The genotypes of the parents are
a. TTGG × TtGg.
b. TtGG × Ttgg.
c. TtGg × ttgg.
d. Ttgg × ttgg.
3. In guinea pigs, the allele for dark color (B) is
dominant to the allele for light color (b). In a
cross between two heterozygous organisms, the
chance for producing a light-colored offspring is
a. 4⁄5.
b. 3⁄4.
c. 1⁄2.
d. 1⁄4.
4. The two factors for each trait separate when gametes form, so that each gamete contains only one
factor for each trait. This statement is part of
Mendel’s law of
a. dominance.
b. independent assortment.
c. random recombination.
d. segregation.
5. Select the incorrect association.
a. alleles—A and p
b. heterozygous—Aa
c. homozygous—DD
d. homozygous—pp
6. From the cross Aa × Aa, the probability of producing a homozygous dominant offspring is
a. 25%.
b. 33%.
c. 50%.
d. 75%.
7. How many kinds of gametes can an organism
with genotype AaBB produce?
a. one
b. two
c. three
d. four
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8. Members of one pair of factors separate independently of the members of another pair of factors. This is a statement of Mendel’s law of
a. dominant factors.
b. independent assortment.
c. random recombination.
d. segregation.
9. Two organisms, each with the genotype TtGg,
mate. The chance of producing an offspring that
has the dominant phenotype for height and the
recessive phenotype for color is
a. 9⁄16.
b. 7⁄16.
c. 6⁄16.
d. 3⁄16.
10. In humans, S = short fingers and s = long fingers, W = widow’s peak and w = straight hairline.
From the cross SsWW × Ssww, the chance of an
offspring having both dominant traits is
a. 9⁄16.
b. 3⁄4.
c. 3⁄8.
d. 1⁄4.
11. If 25% of the offspring of one set of parents show
the recessive phenotype, the parents were probably
a. both homozygous recessive.
b. both homozygous dominant.
c. both heterozygous.
d. one homozygous dominant, one homozygous
recessive.
12. If only one parent is a carrier for a dominant disease,
what is the chance a child will have the condition?
a. 25%
b. 50%
c. 75%
d. no chance
13. Polygenic inheritance can explain
a. a range in phenotypes among the offspring.
b. the occurrence of degrees of dominance.
c. incomplete dominance.
d. Both a and c are correct.
14. Two individuals with medium-brown skin color
could have children who are both darker and
lighter than they are.
a. true
b. false
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15. Which children could not have parents both with
type A blood?
a. type A
b. type O
c. type AB
d. type B
e. Both c and d are correct.
16. Which children could not have a parent with type
AB blood?
a. type A
b. type B
c. type AB
d. type O
17. Inheritance by multiple alleles is illustrated by
the inheritance of
a. skin color.
b. blood type.
c. color blindness.
d. Both b and c are correct.
18. The ABO blood group inheritance ia an example
of multiple alleles.
a. true
b. false
Use the following information for questions 19–20: A
mother with a genetic disease has genotype of Aa. The father, who is normal, has a genotype of aa.
19. This trait is inherited as a _______ disease.
a. dominant
b. recessive
c. incomplete dominant
d. X-linked
20. What percentage of their children could have the
disease?
a. none
b. 25%
c. 50%
d. 100%
21. A woman who is not color blind but has an allele for color blindness reproduces with a man who
has normal color vision. What is the chance they
will have a color-blind daughter?
a. 50%
b. 25%
c. 100%
d. no chance
22. A color-blind woman reproduces with a man who
has normal color vision. Their sons will
a. be like the father because the trait is X-linked.
b. be like the mother because the trait is X-linked.
c. all have normal color vision.
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CRITICAL THINKING QUESTIONS
The introduction to this chapter suggests that both genes and the environment play a role in the development of a genetic disorder.
23. Nutritionists know that a diet rich in sodium (salt) can contribute to high blood pressure. What advice would
you give the members of a family that tends toward high blood pressure? ____________________________________
24. Most everyone knows that smoking contributes to lung cancer. If a person’s father dies of lung cancer, what
advice would you give this person? _________________________________________________________________________
25. Is it helpful to know that both environmental conditions and genes cause genetic disorders? __________________
Explain. _________________________________________________________________________________
26. What is the best approach if you know a certain illness runs in your family? __________________________________
_____________________________________________________________________________________________________________
Test Results: ______ number correct ÷ 26 = ______ × 100 = ______ %
EXPLORING
THE
INTERNET
ARIS, the Essentials of Biology website: http://www.mhhe.com/maderessentials
ARIS, the website for Essentials of Biology, offers access to a wide variety of tools to help students learn biological
concepts and to reinforce their knowledge. Online study aids such as practice quizzes, interactive activities, animations, labeling exercises, flashcards, and much more are organized according to the major sections of each chapter.
There is even an online tutorial service!
ANSWER KEY
CHAPTER KEY TERMS
a. pleiotropy b. X-linked c. heterozygous d. linkage
group e. gene locus f. testcross g. homozygous h. allele
i. Punnett square j. codominance k. multifactorial trait
l. genotype m. carrier n. phenotype o. dihybrid p. chromosome map q. wild-type
STUDY EXERCISES
1. a. 3 b. 2 c. 4 d. 1 2. a. TT, tt, Tt b. T, t c. Tt
3. a. dominant allele. b. recessive allele.
4.
Genotype (in letters)
Phenotype
TT
tall
tt
short
Tt
tall
5. a. 1:1 b. 50% c. 3:1 d. 25% e. Cross 1 6. a. ww
b. Ww c. ww d. ww 7. a. T or t b. G or g c. TG, tg, Tg,
tG 8. a. widow’s peak and short fingers b. widow’s
peak and long fingers c. straight hairline and short fingers
d. straight hairline and long fingers 9. a. WwSs b. widow’s
peak and short fingers
10. a.
Bt
bt
BT
BBTt
BbTt
Bt
BBtt
Bbtt
bT
BbTt
bbTt
bt
Bbtt
bbtt
b. 3 black trotters: 3 black pacers; 1 brown pacer: 1 brown
trotter 11. Punnett 12. meiosis 13. a. C R b. C R C W
c. CW d. Because neither CR nor CW is dominant over the
other. 14. a. There are more than two alleles for the
trait. b. 2 c. Both A and B are expressed equally.
d. I A i, I B i, ii e. I A I A or I A i 15. a. AABBCC b. aabbcc
c. AaBbCc d. intermediate between the two 16. A condition in which a single gene has more than one effect.
17. a. Males have either red or white eyes and only one X
chromosome. b. female c. Inheritance of a recessive allele is offset by inheritance of a dominant allele. d. Appears normal but can pass on the mutant allele.
18. a. female with normal vision b. female who is a carrier c. female who is color blind d. male with normal vision e. male who is color blind 19. If the father is color
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blind and the mother is a carrier or color blind.
20. a. none b. 100% 21. son X b Y, mother X B X b , father
XBY 22. does not 23. a. arched, hitchhiker b. curved,
normal c. arched, hitchhiker d. arched, hitchhiker
24. a. 3 arched, hitchhiker: 1 curved, normal b.9:3:3:1
25. a. 20 units b. 10 units c. 10 units
25. d.
A
B
10 map units
10 map units
C
20 map units
DEFINITIONS WORDSEARCH
P
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L G E N O
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O X L
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E L E
OC U S
T Y P E
I N K E D
N E T T
L L A
a. allele b. phenotype c. locus
nett f. X-linked g. multiple
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d. genotype
e. Pun-
CHAPTER TEST
1. c 2. c 3. d 4. d 5. a 6. a 7. b 8. b 9. d
10. b 11. c 12. b 13. a 14. a 15. e 16. d 17. b
18. a 19. a 20. c 21. d 22. b 23. Go on a lowsalt diet. 24. Don’t smoke. 25. Yes, because we can’t
control what genes we inherit, but we can control certain environmental factors. 26. Learn all you can about
the illness and modify your behavior to help prevent its
occurrence.