1. F1 generation Pink hybrid Rr Gametes R r 2 1 2 1 Figure 9.11A_2
Figure 9.11A_2 Incomplete dominance in snapdragon flower color (part 2) 1

2. F2 generation Sperm R r R RR rR Eggs Rr rr r 1 1 2 2 1 2 1 2
Figure 9.11A_3
F2 generation
Sperm 2 1 2 1 R r 2 1 R RR rR
Eggs 2 1 Rr rr
Figure 9.11A_3 Incomplete dominance in snapdragon flower color (part 3) r 2

3. 9.11 Incomplete dominance results in intermediate phenotypes
Incomplete dominance does not support the blending hypothesis because the original parental phenotypes reappear in the F2 generation.
One example of incomplete dominance in humans is hypercholesterolemia, in which
dangerously high levels of cholesterol occur in the blood and
heterozygotes have intermediately high cholesterol levels.
Student Misconceptions and Concerns
1. After reading the preceding modules, students might expect all traits to be governed by a single gene with two alleles, one dominant over the other. Modules 9.11–9.15 describe deviations from simplistic models of inheritance.
2. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference).
3. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class.
Teaching Tips
1. Incomplete dominance is analogous to a compromise, or a gray shade. The key concept is that both “sides” have input. Complete dominance is analogous to an authoritarian style, overruling others and insisting on things being a certain way. Although these analogies might seem obvious to instructors, many students new to genetics appreciate them.
2. Another analogy for cholesterol receptors is fishing poles. The more fishing poles you use, the more fish you can catch. Heterozygotes for hypercholesterolemia have fewer “fishing poles” for cholesterol. Thus, fewer “fish” are caught and more “fish” remain in the water.
© 2012 Pearson Education, Inc. 3

4. Genotypes Hh Heterozygous
Figure 9.11B
Genotypes Hh Heterozygous
HH Homozygous for ability to make LDL receptors
hh Homozygous for inability to make LDL receptors
Phenotypes
LDL
LDL receptor
Figure 9.11B Incomplete dominance in human hypercholesterolemia
Cell
Normal
Mild disease
Severe disease 4

5. 9.12 Many genes have more than two alleles in the population
Although an individual can at most carry two different alleles for a particular gene, more than two alleles often exist in the wider population.
Human ABO blood group phenotypes involve three alleles for a single gene.
The four human blood groups, A, B, AB, and O, result from combinations of these three alleles.
The A and B alleles are both expressed in heterozygous individuals, a condition known as codominance.
Student Misconceptions and Concerns
1. After reading the preceding modules, students might expect all traits to be governed by a single gene with two alleles, one dominant over the other. Modules 9.11–9.15 describe deviations from simplistic models of inheritance.
2. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference).
3. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class.
Teaching Tips
1. Students can think of blood types as analogous to socks on their feet. You can have socks that match, a sock on one foot but not the other, you can wear two socks that do not match, or you can even go barefoot (type O blood)! Developed further, think of Amber (A) and Blue (B) socks. Type A blood can have an Amber sock with either another Amber sock or a bare foot (or “zero” sock). Blue socks work the same way. One amber and one blue sock represent the AB blood type. No socks, as already noted, represent type O.
2. Consider specifically comparing the principles of codominance (expression of both alleles) and incomplete dominance (expression of one intermediate trait). Students will likely benefit from this direct comparison.
© 2012 Pearson Education, Inc. 5

6. 9.12 Many genes have more than two alleles in the population
In codominance,
neither allele is dominant over the other and
expression of both alleles is observed as a distinct phenotype in the heterozygous individual.
AB blood type is an example of codominance.
Student Misconceptions and Concerns
1. After reading the preceding modules, students might expect all traits to be governed by a single gene with two alleles, one dominant over the other. Modules 9.11–9.15 describe deviations from simplistic models of inheritance.
2. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference).
3. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class.
Teaching Tips
1. Students can think of blood types as analogous to socks on their feet. You can have socks that match, a sock on one foot but not the other, you can wear two socks that do not match, or you can even go barefoot (type O blood)! Developed further, think of Amber (A) and Blue (B) socks. Type A blood can have an Amber sock with either another Amber sock or a bare foot (or “zero” sock). Blue socks work the same way. One amber and one blue sock represent the AB blood type. No socks, as already noted, represent type O.
2. Consider specifically comparing the principles of codominance (expression of both alleles) and incomplete dominance (expression of one intermediate trait). Students will likely benefit from this direct comparison.
© 2012 Pearson Education, Inc. 6

7. Figure 9.12 Multiple alleles for the ABO blood groups
Blood Group (Phenotype)
Antibodies Present in Blood
Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left
Carbohydrates Present on Red Blood Cells
Genotypes O A B AB
IAIA or IAi
Carbohydrate A A
Anti-B
IBIB or IBi
Carbohydrate B B
Anti-A
Carbohydrate A and Carbohydrate B AB
IAIB
None
Figure 9.12 Multiple alleles for the ABO blood groups
Anti-A O ii
Neither
Anti-B
No reaction
Clumping reaction 7

8. Blood Group (Phenotype) Carbohydrates Present on Red Blood Cells
Figure 9.12_1
Blood Group (Phenotype)
Carbohydrates Present on Red Blood Cells
Genotypes
IAIA or IAi
Carbohydrate A A
IBIB or IBi
Carbohydrate B B
Carbohydrate A and Carbohydrate B
Figure 9.12_1 Multiple alleles for the ABO blood groups (part 1) AB
IAIB
Neither O ii 8

9. Blood Group (Phenotype) Antibodies Present in Blood
Figure 9.12_2
Blood Group (Phenotype)
Antibodies Present in Blood
Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB A
Anti-B B
Anti-A AB
None
Figure 9.12_2 Multiple alleles for the ABO blood groups (part 2)
Anti-A O
Anti-B 9

11. 9.13 A single gene may affect many phenotypic characters
Pleiotropy occurs when one gene influences many characteristics.
Sickle-cell disease is a human example of pleiotropy. This disease
affects the type of hemoglobin produced and the shape of red blood cells and
causes anemia and organ damage.
Sickle-cell and nonsickle alleles are codominant.
Carriers of sickle-cell disease are resistant to malaria.
Student Misconceptions and Concerns
1. After reading the preceding modules, students might expect all traits to be governed by a single gene with two alleles, one dominant over the other. Modules 9.11–9.15 describe deviations from simplistic models of inheritance.
2. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference).
3. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class.
Teaching Tips
The American Sickle Cell Anemia Association’s website, is a good place to find additional details.
© 2012 Pearson Education, Inc. 11

12. Figure 9.13A
Figure 9.13A In this micrograph, you can see several jagged sickled cells in the midst of normal red blood cells. 12

13. An individual homozygous for the sickle-cell allele
Figure 9.13B
An individual homozygous for the sickle-cell allele
Produces sickle-cell (abnormal) hemoglobin
The abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped
Sickled cell
The multiple effects of sickled cells
Figure 9.13B Sickle-cell disease, an example of pleiotropy
Damage to organs
Other effects
Kidney failure Heart failure Spleen damage Brain damage (impaired mental function, paralysis)
Pain and fever Joint problems Physical weakness Anemia Pneumonia and other infections 13

14. 9.14 A single character may be influenced by many genes
Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes.
Human skin color is an example of polygenic inheritance.
Student Misconceptions and Concerns
1. After reading the preceding modules, students might expect all traits to be governed by a single gene with two alleles, one dominant over the other. Modules 9.11–9.15 describe deviations from simplistic models of inheritance.
2. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference).
3. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class.
Teaching Tips
1. Polygenic inheritance makes it possible for children to inherit genes to be taller or shorter than either parent. Similarly, skin tones can be darker or lighter than either parent. The environment also contributes significantly to the final phenotype for both of these traits.
2. The authors note that polygenic inheritance is the converse of pleiotropy. This is worth noting in lecture as these concepts are discussed. We often remember concepts better when they are contrasted in pairs.
© 2012 Pearson Education, Inc. 14

15. Figure 9.14 A model for polygenic inheritance of skin color
P generation 
aabbcc (very light)
AABBCC (very dark)
F1 generation 
AaBbCc
AaBbCc
Sperm 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1
F2 generation 8 1 8 1 8 1
Figure 9.14 A model for polygenic inheritance of skin color 8 1
Eggs 8 1 8 1
Fraction of population 8 1 8 1 64 1 64 6 64 15 64 20 64 15 64 6 64 1
Skin color 15

16. aabbcc (very light) AABBCC (very dark)
Figure 9.14_1
P generation 
aabbcc (very light)
AABBCC (very dark)
Figure 9.14_1 A model for polygenic inheritance of skin color (part 1)
F1 generation 
AaBbCc
AaBbCc 16

17. Sperm F2 generation Eggs 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8
Figure 9.14_2
Sperm 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1
F2 generation 8 1 8 1 8 1 8 1
Eggs 8 1 8 1
Figure 9.14_2 A model for polygenic inheritance of skin color (part 2) 8 1 8 1 64 1 64 6 64 15 64 20 64 15 64 6 64 1 17

18. Fraction of population
Figure 9.14_3 64 20 64 15
Fraction of population 64 6
Figure 9.14_3 A model for polygenic inheritance of skin color (part 3) 64 1
Skin color 18

19. 9.15 The environment affects many characters
Many characters result from a combination of heredity and the environment. For example,
skin color is affected by exposure to sunlight,
susceptibility to diseases, such as cancer, has hereditary and environmental components, and
identical twins show some differences.
Only genetic influences are inherited.
Student Misconceptions and Concerns
1. After reading the preceding modules, students might expect all traits to be governed by a single gene with two alleles, one dominant over the other. Modules 9.11–9.15 describe deviations from simplistic models of inheritance.
2. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference).
3. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class.
Teaching Tips
1. The authors note that polygenic inheritance is the converse of pleiotropy. This is worth noting in lecture as these concepts are discussed. We often remember concepts better when they are contrasted in pairs.
2. As the authors are careful to note, although genetics and the environment both contribute to the final phenotypes, only the genetic factors are inherited. This distinction is important to understanding the limitations of Lamarck’s mechanisms of evolution. If you will address principles of evolution soon after this chapter, this may be an important distinction to reinforce in lecture. References to tattoos and piercing may also help to distinguish between environmental influences and inheritance. Students with tattoos will not produce children born with tattoos!
© 2012 Pearson Education, Inc. 19

20. Figure 9.15A
Figure 9.15A The effect of genes and sun exposure on the skin of one of this book’s authors and his family 20

21. Figure 9.15B
Figure 9.15B Varying phenotypes due to environmental factors in genetically identical twins 21

22. THE CHROMOSOMAL BASIS OF INHERITANCE
© 2012 Pearson Education, Inc. 22

23. 9.16 Chromosome behavior accounts for Mendel’s laws
The chromosome theory of inheritance states that
genes occupy specific loci (positions) on chromosomes and
chromosomes undergo segregation and independent assortment during meiosis.
Student Misconceptions and Concerns
This section of the chapter relies upon a good understanding of the chromosome-sorting process of meiosis. If students were not assigned Chapter 8, and meiosis has not otherwise been addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes.
Teaching Tips
Figure 9.16 requires an understanding of meiosis and the general cell cycle from Chapter 8. Students may need to be reminded that chromosomes are duplicated in the preceding interphase, as indicated in the first step. Furthermore, students may not initially notice that this diagram represents four possible outcomes, not stages of any one meiotic cycle.
© 2012 Pearson Education, Inc. 23

24. 9.16 Chromosome behavior accounts for Mendel’s laws
Mendel’s laws correlate with chromosome separation in meiosis.
The law of segregation depends on separation of homologous chromosomes in anaphase I.
The law of independent assortment depends on alternative orientations of chromosomes in metaphase I.
Student Misconceptions and Concerns
This section of the chapter relies upon a good understanding of the chromosome-sorting process of meiosis. If students were not assigned Chapter 8, and meiosis has not otherwise been addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes.
Teaching Tips
Figure 9.16 requires an understanding of meiosis and the general cell cycle from Chapter 8. Students may need to be reminded that chromosomes are duplicated in the preceding interphase, as indicated in the first step. Furthermore, students may not initially notice that this diagram represents four possible outcomes, not stages of any one meiotic cycle.
© 2012 Pearson Education, Inc. 24

25. All yellow round seeds (RrYy) Meta- phase I of meiosis
Figure 9.16_s1
F1 generation
All yellow round seeds (RrYy) R y r Y R r r R
Meta- phase I of meiosis Y y Y y
Figure 9.16_s1 The chromosomal basis of Mendel’s laws (step 1) 25

26. All yellow round seeds (RrYy) Meta- phase I of meiosis
Figure 9.16_s2
F1 generation
All yellow round seeds (RrYy) R y r Y R r r R
Meta- phase I of meiosis Y y Y y R r r R
Anaphase I Y y Y y
Metaphase II R r r R Y y Y y
Figure 9.16_s2 The chromosomal basis of Mendel’s laws (step 2) 26

27. All yellow round seeds (RrYy) Meta- phase I of meiosis
Figure 9.16_s3
F1 generation
All yellow round seeds (RrYy) R y r Y R r r R
Meta- phase I of meiosis Y y Y y R r r R
Anaphase I Y y Y y
Metaphase II R r r R Y y Y y
Gametes
Figure 9.16_s3 The chromosomal basis of Mendel’s laws (step 3) y Y Y y Y Y y y R R r r r r R R 4 1 4 1 4 1 4 1 RY ry rY Ry
Fertilization
F2 generation 9 :3 :3 :1 27

28. Yellow round Green round Yellow wrinkled Green wrinkled
Figure 9.16_4
Sperm 4 1 4 1 4 1 4 1 RY rY Ry ry 4 1 RY
RRYY
RrYY
RRYy
RrYy 4 1 rY
RrYY
rrYY
RrYy
rrYy
Eggs 4 1 Ry
RRYy
RrYy
RRyy
Rryy 4 1 ry
RrYy
rrYy
Rryy
rryy
Figure 9.16_4 The chromosomal basis of Mendel’s laws (Punnett square) 16 9
Yellow round 16 3
Green round 16 3
Yellow wrinkled 16 1
Green wrinkled 28

29. 9.17 SCIENTIFIC DISCOVERY: Genes on the same chromosome tend to be inherited together
Bateson and Punnett studied plants that did not show a 9:3:3:1 ratio in the F2 generation. What they found was an example of linked genes, which
are located close together on the same chromosome and
tend to be inherited together.
Student Misconceptions and Concerns
This section of the chapter relies upon a good understanding of the chromosome-sorting process of meiosis. If students were not assigned Chapter 8, and meiosis has not otherwise been addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes.
Teaching Tips
Building on the shoe analogy developed in Chapter 8, linked genes are like a shoe and its shoelaces. The two are usually transferred together but can be moved separately under special circumstances.
© 2012 Pearson Education, Inc. 29

30. Figure 9.17 The experiment revealing linked genes in the sweet pea
Purple flower
PpLl 
PpLl
Long pollen
Phenotypes
Observed offspring
Prediction (9:3:3:1)
Purple long
284
215
Purple round 21 71
Red long 21 71
Red round 55 24
The Explanation: Linked Genes
Parental diploid cell PpLl
P L
p l
Meiosis
Most gametes
P L
p l
Figure 9.17 The experiment revealing linked genes in the sweet pea
Fertilization
Sperm PL pl
P L
P L PL
Most offspring
P L
p l
Eggs
p l
p l pl
P L
p l
3 purple long : 1 red round Not accounted for: purple round and red long 30

31. Observed offspring Prediction (9:3:3:1)
Figure 9.17_1
The Experiment
Purple flower
PpLl 
PpLl
Long pollen
Phenotypes
Observed offspring
Prediction (9:3:3:1)
Purple long
284
215
Figure 9.17_1 The experiment revealing linked genes in the sweet pea (part 1)
Purple round 21 71
Red long 21 71
Red round 55 24 31

32. The Explanation: Linked Genes
Figure 9.17_2
The Explanation: Linked Genes
Parental diploid cell PpLl
P L
p l
Meiosis
Most gametes
P L
p l
Fertilization
Sperm PL pl
Figure 9.17_2 The experiment revealing linked genes in the sweet pea (part 2)
P L
P L PL
Most offspring
P L
p l
Eggs
p l
p l pl
P L
p l
3 purple long : 1 red round Not accounted for: purple round and red long 32

33. 9.18 SCIENTIFIC DISCOVERY: Crossing over produces new combinations of alleles
Crossing over between homologous chromosomes produces new combinations of alleles in gametes.
Linked alleles can be separated by crossing over, forming recombinant gametes.
The percentage of recombinants is the recombination frequency.
Student Misconceptions and Concerns
1. This section of the chapter relies upon a good understanding of the chromosome-sorting process of meiosis. If students were not assigned Chapter 8, and meiosis has not otherwise been addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes.
2. The nature of linked genes builds upon our natural expectations that items that are closely together are less likely to be separated. Yet, students may find such concepts initially foreign. Whether it is parents holding the hands of children or people and their pets, we generally know that separation is more likely when things are farther apart.
Teaching Tips
1. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth minute of Avatar. Clearly, the closer together two frames of film are, the more likely they are to move or remain together.
2. Challenge students to explain why Sturtevant and Morgan studied the genetics of fruit flies. As the text notes, their small size, ease of care, and ability to produce several generations in a matter of weeks or months were important factors.
© 2012 Pearson Education, Inc. 33

34. Tetrad (pair of homologous chromosomes)
Figure 9.18A
p L
p l
P L
Parental gametes
p l
p L
P l
Tetrad (pair of homologous chromosomes)
Crossing over
Figure 9.18A Review: the production of recombinant gametes
Recombinant gametes 34

35. Figure 9.18B
Figure 9.18B Drosophila melanogaster 35

36. Figure 9.18C
The Experiment
The Explanation
G L
g l
Gray body, long wings (wild type)
Black body, vestigial wings
GgLl Female
ggll Male
g l
g l
Crossing over
GgLl
ggll
Female
Male
G L
g l
G l
g L
g l
Eggs
Sperm
Offspring
Gray long
Black vestigial
Gray vestigial
Black long
Offspring
G L
g l
G l
g L
g l
g l
g l
g l
Figure 9.18C A fruit fly experiment demonstrating the role of crossing over in inheritance
Parental
Recombinant
965
944
206
185
Parental phenotypes
Recombinant phenotypes
391 recombinants
2,300 total offspring
Recombination frequency 
 0.17 or 17% 36

37. Gray body, long wings (wild type) Black body, vestigial wings
Figure 9.18C_1
The Experiment
Gray body, long wings (wild type)
Black body, vestigial wings
GgLl
ggll
Female
Male
Offspring:
Gray long
Black vestigial
Gray vestigial
Black long
Figure 9.18C_1 A fruit fly experiment demonstrating the role of crossing over in inheritance (part 1)
965
944
206
185
Parental phenotypes
Recombinant phenotypes
391 recombinants
2,300 total offspring
Recombination frequency 
 0.17 or 17% 37

38. The Explanation G L g l GgLl Female ggll Male g l g l G L g l G l g L
Figure 9.18C_2
The Explanation
G L
g l
GgLl Female
ggll Male
g l
g l
Crossing over
G L
g l
G l
g L
g l
Eggs
Sperm
Figure 9.18C_2 A fruit fly experiment demonstrating the role of crossing over in inheritance (part 2)
Offspring
G L
g l
G l
g L
g l
g l
g l
g l
Parental
Recombinant 38

39. 9.19 Geneticists use crossover data to map genes
When examining recombinant frequency, Morgan and his students found that the greater the distance between two genes on a chromosome, the more points there are between them where crossing over can occur.
Recombination frequencies can thus be used to map the relative position of genes on chromosomes.
Student Misconceptions and Concerns
1. This section of the chapter relies upon a good understanding of the chromosome-sorting process of meiosis. If students were not assigned Chapter 8, and meiosis has not otherwise been addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes.
2. The nature of linked genes builds upon our natural expectations that items that are closely together are less likely to be separated. Yet, students may find such concepts initially foreign. Whether it is parents holding the hands of children or people and their pets, we generally know that separation is more likely when things are farther apart.
Teaching Tips
1. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth minute of Avatar. Clearly, the closer together two frames of film are, the more likely they are to move or remain together.
2. Challenge students to explain why Sturtevant and Morgan studied the genetics of fruit flies. As the text notes, their small size, ease of care, and ability to produce several generations in a matter of weeks or months were important factors.
© 2012 Pearson Education, Inc. 39

40. Recombination frequencies
Figure 9.19A
Section of chromosome carrying linked genes g c l
17% 9%
9.5%
Recombination frequencies
Figure 9.19A Mapping genes from crossover data 40

41. Long aristae (appendages on head) Gray body (G) Red eyes (C)
Figure 9.19B
Mutant phenotypes
Short aristae
Black body (g)
Cinnabar eyes (c)
Vestigial wings (l)
Brown eyes
Figure 9.19B A partial linkage map of a fruit fly chromosome
Long aristae (appendages on head)
Gray body (G)
Red eyes (C)
Normal wings (L)
Red eyes
Wild-type phenotypes 41

42. SEX CHROMOSOMES AND SEX-LINKED GENES
© 2012 Pearson Education, Inc. 42

43. 9.20 Chromosomes determine sex in many species
Many animals have a pair of sex chromosomes,
designated X and Y,
that determine an individual’s sex.
In mammals,
males have XY sex chromosomes,
females have XX sex chromosomes,
the Y chromosome has genes for the development of testes, and
an absence of the Y allows ovaries to develop.
Teaching Tips
As the text notes, in crocodilians and many turtles, sex is not genetically determined. Instead, the incubation temperature of the eggs determines an animal’s sex. Students may enjoy researching this unique form of sex determination, often identified as TSD (temperature-dependent sex determination).
© 2012 Pearson Education, Inc. 43

44. Figure 9.20A X
Figure 9.20A The human sex chromosomes Y 44

45. Male Female 44  XY 44  XX Parents (diploid) Gametes (haploid) 22  X
Figure 9.20B
Male
Female
44  XY
44  XX
Parents (diploid)
Gametes (haploid)
22  X
22  Y
22  X
Sperm
Egg
Figure 9.20B The X-Y system
Offspring (diploid)
44  XX
44  XY
Female
Male 45

46. Figure 9.20B_1
Figure 9.20B_1 The X-Y system (man) 46

47. 9.20 Chromosomes determine sex in many species
Grasshoppers, roaches, and some other insects have an X-O system, in which
O stands for the absence of a sex chromosome,
females are XX, and
males are XO.
In certain fishes, butterflies, and birds,
the sex chromosomes are Z and W,
males are ZZ, and
females are ZW.
Teaching Tips
As the text notes, in crocodilians and many turtles, sex is not genetically determined. Instead, the incubation temperature of the eggs determines an animal’s sex. Students may enjoy researching this unique form of sex determination, often identified as TSD (temperature-dependent sex determination).
© 2012 Pearson Education, Inc. 47

48. Figure 9.20C
Male
Female
22  X
22  XX
Figure 9.20C The X-O system 48

49. Figure 9.20C_1
Figure 9.20C_1 The X-O system (grasshopper) 49

50. Male Female 76  ZZ 76  ZW Figure 9.20D Figure 9.20D The Z-W system50

51. Figure 9.20D_1
Figure 9.20D_1 The Z-W system (chicken) 51

52. 9.20 Chromosomes determine sex in many species
Some organisms lack sex chromosomes altogether.
In bees, sex is determined by chromosome number.
Females are diploid.
Males are haploid.
Teaching Tips
As the text notes, in crocodilians and many turtles, sex is not genetically determined. Instead, the incubation temperature of the eggs determines an animal’s sex. Students may enjoy researching this unique form of sex determination, often identified as TSD (temperature-dependent sex determination).
© 2012 Pearson Education, Inc. 52

53. Figure 9.20E
Male
Female 16 32
Figure 9.20E Sex determination by chromosome number 53

54. Figure 9.20E_1
Figure 9.20E_1 Sex determination by chromosome number (bee) 54

55. 9.20 Chromosomes determine sex in many species
In some animals, environmental temperature determines the sex.
For some species of reptiles, the temperature at which the eggs are incubated during a specific period of development determines whether the embryo will develop into a male or female.
Global climate change may therefore impact the sex ratio of such species.
Teaching Tips
As the text notes, in crocodilians and many turtles, sex is not genetically determined. Instead, the incubation temperature of the eggs determines an animal’s sex. Students may enjoy researching this unique form of sex determination, often identified as TSD (temperature-dependent sex determination).
© 2012 Pearson Education, Inc. 55

56. 9.21 Sex-linked genes exhibit a unique pattern of inheritance
Sex-linked genes are located on either of the sex chromosomes.
The X chromosome carries many genes unrelated to sex.
The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait.
Student Misconceptions and Concerns
The prior discussion of linked genes addresses a different relationship than the use of the similar term sex-linked genes. Consider emphasizing this distinction for your students.
Teaching Tips
An analogy can be drawn between sex-linked genes and the risk of not having a backup copy of a file on your computer. If you only have one copy, and it is damaged, you have to live with the damaged file. Females, who have two X chromosomes, thus have a “backup copy” that can function if one of the sex-linked genes is damaged.
© 2012 Pearson Education, Inc. 56

57. Figure 9.21A
Figure 9.21A Fruit fly eye color determined by sex-linked gene 57

58. Figure 9.21A_1
Figure 9.21A_1 Fruit fly eye color determined by sex-linked gene (red eye) 58

59. Figure 9.21A_2
Figure 9.21A_2 Fruit fly eye color determined by sex-linked gene (white eye) 59

60. Female Male XRXR XrY Sperm Xr Y Eggs XR XRXr XRY R  red-eye allele
Figure 9.21B
Female
Male
XRXR
XrY
Sperm Xr Y
Figure 9.21B A homozygous, red-eyed female crossed with a white-eyed male
Eggs XR
XRXr
XRY
R  red-eye allele
r  white-eye allele 60

61. Female Male XRXr XRY Sperm xR Y XR XRXR XRY Eggs Xr XrXR XrY
Figure 9.21C
Female
Male
XRXr
XRY
Sperm xR Y XR
XRXR
XRY
Figure 9.21C A heterozygous female crossed with a red-eyed male
Eggs Xr
XrXR
XrY
R  red-eye allele
r  white-eye allele 61

62. Female Male XRXr XrY Sperm Xr Y XR XRXr XRY Eggs Xr XrXr XrY
Figure 9.21D
Female
Male
XRXr
XrY
Sperm Xr Y XR
XRXr
XRY
Figure 9.21D A heterozygous female crossed with a white-eyed male
Eggs Xr
XrXr
XrY
R  red-eye allele
r  white-eye allele 62

63. 9.22 CONNECTION: Human sex-linked disorders affect mostly males
Most sex-linked human disorders are
due to recessive alleles and
seen mostly in males.
A male receiving a single X-linked recessive allele from his mother will have the disorder.
A female must receive the allele from both parents to be affected.
Student Misconceptions and Concerns
The likelihood that at least some students in larger classes are color-blind is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed and not wish to self-identify.
Teaching Tips
1. Female hemophiliacs are very rare because both X chromosomes would need to have the recessive trait. Although very unlikely, female hemophiliacs are known. Students may enjoy searching for details of these rare cases. For additional information about hemophilia, consider visiting the website of the National Hemophilia Foundation at
2. Hemophilia and other genetic diseases may also result from spontaneous mutations in a family with no known history of the disease. Although rare, this possibility should always be considered when tracing the history of an inherited disease.
© 2012 Pearson Education, Inc. 63

64. 9.22 CONNECTION: Human sex-linked disorders affect mostly males
Recessive and sex-linked human disorders include
hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting,
red-green color blindness, a malfunction of light-sensitive cells in the eyes, and
Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination.
Student Misconceptions and Concerns
The likelihood that at least some students in larger classes are color-blind is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed and not wish to self-identify.
Teaching Tips
1. Female hemophiliacs are very rare because both X chromosomes would need to have the recessive trait. Although very unlikely, female hemophiliacs are known. Students may enjoy searching for details of these rare cases. For additional information about hemophilia, consider visiting the website of the National Hemophilia Foundation at
2. Hemophilia and other genetic diseases may also result from spontaneous mutations in a family with no known history of the disease. Although rare, this possibility should always be considered when tracing the history of an inherited disease.
© 2012 Pearson Education, Inc. 64

65. Czar Nicholas II of Russia
Figure 9.22
Queen Victoria
Albert
Alice
Louis
Female
Male
Alexandra
Czar Nicholas II of Russia
Hemophilia
Figure 9.22 Hemophilia in the royal family of Russia
Carrier
Normal
Alexis 65

66. Figure 9.22_1
Figure 9.22_1 Hemophilia in the royal family of Russia (family photo) 66

67. 9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution
The Y chromosome provides clues about human male evolution because
Y chromosomes are passed intact from father to son and
mutations in Y chromosomes can reveal data about recent shared ancestry.
Teaching Tips
Like the Y chromosome, mitochondrial DNA (mtDNA) can be used to trace maternal ancestry (because mitochondria are characteristically inherited from the egg). For a fee, several commercial groups offer to provide information about a person’s ancestry based upon genetic samples. Such groups can be found by searching the Internet using the keywords “genetic ancestry.”
© 2012 Pearson Education, Inc. 67

68. Figure 9.23
Figure 9.23 Genghis Khan 68

69. You should now be able to
Describe pangenesis theory and the blending hypothesis. Explain why both ideas are now rejected.
Define and distinguish between true-breeding organisms, hybrids, the P generation, the F1 generation, and the F2 generation.
Define and distinguish between the following pairs of terms: homozygous and heterozygous; dominant allele and recessive allele; genotype and phenotype. Also, define a monohybrid cross and a Punnett square.
© 2012 Pearson Education, Inc. 69

70. You should now be able to
Explain how Mendel’s law of segregation describes the inheritance of a single characteristic.
Describe the genetic relationships between homologous chromosomes.
Explain how Mendel’s law of independent assortment applies to a dihybrid cross.
Explain how and when the rule of multiplication and the rule of addition can be used to determine the probability of an event.
© 2012 Pearson Education, Inc. 70

71. You should now be able to
Explain how family pedigrees can help determine the inheritance of many human traits.
Explain how recessive and dominant disorders are inherited. Provide examples of each.
Compare the health risks, advantages, and disadvantages of the following forms of fetal testing: amniocentesis, chorionic villus sampling, and ultrasound imaging.
© 2012 Pearson Education, Inc. 71

72. You should now be able to
Describe the inheritance patterns of incomplete dominance, multiple alleles, codominance, pleiotropy, and polygenic inheritance.
Explain how the sickle-cell allele can be adaptive.
Explain why human skin coloration is not sufficiently explained by polygenic inheritance.
Define the chromosome theory of inheritance. Explain the chromosomal basis of the laws of segregation and independent assortment.
© 2012 Pearson Education, Inc. 72

73. You should now be able to
Explain how linked genes are inherited differently from nonlinked genes.
Describe T. H. Morgan’s studies of crossing over in fruit flies. Explain how Sturtevant created linkage maps.
Explain how sex is genetically determined in humans and the significance of the SRY gene.
Describe patterns of sex-linked inheritance and examples of sex-linked disorders.
Explain how the Y chromosome can be used to trace human ancestry.
© 2012 Pearson Education, Inc. 73

74. Homologous chromosomes Alleles, residing at the same locus
Figure 9.UN01
Homologous chromosomes
Alleles, residing at the same locus
Fertilization
Meiosis
Gamete from the other parent
Diploid zygote (containing paired alleles)
Paired alleles, different forms of a gene
Haploid gametes (allele pairs separated)
Figure 9.UN01 Reviewing the Concepts, 9.3 74

75. Incomplete dominance Red RR White rr Pink Rr
Figure 9.UN02
Incomplete dominance
Red RR
White rr
Pink Rr
Figure 9.UN02 Reviewing the Concepts, 9.11 75

76. Pleiotropy Single gene Multiple characters Figure 9.UN03
Figure 9.UN03 Reviewing the Concepts, 9.13 76

77. Polygenic inheritance
Figure 9.UN04
Polygenic inheritance
Multiple genes
Single characters (such as skin color)
Figure 9.UN04 Reviewing the Concepts, 9.14 77

78. Genes chromosomes (a) (b) (c) heterozygous (d) (e) (f) located on
Figure 9.UN05
Genes
located on
alternative versions called
chromosomes
(a)
at specific locations called
if both are the same, the genotype is called
if different, the genotype is called
(b)
(c)
heterozygous
the expressed allele is called
the unexpressed allele is called
Figure 9.UN05 Connecting the Concepts, question 1
(d)
(e)
inheritance when the phenotype is in between is called
(f) 78

79. Figure 9.UN06
Figure 9.UN06 Applying the Concepts, question 16 79