1. Patterns of Inheritance
Chapter 9
Patterns of Inheritance

2. Biology And Society: A Matter of Breeding
Genetics is the scientific study of heredity.
Genetics explains why the offspring of purebred dogs are like their parents.
Inbreeding of dogs makes some genetic disorders common.
A dog’s behavior is determined by its
Genes
Environment

3. Figure 9.00
Figure 9.0 Canine genetics.

4. HERITABLE VARIATION AND PATTERNS OF INHERITANCE
Heredity is the transmission of traits from one generation to the next.
Gregor Mendel
Worked in the 1860s
Was the first person to analyze patterns of inheritance
Deduced the fundamental principles of genetics

5. In an Abbey Garden Mendel studied garden peas because they
Are easy to grow
Come in many readily distinguishable varieties
Are easily manipulated
Can self-fertilize

6. Petal Stamen (makes sperm- producing pollen) Carpel (produces eggs)
Figure 9.2
Figure 9.2 The structure of a pea flower.

7. A character is a heritable feature that varies among individuals.
A trait is a variant of a character.
Each of the characters Mendel studied occurred in two distinct forms.
Mendel
Created true-breeding varieties of plants
Crossed two different true-breeding varieties
Hybrids are the offspring of two different true-breeding varieties.
The parental plants are the P generation.
Their hybrid offspring are the F1 generation.
A cross of the F1 plants forms the F2 generation.

8. Transferred pollen from stamens of white flower to carpel of purple
Removed
stamens
from purple
flower.
White
Stamens
Parents
(P)
Transferred pollen from
stamens of white flower
to carpel of purple
flower.
Carpel
Purple
Pollinated carpel
matured into pod.
Planted seeds
from pod.
Offspring
(F1)
Figure 9.3-3
Figure 9.3 Mendel's technique for cross-fertilizing pea plants. (Step 3)

9. Mendel’s Law of Segregation
Mendel performed many experiments.
He tracked the inheritance of characters that occur as two alternative traits.

10. Dominant Recessive Dominant Recessive Flower color Pod shape Inflated
Constricted
Purple
White
Pod color
Flower position
Green
Yellow
Stem length
Axial
Terminal
Seed color
Yellow
Green
Tall
Dwarf
Seed shape
Round
Wrinkled
Figure 9.4
Figure 9.4 The seven characters of pea plants studied by Mendel.

11. Monohybrid Crosses
A monohybrid cross is a cross between parent plants that differ in only one character.

12. P Generation (true-breading parents) Purple flowers White flowers
F1 Generation
All plants have
purple flowers
Fertilization
among F1 plants
(F1  F1)
F2 Generation 3 4 1 4
of plants
have purple flowers
of plants
have white flowers
Figure 9.5-3
Figure 9.5 Mendel's cross tracking one character (flower color). (Step 3)

13. Mendel developed four hypotheses from the monohybrid cross:
1. There are alternative versions of genes, called alleles.
2. For each character, an organism inherits two alleles, one from each parent.
An organism is homozygous for that gene if both alleles are identical.
An organism is heterozygous for that gene if the alleles are different
3. If two alleles of an inherited pair differ
The allele that determines the organism’s appearance is the dominant allele
The other allele, which has no noticeable effect on the appearance, is the recessive allele
. 4. Gametes carry only one allele for each inherited character.
The two members of an allele pair segregate (separate) from each other during the production of gametes.
This statement is the law of segregation.

14. Do Mendel’s hypotheses account for the 3:1 ratio he observed in the F2 generation?
A Punnett square highlights the four possible combinations of gametes and offspring that result from each cross.

15. Figure 9.6-1 P Generation Genetic makeup (alleles) Purple flowers
White flowers PP pp
Alleles carried
by parents
Gametes
All P
All p
Figure 9.6-1
Figure 9.6 The law of segregation. (Step 1)

16. Figure 9.6-2 P Generation Genetic makeup (alleles) Purple flowers
White flowers PP pp
Alleles carried
by parents
Gametes
All P
All p
F1 Generation
(hybrids)
Purple flowers
Alleles
segregate
All Pp 1 2 1 2
Gametes P p
Figure 9.6-2
Figure 9.6 The law of segregation. (Step 2)

17. Figure 9.6-3 P Generation Genetic makeup (alleles) Purple flowers
White flowers PP pp
Alleles carried
by parents
Gametes
All P
All p
F1 Generation
(hybrids)
Purple flowers
Alleles
segregate
All Pp 1 2 1 2
Gametes P p
F2 Generation
(hybrids)
Sperm from
F1 plant P p P
Eggs from
F1 plant PP Pp p Pp pp
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
Figure 9.6-3
Figure 9.6 The law of segregation. (Step 3)

18. Geneticists distinguish between an organism’s physical traits and its genetic makeup.
An organism’s physical traits are its phenotype.
An organism’s genetic makeup is its genotype.

19. Genetic Alleles and Homologous Chromosomes
Homologous chromosomes have
Genes at specific loci
Alleles of a gene at the same locus

20. Gene loci Dominant allele P a B Homologous chromosomes P a b Recessive
Genotype: PP aa Bb
Homozygous
for the
dominant allele
Homozygous
for the
recessive allele
Heterozygous
Figure 9.7
Figure 9.7 The relationship between alleles and homologous chromosomes.

21. Mendel’s Law of Independent Assortment
A dihybrid cross is the crossing of parental varieties differing in two characters.
What would result from a dihybrid cross? Two hypotheses are possible:
1. Dependent assortment
2. Independent assortment

22. (a) Hypothesis: Dependent assortment
(b) Hypothesis: Independent assortment
P Generation
RRYY
rryy
RRYY
rryy
Gametes RY ry
Gametes RY ry
F1 Generation
RrYy
RrYy
F2 Generation
Sperm 1 4 1 4 1 4 1 4 RY rY Ry ry
Sperm 1 2 1 2 RY ry 1 4 RY
RRYY
RrYY
RRYy
RrYy 1 2 RY 1 4 rY 16 9
Yellow
round
Eggs
RrYY
rrYY
RrYy
rrYy
Eggs 1 2 ry 1 4 Ry 16 3
Green
round
RRYy
RrYy
RRyy
Rryy 16 3
Yellow
wrinkled 1 4 ry
RrYy
rrYy
Rryy
rryy 1 16
Green
wrinkled
Predicted results
(not actually seen)
Actual results
(support hypothesis)
Figure 9.8
Figure 9.8 Testing alternative hypotheses for gene assortment in a dihybrid cross.

23. Figure 9.8
Figure 9.8 Photo of peas

24. Mendel’s dihybrid cross supported the hypothesis that each pair of alleles segregates independently of the other pairs during gamete formation.
Thus, the inheritance of one character has no effect on the inheritance of another.
This is the law of independent assortment.
Independent assortment is also seen in two hereditary characters in Labrador retrievers.

25. Mating of double heterozygotes (black coat, normal vision)
Blind dog
Blind dog
Phenotypes
Black coat,
normal vision
Black coat,
blind (PRA)
Chocolate coat,
normal vision
Chocolate coat,
blind (PRA)
Genotypes
B_N_
B_nn
bbN_
bbnn
(a) Possible phenotypes of Labrador retrievers
Mating of double heterozygotes
(black coat, normal vision)
BbNn
BbNn
Phenotypic
ratio of
offspring
9 black coat,
normal vision
3 black coat,
blind (PRA)
3 chocolate coat,
normal vision
1 chocolate coat,
blind (PRA)
(b) A Labrador dihybrid cross
Figure 9.9
Figure 9.9 Independent assortment of genes in Labrador retrievers.

26. Using a Testcross to Determine an Unknown Genotype
A testcross is a mating between
An individual of dominant phenotype (but unknown genotype)
A homozygous recessive individual
© 2010 Pearson Education, Inc.

27. Two possible genotypes for the black dog:
Testcross
Genotypes B_ bb
Two possible genotypes for the black dog: BB or Bb
Gametes B B b b Bb b Bb bb
Offspring
All black
1 black : 1 chocolate
Figure 9.10
Figure 9.10 A Labrador retriever testcross

28. The Rules of Probability
Mendel’s strong background in mathematics helped him understand patterns of inheritance.
The rule of multiplication states that the probability of a compound event is the product of the separate probabilities of the independent events.

29. F1 Genotypes Bb female Bb male Formation of eggs Formation of sperm
Male gametes 1 2 1 2 B b B B B 1 2 b B 1 4 1 4 1 2 1 2
(  )
Female gametes 1 2 b b B b b 1 4 1 4
Figure 9.11
Figure 9.11 Segregation of alleles and fertilization as chance events.

30. Family Pedigrees
Mendel’s principles apply to the inheritance of many human traits.

31. DOMINANT TRAITS RECESSIVE TRAITS Freckles Widow’s peak Free earlobe
No freckles
Straight hairline
Attached earlobe
Figure 9.12
Figure 9.12 Examples of inherited traits thought to be controlled by a single gene.

32. Dominant traits are not necessarily
Normal or
More common
Wild-type traits are
Those seen most often in nature
Not necessarily specified by dominant alleles
A family pedigree
Shows the history of a trait in a family
Allows geneticists to analyze human traits

33. First generation (grandparents) Ff Ff ff Ff Second generation
(parents, aunts, and
uncles) FF ff ff Ff Ff ff or Ff
Third generation
(brother and
sister) ff FF or Ff
Female Male
Attached
Free
Figure 9.13
Figure 9.13 A family pedigree showing inheritance of free versus attached earlobes.

34. Figure 9.13a
Figure 9.13a A family pedigree showing inheritance of free versus attached earlobes.

35. Human Disorders Controlled by a Single Gene
Many human traits
Show simple inheritance patterns
Are controlled by single genes on autosomes

36. Table 9.1
Table 9.1 Some Autosomal Disorders in Humans

37. Recessive Disorders Most human genetic disorders are recessive.
Individuals who have the recessive allele but appear normal are carriers of the disorder.

38. Parents Hearing Hearing Dd Dd Offspring D d Sperm Dd DD D Hearing
(carrier)
Hearing
Eggs Dd dd d
Hearing
(carrier)
Deaf
Figure 9.14
Figure 9.14 Predicted offspring when both parents are carriers for a recessive disorder.

39. Cystic fibrosis
Is the most common lethal genetic disease in the United States
Is caused by a recessive allele carried by about one in 25 people of European ancestry
Prolonged geographic isolation of certain populations can lead to inbreeding, the mating of close relatives.
Inbreeding increases the chance of offspring that are homozygous for a harmful recessive trait.

40. Dominant Disorders Some human genetic disorders are dominant.
Huntington’s disease, which leads to degeneration of the nervous system, does not begin until middle age.
Achondroplasia is a form of dwarfism.
The homozygous dominant genotype causes death of the embryo.
Thus, only heterozygotes have this disorder.

41. Normal (no achondroplasia) Dwarf (achondroplasia)
Parents
Normal
(no achondroplasia)
Dwarf
(achondroplasia) dd Dd d d
Sperm Dd Dd D
Dwarf
Dwarf
Eggs dd dd d
Normal
Normal
Molly Jo
Matt
Amy
Zachary
Jake
Jeremy
Figure 9.16
Figure 9.16 A family with and without achondroplasia.

42. Normal (no achondroplasia) Dwarf (achondroplasia)
Parents
Normal
(no achondroplasia)
Dwarf
(achondroplasia) dd Dd d d
Sperm Dd Dd D
Dwarf
Dwarf
Eggs dd dd d
Normal
Normal
Figure 9.16a
Figure 9.16a A family with and without achondroplasia.

43. The Process of Science: What Is the Genetic Basis of Hairless Dogs?
Observation: Dogs come in a wide variety of physical types.
Question: What is the genetic basis for the hairless phenotype?
Hypothesis: A comparison of genes of coated and hairless dogs would identify the gene or genes responsible.

44. Prediction: A mutation in a single gene accounts for the hairless appearance.
Experiment: Compared DNA sequences of 140 hairless dogs from 3 breeds with 87 coated dogs from 22 breeds.
Results: Every hairless dog, but no coated dogs, had a single change in a single gene.

45. Figure 9.17
Figure 9.17 Hairless versus coated Chinese crested dogs.

46. Genetic Testing
Today many tests can detect the presence of disease-causing alleles.
Most genetic testing is performed during pregnancy.
Amniocentesis collects cells from amniotic fluid.
Chorionic villus sampling removes cells from placental tissue.
Genetic counseling helps patients understand the results and implications of genetic testing.

47. VARIATIONS ON MENDEL’S LAWS
Some patterns of genetic inheritance are not explained by Mendel’s laws.

48. Incomplete Dominance in Plants and People
In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.

49. P Generation Red White RR rr Gametes R r F1 Generation Pink Rr 1 2 1 2
Sperm 1 2 1 2 R r 1 2 R RR Rr
Eggs 1 2 r Rr rr
Figure
Figure 9.18 Incomplete dominance in snapdragons. (Step 3)

50. Hypercholesterolemia
Is characterized by dangerously high levels of cholesterol in the blood.
Is a human trait that is incompletely dominant.
Heterozygotes have blood cholesterol levels about twice normal.
Homozygotes have blood cholesterol levels about five times normal.

51. HH Hh hh Homozygous for ability to make LDL receptors Heterozygous
for inability to make
LDL receptors
GENOTYPE
LDL
LDL
receptor
PHENOTYPE
Cell
Normal
Mild disease
Severe disease
Figure 9.19
Figure 9.19 Incomplete dominance in human hypercholesterolemia.

52. ABO Blood Groups: An Example of Multiple Alleles and Codominance
The ABO blood groups in humans are an example of multiple alleles.
© 2010 Pearson Education, Inc.

53. Figure 9.20 Blood Group (Phenotype) Antibodies Present in Blood
Reactions When Blood from Groups Below Is
Mixed with Antibodies from Groups at Left
Genotypes
Red Blood Cells O A B AB
Carbohydrate A
IAIA or
IAi A
Anti-B
Carbohydrate B
IBIB or
IBi B
Anti-A AB
IAIB —
Anti-A
Anti-B O ii
Figure 9.20
Figure 9.20 Multiple alleles for the ABO blood groups.

54. The immune system produces blood proteins called antibodies that can bind specifically to blood cell carbohydrates.
Blood cells may clump together if blood cells of a different type enter the body.
The clumping reaction is the basis of a blood-typing lab test.

55. Individual homozygous, for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes into long flexible chains,
causing red blood cells to become sickle-shaped.
Colorized SEM
Sickled cells can lead to a cascade of symptoms, such as
weakness, pain, organ damage, and paralysis.
Figure 9.21
Figure 9.21 Sickle-cell disease, multiple effects of a single human gene.

56. Figure 9.21
Figure 9.21 Sickle-cell disease, multiple effects of a single human gene.

57. The human blood type alleles IA and IB exhibit codominance: Both alleles are expressed in the phenotype.

58. Pleiotropy and Sickle-Cell Disease
Pleiotropy is the impact of a single gene on more than one character.
Sickle-cell disease
Exhibits pleiotropy
Results in abnormal hemoglobin production
Causes disk-shaped red blood cells to deform into a sickle shape with jagged edges

59. Figure 9.22 P Generation aabbcc (very light) AABBCC (very dark)
F1 Generation
AaBbCc
AaBbCc
F2 Generation
Sperm 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 64 20 1 8 1 8 64 15
Eggs 1 8
Fraction of population 1 8 64 6 1 8 1 8 1 64
Skin pigmentation 1 64 64 6 64 15 64 20 64 15 64 6 1 64
Figure 9.22
Figure 9.22 A model for polygenic inheritance of skin color.

60. Polygenic Inheritance
Polygenic inheritance is the additive effects of two or more genes on a single phenotype.

61. Figure 9.23
Figure 9.23 As a result of environmental influences, even identical twins can look different.

62. The Role of Environment
Many human characters result from a combination of heredity and environment.
Only genetic influences are inherited.

63. All round-yellow seeds
P Generation
Round-yellow seeds
(RRYY)
Wrinkled-green seeds
(rryy) Y r y R Y R r y
MEIOSIS
FERTILIZATION
Gametes R Y y r
F1 Generation
All round-yellow seeds
(RrYy) R r y
Law of Segregation: Follow the long
chromosomes (carrying R and r) taking
either the left or right branch.
Law of Independent Assortment:
Follow both the long and the short
chromosomes. Y
MEIOSIS R r
Metaphase I
(alternative
arrangements) r R
They are arranged in either of
two equally likely ways at
metaphase I.
The R and r alleles segregate in
anaphase I of meiosis. Y y Y y
Metaphase II R r r R
Only one long
chromosome ends
up in each gamete.
They sort independently,
giving four gamete types. Y y Y y y Y Y y Y Y y y
Gametes R R r r r r R R 4 1 4 1 4 1 4 1 RY ry rY Ry
Fertilization results in the
9:3:3:1 phenotypic ratio in
the F2 generation.
Fertilization recombines the r
and R alleles at random.
FERTILIZATION AMONG THE F1 PLANTS
F2 Generation 9
: 3
: 3
: 1
Figure
Figure 9.24 The chromosomal basis of Mendel's laws. (Step 4)

64. THE CHROMOSOMAL BASIS OF INHERITANCE
The chromosome theory of inheritance states that
Genes are located at specific positions on chromosomes
The behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns
It is chromosomes that undergo segregation and independent assortment during meiosis and thus account for Mendel’s laws.

65. Recombinant phenotypes 17%
Dihybrid testcross
Gray body,
long wings
(wild-type)
Black body,
short wings
(mutant)
GgLl
ggll
Female
Male
Results
Offspring
Gray-long
GgLl
Black-short
ggll
Gray-short
Ggll
Black-long
ggLl
965
944
206
185
Parental phenotypes 83%
Recombinant phenotypes 17%
Figure
Figure 9.25 Thomas Morgan's experiment and results.

66. Linked Genes Linked genes
Are located close together on a chromosome
May be inherited together
Using the fruit fly Drosophila melanogaster, Thomas Hunt Morgan determined that some genes were linked based on the inheritance patterns of their traits.

67. Pair of homologous chromosomesB a b A B
Parental gametes a b A b a B
Pair of
homologous
chromosomes
Crossing over
Recombinant gametes
Figure 9.26
Figure 9.26 Review: Crossing over can produce recombinant gametes.

68. Genetic Recombination: Crossing Over
Crossing over can
Separate linked alleles
Produce gametes with recombinant chromosomes
Produce offspring with recombinant phenotypes

69. FERTILIZATION G L g l GgLl (female) ggll (male) g l g l Crossing over
Sperm
Parental gametes
Recombinant gametes
Eggs
FERTILIZATION
Offspring
G L
g l
G l
g L
g l
g l
g l
g l
Parental
Recombinant
Figure 9.27
Figure 9.27 Explaining the unexpected results from the dihybrid testcross in Figure 9.25.

70. The percentage of recombinant offspring among the total is called the recombination frequency.

71. Recombination frequencies
Chromosome g c l
17% 9%
9.5%
Recombination
frequencies
Figure 9.28
Figure 9.28 Using crossover data to map genes.

72. Linkage Maps
Early studies of crossing over were performed using the fruit fly Drosophila melanogaster.
Alfred H. Sturtevant, a student of Morgan, developed a method for mapping gene loci, which resulted in the creation of linkage maps.
A diagram of relative gene locations on a chromosome is a linkage map.

73. 44  XY 44  XX Somatic cells 22  X 22  Y 22  X 44  XX 44  XY
Male
Female 44  XY 44  XX
Somatic
cells 22  X 22  Y 22  X
Sperm
Egg 44  XX 44  XY
Offspring
Female
Male
Figure 9.29
Figure 9.29 The chromosomal basis of sex determination.

74. Y X Colorized SEM Figure 9.29
Figure 9.29 The chromosomal basis of sex determination.

75. SEX CHROMOSOMES AND SEX-LINKED GENES
Sex chromosomes influence the inheritance of certain traits.

76. Sex Determination in Humans
Nearly all mammals have a pair of sex chromosomes designated X and Y.
Males have an X and Y.
Females have XX.

77. Sex-Linked Genes
Any gene located on a sex chromosome is called a sex-linked gene.
Most sex-linked genes are found on the X chromosome.
Red-green color blindness is a common human sex-linked disorder.

78. Figure 9.30
Figure 9.30 A test for red-green colorblindness.

79. XNXN XnY XNXn XNY XNXn XnY Xn Y XN Y Xn Y XNXN XNY XN XNXn XNY XN XN
Sperm
Sperm
Sperm Xn Y XN Y Xn Y
XNXN
XNY XN
XNXn
XNY XN XN
XNXn
XNY
Eggs
Eggs
Eggs XN
XNXn
XNY Xn
XNXn
XnY Xn
XnXn
XnY
(a)
(b)
(c)
Normal female 
colorblind male
Carrier female 
normal male
Carrier female 
colorblind male
Key
Unaffected individual
Carrier
Colorblind individual
Figure 9.31
Figure 9.31 Inheritance of colorblindness, a sex-linked recessive trait.

80. Hemophilia
Is a sex-linked recessive blood-clotting trait that may result in excessive bleeding and death after relatively minor cuts and bruises
Has plagued royal families of Europe

81. Queen Victoria Czar Nicholas II of Russia
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.32
Figure 9.32 Hemophilia in the royal family of Russia.

82. Evolution Connection: Barking Up the Evolutionary Tree
About 15,000 years ago in East Asia, humans began to cohabit with ancestral canines that were predecessors of modern wolves and dogs.
As people settled into geographically distinct populations, different canines became separated and inbred.
In 2005 researchers sequenced the complete genome of a dog.
An evolutionary tree of dog breeds was created.
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83. Ancestral canine Alaskan malamute
Wolf
Chinese shar-pei
Ancestral
canine
Siberian husky
Akita
Alaskan
malamute
Basenji
Afghan hound
Saluki
Rottweiler
Sheepdog
Retriever
Figure 9.33
Figure 9.33 An evolutionary tree of dog breeds.

84. (allele pairs separate)
Fertilization
Alleles
Meiosis
Gamete
from other
parent
Diploid zygote
(contains
paired alleles)
Diploid cell
(contains paired
alleles, alternate
forms of a gene)
Haploid gametes
(allele pairs separate)
Figure 9.UN1
Figure 9.UN1 Summary: Law of Segregation

85. Phenotype Dominant Recessive Genotype pp P ? or Phenotype All dominant
Conclusion
Unknown parent
is PP
Unknown parent
is Pp
Figure 9.UN2
Figure 9.UN2 Summary: Testcross

86. Intermediate phenotype (incomplete dominance)
Dominant phenotype
(RR)
Recessive phenotype
(rr)
Intermediate phenotype
(incomplete dominance)
(Rr)
Figure 9.UN3
Figure 9.UN3 Summary: Incomplete Dominance

87. Pleiotropy Single gene Multiple traits (e.g., sickle-cell disease)
Figure 9.UN4
Figure 9.UN4 Summary: Pleiotropy

88. Polygenic inheritance Single trait (e.g., skin color) Multiple genes
Figure 9.UN5
Figure 9.UN5 Summary:Polygenic Inheritance

89. Figure 9.UN7
Figure 9.UN7 Summary: Sex Linked Traits

90. Figure 9.UN8
Figure 9.UN8 Question 18: Curled Ear Cat