1. Patterns of Inheritance
Chapter 9
Patterns of Inheritance

2. Purebreds and Mutts-A Difference of Heredity
Genetics is the science of heredity
A common genetic background will produce offspring with similar physical and behavioral traits
Purebred dogs show less variation than mutts
True-breeding individuals are useful in genetic research
Behavioral characteristics are also influenced by environment

3. 9.1 The science of genetics has ancient roots
MENDEL'S LAWS
9.1 The science of genetics has ancient roots
Early attempts to explain heredity have been rejected by later science
Hippocrates' theory of Pangenesis
Particles from each part of the body travel to eggs or sperm and are passed on
Early 19th-century biologists' blending hypothesis
Traits from both parents mix in the offspring

4. 9.2 Experimental genetics began in an abbey garden
Gregor Mendel hypothesized that there are alternative forms of genes, the units that determine heritable traits
Mendel crossed pea plants that differed in certain characteristics
Could control matings
Developed true-breeding varieties
Traced traits from generation to generation

5. Terminology of Mendelian genetics
Self-fertilization: fertilization of eggs by sperm-carrying pollen of the same flower
Cross-fertilization (cross): fertilization of one plant by pollen from a different plant
True-breeding: identical offspring from self-fertilizing parents
Hybrid: offspring of two different varieties

6. P generation: true-breeding parents
F1 generation: hybrid offspring of true-breeding parents
F2 generation: offspring of self-fertilizing F1 parents

7. Petal Stamen Carpel Removed stamens White from purple flower Stamens
Parents
(P)
Purple
Transferred pollen
from stamens of
white flower to
carpel of purple
Stamens
Pollinated carpel
matured into pod
Planted seeds
from pod
Offspring
(F1)
Petal
Stamen
Carpel

8. LE 9-2d Flower color Purple White Flower position Axial Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Stem length
Tall
Dwarf

9. 9.3 Mendel's law of segregation describes the inheritance of a single characteristic
From his experimental data, Mendel developed several hypotheses
There are alternative forms (alleles) of genes that account for variation in inherited characteristics
For each characteristic, an organism inherits two alleles, one from each parent
Homozygous: two identical alleles
Heterozygous: two different alleles

10. If the two alleles of an inherited pair differ
The dominant allele determines the organism's appearance
The recessive allele has no noticeable effect on the organism's appearance
The law of segregation: A sperm or egg carries only one allele for each inherited trait, because allele pairs separate from each other during gamete production

11. An organism's appearance does not always reveal its genetic composition
Phenotype: Expressed (physical) traits
Genotype: Genetic makeup

12. have purple flowers have white flowers
LE 9-3a
P generation
(true-breeding
parents) 
Purple flowers
White flowers
F1 generation
All plants have
purple flowers
Fertilization
among F1 plants
(F1  F1)
F2 generation 3 4
of plants 1 4
of plants
have purple flowers
have white flowers

13. Genetic makeup (alleles)
LE 9-3b
Genetic makeup (alleles)
P plants PP pp
Gametes
All P
All p
F1 plants
(hybrids)
All Pp
Gametes 1 2 1 2 P p
Sperm P p
F2 plants
Phenotypic ratio
3 purple : 1 white P PP Pp
Eggs
Genotypic ratio
1 PP : 2 Pp : 1 pp p Pp pp

14. 9.4 Homologous chromosomes bear the two alleles for each characteristic
Alternative forms of a gene reside at the same locus on homologous chromosomes
Supports the law of segregation

15. LE 9-4 Gene loci Dominant allele P a B P a b Recessive allele
Genotype: PP aa Bb
Homozygous
for the
dominant allele
Homozygous
for the
recessive allele
Heterozygous

16. 9.5 The law of independent assortment is revealed by tracking two characteristics at once
Dihybrid cross
Mate true-breeding parents differing in two characteristics
The F1 generation exhibits only the dominant phenotype
The F2 generation exhibits a phenotypic ratio of 9:3:3:1
Mendel's law of independent assortment: each pair of alleles segregates independently of other allele pairs during gamete formation

17. contradict hypothesis
LE 9-5a
Hypothesis: Dependent assortment
Hypothesis: Independent assortment
P generation
RRYY
rryy
rryy
RRYY
rryy
Gametes RY ry
Gametes RY  ry
F1 generation
RrYy
RrYy
Sperm
Sperm 1 4 RY 1 4 rY 1 4 Ry 1 4 ry 1 2 RY 1 2 ry 1 4 RY 1 2 RY
RRYY
RrYY
RRYy
RrYy
F2 generation
Eggs 1 4 rY 1 2 ry
RrYY
rrYY
RrYy
rrYy
Eggs
Yellow
round 1 4 Ry 9 16
RRYy
RrYy
RRyy
Rryy 3 16
Green
round
Actual results
contradict hypothesis 1 4 ry
Yellow
wrinkled
RrYy
rrYy
Rryy
rryy 3 16
Actual results
support hypothesis 1 16
Green
wrinkled

18. LE 9-5b Blind Blind Phenotypes Genotypes Mating of heterozygotes
Black coat, normal vision
B_N_
Black coat, blind (PRA)
B_nn
Chocolate coat, normal vision
bbN_
Chocolate coat, blind (PRA)
bbnn
Mating of heterozygotes
(black, 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)

19. 9.6 Geneticists use the testcross to determine unknown genotypes
A testcross can reveal an unknown genotype
Mate an individual of unknown genotype and a homozygous-recessive individual
Each of the two possible genotypes (homozygous or heterozygous) gives a different phenotypic ratio in the F1 generation

20. Two possibilities for the black dog:
LE 9-6
Testcross: 
Genotypes B_ bb
Two possibilities for the black dog: BB or Bb
Gametes B B b b Bb b Bb bb
Offspring
All black
1 black : 1 chocolate

21. 9.7 Mendel's laws reflect the rules of probability
Events that follow probability rules are independent events
One such event does not influence the outcome of a later such event
The rule of multiplication: The probability of two events occurring together is the product of the separate probabilities of the independent events
The rule of addition: The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities of the different ways

22. LE 9-7 F1 genotypes Bb male Formation of sperm Bb female
Formation of eggs 1 2 1 2 B b B B B b 1 2 B 1 4 1 4
F2 genotypes 1 2 b B b b b 1 4 1 4

23. 9.8 Genetic traits in humans can be tracked through family pedigrees
CONNECTION
9.8 Genetic traits in humans can be tracked through family pedigrees
The inheritance of many human traits follows Mendel's laws
The dominant phenotype results from either the heterozygous or homozygous genotype
The recessive phenotype results from only the homozygous genotype
Family pedigrees can be used to determine individual genotypes

24. LE 9-8a Dominant Traits Recessive Traits Freckles No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe

25. Dd Joshua Lambert Dd Abigail Linnell D ? John Eddy D ? Hepzibah
LE 9-8b Dd
Joshua
Lambert Dd
Abigail
Linnell
D ?
John
Eddy
D ?
Hepzibah
Daggett
D ?
Abigail
Lambert dd
Jonathan
Lambert Dd
Elizabeth
Eddy Dd Dd dd Dd Dd Dd dd
Female
Male
Deaf
Hearing

26. 9.9 Many inherited disorders in humans are controlled by a single gene
CONNECTION
9.9 Many inherited disorders in humans are controlled by a single gene
Thousands of human genetic disorders follow simple Mendelian patterns of inheritance
Recessive disorders
Most genetic disorders
Can be carried unnoticed by heterozygotes
Range in severity from mild (albinism) to severe (cystic fibrosis)
More likely to occur with inbreeding

27. Some serious, but nonlethal, disorders (achondroplasia)
Dominant disorders
Some serious, but nonlethal, disorders (achondroplasia)
Lethal conditions less common than in recessive disorders
Cannot be carried by heterozygotes without affecting them
Can be passed on if they do not cause death until later age (Huntington's disease)

28. Parents Normal Dd Normal Dd  Sperm D d Dd Normal (carrier) DD Normal
LE 9-9a
Parents
Normal Dd
Normal Dd 
Sperm D d Dd
Normal
(carrier) DD
Normal D
Offspring
Eggs Dd
Normal
(carrier) d dd
Deaf

30. 9.10 New technologies can provide insight into one's genetic legacy
CONNECTION
9.10 New technologies can provide insight into one's genetic legacy
New technologies can provide insight for reproductive decisions
Identifying carriers
Tests can distinguish parental carriers of many genetic disorders
Fetal testing
Amniocentesis and chorionic villus sampling (CVS) allow removal of fetal cells to test for genetic abnormalities

31. LE 9-10a Amniocentesis Chorionic villus sampling (CVS) Ultrasound
monitor
Needle inserted
through abdomen to
extract amniotic fluid
Suction tube inserted
through cervix to extract
tissue from chorionic villi
Ultrasound
monitor
Fetus
Fetus
Placenta
Placenta
Chorionic
villi
Uterus
Cervix
Cervix
Uterus
Amniotic
fluid
Centrifugation
Fetal
cells
Fetal
cells
Biochemical
tests
Several
weeks
Several
hours
Karyotyping

32. Video: Ultrasound of Human Fetus 1
Fetal imaging
Ultrasound imaging uses sound waves to produce a picture of the fetus
Newborn screening
Some genetic disorders can be detected at birth by routine tests
Ethical considerations
How will genetic testing information be used?
Video: Ultrasound of Human Fetus 1

33. VARIATIONS ON MENDEL'S LAWS
9.11 The relationship of genotype to phenotype is rarely simple
Mendel's principles are valid for all sexually reproducing species
However, most characteristics are inherited in ways that follow more complex patterns

34. 9.12 Incomplete dominance results in intermediate phenotypes
Dominant allele has same phenotypic effect whether present in one or two copies
Incomplete dominance
Heterozygote exhibits characteristics intermediate between both homozygous conditions
Not the same as blending

35. LE 9-12a P generation Red RR White rr  Gametes R r F1 generation Pink
Sperm 1 2 1 2 R r 1 2
Red RR
Pink rR R
F2 generation
Eggs 1 2
Pink Rr
White rr r

36. LE 9-12b Genotypes: HH Homozygous for ability to make LDL receptors Hh
Heterozygous hh
Homozygous
for inability to make
LDL receptors
Phenotypes:
LDL
LDL
receptor
Cell
Normal
Mild disease
Severe disease

37. 9.13 Many genes have more than two alleles in the population
In a population, multiple alleles often exist for a single characteristic
Example: human ABO blood group
Involves three alleles of a single gene
AB blood group is an example of codominance-both alleles are expressed in heterozygotes

38. LE 9-13 Blood Group (Phenotype) Antibodies Present in Blood
Reaction When Blood from Groups Below Is Mixed with
Antibodies from Groups at Left
Genotypes O A B AB
Anti-A
Anti-B O ii
IAIA or
IAi A
Anti-B
IBIB or
IBi B
Anti-A AB
IAIB

39. 9.14 A single gene may affect many phenotypic characteristics
Pleiotropy: a single gene may influence multiple characteristics
Example: sickle cell disease
Allele causes production of abnormal hemoglobin in homozygotes
Many severe physical effects
Heterozygotes generally healthy

40. Most common inherited disorder among people of African descent
Allele persists in population because heterozygous condition protects against malaria

41. LE 9-14 Individual homozygous for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle-cells
5,555
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Accumulation of
sickled cells in spleen
Physical
weakness
Heart
failure
Pain and
fever
Brain
damage
Damage to
other organs
Spleen
damage
Anemia
Impaired
mental
function
Pneumonia
and other
infections
Kidney
failure
Paralysis
Rheumatism

42. 9.15 A single characteristic may be influenced by many genes
Polygenic inheritance is the additive effects of two or more genes on a single phenotypic characteristic
Example: human skin color
Controlled by at least three genes

43. Fraction of population
LE 9-15
P generation 
aabbcc
(very light)
AABBCC
(very dark)
F1 generation 
AaBbCc
AaBbCc 1 64 6 64 15 64 20 64 15 64 6 64 1 64
Sperm 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 20 64
F2 generation 1 8 1 8 15 64 1 8 1 8
Fraction of population
Eggs 1 8 6 64 1 8 1 8 1 64 1 8
Skin color

44. 9.16 The environment affects many characteristics
Many characteristics result from a combination of genetic and environmental factors
Nature vs. nurture is an old and hotly contested debate
Only genetic influences are inherited

45. 9.17 Genetic testing can detect disease-causing alleles
CONNECTION
9.17 Genetic testing can detect disease-causing alleles
Predictive genetic testing may inform people of their risk for developing genetic diseases
Used when there is a family history but no symptoms
Increased use of genetic testing raises ethical, moral, and medical issues

46. THE CHROMOSOMAL BASIS OF INHERITANCE
9.18 Chromosome behavior accounts for Mendel's laws
Chromosome theory of inheritance
Genes occupy specific loci on chromosomes
Chromosomes undergo segregation and independent assortment during meiosis
Thus, chromosome behavior during meiosis and fertilization accounts for inheritance patterns

47. (alternative arrangements) Fertilization among the F1 plants
LE 9-18
All round yellow seeds
(RrYy)
F1 generation R y r Y R r r R
Metaphase I
of meiosis
(alternative arrangements) Y y Y y R r r R
Anaphase I
of meiosis Y y Y y R r r R
Metaphase II
of meiosis Y y Y y Y y Y y Y Y y y
Gametes R R r r r r R R 1 4 RY 1 4 ry 1 4 rY 1 4 Ry
Fertilization among the F1 plants
F2 generation 9 :3 :3 :1
(See Figure 9.5A)

48. 9.19 Genes on the same chromosome tend to be inherited together
Linked genes
Lie close together on the same chromosome
Tend to be inherited together
Generally do not follow Mendel's law of independent assortment

49. Not accounted for: purple round and red long
Experiment
Purple flower
PpLl 
PpLl
Long pollen
Observed
offspring
Prediction
(9:3:3:1)
Phenotypes
Purple long
Purple round
Red long
Red round
284 21 55
215 71 24
Explanation: linked genes
Parental
diploid cell
PpLl
P L
p l
Meiosis
Most
gametes
P L
p l
Fertilization
Sperm
P L
p l
P L
P L
P L
Most
offspring
P L
p l
Eggs
p l
p l
p l
P L
p l
3 purple long : 1 red round
Not accounted for: purple round and red long

50. 9.20 Crossing over produces new combinations of alleles
During meiosis, homologous chromosomes undergo crossing over
Produces new combinations of alleles in gametes
Percentage of recombinant offspring is called the recombination frequency

51. LE 9-20a A B a b A B a b A b a B
Tetrad
Crossing over
Gametes

52. Thomas Hunt Morgan performed some of the most important studies of crossing over in the early 1900s
Used the fruit fly Drosophila melanogaster
Established that crossing over was the mechanism that "breaks linkages" between genes

53. LE 9-20c Offspring Experiment Gray body, long wings (wild type)
Black body,
vestigial wings 
GgLl
ggll
Female
Male
Offspring
Gray long
Black vestigial
Gray vestigial
Black long
965
944
206
185
Parental
phenotypes
Recombinant
phenotypes
Recombination frequency =
391 recombinants
2,300 total offspring
= 0.17 or 17%
Explanation
G L g l
GgLl
(female)
ggll
(male)
g l g l
G L
g l
G l
g L
g l
Eggs
Sperm
G L
g l
G l
g L
g l
g l
g l
g l
Offspring

54. 9.21 Geneticists use crossover data to map genes
Morgan and his students greatly advanced understanding of genetics
Alfred Sturtevant used crossover data to map genes in Drosophila
Used recombination frequencies to map the relative positions of genes on chromosomes

55. LE 9-21b
Chromosome g c l
17% 9%
9.5%
Recombination
frequencies

56. Mutant phenotypes Wild-type phenotypes
LE 9-21c
Mutant phenotypes
Short
aristae
Black
body
(g)
Cinnabar
eyes
(c)
Vestigial
wings
(l)
Brown
eyes
Long aristae
(appendages
on head)
Gray
body
(G)
Red
eyes
(C)
Normal
wings
(L)
Red
eyes
Wild-type phenotypes

57. SEX CHROMOSOMES AND SEX-LINKED GENES
9.22 Chromosomes determine sex in many species
Many animals have a pair of chromosomes that determine sex
Humans: X-Y system
Male is XY; the Y chromosome has genes for the development of testes
Female is XX; absence of a Y chromosome allows ovaries to develop

58. (male) (female) 44 + XY 44 + XX Parents’ diploid cells 22 + X 22 + Y
LE 9-22a
(male)
(female) 44 + XY 44 + XX
Parents’
diploid
cells 22 + X 22 + Y 22 + X
Sperm
Egg 44 + XX 44 + XY
Offspring
(diploid)

59. Other animals have other sex-determination systems
X-O (grasshopper, roaches, some other insects)
Z-W (certain fishes, butterflies, birds)
Chromosome number (ants, bees)
Different plants have various sex-determination systems

60. LE 9-22b 22 + X XX 76 + ZW ZZ 32 16

61. 9.23 Sex-linked genes exhibit a unique pattern of inheritance
Sex-linked genes are genes for characteristics unrelated to sex that are located on either sex chromosome
In humans, refers to a gene on the X chromosome
Because of linkage and location, the inheritance of these characteristics follows peculiar patterns
Example: eye color inheritance in fruit flies follows three possible patterns, depending on the genotype of the parents

63. LE 9-23b Female Male XRXR  Xr Y Sperm Xr Y Eggs XR XRXr XRY
R = red-eye allele
r = white-eye allele

64. LE 9-23c Female Male XRXr  XRY Sperm XR Y XR XRXR XRY Eggs Xr XrXR

65. LE 9-23d Female Male XRXr  Xr Y Sperm Xr Y XR XRXr XRY Eggs Xr Xr Xr

66. CONNECTION 9.24 Sex-linked disorders affect mostly males
In humans, recessive sex-linked traits are expressed much more frequently in men than in women
Most known sex-linked traits are caused by genes (alleles) on the X chromosome
Because the male has only one X chromosome, his recessive X-linked characteristic will always be exhibited
Females with the allele are normally carriers and will exhibit the condition only if they are homozygous
Examples: red-green color blindness, hemophilia, Duchenne muscular dystrophy

68. Queen Victoria Albert Alice Louis Alexandra Czar Nicholas  of Russia
LE 9-24b
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas 
of Russia
Alexis