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Austin Community College
CHAPTER
20
Development
and Inheritance
© 2013 Pearson Education, Inc.
Chapter 20 Learning Outcomes
•
20-1
•
•
Explain the relationship between differentiation and development,
and describe the various stages of development.
20-2
• Describe the process of fertilization.
•
20-3
•
•
List the three stages of prenatal development, and describe the
major events of each.
20-4
• Explain how the three germ layers participate in the formation of
extraembryonic membranes, and discuss the importance of the
placenta as an endocrine organ.
© 2013 Pearson Education, Inc.
Chapter 20 Learning Outcomes
•
20-5
•
•
20-6
•
•
List and discuss the events that occur during labor and delivery.
20-7
•
•
Describe the interplay between maternal organ systems and the
developing fetus, and discuss the structural and functional changes
in the uterus during gestation.
Identify the features and physiological changes of the postnatal
stages of life.
20-8
• Relate the basic principles of genetics to the inheritance of human
traits.
© 2013 Pearson Education, Inc.
Basics of Development (20-1)
• Differentiation is formation of different cell types
• Fertilization (or conception) is fusing of gametes
• Embryological development is first two months
• Fetal development is from the ninth week until birth
• Prenatal is both embryological and fetal development
• Postnatal continues to maturity
• Genetics is the study of mechanisms of inheritance
© 2013 Pearson Education, Inc.
Checkpoint (20-1)
1. Define differentiation.
2. What event marks the beginning of
development?
3. Define inheritance.
© 2013 Pearson Education, Inc.
Fertilization (20-2)
• Fusion of two haploid gametes each with 23
chromosomes
• Produces a zygote with 46 chromosomes
• Sperm provides paternal chromosomes
• Oocyte provides maternal chromosomes,
organelles, and nourishment to support embryo
• Occurs in upper third of uterine tube
© 2013 Pearson Education, Inc.
Fertilization (20-2)
• Sperm are motile when deposited in vagina
• Then must be exposed to peg cells in wall of uterine tube to
complete capacitation
• Requires dozens of sperm to reach oocyte
• Takes more than one to break through corona radiata around
oocyte
© 2013 Pearson Education, Inc.
Figure 20-1a Fertilization.
A secondary oocyte and
numerous sperm at the time
of fertilization. Notice the
difference in size between
the gametes.
© 2013 Pearson Education, Inc.
Figure 20-1b Fertilization.
Oocyte at Ovulation
Corona First polar
radiata
body
Fertilization and Oocyte
Activation
Fertilizing
spermatozoon
Second
polar body
Pronucleus Formation
Begins
Nucleus of
fertilizing
spermatozoon
Female
pronucleus
Zona
pellucida
Cleavage Begins
Amphimixis Occurs
and Cleavage Begins
Metaphase of first
cleavage division
Blastomeres
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Spindle Formation and
Cleavage Preparation
Male
pronucleus
Female
pronucleus
Ovulation (20-2)
• Results in expulsion of an immature secondary
oocyte
• Acrosomal caps of spermatozoa
• Release hyaluronidase
• Penetrate corona radiata, zona pellucida, toward oocyte
surface
© 2013 Pearson Education, Inc.
Oocyte Activation (20-2)
• Contact and fusion of cell membranes of sperm
and oocyte
• Oocyte undergoes last stages of meiosis II
• Female pronucleus
• Nuclear material remaining in ovum after oocyte activation
• Male pronucleus
• Swollen nucleus of spermatozoon
• Migrates to center of cell
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Amphimixis (20-2)
• Female pronucleus and male pronucleus fuse
• Moment of conception
• Cell becomes a zygote with 46 chromosomes
• Fertilization is finalized
© 2013 Pearson Education, Inc.
Checkpoint (20-2)
4. What two important roles do the acrosomal
enzymes of spermatozoa play in fertilization?
5. How many chromosomes are contained within a
human zygote?
© 2013 Pearson Education, Inc.
The Three Stages of Gestation (20-3)
• Also called pregnancy
• First trimester
• Embryological and early fetal development
• Basic components of organ systems appear
• Second trimester
• Organs and organ systems near completion of
development
• Third trimester
• Rapid fetal growth
• Organ systems become fully functional
© 2013 Pearson Education, Inc.
Checkpoint (20-3)
6. Define gestation.
7. Describe the key features of each trimester.
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Cleavage and Blastocyte Formation (20-4)
• Cleavage is a sequence of cell divisions
• Begins immediately after fertilization
• Daughter cells become smaller blastomeres
• Zygote becomes a pre-embryo, a morula
• Develops into multicellular blastocyst
• Outer layer is trophoblast; inner cell mass is to one side
• Ends when blastocyst contacts uterine wall
© 2013 Pearson Education, Inc.
Figure 20-2 Cleavage and Blastocyst Formation.
Blastomeres
Polar bodies
2-cell stage
DAY 1
4-cell stage
DAY 2
DAY 3
Early morula
Advanced
morula
DAY 4
First cleavage
division
Hatching
DAY 0:
Inner cell
mass
Fertilization
DAY 6
Days 7–10:
Implantation in
uterine wall
(See Figure 20-3)
© 2013 Pearson Education, Inc.
Blastocoele
Trophoblast
Blastocyst
Implantation (20-4)
• Begins as blastocyst adheres to endometrium of
uterus
• Occurs in day 6–9
• Inner cell mass develops into syncytial trophoblast
• Sets stage for formation of vital embryonic structures
• Ectopic pregnancy
• Implantation occurs in site other than uterus
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Formation of the Amniotic Cavity (20-4)
• Fluid-filled cavity
• Inner cell mass separates from trophoblast
• Cavity develops by day 9
• Yolk sac forms by day 10
© 2013 Pearson Education, Inc.
Figure 20-3 Events in Implantation.
DAY 6
UTERINE CAVITY
FUNCTIONAL ZONE
OF ENDOMETRIUM
Uterine
glands
Blastocyst
DAY 7
Trophoblast
Blastocoele
Inner cell
mass
DAY 8
Syncytial
trophoblast
Cellular
trophoblast
DAY 9
Developing
villi
Endometrial
capillary
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Amniotic
cavity
Lacuna
Gastrulation and Germ Layer Formation (20-4)
• By day 12
• Inner cell mass develops into germ layers
• Ectoderm
• Endoderm
• Mesoderm
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Figure 20-4 The Inner Cell Mass and Gastrulation.
Day 10: Yolk Sac Formation
Syncytial trophoblast
Cellular trophoblast
Amniotic cavity
Yolk sac
Blastocoele
Lacuna
Superficial layer
Deep layer
Day 12: Gastrulation
Yolk sac
Amnion
Ectoderm
Mesoderm
Primitive
streak
Endoderm
Blastodisc
Embryonic
disc
© 2013 Pearson Education, Inc.
Table 20-1 The Fates of the Germ Layers
© 2013 Pearson Education, Inc.
Four Extraembryonic Membranes (20-4)
1. Yolk sac
•
Site of early nutrients and blood cell formation
2. Amnion
•
Contains amnionic fluid
3. Allantois
•
Gives rise to urinary bladder
4. Chorion
•
Provides rapid transit pathway for nutrients to embryo
© 2013 Pearson Education, Inc.
Figure 20-5 Extraembryonic Membranes and Placenta Formation.
Week 2
Week 3
Amniotic cavity
(containing
amniotic fluid)
Amnion
Allantois
Syncytial
trophoblast
Yolk
sac
Cellular
trophoblast Chorion
Mesoderm
Yolk sac
Chorion
Syncytial
trophoblast
Chorionic villi
of placenta
Blastocoele
Week 5
Head fold
of embryo
Week 4
Tail fold
Uterus
Myometrium
Body stalk
Yolk stalk
Yolk sac
Umbilical stalk
Placenta
Yolk sac
Embryonic gut
Embryonic
head fold
Chorionic villi
of placenta
Uterine cavity
Umbilical cord
Placenta
Amniotic cavity
Week 10
Amnion
Chorion
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Placentation (20-4)
• Occurs as blood vessels form in chorion around
periphery of blastocyst
• Chorionic villi form in contact with maternal tissue
• By week 4 embryo, amnion, and yolk sac are
within fluid-filled chamber
• By week 10 fetus floats in amniotic cavity
• Connected by umbilical cord
© 2013 Pearson Education, Inc.
Placental Circulation (20-4)
• By end of the first trimester circulation is
developed
• Umbilical arteries
• Take deoxygenated blood to placenta
• Umbilical vein
• Returns oxygenated blood from placenta to fetus
© 2013 Pearson Education, Inc.
First Trimester Endocrine Secretions of the
Placenta (20-4)
• Human chorionic gonadotropin (hCG)
• Maintains corpus luteum
• Results in maintenance of endometrial lining
• Presence in urine used as indicator of pregnancy
• Progesterone and estrogens
• Secreted by corpus luteum until placenta takes over
• Prevents menses
© 2013 Pearson Education, Inc.
Third Trimester Endocrine Secretions of
Placenta (20-4)
• Human placental lactogen and placental
prolactin
• Rise at end of third trimester
• Prepare mammary glands for milk production
• Relaxin
• Increases flexibility of pubis symphysis
• Causes dilation of cervix
• Suppresses secretion of oxytocin, delaying onset of labor
contractions
© 2013 Pearson Education, Inc.
Figure 20-6 The Placenta and Placental Circulation.
Amnion
Umbilical cord (cut) Placenta
Yolk sac
Chorion
Umbilical
vein
Umbilical
arteries
Chorionic
villi
Area filled with
maternal blood
Endometrium
Myometrium
Uterine cavity
Cervical (mucous)
plug in
cervical canal
External os
Maternal
blood vessels
Cervix
Vagina
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Amnion
Syncytial trophoblast
Embryogenesis (20-4)
• Formation of viable embryo
• Folding and differential growth
• By week 4 dorsal and ventral surfaces are
apparent
• Organogenesis
• Organ formation in first trimester
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Figure 20-7a Development during the First Trimester.
Future head of embryo
Thickened neural plate
(will form brain)
Axis of future spinal
cord
Somites
Neural folds
Cut wall of amniotic
cavity
Future tail of embryo.
© 2013 Pearson Education, Inc.
Week 2. An SEM of the superior surface of a
monkey embryo at 2 weeks of development. A
human embryo at this stage would look essentially
the same.
Figure 20-7b Development during the First Trimester.
Medulla
oblongata
Ear
Forebrain
Pharyngeal
arches
Eye
Heart
Body
stalk
Tail
Somites
Arm bud
Leg bud
Week 4. Fiberoptic view of human development
at week 4.
© 2013 Pearson Education, Inc.
Figure 20-7c Development during the First Trimester.
Chorionic
villi
Amnion
Umbilical
cord
Placenta
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Week 8. Fiberoptic view of
human development at week 8.
Figure 20-7d Development during the First Trimester.
Amnion
Umbilical
cord
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Week 12. Fiberoptic view of human
development at week 12.
Checkpoint (20-4)
8. What is the developmental fate of the inner cell
mass of the blastocyst?
9. Sue's pregnancy test indicates elevated levels of
the hormone hCG (human chorionic
gonadotropin). Is she pregnant?
10. What are two important functions of the
placenta?
© 2013 Pearson Education, Inc.
Second and Third Trimester Development
(20-5)
• Second trimester
• Fetus grows faster than placenta
• Third trimester
• Basic components of organ systems appear
• Most are ready to perform functions
• Largest fetal weight gain occurs
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Figure 20-8 The Fetus during the Second and Third Trimesters.
A four-month-old fetus, seen through a fiberoptic endoscope
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Head of a six-month-old fetus, revealed
through ultrasound
Table 20-2 An Overview of Prenatal and Early Postnatal Development* (1 of 4)
© 2013 Pearson Education, Inc.
Table 20-2 An Overview of Prenatal and Early Postnatal Development* (2 of 4)
© 2013 Pearson Education, Inc.
Table 20-2 An Overview of Prenatal and Early Postnatal Development* (3 of 4)
© 2013 Pearson Education, Inc.
Table 20-2 An Overview of Prenatal and Early Postnatal Development* (4 of 4)
© 2013 Pearson Education, Inc.
Figure 20-9 Changes in Body Form and Proportion during Development.
Prenatal Development
Embryological Development
Fetal Development
4 weeks
8 weeks
16 weeks
Postnatal Development
Neonatal Infancy
Childhood
Adolescence
Maturity
5 ft
4 ft
3 ft
2 ft
1 ft
0
© 2013 Pearson Education, Inc.
1 month
2 years
Puberty
(between 9–14 years)
18 years
Changes in Maternal Systems (20-5)
• Respiratory rate and tidal volume increase
• Blood volume increases
• Nutrient requirements increase
• GFR increases
• Uterus increases in size
• Mammary glands increase in size and activity
© 2013 Pearson Education, Inc.
Structural and Functional Uterine Changes
(20-5)
• At end of gestation the uterus:
• Has grown from 3 to 12 inches in length
• Contains 2 liters of fluid, fetus, and placenta
• Labor contractions
• Fetal oxytocin triggers positive feedback mechanism
• Increases myometrial contractions
© 2013 Pearson Education, Inc.
Checkpoint (20-5)
11. List the major changes that occur in maternal
systems during pregnancy.
12. Why does a woman's blood volume increase
during pregnancy?
13. By what means does the uterus greatly increase
in size and weight during pregnancy?
14. Identify three major factors opposing the
calming action of progesterone on the uterus.
© 2013 Pearson Education, Inc.
The Three Stages of Labor (20-6)
•
Also called parturition
1. Dilation stage
2. Expulsion stage
3. Placental stage
© 2013 Pearson Education, Inc.
The Dilation Stage (20-6)
• Fetus shifts toward cervix
• This stage is highly variable in length, but typically
lasts 8 or more hours
• Amnion ruptures, "water breaks"
© 2013 Pearson Education, Inc.
The Expulsion Stage (20-6)
• Fetus pushed through cervix and vagina
• Referred to as delivery
• Episiotomy
• Incision in perineal musculature to enlarge birth canal
• Cesarean section
• Incision in abdominal wall to deliver fetus if vaginal delivery
not possible
© 2013 Pearson Education, Inc.
The Placental Stage (20-6)
• Uterine contractions tear connections between
endometrium and placenta
• Placenta is ejected from body as "afterbirth"
• Retained placenta can result in infection
© 2013 Pearson Education, Inc.
Figure 20-10 Factors Involved in Initiating and Sustaining Labor and Delivery.
Placental Factors
Fetal Factors
Placental estrogens increase the sensitivity of the smooth
muscle cells of the myometrium and make contractions
more likely. As delivery approaches, the production of
estrogens accelerates. Estrogens also increase the
sensitivity of smooth muscle fibers to oxytocin.
Relaxin produced
by the placenta
relaxes the pelvic
articulations and
dilates the cervix.
Growth and the
increase in fetal
weight stretch
and distort the
myometrium.
Fetal pituitary
releases
oxytocin in
response to
estrogens.
Distortion of Myometrium
Distortion of the myometrium increases
the sensitivity of the smooth muscle
layers, promoting spontaneous contractions that get stronger and more
frequent as the pregnancy advances.
Maternal Oxytocin Release
Maternal oxytocin release is
stimulated by high estrogen levels
and by distortion of the cervix.
Prostaglandin Production
Estrogens and oxytocin stimulate the production of
prostaglandins in the endometrium. These local
hormones further stimulate smooth muscle contractions.
Increased Excitability of the Myometrium
Oxytocin and prostaglandins both stimulate the myometrium. In addition, the sensitivity of the uterus to oxytocin
increases dramatically; the smooth muscle in a late-term uterus is 100 times more sensitive to oxytocin than the
smooth muscle in a nonpregnant uterus.
LABOR CONTRACTIONS OCCUR
© 2013 Pearson Education, Inc.
Labor contractions
move the fetus and
further distort the
myometrium. This
distortion stimulates
additional oxytocin
and prostaglandin
release. This positive
feedback continues
until delivery is
completed.
Figure 20-11 The Stages of Labor.
Fully developed fetus before labor begins
The Dilation Stage
Pubic
symphysis
Placenta Umbilical
cord
Cervical
canal
Sacral
promontory
Vagina
Cervix
The Placental Stage
Ejection of the
Uterus
placenta
© 2013 Pearson Education, Inc.
The Expulsion Stage
Premature Labor (20-6)
• Contractions occur before fetus completes development
• Miscarriage or spontaneous abortion
• Prior to end of second trimester, fetal weight under 500 g
• Immature delivery
• Fetal weight above 500 g
• Most born at 25–27 weeks of gestation die or have complications
• Premature delivery
• Birth at 28–36 weeks requires extra care, infants usually survive
© 2013 Pearson Education, Inc.
Multiple Births (20-6)
• "Fraternal" or dizygotic
• Two separate oocytes are fertilized at same time
• "Identical" or monozygotic
• Blastomeres separate early in cleavage
• Conjoined twins
• When splitting of blastomeres is incomplete
• Shared skin and organs
© 2013 Pearson Education, Inc.
Checkpoint (20-6)
15. Name the three stages of labor.
16. What is the difference between immature
delivery and premature delivery?
17. What are the biological terms for fraternal twins
and identical twins?
© 2013 Pearson Education, Inc.
Postnatal Stages (20-7)
• Life stages
• Neonatal period
• Infancy
• Childhood
• Adolescence
• Maturity
© 2013 Pearson Education, Inc.
The Neonatal Period (20-7)
•
Newborn is also called a neonate
1. Filling collapsed lungs with powerful inhalation
2. Changes in blood pressure and flow rates
•
Triggers separation of systemic and pulmonary circuits
3. Heart rate slows from 150 bpm to 120–140 bpm
4. Digestive system becomes active with nursing
© 2013 Pearson Education, Inc.
The Neonatal Period (20-7)
5. Kidneys eliminate urine
•
Lack ability to concentrate urine
•
Neonates require high fluid intake
6. Mechanisms for controlling body temperature
•
Develops subcutaneous fat layer
•
Increases metabolic activity
© 2013 Pearson Education, Inc.
Lactation and the Mammary Glands (20-7)
• By month 6 of gestation, mammary glands fully
developed
• Colostrum
• Early secretion includes:
• Higher proteins, lower fat than breast milk
• Proteins are mostly antibodies for short-term immunity
• Milk let-down reflex
• Initiated by suckling
• Functions until weaning
© 2013 Pearson Education, Inc.
Figure 20-12 The Milk Let-Down Reflex.
Stimulation of hypothalamic nuclei
Posterior
lobe of the
pituitary
gland
Oxytocin Release
Milk Ejected
Start
Tactile receptors
in nipples
stimulated
© 2013 Pearson Education, Inc.
Neural impulses are
propagated to the
spinal cord.
Infancy and Childhood (20-7)
• Growth
• Directed by circulating hormones
• GH, adrenal steroids, TH
• Specific effects
• Are unique from organ to organ
• Results in nonuniformity of growth patterns
© 2013 Pearson Education, Inc.
Adolescence (20-7)
• Begins at onset of puberty
• Increase in GnRH
• Increase in LH and FSH
• Gamete formation
• Secretion of sex hormones
• Development of secondary sex characteristics
• Rapid growth spurt
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Maturity (20-7)
• Often identified as starting when growth stops
• Physiological changes continue
• Menopause and male climacteric
• Senescence
• The aging process
• Ultimately leads to death
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Checkpoint (20-7)
18. Name the postnatal stages of development.
19. What is the difference between colostrum and
breast milk?
20. Increases in the blood levels of GnRH, FSH, LH,
and sex hormones mark the onset of which
stage of development?
© 2013 Pearson Education, Inc.
Genes and Chromosomes (20-8)
• DNA
• Contains chromosomes, which contain genes
• Segments of DNA with peptide synthesis information
• Genotype
• Original 46 chromosomes formed in zygote retained in every
cell
• Determine unique characteristics of your phenotype
© 2013 Pearson Education, Inc.
Patterns of Inheritance (20-8)
• Homologous chromosomes
• Members of each pair of chromosomes
• One member contributed by sperm, other by ovum
• Autosomal chromosomes
• 22 pairs of homologous chromosomes
• Affect somatic characteristics like hair color
© 2013 Pearson Education, Inc.
Patterns of Inheritance (20-8)
• Sex chromosomes
• 23rd pair of homologous chromosomes
• Determine genetic male or genetic female
• Karyotype
• Entire set of chromosomes
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Patterns of Inheritance (20-8)
• Alleles are forms of a particular gene
• Homozygous
• When both alleles are the same for a specific trait
• Heterozygous
• Alleles are not identical
• Dominant will be expressed phenotypically
• Recessive will not be expressed unless on both
chromosomes of pair
© 2013 Pearson Education, Inc.
Figure 20-13 A Human Karyotype.
© 2013 Pearson Education, Inc.
Predicting Inheritance (20-8)
• Simple inheritance
• Phenotypes determined by interactions of single pair of
alleles
• Fairly easy to predict
• Polygenic inheritance
• Phenotypes determined by interaction of multiple alleles
• Difficult to predict
© 2013 Pearson Education, Inc.
Table 20-3 The Inheritance of Selected Phenotypic Characteristics
© 2013 Pearson Education, Inc.
Predicting Inheritance (20-8)
• Genotype for specific trait indicated by letters
• Dominant trait uses capital letter
• Recessive trait uses lowercase letter
• Example: AA is homozygous dominant, Aa is heterozygous,
aa is homozygous recessive
• Combinations of parental alleles determine outcome
• Can be predicted using Punnett square
© 2013 Pearson Education, Inc.
Figure 20-14a Predicting Genotypes and Phenotypes with Punnett Squares.
Maternal alleles (contributed by
the ovum). Every ovum will carry
the recessive gene a.
a
Paternal alleles
(contributed by
the spermatozoon).
Every sperm produced by a homozygous dominant (AA)
father will carry the A
allele.
A
Aa
a
Aa
All have normal skin
pigmentation
A
Aa
Aa
If the father is homozygous for normal pigmentation, all of the
children will have the genotype Aa, and all will have normal
skin pigmentation.
© 2013 Pearson Education, Inc.
Figure 20-14b Predicting Genotypes and Phenotypes with Punnett Squares.
a
Half of the
sperm produced
by a heterozygous
A
(Aa) father will carry
the dominant allele
A, and the other half
will carry the recessive
allele a.
a
Maternal
alleles
a
Aa
Aa
50% of the children are heterozygous and have normal
pigmentation
aa
aa
50% of the children are homozygous recessive and
exhibit albinism.
If the father is heterozygous for normal skin pigmentation,
the probability that a child will have normal pigmentation is
reduced to 50%.
© 2013 Pearson Education, Inc.
Table 20-4 Fairly Common Inherited Disorders
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Sex-Linked Inheritance (20-8)
• X chromosome
• Larger with more genes
• Carried by all oocytes
• Y chromosome
• Includes dominant alleles for male genotype
• X-linked traits
• Alleles for somatic traits on the X chromosome
© 2013 Pearson Education, Inc.
Figure 20-15 Inheritance of an X-Linked Trait.
A woman—who has two X
chromosomes—can be either homozygous dominant (XCXC) or
heterozygous (XC Xc) and still have
normal color vision. She will be
unable to distinguish reds from
greens only if she carries two
recessive alleles, XcXc.
A man has only
one X chromoXC
some, so whichever
allele that chromosome
carries determines
whether he has normal
color vision or is red–
green color-blind.
Y
XC
Xc
XC XC
XC Xc
Normal female
Normal female
(carrier)
XC Y
XC Y
Normal male
© 2013 Pearson Education, Inc.
Color-blind
male
Human Genome Project (20-8)
• Genome is the full set of DNA in chromosomes
mapped through karyotyping
• All human chromosomes have been sequenced
• Total number of genes estimated at 20,000–25,000
• 99 percent of all nucleotide bases same in all people
• Single nucleotide polymorphisms locate disease
sequences on chromosomes
• 10,000 single-gene disorders have been described
© 2013 Pearson Education, Inc.
Figure 20-16 A Map of Human Chromosomes.
Color Blindness (multiple forms)
Prostate Cancer
p. 324
Muscular Dystrophy p. 667
p. 236
Hemophilia
p. 399
Down Syndrome
p. 700
Huntington’s Disease
p. 295
Familial Polyposis of the Colon
Abnormal tissue growths that
commonly lead to colon cancer
Familial Hypercholesterolemia
Extremely high cholesterol
Spinocerebellar Ataxia
Destroys neurons in the brain
and spinal cord, resulting in
loss of muscle control
Y1 2
3
22 X
4
21
5
20
19
6
CHROMOSOME
18
7
PAIRS
17
8
16
9
10
15
14 13 12 11
Breast Cancer
(one form)
p. 658
Cystic Fibrosis
p. 506
Malignant Melanoma
p. 125
Multiple Endocrine Neoplasia, Type 2
Tumors in endocrine glands and
other tissues
Marfan Syndrome
p. 103
Sickle Cell Anemia
p. 385
Alzheimer’s Disease
(one form) p. 294
PKU
(phenylketonuria)
p. 587
Retinoblastoma
A relatively common tumor of the eye,
accounting for 2% of childhood malignancies
© 2013 Pearson Education, Inc.
Checkpoint (20-8)
21. Describe the relationship between genotype and
phenotype.
22. Curly hair is an autosomal dominant trait. What
would be the phenotype of a person who is
heterozygous for this trait?
23. Joe has three daughters and complains that it's
his wife's "fault" that he has no sons. What
would you tell him?
© 2013 Pearson Education, Inc.
Checkpoint (20-8)
24. The human genome consists of approximately
3200 Mb. What is a genome, and how many
nucleotide base pairs does 3200 Mb represent?
© 2013 Pearson Education, Inc.