BDOL Interactive Chalkboard

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Table of Contents pages iv-v
Unit 1 What is Biology? Unit 2 Ecology Unit
3 The Life of a Cell Unit 4 Genetics Unit 5
Change Through Time Unit 6 Viruses, Bacteria,
Protists, and Fungi Unit 7 Plants Unit 8
Invertebrates Unit 9 Vertebrates Unit 10 The
Human Body
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Table of Contents pages iv-v
Unit 1 What is Biology? Chapter 1
Biology The Study of Life Unit 2 Ecology
Chapter 2 Principles of Ecology Chapter
3 Communities and Biomes Chapter 4
Population Biology Chapter 5 Biological
Diversity and Conservation Unit 3 The Life of a
Cell Chapter 6 The Chemistry of Life
Chapter 7 A View of the Cell Chapter 8
Cellular Transport and the Cell Cycle
Chapter 9 Energy in a Cell
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Table of Contents pages iv-v
Unit 4 Genetics Chapter 10 Mendel and
Meiosis Chapter 11 DNA and Genes
Chapter 12 Patterns of Heredity and Human
Genetics Chapter 13 Genetic Technology Unit
5 Change Through Time Chapter 14 The
History of Life Chapter 15 The Theory of
Evolution Chapter 16 Primate Evolution
Chapter 17 Organizing Lifes Diversity
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Table of Contents pages iv-v
Unit 6 Viruses, Bacteria, Protists, and Fungi
Chapter 18 Viruses and Bacteria Chapter
19 Protists Chapter 20 Fungi Unit 7
Plants Chapter 21 What Is a Plant?
Chapter 22 The Diversity of Plants
Chapter 23 Plant Structure and Function
Chapter 24 Reproduction in Plants
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Table of Contents pages iv-v
Unit 8 Invertebrates Chapter 25 What Is
an Animal? Chapter 26 Sponges,
Cnidarians, Flatworms, and
Roundworms Chapter 27
Mollusks and Segmented Worms Chapter 28
Arthropods Chapter 29 Echinoderms and
Invertebrate
Chordates
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Table of Contents pages iv-v
Unit 9 Vertebrates Chapter 30 Fishes
and Amphibians Chapter 31 Reptiles and
Birds Chapter 32 Mammals Chapter 33
Animal Behavior Unit 10 The Human Body
Chapter 34 Protection, Support, and
Locomotion Chapter 35 The Digestive and
Endocrine Systems Chapter 36 The Nervous
System Chapter 37 Respiration,
Circulation, and Excretion Chapter 38
Reproduction and Development Chapter 39
Immunity from Disease
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Unit Overview pages 250-251
Genetics
Mendel and Meiosis
DNA and Genes
Patterns of Heredity and Human Genetics
Genetic Technology
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Chapter Contents page viii
Chapter 12 Patterns of Heredity and Human
Genetics 12.1 Mendelian Inheritance of
Human Traits 12.1 Section Check 12.2 When
Heredity Follows Different Rules 12.2 Section
Check 12.3 Complex Inheritance of Human
Traits 12.3 Section Check Chapter 12
Summary Chapter 12 Assessment
10
Chapter Intro-page 308
What Youll Learn
You will compare the inheritance of recessive and
dominant traits in humans.
You will analyze the inheritance patterns of
traits with incomplete dominance and codominance.
You will determine the inheritance of sex-linked
traits.
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12.1 Section Objectives page 309
Section Objectives

• Interpret a pedigree.

• Identify human genetic disorders caused by
  inherited recessive alleles.

• Predict how a human trait can be determined by a
  simple dominant allele.

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Section 12.1 Summary pages 309 - 314
Making a Pedigree

• A family tree traces a family name and various
  family members through successive generations.

• Through a family tree, you can identify the
  relationships among your cousins, aunts, uncles,
  grandparents, and great-grandparents.

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Section 12.1 Summary pages 309 - 314
Pedigrees illustrate inheritance

• A pedigree is a graphic representation of genetic
  inheritance.

• It is a diagram made up of a set of symbols that
  identify males and females, individuals affected
  by the trait being studied, and family
  relationships.

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Section 12.1 Summary pages 309 - 314
Male
Parents
Siblings
Female
Pedigrees illustrate inheritance
Affected male
Known heterozygotes for recessive allele
Affected female
Mating
Death
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Section 12.1 Summary pages 309 - 314
Pedigrees illustrate inheritance
Female
Male
I
1
2
II
2
1
3
4
5

• In a pedigree, a circle represents a female a
  square represents a male.

III
1
4
2
3
?
IV
5
3
4
2
1
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Section 12.1 Summary pages 309 - 314
Pedigrees illustrate inheritance
I
1
2
II
3
2
1
4
5

• Highlighted circles and squares represent
  individuals showing the trait being studied.

III
1
4
2
3
?
IV
2
3
5
1
4
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Section 12.1 Summary pages 309 - 314
Pedigrees illustrate inheritance
I
1
2
II

• Circles and squares that are not highlighted
  designate individuals that do not show the trait.

2
3
1
4
5
III
1
4
2
3
?
IV
3
5
2
4
1
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Section 12.1 Summary pages 309 - 314
Pedigrees illustrate inheritance

• A half-shaded circle or square represents a
  carrier, a heterozygous individual.

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Section 12.1 Summary pages 309 - 314
Pedigrees illustrate inheritance

• A horizontal line connecting a circle and a
  square indicates that the individuals are
  parents, and a vertical line connects parents
  with their offspring.

I
1
2
II
4
3
2
1
5
III
1
4
2
3
?
IV
2
3
5
1
4
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Section 12.1 Summary pages 309 - 314
Pedigrees illustrate inheritance

• Each horizontal row of circles and squares in a
  pedigree designates a generation, with the most
  recent generation shown at the bottom.

I
1
2
II
1
3
2
4
5
III
1
2
4
3
?
IV
3
5
1
2
4
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Section 12.1 Summary pages 309 - 314
Pedigrees illustrate inheritance

• The generations are identified in sequence by
  Roman numerals, and each individual is given an
  Arabic number.

I
1
2
II
1
3
2
4
5
III
1
2
4
3
?
IV
3
5
1
2
4
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Section 12.1 Summary pages 309 - 314
Simple Recessive Heredity

• Most genetic disorders are caused by recessive
  alleles.

Cystic fibrosis

• Cystic fibrosis (CF) is a fairly common genetic
  disorder among white Americans.

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Section 12.1 Summary pages 309 - 314
Cystic fibrosis

• Approximately one in 28 white Americans carries
  the recessive allele, and one in 2500 children
  born to white Americans inherits the disorder.

• Due to a defective protein in the plasma
  membrane, cystic fibrosis results in the
  formation and accumulation of thick mucus in the
  lungs and digestive tract.

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Section 12.1 Summary pages 309 - 314
Tay-Sachs disease

• Tay-Sachs (tay saks) disease is a recessive
  disorder of the central nervous system.

• In this disorder, a recessive allele results in
  the absence of an enzyme that normally breaks
  down a lipid produced and stored in tissues of
  the central nervous system.

• Because this lipid fails to break down properly,
  it accumulates in the cells.

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Section 12.1 Summary pages 309 - 314
I
1
2
Typical Pedigree for
II
1
2
3
4
Tay-Sachs
III
3
1
2
IV
1
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Section 12.1 Summary pages 309 - 314
Phenylketonuria

• Phenylketonuria (fen ul kee tun YOO ree uh), also
  called (PKU), is a recessive disorder that
  results from the absence of an enzyme that
  converts one amino acid, phenylalanine, to a
  different amino acid, tyrosine.

• Because phenylalanine cannot be broken down, it
  and its by-products accumulate in the body and
  result in severe damage to the central nervous
  system.

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Section 12.1 Summary pages 309 - 314
Phenylketonuria

• A PKU test is normally performed on all infants a
  few days after birth.

• Infants affected by PKU are given a diet that is
  low in phenylalanine until their brains are fully
  developed.

• Ironically, the success of treating
  phenylketonuria infants has resulted in a new
  problem.

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Section 12.1 Summary pages 309 - 314
Phenylketonuria

• If a female who is homozygous recessive for PKU
  becomes pregnant, the high phenylalanine levels
  in her blood can damage her fetusthe developing
  baby.

• This problem occurs even if the fetus is
  heterozygous and would be phenotypically normal.

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Section 12.1 Summary pages 309 - 314
Phenylketonuria
Phenylketonurics Contains Phenylalanine
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Section 12.1 Summary pages 309 - 314
Simple Dominant Heredity

• Many traits are inherited just as the rule of
  dominance predicts.

• Remember that in Mendelian inheritance, a single
  dominant allele inherited from one parent is all
  that is needed for a person to show the dominant
  trait.

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Section 12.1 Summary pages 309 - 314
Simple dominant traits

• A cleft chin, widows peak hairline, hitchhikers
  thumb, almond shaped eyes, thick lips, and the
  presence of hair on the middle section of your
  fingers all are examples of dominant traits.

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Section 12.1 Summary pages 309 - 314
Huntingtons disease

• Huntingtons disease is a lethal genetic disorder
  caused by a rare dominant allele.

• It results in a breakdown of certain areas of the
  brain.

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Section 12.1 Summary pages 309 - 314
Huntingtons disease

• Ordinarily, a dominant allele with such severe
  effects would result in death before the affected
  individual could have children and pass the
  allele on to the next generation.

• But because the onset of Huntingtons disease
  usually occurs between the ages of 30 and 50, an
  individual may already have had children before
  knowing whether he or she is affected.

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Section 12.1 Summary pages 309 - 314
Typical Pedigree of Huntingtons Disease
I
1
2
II
1
2
5
4
3
III
1
2
3
4
5
35
Section 1 Check
Question 1
I
1
2
What does this pedigree tell you about
those who show the recessive phenotype for the
disease?
II
1
2
3
4
III
3
1
2
IV
1
36
Section 1 Check
I
The pedigree indicates that showing the recessive
phenotype for the disease is fatal.
1
2
II
1
2
3
4
III
3
1
2
IV
1
37
Section 1 Check
Question 2
What must happen for a person to show a
recessive phenotype?
Answer
The person must inherit a recessive allele for
the trait from both parents.
38
Section 1 Check
Question 3
Which of the following diseases is the
result of a dominant allele?
A. Huntingtons disease
B. Tay-Sachs disease
C. cystic fibrosis
D. phenylketonuria
The answer is A.
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12.2 Section Objectives page 315
Section Objectives

• Distinguish between alleles for incomplete
  dominance and codominance.

• Explain the patterns of multiple allelic and
  polygenic inheritance.

• Analyze the pattern of sex-linked inheritance.

• Summarize how internal and external environments
  affect gene expression.

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Section 12.2 Summary pages 315 - 322
Complex Patterns of Inheritance

• Patterns of inheritance that are explained by
  Mendels experiments are often referred to as
  simple.

• However, many inheritance patterns are more
  complex than those studied by Mendel.

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Section 12.2 Summary pages 315 - 322
Incomplete dominance Appearance of a third
phenotype

• When inheritance follows a pattern of dominance,
  heterozygous and homozygous dominant individuals
  both have the same phenotype.

• When traits are inherited in an incomplete
  dominance pattern, however, the phenotype of
  heterozygous individuals is intermediate between
  those of the two homozygotes.

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Section 12.2 Summary pages 315 - 322
Incomplete dominance Appearance of a third
phenotype

• For example, if a homozygous red-flowered
  snapdragon plant (RR) is crossed with a
  homozygous white-flowered snapdragon plant (R'
  R'), all of the F1 offspring will have pink
  flowers.

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Section 12.2 Summary pages 315 - 322
Incomplete dominance Appearance of a third
phenotype
Red
White
All pink
Red (RR)
Pink (RR)
White (RR)
Pink (RR)
All pink flowers
1 red 2 pink 1 white
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Section 12.2 Summary pages 315 - 322
Incomplete dominance Appearance of a third
phenotype

• The new phenotype occurs because the flowers
  contain enzymes that control pigment production.

• The R allele codes for an enzyme that produces a
  red pigment. The R allele codes for a defective
  enzyme that makes no pigment.

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Section 12.2 Summary pages 315 - 322
Incomplete dominance Appearance of a third
phenotype

• Because the heterozygote has only one copy of the
  R allele, its flowers appear pink because they
  produce only half the amount of red pigment that
  red homozygote flowers produce.

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Section 12.2 Summary pages 315 - 322
Incomplete dominance Appearance of a third
phenotype
Red
White
All pink
Red (RR)
Pink (RR)
White (RR)
Pink (RR)
All pink flowers
1 red 2 pink 1 white
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Section 12.2 Summary pages 315 - 322
Codominance Expression of both alleles

• Codominant alleles cause the phenotypes of both
  homozygotes to be produced in heterozygous
  individuals. In codominance, both alleles are
  expressed equally.

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Section 12.2 Summary pages 315 - 322
Multiple phenotypes from multiple alleles

• Although each trait has only two alleles in the
  patterns of heredity you have studied thus far,
  it is common for more than two alleles to control
  a trait in a population.

• Traits controlled by more than two alleles have
  multiple alleles.

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Section 12.2 Summary pages 315 - 322
Sex determination

• In humans the diploid number of chromosomes is
  46, or 23 pairs.

• There are 22 pairs of homologous chromosomes
  called autosomes. Homologous autosomes look
  alike.

• The 23rd pair of chromosomes differs in males and
  females.

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Section 12.2 Summary pages 315 - 322
Sex determination

• These two chromosomes, which determine the sex of
  an individual, are called sex chromosomes and are
  indicated by the letters X and Y.

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Section 12.2 Summary pages 315 - 322
Sex determination

• If you are female, your 23rd pair of chromosomes
  are homologous, XX.

X
X
Female

• If you are male, your 23rd pair of chromosomes
  XY, look different.

Y
X
Male
52
Section 12.2 Summary pages 315 - 322
Sex determination

• Males usually have one X and one Y chromosome and
  produce two kinds of gametes, X and Y.

• Females usually have two X chromosomes and
  produce only X gametes.

• It is the male gamete that determines the sex of
  the offspring.

53
Section 12.2 Summary pages 315 - 322
XY Male
Sex determination
X
Y
X
XX Female
XY Male
XX Female
X
XY Male
XX Female
54
Section 12.2 Summary pages 315 - 322
Sex-linked inheritance

• Traits controlled by genes located on sex
  chromosomes are called sex-linked traits.

• The alleles for sex-linked traits are written as
  superscripts of the X or Y chromosomes.

• Because the X and Y chromosomes are not
  homologous, the Y chromosome has no corresponding
  allele to one on the X chromosome and no
  superscript is used.

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Section 12.2 Summary pages 315 - 322
Sex-linked inheritance

• Also remember that any recessive allele on the X
  chromosome of a male will not be masked by a
  corresponding dominant allele on the Y chromosome.

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Section 12.2 Summary pages 315 - 322
Sex-linked inheritance
White-eyed male (XrY)
F2
Females
Red-eyed female (XRXR)
all red eyed
Males
1/2 red eyed
1/2 white eyed
F1 All red eyed
57
Section 12.2 Summary pages 315 - 322
Sex-linked inheritance

• The genes that govern sex-linked traits follow
  the inheritance pattern of the sex chromosome on
  which they are found.

Click here to view movie.
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Section 12.2 Summary pages 315 - 322
Polygenic inheritance

• Polygenic inheritance is the inheritance pattern
  of a trait that is controlled by two or more
  genes.

• The genes may be on the same chromosome or on
  different chromosomes, and each gene may have two
  or more alleles.

• Uppercase and lowercase letters are used to
  represent the alleles.

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Section 12.2 Summary pages 315 - 322
Polygenic inheritance

• However, the allele represented by an uppercase
  letter is not dominant. All heterozygotes are
  intermediate in phenotype.

• In polygenic inheritance, each allele represented
  by an uppercase letter contributes a small, but
  equal, portion to the trait being expressed.

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Section 12.2 Summary pages 315 - 322
Polygenic inheritance

• The result is that the phenotypes usually show a
  continuous range of variability from the minimum
  value of the trait to the maximum value.

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Section 12.2 Summary pages 315 - 322
Environmental Influences

• The genetic makeup of an organism at
  fertilization determines only the organisms
  potential to develop and function.

• As the organism develops, many factors can
  influence how the gene is expressed, or even
  whether the gene is expressed at all.

• Two such influences are the organisms external
  and internal environments.

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Section 12.2 Summary pages 315 - 322
Influence of external environment

• Temperature, nutrition, light, chemicals, and
  infectious agents all can influence gene
  expression.

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Section 12.2 Summary pages 315 - 322
Influence of external environment

• In arctic foxes temperature has an effect on the
  expression of coat color.

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Section 12.2 Summary pages 315 - 322
Influence of external environment

• External influences can also be seen in leaves.
  Leaves can have different sizes, thicknesses, and
  shapes depending on the amount of light they
  receive.

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Section 12.2 Summary pages 315 - 322
Influence of internal environment

• The internal environments of males and females
  are different because of hormones and structural
  differences.

• An organisms age can also affect gene function.

66
Section 2 Check
Question 1
What is the difference between simple
Mendelian inheritance and codominant inheritance?
67
Section 2 Check
In Mendelian inheritance, heterozygous
individuals will display the inherited dominant
trait of the homozygotes. When traits are
inherited in a codominant pattern the phenotypes
of both homozygotes are displayed equally in the
heterozygotes.
68
Section 2 Check
Question 2
Which of the following does NOT have an
effect on male-pattern baldness?
A. hormones
B. internal environment
C. sex-linked inheritance
D. incomplete dominance
The answer is D.
69
Section 2 Check
Question 3
If the offspring of human mating have a
50-50 chance of being either male or female, why
is the ratio not exactly 11 in a small
population?
Answer
The ratio is not exactly 11 because the laws of
probability govern fertilization.
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12.3 Section Objectives page 323
Section Objectives

• Identify codominance, multiple allelic,
  sex-linked and polygenic patterns of inheritance
  in humans.

• Distinguish among conditions that result from
  extra autosomal or sex chromosomes.

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Section 12.3 Summary pages 323 - 329
Codominance in Humans

• Remember that in codominance, the phenotypes of
  both homozygotes are produced in the heterozygote.

• One example of this in humans is a group of
  inherited red blood cell disorders called
  sickle-cell disease.

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Section 12.3 Summary pages 323 - 329
Sickle-cell disease

• In an individual who is homozygous for the
  sickle-cell allele, the oxygen-carrying protein
  hemoglobin differs by one amino acid from normal
  hemoglobin.

• This defective hemoglobin forms crystal-like
  structures that change the shape of the red blood
  cells. Normal red blood cells are disc-shaped,
  but abnormal red blood cells are shaped like a
  sickle, or half-moon.

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Section 12.3 Summary pages 323 - 329
Sickle-cell disease

• The change in shape occurs in the bodys narrow
  capillaries after the hemoglobin delivers oxygen
  to the cells.

Normal red blood cell
Sickle cell
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Section 12.3 Summary pages 323 - 329
Sickle-cell disease

• Abnormally shaped blood cells, slow blood flow,
  block small vessels, and result in tissue damage
  and pain.

Normal red blood cell
Sickle cell
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Section 12.3 Summary pages 323 - 329
Sickle-cell disease

• Individuals who are heterozygous for the allele
  produce both normal and sickled hemoglobin, an
  example of codominance.

• Individuals who are heterozygous are said to have
  the sickle-cell trait because they can show some
  signs of sickle-cell-related disorders if the
  availability of oxygen is reduced.

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Section 12.3 Summary pages 323 - 329
Multiple Alleles Govern Blood Type

• Mendels laws of heredity also can be applied to
  traits that have more than two alleles.

• The ABO blood group is a classic example of a
  single gene that has multiple alleles in humans.

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Section 12.3 Summary pages 323 - 329
Multiple Alleles Govern Blood Type
Human Blood Types
Genotypes
Surface Molecules
Phenotypes
A
A
lA lA or lAli
B
B
lB lB or lBi
lA lB
A and B
AB
None
ii
O
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Section 12.3 Summary pages 323 - 329
The importance of blood typing

• Determining blood type is necessary before a
  person can receive a blood transfusion because
  the red blood cells of incompatible blood types
  could clump together, causing death.

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Section 12.3 Summary pages 323 - 329
The ABO Blood Group

• The gene for blood type, gene l, codes for a
  molecule that attaches to a membrane protein
  found on the surface of red blood cells.

• The lA and lB alleles each code for a different
  molecule.

• Your immune system recognizes the red blood cells
  as belonging to you. If cells with a different
  surface molecule enter your body, your immune
  system will attack them.

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Section 12.3 Summary pages 323 - 329
Phenotype A
Surface molecule A

• The lA allele is dominant to i, so inheriting
  either the lAi alleles or the lA lA alleles from
  both parents will give you type A blood.

• Surface molecule A is produced.

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Section 12.3 Summary pages 323 - 329
Phenotype B
Surface molecule B

• The lB allele is also dominant to i.

• To have type B blood, you must inherit the lB
  allele from one parent and either another lB
  allele or the i allele from the other.

• Surface molecule B is produced.

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Section 12.3 Summary pages 323 - 329
Phenotype AB
Surface molecule B

• The lA and lB alleles are codominant.

• This means that if you inherit the lA allele from
  one parent and the lB allele from the other, your
  red blood cells will produce both surface
  molecules and you will have type AB blood.

Surface molecule A
83
Section 12.3 Summary pages 323 - 329
Phenotype O

• The i allele is recessive and produces no surface
  molecules.

• Therefore, if you are homozygous ii, your blood
  cells have no surface molecules and you have
  blood type O.

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Section 12.3 Summary pages 323 - 329
Sex-Linked Traits in Humans

• Many human traits are determined by genes that
  are carried on the sex chromosomes most of these
  genes are located on the X chromosome.

• The pattern of sex-linked inheritance is
  explained by the fact that males, who are XY,
  pass an X chromosome to each daughter and a Y
  chromosome to each son.

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Section 12.3 Summary pages 323 - 329
Sex-Linked Traits in Humans

• Females, who are XX, pass one of their X
  chromosomes to each child.

Female
Male
Male
Female
Sperm
Eggs
Eggs
Sperm
Male
Male
Female
Female
Female
Female
Male
Male
86
Section 12.3 Summary pages 323 - 329
Sex-Linked Traits in Humans

• If a son receives an X chromosome with a
  recessive allele, the recessive phenotype will be
  expressed because he does not inherit on the Y
  chromosome from his father a dominant allele that
  would mask the expression of the recessive allele.

• Two traits that are governed by X-linked
  recessive inheritance in humans are red-green
  color blindness and hemophilia.

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Section 12.3 Summary pages 323 - 329
Red-green color blindness

• People who have red-green color blindness cant
  differentiate these two colors. Color blindness
  is caused by the inheritance of a recessive
  allele at either of two gene sites on the X
  chromosome.

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Section 12.3 Summary pages 323 - 329
Hemophilia An X-linked disorder

• Hemophilia A is an X-linked disorder that causes
  a problem with blood clotting.

• About one male in every 10 000 has hemophilia,
  but only about one in 100 million females
  inherits the same disorder.

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Section 12.3 Summary pages 323 - 329
Hemophilia An X-linked disorder

• Males inherit the allele for hemophilia on the X
  chromosome from their carrier mothers. One
  recessive allele for hemophilia will cause the
  disorder in males.

• Females would need two recessive alleles to
  inherit hemophilia.

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Section 12.3 Summary pages 323 - 329
Polygenic Inheritance in Humans

• Although many of your traits were inherited
  through simple Mendelian patterns or through
  multiple alleles, many other human traits are
  determined by polygenic inheritance.

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Section 12.3 Summary pages 323 - 329
Skin color A polygenic trait

• In the early 1900s, the idea that polygenic
  inheritance occurs in humans was first tested
  using data collected on skin color.

• Scientists found that when light-skinned people
  mate with dark-skinned people, their offspring
  have intermediate skin colors.

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Section 12.3 Summary pages 323 - 329
Skin color A polygenic trait

• This graph shows the expected distribution of
  human skin color if controlled by one, three, or
  four genes.

Number of Genes Involved in Skin Color
Expected distribution- 4 genes
Observed distribution of skin color
Expected distribution- 1 gene
Number of individuals
Expected distribution- 3 genes
Light
Right
Range of skin color
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Section 12.3 Summary pages 323 - 329
Changes in Chromosome Numbers

• What would happen if an entire chromosome or part
  of a chromosome were missing from the complete
  set?

• As you have learned, abnormal numbers of
  chromosomes in offspring usually, but not always,
  result from accidents of meiosis.

• Many abnormal phenotypic effects result from such
  mistakes.

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Section 12.3 Summary pages 323 - 329
Abnormal numbers of autosomes

• Humans who have an extra whole or partial
  autosome are trisomicthat is, they have three of
  a particular autosomal chromosome instead of just
  two. In other words, they have 47 chromosomes.

• To identify an abnormal number of chromosomes, a
  sample of cells is obtained from an individual or
  from a fetus.

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Section 12.3 Summary pages 323 - 329
Abnormal numbers of autosomes

• Metaphase chromosomes are photographed the
  chromosome pictures are then enlarged and
  arranged in pairs by a computer according to
  length and location of the centromere.

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Section 12.3 Summary pages 323 - 329
Abnormal numbers of autosomes

• This chart of chromosome pairs is called a
  karyotype, and it is valuable in identifying
  unusual chromosome numbers in cells.

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Section 12.3 Summary pages 323 - 329
Down syndrome Trisomy 21

• Down syndrome is the only autosomal trisomy in
  which affected individuals survive to adulthood.

• It occurs in about one in 700 live births.

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Section 12.3 Summary pages 323 - 329
Down syndrome Trisomy 21

• Down syndrome is a group of symptoms that results
  from trisomy of chromosome 21.

• Individuals who have Down syndrome have at least
  some degree of mental retardation.

• The incidence of Down syndrome births is higher
  in older mothers, especially those over 40.

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Section 12.3 Summary pages 323 - 329
Abnormal numbers of sex chromosomes

• Many abnormalities in the number of sex
  chromosomes are known to exist.

• An X chromosome may be missing (designated as XO)
  or there may be an extra one (XXX or XXY). There
  may also be an extra Y chromosome (XYY).

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Section 12.3 Summary pages 323 - 329
Abnormal numbers of sex chromosomes

• Any individual with at least one Y chromosome is
  a male, and any individual without a Y chromosome
  is a female.

• Most of these individuals lead normal lives, but
  they cannot have children and some have varying
  degrees of mental retardation.

101
Section 3 Check
Question 1
Which of the following inherited diseases
would a black American be most likely to inherit?
A. cystic fibrosis
B. Tay-Sachs disease
C. phenylketonuria
D. sickle-cell disease
The answer is D.
102
Section 3 Check
Question 2
Trisomy usually results from _______.
A. polygenic inheritance
B. incomplete dominance
C. nondisjunction
D. twenty-two pairs of chromosomes
The answer is C.
103
Section 3 Check
Question 3
How do red blood cells of phenotype O
differ from the cells of the other phenotypes?
Answer
Red blood cells of phenotype O display no surface
molecules.
104
Chapter Summary 12.1
Mendelian Inheritance of Human Traits

• A pedigree is a family tree of inheritance.

• Most human genetic disorders are inherited as
  rare recessive alleles, but a few are inherited
  as dominant alleles.

105
Chapter Summary 12.2
When Heredity Follows Different Rules

• Some alleles can be expressed as incomplete
  dominance or codominance.

• There may be many alleles for one trait or many
  genes that interact to produce a trait.

• Cells have matching pairs of homologous
  chromosomes called autosomes.

• Sex chromosomes contain genes that determine the
  sex of an individual.

106
Chapter Summary 12.2
When Heredity Follows Different Rules

• Inheritance patterns of genes located on sex
  chromosomes are due to differences in the number
  and kind of sex chromosomes in males and in
  females.

• The expression of some traits is affected by the
  internal and external environments of the
  organism.

107
Chapter Summary 12.3
Complex Inheritance of Human Traits

• The majority of human traits are controlled by
  multiple alleles or by polygenic inheritance.
  The inheritance patterns of these traits are
  highly variable.

• Sex-linked traits are determined by inheritance
  of sex chromosomes. X-linked traits are usually
  passed from carrier females to their male
  offspring. Y-linked traits are passed only from
  male to male.

108
Chapter Summary 12.3
Complex Inheritance of Human Traits

• Nondisjunction may result in an abnormal number
  of chromosomes. Abnormal numbers of autosomes
  usually are lethal.

• A karyotype can identify unusual numbers of
  chromosomes in an individual.

109
Chapter Assessment
Question 1
Which of the following is NOT a sex-linked trait?
A. hemophilia
B. sickle-cell disease
C. male patterned baldness
D. red-green color blindness
The answer is B.
110
Chapter Assessment
Question 2
Human eye color is determined by _______.
A. the influence of hormones
B. sex-linked inheritance
C. codominance
D. polygenic inheritance
The answer is D.
111
Chapter Assessment
Question 3
What are blood phenotypes based on?
Answer
Blood phenotypes are based on a molecule that
attaches to a membrane protein found on the
surface of red blood cells.
112
Chapter Assessment
Question 4
Cob length in corn is the result of _______.
A. sex-linked inheritance
B. incomplete dominance
C. polygenic inheritance
D. simple dominance
The answer is C.
113
Chapter Assessment
Question 5
A cleft chin is the result of _______.
A. simple dominance
B. incomplete dominance
C. polygenic inheritance
D. sex-linked inheritance
The answer is A.
114
Chapter Assessment
Question 6
What is the difference between simple Mendelian
inheritance and inheritance by incomplete
dominance?
115
Chapter Assessment
In Mendelian inheritance, heterozygous
individuals will display the inherited dominant
trait of the homozygotes. However, when traits
are inherited in an incomplete dominance pattern,
the phenotype of heterozygous individuals is
intermediate between those of the two homozygotes.
116
Chapter Assessment
Question 7
If a trait is Y-linked, males pass the Y-linked
allele to _______ of their daughters.
A. a quarter
B. half
C. all
D. none
117
Chapter Assessment
The answer is D. Y-linked traits are only passed
to males.
118
Chapter Assessment
Question 8
What is necessary for a person to show a dominant
trait?
Answer
The person must inherit at least a single
dominant allele from one parent for the trait to
appear.
119
Chapter Assessment
Question 9
Why is sickle-cell disease considered to be an
example of codominant inheritance?
Answer
Individuals who are heterozygous for the
sickle-cell allele produce both normal and
sickled hemoglobin. This is an example of
codominance.
120
Chapter Assessment
Question 10
What sex is an XXY individual?
Answer
Any individual with at least one Y chromosome is
a male.
121
Chapter Assessment
Photo Credits

• Aaron Haupt
• Digital Stock
• Horizons Companies
• Russ Lappa
• Scott Cunningham
• Alton Biggs

122
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End of Chapter 12 Show