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Chapter 10
Patterns of Inheritance
Lecture Outlines by Gregory Ahearn,
University of North Florida
Copyright © 2011 Pearson Education Inc.
Chapter 10 At a Glance
 10.1 What Is the Physical Basis of Inheritance?
 10.2 How Were the Principles of Inheritance
Discovered?
 10.3 How Are Single Traits Inherited?
 10.4 How Are Multiple Traits Inherited?
 10.5 How Are Genes Located on the Same
Chromosome Inherited?
 10.6 How Is Sex Determined?
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Chapter 10 At a Glance (continued)
 10.7 How Are Sex-Linked Genes Inherited?
 10.8 Do the Mendelian Rules of Inheritance
Apply to All Traits?
 10.9 How Are Human Genetic Disorders
Investigated?
 10.10 How Are Human Disorders Caused by
Single Genes Inherited?
 10.11 How Do Errors in Chromosome Number
Affect Humans?
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10.1 What Is the Physical Basis of Inheritance?
 Genes are sequences of nucleotides at specific
locations on chromosomes
– Inheritance is the process by which the characteristics of
individuals are passed to their offspring
– A gene is a unit of heredity that encodes information
needed to produce proteins, cells, and entire organisms
– Genes comprise segments of DNA ranging from a few
hundred to many thousands of nucleotides in length
– The location of a gene on a chromosome is called its
locus (plural, loci)
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10.1 What Is the Physical Basis of Inheritance?
 Genes are sequences of nucleotides at specific
locations on chromosomes (continued)
– Homologous chromosomes carry the same kinds
of genes for the same characteristics
– Genes for the same characteristic are found at
the same loci on both homologous
chromosomes
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10.1 What Is the Physical Basis of Inheritance?
 Genes are sequences of nucleotides at specific
locations on chromosomes (continued)
– Genes for a characteristic found on homologous
chromosomes may not be identical
– Alternative versions of genes found at the same
gene locus are called alleles
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10.1 What Is the Physical Basis of Inheritance?
 Mutations are the source of alleles
– Alleles arise as mutations in the nucleotide
sequence in genes
– If a mutation occurs in the cells that become
sperm or eggs, it can be passed on from parent
to offspring
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10.1 What Is the Physical Basis of Inheritance?
 An organism’s two alleles may be the same or
different
– Each cell carries two alleles per characteristic,
one on each of the two homologous
chromosomes
– If both homologous chromosomes carry the
same allele (gene form) at a given gene locus,
the organism is homozygous at that locus
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10.1 What Is the Physical Basis of Inheritance?
 An organism’s two alleles may be the same or
different (continued)
– If two homologous chromosomes carry different
alleles at a given locus, the organism is
heterozygous at that locus (a hybrid)
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The Relationship Among Genes, Alleles, and
Chromosomes
a pair of
homologous
chromosomes
Both chromosomes carry the same allele
of the gene at this locus; the organism is
homozygous at this locus
gene loci
This locus contains another gene for which
the organism is homozygous
Each chromosome carries a different allele
of this gene, so the organism is
heterozygous at this locus
the chromosome
from the male
parent
Biology: Life on Earth, 9e
the chromosome
from the female
parent
Fig. 10-1
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10.2 How Were the Principles of Inheritance
Discovered?
 Doing it right: The secrets of Mendel’s success
– Who was Gregor Mendel?
– Mendel was an Austrian monk in a monastery in the
late 1800s
– He discovered the common patterns of inheritance
and many essential facts about genes, alleles, and the
distribution of alleles in gametes and zygotes during
sexual reproduction
– He chose the edible pea plant for his experiments,
which took place in the monastery garden
– Mendel’s background allowed him to see patterns in
the way plant characteristics were inherited
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Gregor Mendel
Fig. 10-2
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10.2 How Were the Principles of Inheritance
Discovered?
 Doing it right: The secrets of Mendel’s success
(continued)
– Mendel was the first to perform experiments by
correctly applying three key scientific steps to his
research:
–Choosing the right organism
–Designing and performing the experiment
correctly
–Analyzing the data properly
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10.2 How Were the Principles of Inheritance
Discovered?
 Doing it right: The secrets of Mendel’s success
(continued)
– Pea plants have qualities that make it a good organism
for studying inheritance
– Pea flowers have stamens, the male structures that
produce pollen, that in turn contain the sperm (male
gametes); sperm are gametes and pollen is the
vehicle
– Pea flowers have carpels, female structures housing
the ovaries, which produce the eggs (female gametes)
– Pea flower petals enclose both male and female
flower parts and prevent entry of pollen from another
pea plant
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Flowers of the Edible Pea
intact pea flower
flower dissected to show
its reproductive structures
Carpel (female,
produces eggs)
Stamen (male, produces
pollen that contain sperm)
Fig. 10-3
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10.2 How Were the Principles of Inheritance
Discovered?
 Doing it right: The secrets of Mendel’s success
(continued)
– Because of their structure, pea flowers naturally
self-fertilize
–Pollen from the stamen of a plant transfers to
the carpel of the same plant, where the sperm
then fertilizes the plant’s eggs
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10.2 How Were the Principles of Inheritance
Discovered?
 Doing it right: The secrets of Mendel’s success
(continued)
– Mendel was able to mate two different plants by
hand (cross-fertilization)
–Female parts (carpels) were dusted with
pollen from other selected plants
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10.2 How Were the Principles of Inheritance
Discovered?
 Doing it right: The secrets of Mendel’s success
(continued)
– Unlike previous researchers, Mendel chose a
simple experimental design
–He chose to study individual characteristics
(called traits) that had unmistakably different
forms, such as white versus purple flowers
–He started out by studying only one trait at a
time
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10.2 How Were the Principles of Inheritance
Discovered?
 Doing it right: The secrets of Mendel’s success
(continued)
– Mendel employed numerical analysis in studying
the traits
–He followed the inheritance of these traits for
several generations, counting the numbers of
offspring with each type of trait
–By analyzing these numbers, he saw the basic
patterns of inheritance emerge
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10.3 How Are Single Traits Inherited?
 Research into inheritance begins with parental
organisms that have easily identified traits that
are inherited consistently from generation to
generation
 Pea plants that are homozygous for a particular
characteristic always produce the same physical
forms
– If a plant is homozygous for purple flowers, it will
always produce offspring with purple flowers
– Plants homozygous for a characteristic are truebreeding
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10.3 How Are Single Traits Inherited?
 The language of a genetic cross
– A genetic cross is the mating of pollen and eggs
(from same or different parents)
– The parents used in a cross are part of the
parental generation (known as P)
– The offspring of the P generation are members
of the first filial generation (F1)
– Offspring of the F1 generation are members of
the F2 generation
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10.3 How Are Single Traits Inherited?
 Mendel’s flower color experiments
– Mendel crossed a true-breeding purple flower
plant with a true-breeding white-flower plant (the
P generation)
– The F1 generation consisted of all purpleflowered plants
–What had happened to the white-flowered
trait?
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Cross of Pea Plants True-Breeding for White or
Purple Flowers
pollen
Parental
generation (P)
pollen
cross-fertilize
true-breeding,
purple-flowered
plant
true-breeding,
white-flowered
plant
First-generation
offspring (F1)
all purple-flowered
plants
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Fig. 10-4
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10.3 How Are Single Traits Inherited?
 Mendel’s flower color experiments (continued)
– Mendel allowed the F1 generation to self-fertilize
– The F2 was composed of 3/4 purple-flowered
plants and 1/4 white-flowered plants, a ratio of
3:1
– The results showed that the white trait had not
disappeared in the F1 but merely was hidden
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Author Animation: Self- and Cross-Pollination of
Pea Plants
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Self-Fertilization of F1 Pea Plants with Purple
Flowers
Firstgeneration
offspring (F1)
self-fertilize
Secondgeneration
offspring (F2)
3/4 purple
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1/4 white
Fig. 10-5
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10.3 How Are Single Traits Inherited?
 Mendel’s flower color experiments (continued)
– Mendel then self-fertilized the F2 generation
– In the F3 generation, all the white-flowered F2
plants produced white-flowered offspring
–These proved to be true-breeding
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10.3 How Are Single Traits Inherited?
 Mendel’s flower color experiments (continued)
– In the F3 generation, self-fertilized purple-flowered F2
plants produced two types of offspring
– About 1/3 were true-breeding for purple
– The other 2/3 were hybrids that produced both purpleand white-flowered offspring, again, in the ratio of 3
purple to 1 white
– Therefore, the F2 generation included 1/4 truebreeding purple-flowered plants, 1/2 hybrid purple,
and 1/4 true-breeding white-flowered plants
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10.3 How Are Single Traits Inherited?
 The inheritance of dominant and recessive alleles on
homologous chromosomes can explain the results of
Mendel’s crosses
– A five-part hypothesis explains the inheritance of single traits
1. Each trait is determined by pairs of genes; each organism
has two alleles for each gene, one on each homologous
chromosome
– True-breeding white-flowered plants have different
alleles than true-breeding purple-flowered plants
2. When two different alleles are present in an organism, the
dominant allele may mask the recessive allele, even
though the recessive allele is still present
– In edible peas the purple-flower trait is dominant to the
white-flower trait
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10.3 How Are Single Traits Inherited?
 A five-part hypothesis explains the inheritance
of single traits (continued)
3. The pairs of alleles on homologous
chromosomes separate, or segregate, from
each other during meiosis, which is known as
Mendel’s law of segregation
4. Chance determines which allele is included in
a given gamete—because homologous
chromosomes separate at random during
meiosis; the distribution of alleles to the
gametes is also random
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10.3 How Are Single Traits Inherited?
 A five-part hypothesis explains the inheritance
of single traits (continued)
5. True-breeding organisms have two copies of
the same allele for a given gene and are
homozygous for that gene; hybrid organisms
have two different alleles for a given gene
and are heterozygous for that gene
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The Distribution of Alleles in Gametes
homozygous parent
A
A
gametes
A
A
(a) Gametes produced by a homozygous parent
heterozygous parent
A
a
gametes
A
a
(b) Gametes produced by a heterozygous parent
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Fig. 10-6
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10.3 How Are Single Traits Inherited?
 The hypothesis explains Mendel’s results with
peas
– The particular combination of the two alleles
carried by an individual is called the genotype
– The physical expression of the genotype is
known as the phenotype (for example, purple or
white flowers)
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10.3 How Are Single Traits Inherited?
 The hypothesis explains Mendel’s results with peas
(continued)
– There are two alleles for a given gene characteristic
(such as flower color)
– Let P stand for the dominant purple-flowered allele: A
homozygous purple-colored plant has two alleles for
purple flower color (PP) and produces only P gametes
– Let p stand for the recessive white-flowered allele: A
homozygous white-colored plant has two alleles for
white flower color (pp) and produces only p gametes
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10.3 How Are Single Traits Inherited?
 The hypothesis explains Mendel’s results with
peas (continued)
– A cross between a purple-flowered plant (PP)
and a white-flowered plant (pp) produces all
purple-flowered F1 offspring, with a Pp genotype
–Dominant P gametes from purple-flowered
plants combined with recessive p gametes
from white-flowered plants to produce hybrid
purple-flowered plants (Pp)
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10.3 How Are Single Traits Inherited?
 The hypothesis explains Mendel’s results with
peas (continued)
– The F1 offspring were all heterozygous (Pp) for
flower color
– When the F1 offspring were allowed to selffertilize, four types of gametes were produced
from the Pp parents
–Sperm: Pp
–Eggs: Pp
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10.3 How Are Single Traits Inherited?
 The hypothesis explains Mendel’s results with peas
(continued)
– A heterozygous plant produces equal numbers of P and
p sperm and equal numbers of P and p eggs
– When a Pp plant self-fertilizes, each type of sperm has
an equal chance of fertilizing each type of egg
– Combining these four gametes into genotypes in every
possible way produces offspring PP, Pp, Pp, and pp
– The probabilities of each combination (and therefore
the genotypic fraction each genotype is of the total
offspring) are 1/4 PP, 1/2 Pp, and 1/4 pp
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Segregation of Alleles and Fusion of Gametes
purple parent
P
PP
+
P
all P sperm and eggs
white parent
pp
p
+
p
all p sperm and eggs
(a) Gametes produced by homozygous parents
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Fig. 10-7a
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Segregation of Alleles and Fusion of Gametes
F1 offspring
sperm
P
eggs
+
p
Pp
P
Pp
or
p
+
(b) Fusion of gametes produces F1 offspring
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Fig. 10-7b
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Segregation of Alleles and Fusion of Gametes
gametes from F1 Pp plants
sperm
eggs
F2 offspring
P
+
P
PP
P
+
p
Pp
p
+
P
Pp
p
+
p
pp
(c) Fusion of gametes from the F1 generation produces
F2 offspring
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Fig. 10-7c
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10.3 How Are Single Traits Inherited?
 Simple “genetic bookkeeping” can predict genotypes
and phenotypes of offspring
– The Punnett square method predicts offspring
genotypes and phenotypes from combinations of
parental gametes
1. First, assign letters to the different alleles of the
characteristic under consideration (uppercase for
dominant, lowercase for recessive)
2. Determine the gametes and their fractional
proportions (out of all the gametes) from both parents
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10.3 How Are Single Traits Inherited?
 The Punnett square method predicts offspring
genotypes from combinations of parental
gametes (continued)
3. Write the gametes from each parent, together
with their fractional proportions, along each
side of a 2 x 2 grid (Punnett square)
4. Fill in the genotypes of each pair of combined
gametes in the grid, including the product of
the fractions of each gamete (e.g., 1/4 PP,
1/4 Pp and 1/4 pP, and 1/4 pp)
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10.3 How Are Single Traits Inherited?
 The Punnett square method predicts offspring
genotypes from combinations of parental
gametes (continued)
5. Add together the fractions of any genotypes
of the same kind (1/4 Pp + 1/4 pP = 1/2 Pp
total)
6. From the sums of all the different kinds of
offspring genotypes, create a genotypic
fraction
– 1/4 PP, 1/2 Pp, 1/4 pp is in the ratio
1 PP : 2 Pp : 1 pp
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10.3 How Are Single Traits Inherited?
 The Punnett square method predicts offspring
genotypes from combinations of parental
gametes (continued)
7. Based on dominant and recessive rules,
determine the phenotypic fraction
– A genotypic ratio of 1 PP : 2 Pp : 1 pp
yields 3 purple-flowered plants : 1 whiteflowered plant
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Author Animation: The Inheritance of Single Traits
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Determining the Outcome of a Single-Trait Cross
Pp
self-fertilize
1
2
P
eggs
p
1
2
1
2
genotypic phenotypic
ratio
ratio
(1:2:1)
(3:1)
P
sperm
1
2
offspring
genotypes
eggs
sperm
1
4
PP
1
4
pP
1
4
pp
(a) Punnett square of a single-trait cross
Biology: Life on Earth, 9e
P

1
2
P

1
4
PP
1
2
P

1
2
p

1
4
Pp
1
4
PP
Pp
p
1
4
1
2
1
2
p

1
2
P

1
4
pP
1
2
p

1
2
p

1
4
pp
1
2
Pp
1
4
pp
3
4
purple
1
4
white
(b) Using probabilities to determine the offspring of a
single-trait cross
Fig. 10-8
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10.3 How Are Single Traits Inherited?

Mendel’s hypothesis can be used to predict the
outcome of new types of single-trait crosses
– A test cross is used to deduce whether an organism
with a dominant phenotype is homozygous for the
dominant allele or heterozygous
1. Cross the unknown dominant-phenotype organism
(P_) with a homozygous recessive organism (pp)
2. If the dominant-phenotype organism is homozygous
dominant (PP), only dominant-phenotype offspring
will be produced (Pp)
3. If the dominant-phenotype organism is
heterozygous (Pp), approximately half the offspring
will be of recessive phenotype (pp)
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Punnett Square of a Test Cross
pollen
pp
all eggs p
PP or Pp
sperm unknown
if Pp
if PP
p
p
eggs
eggs
all Pp
sperm
1
2 P
all
P
sperm
1
2 Pp
1
p
2
1
2 pp
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Fig. 10-9
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10.4 How Are Multiple Traits Inherited?
 Mendel next tested his hypothesis that multiple
traits are inherited independently
– Mendel performed genetic crosses in which he
followed the inheritance of two traits at the same
time
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Traits of Pea Plants Studied by Gregor Mendel
Trait
Dominant form
Recessive form
Seed
shape
smooth
wrinkled
Seed
color
yellow
green
Pod
shape
inflated
constricted
Pod
color
green
yellow
purple
white
Flower
color
Flower
location at leaf
junctions
Plant
size
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tall
(about
6 feet)
at tips of
branches
dwarf
(about 8 to
16 inches)
Fig. 10-10
Copyright © 2011 Pearson Education Inc.
10.4 How Are Multiple Traits Inherited?
 Mendel next tested his hypothesis that traits are
inherited independently (continued)
– From the many pea plant phenotypes, he chose
seed color (yellow vs. green peas) and seed
shape (smooth vs. wrinkled peas)
–Yellow color is dominant to green color
–Smooth shape is dominant to wrinkled
– The allele symbols were assigned, as follows:
–Y = yellow (dominant), y = green (recessive)
–S = smooth (dominant), s = wrinkled
(recessive)
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10.4 How Are Multiple Traits Inherited?
 Mendel next tested his hypothesis that traits are
inherited independently (continued)
– The two-trait cross was between two true-breeding
varieties for each characteristic, one dominant for both
traits, the other recessive for both traits
– P: SSYY (smooth, yellow)  ssyy (wrinkled, green
– The SSYY plant produced only SY gametes, and the
ssyy plant produced only sy gametes
– Therefore, the F1 consisted solely of SsYy individuals,
with smooth skins and yellow coloring
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10.4 How Are Multiple Traits Inherited?
 Mendel next tested his hypothesis that traits are
inherited independently (continued)
– Mendel next allowed the F1 individuals to selffertilize: SsYy  SsYy
– Crossing the F1 plants yielded 315 plants with
smooth, yellow seeds; 101 with wrinkled, yellow
seeds; 108 with smooth, green seeds; and 32
with wrinkled, green seeds
–This is a ratio of approximately 9:3:3:1
– Two-trait crosses of other traits produced similar
proportions of phenotype combinations
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Predicting Genotypes and Phenotypes for a Cross
between Parents That Are Heterozygous for Two
Traits
SsYy
self-fertilize
eggs
1 SY
4
sperm
1 SY
4
1 Sy
4
1 sY
4
1 sy
4
1
4
Sy
1 sY
4
1
4
sy
1
16 SSYY
1
16 SSYy
1
16 SsYY
1
16 SsYy
1
16 SSyY
1
16 SSyy
1
16 SsyY
1
16 Ssyy
1
16 sSYY
1
16 sSyY
1
16 sSYy
1
16 sSyy
1
16 ssYY
1
16 ssyY
(a) Punnett square of a two-trait cross
1
16 ssYy
1
16 ssyy
seed shape
seed color
3
4 smooth

3
4 yellow

3
4 smooth

1
4 green

1
4 wrinkled

3
4 yellow

phenotypic ratio
(9:3:3:1)
9
16 smooth yellow
3
16 smooth green
3
16 wrinkled yellow
1
1
1
 16 wrinkled green
4 wrinkled  4 green
(b) Using probabilities to determine the
offspring of a two-trait cross
Fig. 10-11
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10.4 How Are Multiple Traits Inherited?
 Mendel’s results supported his hypothesis that
traits are inherited independently
– Mendel predicted that if the two traits were
inherited independently, then for each trait, threequarters of the offspring should show the
dominant phenotype and one-quarter should
show the recessive phenotype — a 3:1 ratio, as
he had found for the single trait flower color
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10.4 How Are Multiple Traits Inherited?
 Mendel’s results supported his hypothesis that
traits are inherited independently (continued)
– He found 423 plants with smooth seeds (of either
color) and 133 with wrinkled seeds (a ratio of
about 3:1)
– He found 416 plants produced yellow seeds (of
either shape) and 140 produced green seeds
(also about 3:1)
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10.4 How Are Multiple Traits Inherited?
 Mendel’s results supported his hypothesis that traits are
inherited independently (continued)
– The independent inheritance of two or more traits is
called the law of independent assortment
– Multiple traits are inherited independently because the
alleles of one gene are distributed to gametes
independently of the alleles for other genes
– Independent assortment will occur when the traits being
studied are controlled by genes on different pairs of
homologous chromosomes
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Author Animation: The Inheritance of Multiple
Traits
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10.4 How Are Multiple Traits Inherited?
 Mendel’s results supported his hypothesis that
traits are inherited independently (continued)
– The physical basis of independent assortment
has to do with the way homologous pairs line up
during meiosis
– Which of the two homologues is “on top” occurs
randomly for all pairs, so the homologues assort
randomly and independently of one another at
anaphase I
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Independent Assortment of Alleles
S
s
pairs of alleles on homologous
chromosomes in diploid cells
Y
y
chromosomes replicate
S
Y
s
y
replicated homologous
pair during metaphase of
meiosis I, orienting
like this
or like this
y
S
s
Y
meiosis I
S
Y
s
y
S
y
s
Y
S
Y
s
y
S
y
s
Y
meiosis II
S
S
Y
s
Y
SY
S
s
y
y
sy
y
y
Sy
independent assortment produces four equally
likely allele combinations during meiosis
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s
s
S
Y
Y
sY
Fig. 10-12
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10.4 How Are Multiple Traits Inherited?
 In an unprepared world, genius may go
unrecognized
– Mendel’s work was published in 1865 but went
unnoticed
– Three biologists—Carl Correns, Hugo de Vries,
and Erich Tschermak—independently (of Mendel
and each other) rediscovered Mendel’s
principles of inheritance in 1900
– Mendel was credited in new papers as laying the
groundwork of genetics 30 years previously
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.5 How Are Genes Located on the Same
Chromosome Inherited?
 Genes on the same chromosome tend to be
inherited together
– Mendel’s law of independent assortment works
only for genes whose loci are on different pairs
of homologous chromosomes
– Alleles that are on the same chromosome do not
line up independently of one another on the
metaphase plate and are not separated at
anaphase I
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.5 How Are Genes Located on the Same
Chromosome Inherited?
 Genes on the same chromosome tend to be
inherited together (continued)
– Different gene loci located on the same
chromosome tend to be inherited together
– Characteristics whose genes tend to assort
together are said to be linked
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.5 How Are Genes Located on the Same
Chromosome Inherited?
 Genes on the same chromosome tend to be
inherited together (continued)
– An example of genetic linkage is flower color
and pollen in sweet peas
–The genes for flower color and pollen shape
are linked; that is, their loci are on the same
chromosome
–Purple flower color is dominant to red; long
pollen shape is dominant to round
–Let P = purple flowers and p = red flowers
–Let L = long pollen shape and l = round shape
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.5 How Are Genes Located on the Same
Chromosome Inherited?
 Genes on the same chromosome tend to be
inherited together (continued)
– The pattern of inheritance for linked genes is
different from genes that assort independently
–What are the expected gametes from parent
PpLl, where P is linked with L and p is linked
with l ?
–Independent assortment would yield gametes
in a genetic proportion of 1/4 PL, 1/4 Pl, 1/4
pL, 1/4 pl
–Instead, the gametes are mostly PL and pl
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Linked Genes on Homologous Chromosomes
flower-color gene
pollen-shape gene
purple
allele, P
long
allele, L
red
allele, p
round
allele, l
Fig. 10-13
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.5 How Are Genes Located on the Same
Chromosome Inherited?
 Crossing over creates new combinations of linked
alleles
– Genes on the same chromosome do not always sort
together
– Crossing over, or genetic recombination, in prophase I
of meiosis creates new gene combinations
– Crossing over involves the exchange of DNA between
chromatids of paired homologous chromosomes in
synapsis (the tight association of homologous
chromosomes on the metaphase plate)
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.5 How Are Genes Located on the Same
Chromosome Inherited?
 Crossing over creates new combinations of linked
alleles (continued)
– The farther apart two linked gene loci are on a
chromosome, the more likely crossing over is to occur
between them
– Crossing over occurs so often between loci far apart on a
chromosome that they appear to assort independently
– They appear to assort randomly because roughly as
many gametes are produced with the genes
exchanged by crossing over as are produced in which
the original, linked combination of alleles occurs
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Crossing Over Recombines Alleles on Homologous
Chromosomes
flower-color gene
pollen-shape gene
sister
chromatids
purple allele, P
long allele, L
homologous
chromosomes
(duplicated)
at meiosis I
sister
chromatids
red allele, p
round allele, l
(a) Replicated chromosomes in prophase of meiosis I
P
L
P
L
p
l
p
(b) Crossing over during prophase I
l
Fig. 10-14a, b
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Copyright © 2011 Pearson Education Inc.
Crossing Over Recombines Alleles on Homologous
Chromosomes
P
p
recombined
chromatids
L
P
L
p
L
P
l
p
l
L
P
l
p
l
unchanged
chromatids
(c) Homologous chromosomes separate at anaphase I
recombined
chromosomes
unchanged
chromosomes
(d) Unchanged and recombined chromosomes after meiosis II
Fig. 10-14c, d
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Copyright © 2011 Pearson Education Inc.
10.6 How Is Sex Determined?
 Mammals have a set of sex chromosomes that dictate
gender
– Females have two X chromosomes
– Males have an X chromosome and a Y chromosome
– The Y chromosome is much smaller than the X
chromosome
– A small section of the X and Y chromosomes is
homologous, allowing them to pair in prophase I and
segregate during meiosis
– The rest of the (non-sex) chromosomes occur in identical
pairs and are called autosomes
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Copyright © 2011 Pearson Education Inc.
Photomicrograph of Human Sex Chromosomes
Y chromosome
X chromosome
Biology: Life on Earth, 9e
Fig. 10-15
Copyright © 2011 Pearson Education Inc.
10.6 How Is Sex Determined?
 Mammals have a set of sex chromosomes that dictate
gender (continued)
– For organisms in which males are XY and females are
XX, the sex chromosome carried by the sperm
determines the sex of the offspring
– During sperm formation, each sperm receives either
the X or the Y chromosome, along with a copy of each
of the autosomes
– Because the female has only X sex chromosomes, the
unfertilized egg must have an X chromosome
– If the egg is fertilized by a sperm with a Y
chromosome, a male results; if fertilized by an Xbearing sperm, a female is produced
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Sex Determination in Mammals
female parent
X1
X2
eggs
X1
X1
male parent
Y
Xm
X2
Xm
Xm
female offspring
sperm
Xm
X2
X1
Y
X2
Y
Y
male offspring
Biology: Life on Earth, 9e
Fig. 10-16
Copyright © 2011 Pearson Education Inc.
10.7 How Are Sex-Linked Genes Inherited?
 Sex-linked genes are found only on the X or
only on the Y chromosome
– Genes carried on one sex chromosome, but not
on the other, are sex-linked
–In humans, the X chromosome is much larger
than the Y and carries over 1,000 genes
–In contrast, the human Y chromosome is
smaller and carries only 78 genes
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.7 How Are Sex-Linked Genes Inherited?
 Sex-linked genes are found only on the X or
only on the Y chromosomes (continued)
– During embryonic life, the action of the Y-linked
gene SRY sets in motion the entire male
developmental pathway
–Under normal conditions, SRY causes the
male gender to be linked 100 percent to the Y
chromosome
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.7 How Are Sex-Linked Genes Inherited?
 Sex-linked genes are found only on the X or
only on the Y chromosomes (continued)
– Few of the genes on the X chromosome have a
specific role in female reproduction
– Most of the genes on the X chromosome have
no counterpart on the Y chromosome
–Some genes found only on the X chromosome
are important to both sexes, such as genes for
color vision, blood clotting, and certain
structural proteins in muscles
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.7 How Are Sex-Linked Genes Inherited?
 Sex-linked genes are found only on the X or
only on the Y chromosomes (continued)
– The X and the Y have very few genes in
common
– Females (XX) can be homozygous or
heterozygous for a characteristic
– Males (XY) have only one copy of the genes on
the X or the Y
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.7 How Are Sex-Linked Genes Inherited?
 Sex-linked genes are found only on the X or
only on the Y chromosomes (continued)
– Because females have two X chromosomes,
recessive sex-linked genes on an X
chromosome may or may not be expressed
– Because males, with only one X chromosome,
have no second copy to mask recessive genes,
they fully express all the X-linked alleles they
have, whether those alleles are dominant or
recessive
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.7 How Are Sex-Linked Genes Inherited?
 Sex-linked genes are found only on the X or
only on the Y chromosomes (continued)
– Red-green color blindness in humans is a sexlinked trait
– Color blindness is caused by recessive alleles of
either of two genes located on the X
chromosome
– The normal, dominant alleles of these genes
(called C) encode proteins that allow one set of
eye cones to be most sensitive to red light and
another to be most sensitive to green light
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.7 How Are Sex-Linked Genes Inherited?
 Sex-linked genes are found only on the X or
only on the Y chromosomes (continued)
– There are several defective recessive alleles of
these genes, called c
–The afflicted person cannot distinguish
between red and green
– A man can have the genotype CY or cY, which
means that he has a color-vision allele C or c on
his X chromosome and no corresponding gene
on his Y chromosome
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.7 How Are Sex-Linked Genes Inherited?
 Sex-linked genes are found only on the X or
only on the Y chromosomes (continued)
– He will have normal color vision if his X
chromosome bears the C allele, or be color-blind
if his X chromosome bears the c allele
– A woman may be CC, Cc, or cc because she has
two X chromosomes that each can carry an
allele for the trait and will only be color-blind if
her genotype is cc
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.7 How Are Sex-Linked Genes Inherited?
 Sex-linked genes are found only on the X or
only on the Y chromosomes (continued)
– A color-blind man (cY) will pass his defective
allele only to his daughters because only his
daughters inherit his X chromosome
– A heterozygous woman (Cc), although she has
normal color vision, will pass her defective allele
to half her sons, who will be color-blind
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Sex-Linked Inheritance of Color Blindness
The individual cannot distinguish red from green
female parent
XC
(a) Normal color vision
Xc
(b) Red-green color blindness
eggs
XC
XC
male parent
Y
XC
Xc
XC
XC
female offspring
sperm
XC
Xc
XC
Y
Xc
Y
Y
male offspring
(c) Expected children of a man with normal color vision (CY), and
a heterozygous woman (Cc)
Biology: Life on Earth, 9e
Fig. 10-17
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 Many traits do not follow simple Mendelian rules
of inheritance
– Not all traits are completely controlled by a single
gene
– A trait may not be completely dominant to
another
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 Incomplete dominance
– In the genes studied by Mendel, one allele was
dominant over the other, which was recessive
– Some alleles, however, are incompletely
dominant over others
– When the heterozygous phenotype is
intermediate between the two homozygous
phenotypes, the pattern of inheritance is called
incomplete dominance
Biology: Life on Earth, 9e
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10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 Incomplete dominance (continued)
– Human hair texture is influenced by a gene with
two incompletely dominant alleles, C1 and C2
–A person with two copies of the C1 allele has
curly hair
–Someone with two copies of the C2 allele has
straight hair
–Heterozygotes (with the C1C2 genotype) have
wavy hair
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 Incomplete dominance (continued)
– If two wavy-haired people marry, their children
could have any of the three hair types: curly
(C1C1), wavy (C1C2), or straight (C2C2)
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Incomplete Dominance
mother
C1C2
C1
C1C2
Biology: Life on Earth, 9e
C2
C1
sperm
father
eggs
C1C1
C1C2
C1C2
C2C2
C2
Fig. 10-18
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 A single gene may have multiple alleles
– An individual may have at most two different
gene alleles
– A species may have multiple alleles for a given
characteristic
–However, each individual still carries two
alleles for this characteristic
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 A single gene may have multiple alleles
(continued)
– The human blood types are an example of
multiple alleles of a single gene
– Human blood group genes produce blood types
A, B, AB, and O
–There are three alleles in this system: A, B,
and o
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 A single gene may have multiple alleles
(continued)
– Alleles A and B code for enzymes that add
different sugar molecules to the ends of
glycoproteins that protrude from red blood cells
– Allele o codes for a nonfunctional enzyme that
doesn’t add any sugar molecules
– Blood types A, B, AB, and O arise as a result of
the actions of these alleles
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 A single gene may have multiple alleles (continued)
– Alleles A and B are dominant to allele o
– People with AA or Ao genotypes have blood type A;
people with BB or Bo genotypes have blood type B;
people with oo genotypes have blood type O
– AB individuals have both the A and the B allele, so they
produce both types of enzymes
– Consequently, the plasma membranes of their red
blood cells have both A and B glycoproteins
– When heterozygotes express the phenotypes of both
of the homozygotes (in this case, both A and B
glycoproteins), the pattern of inheritance is called
codominance
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 A single gene may have multiple alleles
(continued)
– People make antibodies to the type of
glycoproteins they lack
–People with type A blood make B antibodies;
people with type B blood make A antibodies
–People with type O blood make both type A
and type B antibodies; type AB blood groups
make no antibodies
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 A single gene may have multiple alleles
(continued)
– The antibodies cause red blood cells that bear
foreign glycoproteins to clump together and
rupture
– The presence of such antibodies dictates that
blood type must be determined and matched
carefully before a blood transfusion is made
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 A single gene may have multiple alleles (continued)
– Type O blood, lacking any sugars, is not attacked by
antibodies in A, B, or AB blood, so it can be transfused
safely to all
– Type O blood is called the universal donor
– The A and B antibodies in type O blood become too
dilute to cause problems in the recipient of transfused
type O blood
– Because people with type O blood produce both A and B
antibodies, they can receive blood only from other type O
donors
– Their antibodies would attack any donated blood cells
bearing A or B glycoproteins
Biology: Life on Earth, 9e
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Table 10-1
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 Polygenic inheritance
– Some characteristics show a range of
continuous phenotypes instead of discrete,
defined phenotypes
–Examples of this include human height, skin
color, and body build, and in wheat, grain color
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 Polygenic inheritance (continued)
– Phenotypes produced by polygenic inheritance
are governed by the interaction of more than two
genes at multiple loci
– Human skin color is controlled by at least three
genes, each with pairs of incompletely dominant
alleles
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Polygenic Inheritance of Skin Color in Humans
Fig. 10-19
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 Single genes typically have multiple effects on
phenotype
– Some alleles of a characteristic may have multiple
phenotypic effects (pleiotropy) influencing a number of
gene loci
– The SRY gene on the Y chromosome in male humans
encodes a protein that activates other genes
– The SRY gene stimulates development of gonads into
testes, which in turn stimulate development of the
prostate, seminal vesicles, penis, and scrotum
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 The environment influences the expression of
genes
– The environment in which an organism lives
profoundly affects its phenotype
– Newborn Siamese cats demonstrate the effect of
environment on phenotype
–A Siamese cat has the genotype for dark fur
all over its body
–However, the enzyme that produces the dark
pigment is inactive at temperatures above
93°F (34°C)
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.8 Do the Mendelian Rules of Inheritance Apply
to All Traits?
 Newborn Siamese cats demonstrate the effect
of environment on phenotype (continued)
– When kittens are in the all-encompassing
warmth of their mother’s uterus, the enzyme is
inactive and they are born with pale fur
everywhere on their bodies
– After birth, the ears, nose, paws, and tail become
cooler than the rest of the body, and dark
pigment is produced there in the pattern
characteristic of the breed
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Environmental Influence on Phenotype
Fig. 10-20
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.9 How Are Human Genetic Disorders
Investigated?
 Many human diseases are influenced by genetics
– Human geneticists trying to understand the relationship
between genetics and disease search medical, historical,
and family records to study past crosses
– Geneticists studying humans are proscribed from
using breeding techniques employed with plants and
other animals
– Records of gene expression over several generations of
a family can be diagrammed
– Records extending across several generations can be
arranged in the form of family pedigrees, diagrams that
show the genetic relationships between a set of related
individuals
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.9 How Are Human Genetic Disorders
Investigated?
 Many human diseases are influenced by
genetics (continued)
– Pedigree analysis is often combined with
molecular genetics technology to elucidate gene
action and expression
– As a result, scientists now know the genes
responsible for sickle-cell anemia, hemophilia,
muscular dystrophy, Marfan syndrome, and
cystic fibrosis
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Family Pedigrees
How to read pedigrees
 generations
 male
 female
 parents
(a) A pedigree for a dominant trait
 offspring
?
?
?
?
?
or
 shows trait
or
 does not show trait
or
 known carrier (heterozygote) for
recessive trait
?
?
(b) A pedigree for a recessive trait
?
or
?
 cannot determine the genotype from this
pedigree
Fig. 10-21
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Some human genetic disorders are caused by
recessive alleles
– New alleles produced by mutation usually code
for nonfunctional proteins
– Alleles coding for nonfunctional proteins are
recessive to those coding for functional ones
–The presence of one normal allele may
generate enough functional protein to enable
heterozygotes to be phenotypically
indistinguishable from homozygotes with two
normal alleles
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Some human genetic disorders are caused by
recessive alleles (continued)
– Heterozygous individuals are carriers of a recessive
genetic trait (but otherwise have a normal phenotype)
– Recessive genes are more likely to occur in a
homozygous combination (expressing the defective
phenotype) when related individuals have children
– Close relatives are more likely than the general
population to each be heterozygous for a particular
recessive allele and, so, are more likely to produce the
homozygous recessive phenotype
Biology: Life on Earth, 9e
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10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Albinism results from a defect in melanin
production
– Melanin is the dark pigment that colors skin cells
– Melanin is produced by the enzyme tyrosinase
– An allele known as TYR (for tyrosinase) encodes
a defective tyrosinase protein in skin cells,
producing no melanin and a condition called
albinism
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Albinism results from a defect in melanin
production (continued)
– Humans and other mammals who are
homozygous for TYR have no color in their skin,
fur, or eyes (the skin and hair appear white, and
the eyes are pink)
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Albinism
Fig. 10-22
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Sickle-cell anemia is caused by a defective allele for
hemoglobin synthesis
– Hemoglobin is an oxygen-transporting protein found in
red blood cells
– A mutant hemoglobin gene causes hemoglobin
molecules in blood cells to clump together
– Red blood cells take on a sickle (crescent) shape and
easily break
– Blood clots can form, leading to oxygen starvation of
downstream tissues and paralysis
– The condition is known as sickle-cell anemia
Biology: Life on Earth, 9e
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10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Sickle-cell anemia is caused by a defective allele for
hemoglobin synthesis (continued)
– People homozygous for the sickle-cell allele synthesize
only defective hemoglobin and therefore produce mostly
sickled cells
– Although heterozygotes have about half normal and half
abnormal hemoglobin, they usually have few sickled cells
and are not seriously affected
– Because only people who are homozygous for the sicklecell allele usually show symptoms, sickle-cell anemia is
considered a recessive disorder
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Sickle-cell anemia is caused by a defective allele for
hemoglobin synthesis (continued)
– About 20 to 40 percent of sub-Saharan Africans are
heterozygous for sickle-cell anemia, but the allele is very
rare in Caucasians
– The large proportion of heterozygotes in Africa exists
because heterozygotes have some resistance to the
parasite that causes malaria
– The rarity of heterozygotes in Caucasians
corresponds with the rarity of malaria in northern
climes, where immunity (and therefore,
heterozygosity) has no selective advantage
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Sickle-Cell Anemia
Fig. 10-23
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Some human genetic disorders are caused by
dominant alleles
– A dominant disease can be transmitted to
offspring if at least one parent suffers from the
disease and lives long enough to reproduce
–Dominant disease alleles also arise as new
mutations in the DNA of eggs or sperm of
unaffected parents
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Some human genetic disorders are caused by
dominant alleles (continued)
– Various mechanisms create an allele’s dominance over
the normal allele
– Some dominant alleles encode an abnormal protein
that interferes with the function of the normal protein
– Some dominant alleles encode proteins that carry out
toxic reactions
– An allele may be dominant if it encodes a protein that
is overactive or is active at inappropriate times and
places
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Some human genetic disorders are caused by
dominant alleles (continued)
– Huntington disease is a dominant disorder that causes
a slow, progressive deterioration of parts of the brain
– The disease results in a loss of coordination, flailing
movements, personality disturbances, and eventual
death
– The disease becomes manifest in adulthood, ensuring its
maintenance in the population
– The gene encodes for a protein, called huntingtin, of
unknown function
– Mutant huntington seems both to interfere with the
action of normal huntington and to form large
aggregates in nerve cells that ultimately kill the cells
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Some human genetic disorders are sex-linked
– The X chromosome contains many genes that
have no counterpart on the Y chromosome
– Because males have only one X chromosome,
they have no other allele to exert dominance
over a sex-linked (X-linked) allele causing
disease
–Consequently, sex-linked diseases tend to
occur in males
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Some human genetic disorders are sex-linked
(continued)
– Sex-linked disorders caused by a recessive allele have a
unique pattern of inheritance
– A son receives his X chromosome from his mother
and passes it on only to his daughters, since the gene
doesn’t exist on his Y chromosome
– Sex-linked genes typically skip generations because
the affected male passes the trait to a phenotypically
normal carrier daughter, who in turn bears affected
sons
– Several defective alleles for characteristics encoded
on the X chromosome are known, including red-green
color blindness and hemophilia
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.10 How Are Human Disorders Caused by Single
Genes Inherited?
 Some human genetic disorders are sex-linked
(continued)
– Hemophilia is caused by a recessive allele on
the X chromosome that results in a deficiency in
one of the proteins needed for blood clotting
–Hemophiliacs often have anemia owing to
blood loss and bruise easily
–The hemophilia gene in Queen Victoria of
England was passed among the royal families
of Europe
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Hemophilia Among the Royal Families of Europe
unaffected male
hemophiliac male
unaffected female
carrier female
Edward
Victoria
Duke of Kent Princess of
Saxe-Coburg
Albert
Prince
of SaxeCoburg-Gotha
Edward VII
King of
England
Victoria
Queen
of England
Alexandra
of Denmark
Leopold
Duke
of Albany
Helen
Louis IV
Princess of
Grand Duke of
Waldeck-Pyrmont Hesse-Darmstadt
Alice
Princess
of Hesse
several
unaffected
chidren
Beatrice
Henry
Prince of
Battenburg
present British
royal family
(unaffected)
Victoria Elizabeth Alexandra
Tsarina
Mary
carrier
daughter
and
hemophiliac
grandson
Nicholas II Frederick Ernest Mary Irene
of Russia
Victoria
?
?
?
?
Olga
Tatiana
Maria
Anastasia
Alexander Alfonso
Albert
XII
Victoria Leopold Maurice
Queen
of Spain
?
Alexis
Tsarevitch
Alfonso
Crown
Prince
Juan
Beatrice
?
Marie Jaime Gonzalo
died
in
infancy
Fig. 10-24
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.11 How Do Errors in Chromosome Number
Affect Humans?
 The incorrect separation of chromosomes or
chromatids in meiosis is known as
nondisjunction
– Nondisjunction causes gametes to have too
many and too few chromosomes
– Most embryos that arise from fusion of gametes
with abnormal chromosome numbers
spontaneously abort, but some survive to birth
and beyond
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Effects of Nondisjunction during Meiosis
Normal meiosis
Nondisjunction
during meiosis I
Nondisjunction
during meiosis II
Parent cell
Meiosis I
Meiosis II
n
n
n
n
n1 n1 n1 n1
n1 n1
n
n
Fig. 10-25
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.11 How Do Errors in Chromosome Number
Affect Humans?
 Some genetic disorders are caused by abnormal
numbers of sex chromosomes
– Nondisjunction of sex chromosomes in males or females
produces abnormal numbers of X and Y chromosomes
– Nondisjunction of sex chromosomes in males produces
sperm with either no sex chromosomes (called “O”
sperm), or two sex chromosomes (sperm may be XX, YY,
or XY)
– Nondisjunction of sex chromosomes in females can
produce eggs that are O or XX eggs instead of eggs with
one X chromosome
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.11 How Do Errors in Chromosome Number
Affect Humans?
 Some genetic disorders are caused by abnormal
numbers of sex chromosomes (continued)
– When normal gametes fuse with these defective sperm
or eggs, the zygotes have normal numbers of autosomes
but abnormal numbers of sex chromosomes
– The most common abnormalities are XO, XXX, XXY, and
XYY
– Some sex chromosome abnormalities allow affected
individuals to survive
– The genes on the X chromosome are so essential to
survival, any embryo without at least one X
chromosome spontaneously aborts very early in
development
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.11 How Do Errors in Chromosome Number
Affect Humans?
 Some genetic disorders are caused by abnormal numbers of sex
chromosomes (continued)
– Turner syndrome (XO) occurs in females with only one X
chromosome
– At puberty, hormone deficiencies prevent XO females from
menstruating or developing secondary sexual characteristics
– Hormone treatment promotes physical development, but because
affected women lack mature eggs, they remain infertile
– Additional symptoms include short stature, folds of skin around
the neck, and increased risk of cardiovascular disease, kidney
defects, and hearing loss
– Because they have only one X chromosome, women with
Turner’s syndrome are more susceptible to recessive disorders
such as red-green color blindness and hemophilia
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.11 How Do Errors in Chromosome Number
Affect Humans?
 Some genetic disorders are caused by abnormal
numbers of sex chromosomes (continued)
– Trisomy X (XXX) results in a fertile “normal” woman with
an extra X chromosome
– Most affected women show no abnormal symptoms
– There is an increased chance of learning disabilities
and a tendency toward tallness associated with
trisomy X
– By some unknown mechanism that prevents an extra
X chromosome from being included in their eggs,
women with trisomy X bear normal XX and XY
children
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.11 How Do Errors in Chromosome Number
Affect Humans?
 Some genetic disorders are caused by abnormal
numbers of sex chromosomes (continued)
– Men with Klinefelter syndrome (XXY) have an extra X
chromosome
– Most afflicted males show no symptoms, although
some may show mixed secondary sexual
characteristics, including partial breast development,
broadening of the hips, and small testes
– XXY men are often infertile because of low sperm
count but are not impotent
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.11 How Do Errors in Chromosome Number
Affect Humans?
 Males with Jacob syndrome (XYY) have an
extra Y chromosome (XYY)
– Men with this malady have high levels of
testosterone, tend to develop severe acne, and
may be exceptionally tall
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Table 10-2
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.11 How Do Errors in Chromosome Number
Affect Humans?
 Some genetic disorders are caused by abnormal numbers of
autosomes
– Nondisjunction of autosomes can occur during meiosis in the father
or mother, resulting in eggs or sperm that are missing an autosome
or that have two copies of an autosome
– Fusion of these gametes with a normal sperm or egg results in a
zygote with one or three copies of the affected autosome
– Single-copy autosome embryos usually abort very early in
development
– Embryos with three copies of an autosome (trisomy) also usually
spontaneously abort; however, a small fraction of embryos with
three copies of chromosomes 13, 18, or 21 survive to birth
– The frequency of nondisjunction increases with the age of the
parents
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
10.11 How Do Errors in Chromosome Number
Affect Humans?
 Some genetic disorders are caused by abnormal
numbers of autosomes (continued)
– In trisomy 21 (Down syndrome), afflicted individuals
have three copies of chromosome 21
– Down syndrome includes several distinctive physical
characteristics, including weak muscle tone, a small
mouth held partially open because it cannot
accommodate the tongue, and distinctively shaped
eyelids
– Down syndrome is also characterized by low
resistance to infectious diseases, heart malformations,
and varying degrees of mental retardation, often
severe
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Trisomy 21, or Down Syndrome
Fig. 10-26
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.