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Anu Singh-Cundy • Gary Shin
Discover Biology
SIXTH EDITION
CHAPTER 9
Patterns of Inheritance
© 2015 W. W. Norton & Company, Inc.
CHAPTER 9
Patterns of Inheritance, Part 1
THE LOST PRINCESS
9.1 Principles of Genetics: An Overview
Genes determine traits
Diploid cells have two copies of every gene
Genotype directs phenotype
Some phenotypes are controlled by dominant alleles
Gene mutations are the source of new alleles
Controlled crosses help us understand patterns of inheritance
9.2 Basic Patterns of Inheritance
Mendel’s genetic experiments began with true-breeding pea plants
Mendel inferred that inherited traits are determined by genes
9.3 Mendel’s Laws of Inheritance
Mendel’s single-trait crosses revealed the law of segregation
Mendel’s two-trait experiments led to the law of independent assortment
Mendel’s insights rested on a sound understanding of probability
CHAPTER 9
Patterns of Inheritance, Part 2
9.4 Extensions of Mendel’s Laws
Many alleles display incomplete dominance
The alleles of some genes are codominant
A pleiotropic gene affects multiple traits
Alleles for one gene can alter the effects of another gene
The environment can alter the effects of a gene
Most traits are determined by two or more genes
Complex traits are polygenic and potentially influenced by the environment
BIOLOGY MATTERS: KNOW YOUR TYPE
APPLYING WHAT WE LEARNED: SOLVING THE MYSTERY OF THE LOST
PRINCESS
The Lost Princess
• The last Russian czar and his family were executed in 1918.
Could Mendelian genetics help
settle persistent rumors that
one of his daughters,
Anastasia, had escaped the
murder of the Romanov family?
Humans Have Used the Principles of Inheritance
for Thousands of Years to Domesticate Plants
and Animals
Shar-pei, one of the
oldest dog breeds
• The field of genetics originated in 1866 after
Gregor Mendel published a paper on
inheritance in pea plants.
• Mendel’s work was largely ignored for 30 years
before it was adopted as the foundation for
modern genetics.
• Genetics is the study of inherited
characteristics (genetic traits) and the genes
that affect those traits.
Gregor Mendel (1822–1884)
A Genetic Trait Is Any Inherited Characteristic of
an Organism That Can Be Observed or Detected
• Invariant genetic traits are the same in all individuals in a population.
• Variant genetic traits come in two or more different versions, or phenotypes.
• The display of a particular version of a genetic trait in a specific individual is the
phenotype of that genetic trait in that individual.
Diploid Cells Have Two Copies of Every Gene
•
•
•
Somatic cells (nonsex cells) in the body of plants and animals are diploid: they contain two
copies of each type of chromosome, which together make up a homologous pair.
Each homologous pair contains one paternal homologue and one maternal homologue.
Humans have 23 pairs of homologous chromosomes, making a total of 46 chromosomes.
An Allele Is a Variant Form of a Gene
• Alleles are different versions of a given gene.
• Allelic variation is what makes a population
genetically diverse.
• Mutations are the source of new alleles.
• A mutation is a change in the DNA sequence
of a gene.
• Gene mutations occur at random.
• Mutations are commonly neutral, sometimes
harmful, and occasionally beneficial, to the
individual.
• Only mutations that are present in the
gametes, or the cells that produce gametes,
can be passed to offspring.
Genotype Directs Phenotype
• The genotype of an individual is the
allelic makeup of that individual with
respect to the specified genetic trait(s):
– The genotype completely or partially
controls an individual’s phenotype.
• An individual who carries two copies of
the same allele is said to be
homozygous for that gene.
• An individual whose genotype consists
of two different alleles for a given
phenotype is said to be heterozygous
for that gene.
The baby’s genotype for the genes shown
here is rrHhDDeeGg.
Some Phenotypes Are Controlled by
Dominant Alleles
• The allele that exerts a controlling influence on the phenotype in a
heterozygote is said to be dominant.
• An allele that has no effect on the phenotype when paired with a dominant
allele in a heterozygote is said to be recessive.
Breeding Trials Help Us Understand
Patterns of Inheritance
• A genetic cross is a controlled mating
experiment performed to examine how
a particular trait is inherited.
• The parents, or P generation, are
crossed to produce offspring, called the
F1 generation.
• Two individuals from the F1 generation
are then crossed to produce the F2
generation.
Mendel’s Genetic Experiments Began with
True-Breeding Pea Plants
• Mendel proposed that offspring
inherit two separate units of genetic
information (two copies of each
gene), one from each parent.
• Mendel used true-breeding lines of
pea plants to conduct highly
controlled experiments .
• True-breeding, or purebred,
individuals have a homogenous
genotype.
• Mendel crossed two lines of purebreeding plants to produce two
generations of hybrid plants and
recorded the phenotypic data.
Mendel Began By Studying the Inheritance
of Single Traits
• In a single-trait
cross, the experimenter
tracks the inheritance
of the two alleles of a
single gene.
• If all F2 offspring are hybrids for
that one trait, as they were in all
of Mendel’s experiments, this
type of cross is a monohybrid
cross.
• For the P generation: Mendel crossed true-breeding pea plants with contrasting
phenotypes for a particular genetic trait, such as flower color.
• He performed many such crosses and recorded the phenotypes of the resulting
F1 generation; next he crossed individuals of the F1 generation to raise the F2
generation.
Mendel Observed a 3:1
Ratio of Dominant to
Recessive Phenotypes in
the F2 Generation
Mendel’s observations:
• Odds that the dominant phenotype
will be seen in the F1 generation: 100
percent (4 in 4)
• Odds that the recessive phenotype
will reappear in the F2 generation:
1 in 4 (25 percent)
• Phenotypic ratio (dominant to
recessive phenotype) is 3:1
•
Genotypic ratios:
1:4 (25 percent) PP
1:2 (50 percent) Pp
1:4 (25 percent) pp
A Punnett Square
Can Be Used to Show
All the Possible Ways
in Which Two Alleles
Can Recombine
Through Fertilization
Mendel’s Single-Trait Crosses Revealed the
Law of Segregation
• Mendel concluded that the results
were best explained by assuming that
the two copies of a gene separate into
different cells during the formation of
egg or sperm (meiosis).
• Thus, according to Mendel’s law of
segregation, the two copies of a gene
are separated during meiosis and end
up in different gametes.
• Therefore, each offspring receives one
copy of the gene (one allele) from the
egg and the other copy (other allele)
from the sperm.
Mendel’s Experiments with Two Traits
(Dihybrid Crosses)
• Next, Mendel sought to determine if a
particular phenotype of one trait is
always inherited together with a
particular phenotype of a different
trait.
• Is the yellow seed color always
inherited with the round seed shape?
Or would he find combinations of
phenotypes (yellow color and wrinkled
shape) among the offspring that were
not present in the P generation?
Mendel’s question was rather like asking
whether a particular eye color phenotype (say,
blue eyes) always goes with a particular hair
color phenotype (say, blond hair).
Two Trait Cross between
True-Breeding Parents:
Recombinant Phenotypes
Appear in the F2 Generation
• Mendel crossed dihybrids, individuals
that are heterozygous for two traits.
• He observed two nonparental
combinations of phenotypes
(recombinant phenotypes) in the F2 :
- Round shape; green color (RRyy
and Rryy)
- Wrinkled shape, yellow color (rrYY
and rrYy)
• The phenotypic ratios were 9:3:3:1
-9/16 dominant for both traits
-3/16 dominant for seed shape, recessive
for seed color
-3/16 recessive for seed shape, dominant
for seed color
-1/16 recessive for both seed shape and
color
Predicting Genotypes of Gametes Produced by
Dihybrids of the F1 Generation
• Mendel deduced that the alleles of one gene (R/r) are sorted into gametes independently of
the alleles of the other gene (Y/y).
• The gametes that a dihybrid produces have genotypes that include all possible combinations:
• RY, Ry, rY, and ry.
Mendel’s Two-Trait Experiments Led to the
Law of Independent Assortment
• The law of independent assortment
states that when gametes form, the
two copies of any given allele are
sorted independently of any two
alleles of other genes.
• The law of independent assortment
applies to the inheritance of two
genes that are physically separated
on different chromosomes.
The 9:3:3:1 phenotypic ratio is best
explained by assuming that the
alleles of one gene (RR/Rr/rr) are
sorted into gametes independently of
the alleles of the other gene
(YY/Yy/yy).
Chromosomal Basis of Mendel’s Law of Independent Assortment
The random assortment of different homologous pairs explains Mendel’s law of
independent assortment: the alleles (R/r) of one genetic locus on a certain pair of
homologues are sorted independently of the alleles (Y/y) of another genetic locus that
is located on a different pair of homologous chromosomes.
What Mendel Inferred from His
Breeding Experiments: A Summary
1. Alternative versions of genes (alleles) cause variation in
inherited traits.
2. Offspring inherit one copy (one allele) of a gene from each
parent.
3. An allele is dominant if, when paired with a different allele, it
has exclusive control over an individual’s phenotype.
4. The two copies (alleles) of a gene segregate during meiosis and
end up in different gametes.
5. The alleles of one gene (such as R/r) are sorted independently of
the alleles of another gene (Y/y).*
6. Gametes fuse randomly, without regard to the particular alleles
they carry.
*A modern caveat: this principle will hold if the two genes
are located on a different pair of homologous chromosomes.
Mendel’s Insights Rested on a Sound
Understanding of Probability
• To deduce the patterns of inheritance, Mendel used probability to
analyze the data he collected from the offspring of the genetic
crosses.
• We can predict the probability that a particular offspring will have
a certain phenotype or genotype, but we cannot predict the
actual phenotype or genotype of a particular individual.
• The probability that a particular offspring will display a specific
phenotype is completely unaffected by the number of offspring.
Extensions of Mendel’s Laws of Inheritance
• Mendel’s work was based on
genetic traits controlled by a
single gene with a dominant
and a recessive allele.
• Mendel’s laws have been
expanded to help explain
more complex patterns of
inheritance.
Most human traits are non-Mendelian (their inheritance
patterns cannot be explained by Mendel’s laws alone).
Modern Variations on the Theme by Mendel
• Incomplete dominance
of alleles produces
an intermediate
phenotype in the
heterozygote.
We can still predict the
genotypes and phenotypes of
F1 and F2 offspring using
Mendelian laws of inheritance.
Another Example of Incomplete Dominance:
Coat Color in Horses and Other Mammals
Intermediate
phenotype
The Alleles of Some Genes Are Codominant
• Codominance occurs when the
effect of both alleles is equally
visible in the phenotype of the
heterozygote.
• Neither allele is diminished or
diluted in a heterozygote that
displays codominance.
• The ABO blood groups provide an
example of codominance:
-The I/i gene controls the type of cell
surface sugars that are found on a
person’s red blood cells
– The I allele is dominant over the i allele
– However, the IA and IB alleles are
codominant
A Pleiotropic Gene Affects Multiple Traits
• The situation in which a single
gene influences two or more
distinctly different traits is called
pleiotropy.
• A mutation in a pleiotropic gene
can cause changes in many
different traits.
• Albinism is an example of a
pleiotropic disorder.
Albinism is caused by a single recessive
allele affecting pigment formation, but
other traits such as vision are also
affected.
Another Example Of Pleitropy: Marfan Syndrome
[Marfan syndrome is a connective tissue disorder with a
dominant pattern of inheritance. Connective tissue is a
mixture of cell types that binds and strengthens organs and
other tissue types. Bone, cartilage, tendons, and the
sheath surrounding blood vessels are examples of
specialized connective tissue.]
Flo Hyman was diagnosed with Marfan
syndrome only after her death, shortly
after she and her teammates won the
silver at the 1984 Olympic games.
Alleles for One Gene Can Alter the
Effects of Another Gene
•
The term epistasis applies when the phenotypic effect of the alleles of one gene
depends on the presence of certain alleles for another, independently inherited gene.
•
Epistasis can be seen in the coat color of numerous animals, whose many genes code
for enzymes that convert the amino acid tyrosine into melanin in a multistep pathway.
The Environment Can Alter the
Phenotype
• Chemicals, nutrition, sunlight, and
other internal and external
environmental factors can alter the
effects of certain genes.
• The production of melanin in Siamese
cats is sensitive to temperature—
cooler temperatures produce dark fur
on the extremities.
Polygenic Inheritance of a Genetic Trait Leads to
a Range of Phenotypes in the Population
• Traits governed by the action of more
than one gene are polygenic traits.
• Skin color, running speed, blood
pressure, and body size are all polygenic
traits in humans.
• Skin color in humans, and many other
mammals, is controlled by multiple
genes.
• Assuming control by three genes, each
with two incompletely dominant alleles,
seven different phenotypes are possible
in the offspring of triple-heterozygote
parents.
A Three-Trait Punnett
Square to Predict the
Phenotypic and
Genotypic Outcomes
with Respect to the Skin
Color Trait
If you imagine that skin color is
controlled by just three genetic
loci, with only two alleles that
display incomplete dominance,
you would predict seven possible
phenotypes in the offspring of a
couple that both have an
intermediate phenotype (are
heterozygotes for each of the
three genetic loci hypothetically
controlling skin color).
Polygenic Traits, Combined with Environmental
Influences, Produce a Smoothly Graded Range of
Phenotypic Classes or Continuous Variation
Now, consider the
influence of sun
exposure on the seven
Predicted phenotypes.
An even broader range
of phenotypes,
from very pale to very
dark, is possible.
• Geneticists estimate there are more than a dozen genes
that control melanin production in our skin, which,
when coupled with environmental influences, results in
continuous variation in the trait.
Most Traits That Are Essential for
Survival Are Complex Traits
• Complex traits are those that cannot be predicted using Mendel’s laws of
inheritance; complex traits display often display continuous variation in a
population.
• According to one hypothesis, the evolutionary benefit of continuous variation in
phenotypes is that if the environment changes, there are good odds that one out of
the many phenotypes will be adaptive under the new conditions.
BIOLOGY MATTERS: KNOW YOUR TYPE
• Molecules foreign to the body
are recognized as antigens
by the immune system.
• The cell surface sugars
responsible for the A/B/AB
blood groups are potential
antigens.
• If type A whole blood is given
to a patient with blood type B
or O, the recipient’s immune
system produces specific
antigen-fighting proteins
(antibodies) that attack their
target antigen (red blood cells
with A-type sugars, in this
example). The transfused cells
clump together when attacked,
leading to life-threatening clots.
In a blood transfusion, the
donor and recipient blood types
must match.
APPLYING WHAT WE LEARNED:
SOLVING THE MYSTERY OF THE LOST PRINCESS
• In 1991, the grave of Czar Nicholas and some of
his family members was discovered.
• Five alleles of one gene—A1, A2, A3, A4, and A5—
are common in people of European descent.
Anna Anderson, a Polish factory worker,
claimed she was Anastasia.
• The czar’s genotype: A1A2; czarina’s genotype
:A2A3
• Anna Anderson’s genotype: A4A5
• Mendelian genetics shows that Anna
Anderson could not be the offspring of
Czar Nicholas and Czarina Alexandra.
• In 2009, DNA recovered from a skeleton buried
near the Romanov family was confirmed to be
that of Anastasia and her brother Alexei.
Grand Duchesses Maria (left) and Anastasia
visiting a hospital for soldiers.
List of Key Terms: Chapter 9
allele (p. 194)
codominance (p. 204)
complex trait (p. 210)
dihybrid (p. 200)
dominant allele (p. 195)
epistasis (p. 207)
F¹ generation (p. 196)
F² generation (p. 196)
gene (p. 192)
genetic cross (p. 196)
genetic trait (p. 192)
genetics (p. 192)
genotype (p. 194)
heterozygote (p. 195)
homozygote (p. 195)
hybrid (p. 198)
incomplete dominance (p. 203)
law of independent assortment (p.
202)
law of segregation (p. 199)
monohybrid (p. 199)
mutation (p. 195)
P generation (p. 196)
phenotype (p. 193)
pleiotropy (p. 206)
polygenic (p. 208)
Punnett square (p. 199)
recessive allele (p. 195)
Phenotypic variation in guppies
Class Quiz, Part 1
The orange gene (O) controls the production of
a pigment (pheomelanin) that gives Bengal
tigers their orange coat color. Felines that are
homozygous for the inactive version of the
gene (oo) fail to make pheomelanin and have
white fur. If a white tiger (oo) mates with a
carrier for the trait (Oo), what are the odds that
they will have a white tiger cub?
A. 4/4
B. 1/2
C. 1/4
D. 0/4
(The O gene controls orange fur.
Black pigment is controlled by other
genes, expressed in a stripelike
pattern of skin cells; in white tigers
(oo), no orange pigment is made
anywhere, but the black pigment is
produced in the normal pattern, so
these tigers keep their stripes.)
Class Quiz, Part 2
A.
B.
C.
D.
A red carnation and a white carnation
produce offspring that are all pink. The
alleles controlling these flower
phenotypes show
complete dominance.
incomplete dominance.
codominance.
epistasis.
Class Quiz, Part 3
Fur color in rabbits shows incomplete dominance.
FBFB individuals are brown, FBFW individuals are
cream, FWFW individuals are white. What is the
expected ratio of a FBFW x FWFW cross?
A. 3 white : 1 brown
B. 3 white : 1 cream
C. 2 white : 2 cream
Class
Quiz,
Part
4
Marfan syndrome is caused by a single allele, M, that codes
for an abnormal version of a protein called fibrillin-1 that is
produced by many different cell types and plays an important
role in gluing cells together. The symptoms of Marfan
syndrome include unusual tallness, with long arms, legs,
fingers, and toes; weakening of the aorta, the largest blood
vessel carrying blood away from the heart; and eye
problems. From this information we can conclude that
A. the M allele is codominant with m allele.
B. the M and m allele is incompletely dominant.
C. Marfan syndrome shows polygenic inheritance.
D. the gene that codes for fibrillin-1 displays
pleitropy.
E. the M and m alleles display epistasis.
Relevant Art from Other
Chapters
• All art files from the book are available
in JPEG and PPT formats online and on
the Instructor Resource Disc
A fluorescence in situ
hybridization of
chromosomes
Genes Are Located on
Chromosomes
Autosomes and Sex Chromosomes
Most Chronic Diseases Are Complex Traits
Crossing-Over
The Sorting of One Pair
of Homologous
Chromosomes Is
Independent of the
Sorting of Any Other
Pair of Homologous
Chromosomes
• Which homologue of each pair
goes to which pole is essentially
random.
The Flow of Genetic
Information
Messenger RNA Directs Protein
Synthesis
9.1 Concept Check, Part 1
1. What is an allele? How do new alleles arise?
How many different alleles can a single person
carry for a particular gene?
ANSWER: Alleles are different versions of a particular
gene. New alleles arise by mutation, a change to the
gene’s DNA code. Normally, an individual carries only
two alleles for any gene.
9.1 Concept Check, Part 2
2. For a single genetic trait, what is the
difference between a phenotype and a
genotype?
ANSWER: The phenotype is the particular version of a
genetic trait that is displayed by an individual, such as
wiry hair or smooth hair in dogs. The genotype
specifies the allelic makeup that determines the
phenotype. Dogs with wiry hair (a dominant trait) have
either the genotype WW or Ww; those with smooth hair
(recessive) have the genotype ww.
9.2 Concept Check, Part 1
1. Hair length in cats is controlled by a gene that has at least two
alleles, L and l. Short-haired cats are LL or Ll. Long hair is a breed
standard for Maine coon cats. Are these cats homozygous for the
hair length allele? What is their genotype?
ANSWER: Purebred Maine coon cats are homozygous
recessive for the hair length gene, with the ll genotype.
9.2 Concept Check, Part 2
2. Which of Mendel’s observations
demonstrated that the theory of blending
inheritance was false?
ANSWER: The F1 generation did not have an
intermediate phenotype, and moreover, the parental
phenotypes reappeared in the F2 generation instead of
vanishing forever into a blended phenotype.
9.3 Concept Check, Part 1
1. For the offspring of a cross between an Rr plant and an
rr plant, with R being dominant, predict the number and
ratio of genotypes and phenotypes.
ANSWER: Two genotypes (Rr and rr) and two
phenotypes are predicted, each in a ratio of 1:1.
Constructing a Punnett square confirms this.
9.3 Concept Check, Part 2
2. Explain Mendel’s law of segregation and law
of independent assortment.
ANSWER: The law of segregation states that during
gamete formation, two alleles separate into different
gametes so that each carries only one allele; the law of
independent assortment states that alleles of two
different genes are sorted into gametes independently
of each other.
9.4 Concept Check, Part 1
1. Allele H produces straight hair; allele HH' produces curly
hair. Individuals with the HH' genotype have wavy hair,
somewhere between straight and curly. Are alleles H and
HH' codominant?
ANSWER: No. They display incomplete dominance
because they produce an intermediate phenotype in
the heterozygote.
9.4 Concept Check, Part 2
2. What is pleiotropy?
ANSWER: In pleiotropy, a single gene affects several to
many different genetic traits.
9.4 Concept Check, Part 3
3. The ABO blood groups are determined by
several alleles of the I gene. Is ABO blood type a
polygenic trait?
ANSWER: No. Polygenic means “many genes.” ABO
blood groups are controlled by a single gene with
multiple alleles.