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Chapter Introduction
Genes and Chromosomes
13.1 Heredity and Environment
13.2 Mendel and the Idea of Alleles
13.3 Genes and Chromosomes
Mendelian Patterns of Inheritance
13.4 Probability and Genetics
13.5 Inheritance of Alleles
13.6 Sex Determination
Other Patterns of Inheritance
13.7 Multiple Alleles and
Alleles without Dominance
13.8 Linked Genes
13.9 X-Linked Traits
13.10 Nondisjunction
13.11 Multigene Traits
Chapter Highlights
Chapter Animations
Learning Outcomes
By the end of this chapter you will be able to:
A Explain the roles of heredity and environment in
organism development.
B Explain the relationship among alleles, genes,
and chromosomes.
C Apply the principles of probability to genetics.
D Explain Mendel’s principles of segregation and
independent assortment.
Learning Outcomes
By the end of this chapter you will be able to:
E Give examples of how sex is determined in
several organisms.
F Explain the role of multiple alleles in inheritance.
G Explain linked genes and recognize how X-linked
traits differ from other linked traits.
H Summarize the effects of nondisjunction.
Patterns of Inheritance
 What important biological
concept is illustrated in
this photo?
 What differences among
these starfish do you see?
This photo shows a group of starfish in a tide
pool in Big Sur, California.
Patterns of Inheritance
• Genetics is the branch of
biology that seeks to explain
inherited variation.
• Some of the differences
among individuals are
passed from parent to
offspring.
• Such inherited features
are the building blocks of
evolution.
This photo shows a group of starfish in a tide
pool in Big Sur, California.
Genes and Chromosomes
13.1 Heredity and Environment
• Organisms are products of their heredity and
of their surroundings.
Siamese cats have a gene
for dark fur, but the
enzyme produced by the
gene functions best at low
temperatures. The cat’s
extremities—its ears,
paws, and tail—are cooler
than the rest of its body.
Genes and Chromosomes
13.1 Heredity and Environment (cont.)
• Studies of twins can help separate the effects of
inheritance and the environment.
– Identical twins have the same genetic information
while fraternal twins are no more genetically
similar than other siblings.
– If identical twins exhibit the same trait more often
than fraternal twins, the trait is probably influenced
by genetic factors.
– If the trait differs in identical twins, the
environment must have a strong influence on it.
Genes and Chromosomes
13.1 Heredity and Environment (cont.)
• According to the once popular theory of “blending”,
an individual’s genetic makeup was formed when
the parents’ genes mixed at fertilization.
• The result was a sort of averaging of the
parents’ genes.
• Genetics no longer includes the theory of blending
inheritance, but it was a logical explanation for many
visible differences.
Genes and Chromosomes
13.2 Mendel and the Idea of Alleles
• In the 1860s, Gregor Mendel used garden peas to
study heredity.
• Peas are very easy to grow, are self-fertilizing, and
Mendel could study many generations during
his eight-years of experiments.
Gregor Mendel (1822–1884) By experimenting with
peas in his monastery garden, Mendel developed
the fundamental principles of heredity that became
the foundation of modern genetics.
Self-fertilization in a pea flower
Genes and Chromosomes
13.2 Mendel and the Idea of Alleles (cont.)
• Mendel concentrated on traits that did not fit the
blending theory.
• Mendel found
seven different
characteristics of
pea plants that he
could study in an
either-or form.
Genes and Chromosomes
13.2 Mendel and the Idea of Alleles (cont.)
• Mendel classified and isolated true-breeding plants
which produce offspring identical to themselves
generation after generation.
• Mendel then crossbred his plants, classified all the
offspring, and looked for patterns of inheritance.
Genes and Chromosomes
13.2 Mendel and the Idea of Alleles (cont.)
• When Mendel mated a plant
with round seeds with a plant
with wrinkled seeds, the next
generation of plants produced
only round seeds.
• When those plants were
allowed to self-fertilize,
however, some plants in
the next generation
produced round seeds
and other plants produced
wrinkled seeds.
Genes and Chromosomes
13.2 Mendel and the Idea of Alleles (cont.)
• Mendel demonstrated with pea plants that both
parents pass on to their offspring genetic factors that
remain separate generation after generation.
• Today the concept of genes has replaced Mendel’s
vague idea of factors.
• A gene is now defined as a segment of DNA whose
sequence of nucleotides codes for a specific
functional product.
Genes and Chromosomes
13.2 Mendel and the Idea of Alleles (cont.)
• Most genes exist in more than one form, or allele.
• Each allele of a particular gene has a different
base sequence.
• All organisms have genes that exist as several
different alleles.
• Most traits—such as hair color, skin color, nose
shape, and handedness—result from the complex
interaction of several genes with each other and
the environment.
Genes and Chromosomes
13.3 Genes and Chromosomes
• The arrangement of genes in chromosomes differs
in eukaryotes and prokaryotes.
• Eukaryotic chromosomes are long molecules of
DNA wrapped around proteins.
• Only part of this DNA codes for proteins; noncoding
DNA is not translated.
Genes and Chromosomes
13.3 Genes and Chromosomes (cont.)
• Prokaryotes have a single circular chromosome with
little associated protein.
• An estimated 90% of prokaryotic DNA is translated.
• Many bacteria also have plasmids—small circles of
DNA that contain additional genes.
A scanning electron micrograph (x79,000)
of a bacterial plasmid. These small circular
DNA molecules are common in some
types of bacteria. Their ability to transfer
genes between cells makes plasmids
useful in genetic research.
Genes and Chromosomes
13.3 Genes and Chromosomes (cont.)
• Homologous chromosomes carry the same genes,
although their genes may be present as different
alleles.
• Stains help identify homologous chromosomes by
binding to specific regions of chromosomes to create
unique banding patterns.
• Chromosome painting can also be used to identify
all of an organism’s chromosomes.
Genes and Chromosomes
13.3 Genes and Chromosomes (cont.)
Human chromosomes 7 and X are similar
in size and shape. The banding patterns
evident in the photographs show more
clearly in the drawings. These two
chromosomes, which have almost the
same length and centromere location, can
be distinguished by their banding patterns.
In chromosome painting,
fluorescent dyes of different
colors are chemically bonded to
short pieces of DNA (probes)
that bind to genes on different
chromosomes. This process
stains each homologous pair of
chromosomes a different color.
Genes and Chromosomes
13.3 Genes and Chromosomes (cont.)
• Chromosomes are easiest to study in the condensed
form during cell division.
– Chemicals are added to stop cell division during
metaphase.
– The cells are treated with water on a microscope
slide causing them to swell and their
chromosomes to spread apart.
– Stains applied to the resulting chromosome
spread produce the banding patterns.
Genes and Chromosomes
13.3 Genes and Chromosomes (cont.)
In the human chromosome
spread, the banding patterns
result from differences in stain
absorption by different regions
of the chromosomes. Banded
chromosomes enable biologists
to detect missing or extra
chromosome parts more easily
than uniformly stained
chromosomes do. The banding
patterns also have made the
mapping of genes on
chromosomes more accurate.
Genes and Chromosomes
13.3 Genes and Chromosomes (cont.)
• Stained chromosomes can be photographed under
the microscope.
• Individual chromosomes can be cut out of an
enlarged photograph and arranged by size and
shape to form a display called a karyotype.
• Karyotypes of fetal cells can be used to check
for suspected chromosomal abnormalities in
developing fetuses.
In this karyotype of a human cell, the chromosomes are arranged as
homologous pairs. The larger groups (for example, pairs 1–3) include
chromosome pairs with similar size and centromere position. A karyotype
is prepared by cutting out individual chromosomes from a photograph
and matching them, pair by pair. The pair of X chromosomes determines
this individual’s sex.
Mendelian Patterns of Inheritance
13.4 Probability and Genetics
• Diploid organisms usually carry different alleles of
many genes.
• Geneticists use probability, a branch of mathematics
that predicts the chances that a certain event will
occur, to predict the results of matings.
• Probability is usually expressed as a fraction and
works when each event is independent.
• The chance that two or more independent events will
occur together is the multiplication product of their
chances of occurring separately.
Mendelian Patterns of Inheritance
13.4 Probability and Genetics (cont.)
• Geneticists use probability to predict the alleles of
the offspring of various crosses, or matings.
• Mathematical tests can help show whether a
difference between observed and predicted results
is significant.
• Genetic ratios are estimates of probability, not
absolute numbers.
• The larger the sample size, the less deviation is
expected from predicted ratios.
Mendelian Patterns of Inheritance
13.5 Inheritance of Alleles
• The principles of probability enable you to explain
the results of Mendel’s experiment.
• Mendel performed a monohybrid cross, or crossed
plants that differed in only one trait: seed shape.
• The plants involved in this first cross are called the
parental (P) generation.
Mendelian Patterns of Inheritance
13.5 Inheritance of Alleles (cont.)
• The generation of plants that grew from the seeds
of the parental crosses is called the first filial, or
F1, generation.
• The F1 generation plants self-fertilized producing
the second filial, or F2, generation.
Mendelian Patterns of Inheritance
13.5 Inheritance of Alleles (cont.)
• Only one trait of each pair was visible in the
F1 generation.
– The visible trait is the dominant trait.
– The alternative trait, which is not visible in the
F1 generation, is called the recessive trait.
• In the F2 generation produced by self-fertilization
of F1 plants, the two traits appear in a ratio of
approximately 3:1, dominant to recessive.
Mendelian Patterns of Inheritance
13.5 Inheritance of Alleles (cont.)
• Mendel used mathematics to conclude that each
true-breeding plant has two identical copies of the
factor for a particular trait.
– When gametes formed during meiosis, only
one copy of the factor went into each pollen
or egg cell.
– At fertilization, the F1 generation received a
factor from each parent.
– Only one of these factors went into each gamete
formed by the F1 plants.
Mendelian Patterns of Inheritance
13.5 Inheritance of Alleles (cont.)
• Mendel called this separation of factors the
principle of segregation.
• The alleles (the modern term for factors) of each
gene separate during meiosis when homologous
chromosomes are divided among the gametes.
This example of a
monohybrid cross
demonstrates
Mendel’s principle of
segregation. Each
plant has two factors
(alleles) for each trait,
which segregate
during the formation
of gametes.
Fertilization restores
paired alleles. The
grid, or Punnett
square, shows how
gametes of the F1
generation combine
to form the genotypes
of the F2 generation.
Mendelian Patterns of Inheritance
13.5 Inheritance of Alleles (cont.)
• The allele for a dominant trait is commonly
represented by a capital letter, and the allele for the
recessive trait is represented by the lowercase letter.
• Most multicellular organisms are diploid, so their
genotype, or genetic makeup, has two alleles for
each gene.
– In Mendel’s experiment, round-seeded plant in the
P generation would have been RR.
– The wrinkled-seeded parental plant had the
genotype rr.
Mendelian Patterns of Inheritance
13.5 Inheritance of Alleles (cont.)
• If both alleles are the same (RR or rr), the genotype
is homozygous.
• If the alleles are different (Rr), the genotype
is heterozygous.
• The genotype of each individual is responsible for
its phenotype—its appearance or observable
characteristics.
• Because the round-seeded phenotype is dominant,
both the homozygous RR and the heterozygous Rr
genotypes produce plants with round seeds.
Mendelian Patterns of Inheritance
13.5 Inheritance of Alleles (cont.)
• A Punnett square is useful for
calculating probable ratios of
genotypes and phenotypes.
• All the possible genotypes of
the gametes one parent can
produce are at the top of the
square, and the genotypes of
the other parent’s gametes are
at the side.
• The square is then filled in like a multiplication
table with the genotypes that would result from
the union of those gametes.
Mendelian Patterns of Inheritance
13.5 Inheritance of Alleles (cont.)
• To follow the inheritance of two characteristics at
once, Mendel made dihybrid crosses—crosses
between individuals that differ in two traits.
• The ratio 9:3:3:1 is characteristic of the F2 generation
in a dihybrid cross.
• The results of dihybrid crosses formed the basis for
Mendel’s principle of independent assortment.
Alleles for one characteristic assort, or divide up
among the gametes during meiosis, independently
of alleles for other characteristics.
A dihybrid cross illustrates
the principle of independent
assortment. Note that there
are 12 round to every 4
wrinkled seeds and 12 yellow
to every 4 green seeds. Each
trait independently shows the
3:1 ratio typical of a
monohybrid cross.
R = round seed;
r = wrinkled seed;
Y = yellow embryo;
y = green embryo.
Mendelian Patterns of Inheritance
13.6 Sex Determination
• Chromosomes come in matching pairs except for
the sex chromosomes, which may be different.
• This sex chromosomes determine the sex of
the individual.
• In humans, other mammals, and the fruit fly
Drosophila melanogaster, the sex chromosomes
are labeled X and Y.
• Females have two X chromosomes; males have one
X chromosome and one Y chromosome.
Sex determination
Mendelian Patterns of Inheritance
13.6 Sex Determination (cont.)
• Some insects, such as grasshoppers, crickets, and
roaches, females have two X chromosomes, males
have one; there is no Y chromosome.
• Birds, some fish, and some insects have what is
called a Z-W system where the male has two
matching sex chromosomes (ZZ), and the female
has one Z chromosome and one W chromosome.
Mendelian Patterns of Inheritance
13.6 Sex Determination (cont.)
• Some plants, such as spinach and date palms, have
separate female and male plants and have sex
chromosomes that follow the X-Y system of sex
determination.
• Most plants and some animals have no sex
chromosomes.
Other Patterns of Inheritance
13.7 Multiple Alleles and Alleles without Dominance
• Some genes do not follow an either dominant or
recessive pattern.
• A type of inheritance is known as incomplete
dominance occurs when the phenotype is
intermediate between those of the parents.
Incomplete dominance of flower color in snapdragons
Other Patterns of Inheritance
13.7 Multiple Alleles and Alleles without Dominance
(cont.)
• Human blood type depends on the presence or
absence of type A or type B carbohydrates on the
surface of red blood cells.
– The alleles IA or IB code for different forms of an
enzyme that add different sugars to the
carbohydrate bound to the plasma membrane.
– When a person has the IAIB genotype, both types
of carbohydrates are produced and the individual’s
blood type is AB.
• When alleles are codominant, both phenotypes
appear in heterozygous individuals.
Other Patterns of Inheritance
13.7 Multiple Alleles and Alleles without Dominance
• Blood types also involve multiple alleles.
(cont.)
– In addition to the IA and IB alleles, the allele
i codes for no active enzyme.
– An ii genotype produces type O blood.
– The symbols IA , IB, and i are used to show that
the A and B traits are codominant and the O trait is
recessive.
Other Patterns of Inheritance
13.7 Multiple Alleles and Alleles without Dominance
(cont.)
• If someone receives a transfusion of a different
type of blood, antibodies, which are defensive
proteins found in the blood, cause the donated red
cells to clump together, clogging the blood vessels.
• If a person with type B blood receives a transfusion
of type A blood, the anti-A antibodies that are
produced attack the A surface carbohydrates
and vice versa.
Other Patterns of Inheritance
13.7 Multiple Alleles and Alleles without Dominance
(cont.)
Other Patterns of Inheritance
13.8 Linked Genes
• Genes on the same chromosome are
said to be linked and are often
inherited together.
• Alleles of linked genes do not always
stay together.
Crossing-over frequently occurs between homologous
chromosomes during the early stages of meiosis. This results in
genetic recombination (new combinations of genes). Here the
symbols E and e stand for the alleles of one gene, and F and f
are the alleles of another gene on the same chromosome.
Because these two genes are close together, crossing-over
probably would not occur between them very often.
Other Patterns of Inheritance
13.8 Linked Genes (cont.)
• The frequency with which linked traits become
separated reflects how far apart on the chromosome
the genes for those traits are.
• This information can be used to map the positions
of genes on chromosomes.
• Important genes that are hard to identify can also
be mapped by observing traits produced by nearby
marker genes.
Other Patterns of Inheritance
13.8 Linked Genes (cont.)
A few of the genes that have been mapped to
human chromosomes number 7 (left) and 4 (right).
Linked genes are inherited together because they
are on the same chromosome.
Other Patterns of Inheritance
13.9 X-Linked Traits
• An understanding of linked genes came from
studies of fruit flies by Thomas Hunt Morgan in
the 1910s.
Adult fruit flies, Drosophila
melanogaster (x10). The top fly is
male; the bottom fly is female. Note
the difference in their coloring.
Other Patterns of Inheritance
13.9 X-Linked Traits (cont.)
When Morgan crossed a white-eyed
male with a normal red-eyed female,
all the F1 flies had red eyes.
The F2 flies showed the expected
3:1 ratio of red to white eyes, but
only the male flies showed the
recessive, white-eye trait.
Other Patterns of Inheritance
13.9 X-Linked Traits (cont.)
In the reciprocal cross, a white-eyed
female was crossed with a red-eyed
male. All male offspring had white
eyes because they inherited their X
chromosome from their mother.
The F2 flies contained a 1:1 ratio of
white eyes and red eyes.
Other Patterns of Inheritance
13.9 X-Linked Traits (cont.)
• Morgan’s explanation for these results was that the
gene for eye color is carried on the X chromosome.
• White eye color was the first known example of
an X-linked trait, a trait whose gene is carried
only on the X chromosome.
• Well-known examples of human X-linked,
recessive alleles include those responsible for
red-green color blindness and hemophilia.
Other Patterns of Inheritance
13.9 X-Linked Traits (cont.)
• The British geneticist Mary Lyon suggested that,
early in the development of a normal female, one X
chromosome becomes inactivated in each body cell.
• According to Lyon’s theory, the cells of a female
express a mixture of X-linked traits.
The coloring of this calico female cat is a
visible indication of X inactivation. The cat
carries two different alleles for coat color on its
two X chromosomes. Random X inactivation
during embryonic development results in this
patchwork-colored coat in some female cats.
Other Patterns of Inheritance
13.9 X-Linked Traits (cont.)
• Lyon proposed that a darkly staining mass called a
Barr body that normally appears in the nucleus of
female cells, is the inactive X chromosome.
Other Patterns of Inheritance
13.9 X-Linked Traits (cont.)
• Errors in meiosis cause some females to receive
an extra X chromosome resulting in an XXX
configuration.
• Cells of these XXX females have two Barr bodies.
• Lyon interpreted this observation to mean that only
one X chromosome in a cell remains active.
• Lyon’s X-inactivation hypothesis may help to explain
why an extra X chromosome is not as disruptive as
an extra copy of another chromosome.
Other Patterns of Inheritance
13.10 Nondisjunction
• The failure of homologous chromosomes to separate
in meiosis nondisjunction.
• If for some reason the sex chromosomes fail to
separate in meiosis the offspring developing from
those gametes would have unusual numbers of sex
chromosomes.
• Nondisjunction also can occur between the X and Y
chromosomes in males.
Other Patterns of Inheritance
13.10 Nondisjunction (cont.)
(a), The gene for eye color is carried on the X chromosome. V represents
normal red eye color; v represents vermilion. Males inherit their eye-color
gene on the X chromosome they receive from their mothers.
Other Patterns of Inheritance
13.10 Nondisjunction (cont.)
(b), Nondisjunction of the X chromosome can produce red-eyed, sterile
males and vermilion-eyed females. Bridges explained these rare individuals
by suggesting that sometimes the X chromosomes fail to separate during
meiosis in egg cell development. When Bridges examined the cells of these
exceptional flies, he found the abnormal number of chromosomes that he
had predicted. Nondisjunction also can occur in males.
Other Patterns of Inheritance
13.10 Nondisjunction (cont.)
• Nondisjunction also occurs in the sex chromosomes
and other chromosomes of humans with possibly
severe effects.
• Certain syndromes, or typical combinations of
symptoms, can result from abnormal numbers of
sex chromosomes.
Other Patterns of Inheritance
13.10 Nondisjunction (cont.)
• Nondisjunction can produce an individual with
three copies of a chromosome in a condition
known as trisomy.
• Trisomy of human chromosome 21 (Down
syndrome) causes limited mental abilities, short
stature, characteristic facial features, and heart
defects, although severity varies.
Other Patterns of Inheritance
13.10 Nondisjunction (cont.)
(a), Chromosome number 21 identifies this male Down syndrome karyotype.
(b), This girl shows typical facial features of Down syndrome.
Other Patterns of Inheritance
13.11 Multigene Traits
• Most human traits are multifactorial—they are
affected by several genes and environmental factors.
• Multifactorial traits, such as height, are also known as
quantitative traits.
• The environment includes both external factors, such
as light and temperature, and internal ones, such as
the organism’s metabolic activities.
• Most multifactorial traits, such as height, intelligence,
color, and the control of metabolic processes, vary in
finely graded steps.
Other Patterns of Inheritance
13.11 Multigene Traits (cont.)
• A graph of a multifactorial trait usually resembles a
bell-shaped curve.
• A discontinuous distribution is typical of
characteristics such as sex that exist in only
two forms and are affected by few genes.
Other Patterns of Inheritance
13.11 Multigene Traits (cont.)
(a), This graph of the heights of students in a high school biology class
shows a continuous distribution. Height is a quantitative, or continuous,
characteristic. (b), This graph shows the distribution of the sexes in the same
class. Sex is a qualitative, or discontinuous, characteristic.
Summary
• The concept of a gene has changed during the past century as
research techniques have allowed more detailed analysis.
• A gene is a region of DNA that codes for a product.
• Most genes exist as several alleles that code for different
products.
• Studies of twins demonstrate the effects of environment on
gene expression.
• Chromosomes are complex structures, each containing
many genes.
• Using techniques such as staining to produce banding
patterns, scientists can identify homologous chromosomes.
Summary (cont.)
• Homologous chromosomes carry the same genes, but these
genes may be different alleles.
• Mendel’s work forms the basis of modern genetics.
• Probability can be used to predict the outcome of breeding
experiments.
• Working with Drosophila, Morgan and his students identified
many X-linked traits; helped to establish the chromosomal
basis of inheritance; and explained nondisjunction, crossingover, and recombination.
Reviewing Key Terms
Match the term on the left with the correct description.
___
allele
d
___
karyotype
b
a. the genetic makeup of an
organism
___
probability
c
b. a display of the image of a set of
chromosomes sorted by number
___
genotype
a
c. the chance that an event
will occur
___
phenotype
e
___
antibodies
f
d. one of two or more possible
forms of a gene
e. the expression of a genotype in
the appearance or function of
an organism
f.
a blood protein produced in
response to an antigen
Reviewing Ideas
1. What is the principle of independent
assortment?
Alleles for one characteristic assort, or divide up
among the gametes during meiosis, independently
of alleles for other characteristics.
Reviewing Ideas
2. What is an X-linked trait? What are some
human examples?
An X-linked trait is a trait whose gene is carried
only on the X chromosome. More than 300
X-linked traits have been identified in humans.
Well-known examples of human X-linked,
recessive alleles included those responsible for
red-green color blindness and hemophilia, a
disease in which blood does not clot normally.
Using Concepts
3. How are linked genes useful in locating the
positions of genes on chromosomes?
The frequency with which linked traits become
separated reflects how far apart on the
chromosome the genes for those traits are. This
information can be used to map the positions of
genes on chromosomes.
Using Concepts
4. A person with which blood type would be able
to accept all blood type in a transfusion? Why?
A person with blood type AB would be able to
accept any of the four blood types. Their blood does
not produce anti-A or anti-B antibodies which would
cause rejection. Type O is universally accepted
among all blood types.
Synthesize
5. Why is an extra X chromosome (XXX) in females
not as disruptive as having an extra copy of
another chromosome?
Cells of XXX females have two Barr bodies
which means that only one X chromosome in
a cell remains active.
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Chapter Animations
Self-fertilization in a pea flower
Sex determination
Incomplete dominance of flower
color in snapdragons
Self-fertilization in a pea flower
Sex determination
Incomplete dominance of flower color in snapdragons
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