Transcript Document

DNA is composed of four nucleotides
DNA is made of chains of small subunits called
nucleotides
Each nucleotide has three components
1. A phosphate group
2. A deoxyribose sugar
3. One of four nitrogen-containing bases
1. Thymine (T)
2. Cytosine (C)
3. Adenine (A)
4. Guanine (G)
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© 2014 Pearson Education, Inc.
nucleotide
Phosphate-yellow
Sugar-blue
Base-A,C,T,G
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DNA is a double helix of two nucleotide strands (continued)
James Watson and Francis Crick combined the
X-ray data with bonding theory to deduce the structure of
DNA
They proposed that a single strand of DNA is a
polymer consisting of many nucleotide subunits
Within each DNA strand, the phosphate group of one
nucleotide bonds to the sugar of the next nucleotide in
the same strand
The deoxyribose and phosphate portions make up the
sugar-phosphate backbone
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Hydrogen bonds between complementary bases hold two
DNA strands together in a double helix (continued)
Because of their structures and the way they face each
other, adenine (A) bonds only with thymine (T) and
guanine (G) bonds only with cytosine (C)
Bases that bond with each other are called
complementary base pairs
Thus, if one strand has the base sequence
CGTTTAGCCC, the other strand must have the
sequence GCAAATCGGG
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Parental DNA
double helix
The parental DNA
is unwound
New DNA strands
are synthesized with
bases complementary
to the parental
strands
Each new double helix is composed
of one parental strand (blue) and one
new strand (red)
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10.1 What Is the Physical Basis of Inheritance?
 Inheritance is the process by which the traits of
organisms are passed to their offspring
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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
– 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—changes in the
nucleotide sequence in genes
– If a mutation occurs in a cell that becomes a sperm or
egg, it can be passed on from parent to offspring
– Most mutations occurring in the DNA of an organism
initially appeared in the reproductive cells
– New mutations may have occurred in reproductive
cells of the organism’s own parents, but this is quite a
rarity
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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
– If two homologous chromosomes carry different alleles
at a given locus, the organism is heterozygous at that
locus (a hybrid)
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Figure 10-1 The relationships 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
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the chromosome
from the female
parent
10.2 How Were the Principles of Inheritance
Discovered?
 Gregor Mendel, an Austrian monk, 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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Figure 10-2 Gregor Mendel
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10.2 How Were the Principles of Inheritance
Discovered?
– Pea plants have qualities that make them a good
organism for studying inheritance
– Pea flowers have stamens, the male structures that
produce pollen, which 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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Figure 10-3 Flowers of the edible pea
intact pea
flower
flower dissected
to show its
reproductive structures
Carpel (female,
produces eggs)
Stamens (male, produce pollen
grains that contain sperm)
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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 selffertilize
– 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?
 True-breeding organisms possess traits that remain
inherited unchanged by all offspring produced by selffertilization
 Mendel’s cross-fertilization of pea plants used true-breeding
organisms
 Mendel cross-fertilized true-breeding, white-flowered plants
with true-breeding, purple-flowered plants
– 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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Figure 10-4 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
plant
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10.3 How Are Single Traits Inherited?
 Mendel’s flower color experiments
– 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
– 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?
 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
purple- and 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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Figure 10-5 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
Figure 10-6 The distribution of alleles in gametes
homozygous parent
A
A
gametes
A
A
Gametes produced by a homozygous parent
heterozygous parent
A
a
gametes
A
a
Gametes produced by a heterozygous parent
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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 (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
© 2014 Pearson Education, Inc.
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 (continued)
– A cross between a purple-flowered plant (PP) and a
white-flowered plant (pp) produces all purpleflowered F1 offspring, with a Pp genotype
– Dominant P gametes from purple-flowered plants
combined with recessive p gametes from whiteflowered plants to produce hybrid purple-flowered
plants (Pp)
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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 (continued)
– The F1 offspring were all heterozygous (Pp) for flower
color
– When the F1 offspring were allowed to self-fertilize,
four types of gametes were produced from the Pp
parents
– Sperm: Pp
– Eggs: Pp
© 2014 Pearson Education, Inc.
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 (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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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 (continued)
– The particular combination of the two alleles carried by
an individual is called the genotype
– For example, PP or Pp
– The physical expression of the genotype is known as
the phenotype (for example, purple or white flowers)
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Figure 10-7a Gametes produced by homozygous parents
purple parent
P
PP
P
all P sperm and eggs
white parent
pp
p
p
all p sperm and eggs
Gametes produced by homozygous parents
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Figure 10-7b Fusion of gametes produces F1 offspring
F1 offspring
sperm
eggs
P
p
Pp
P
pP
or
p
Fusion of gametes produces F1 offspring
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Figure 10-7c Fusion of gametes from the F1 generation produces F2 offspring
gametes from F1 Pp plants
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F2 offspring
sperm
eggs
P
P
P
p
Pp
p
P
pP
p
p
pp
Fusion of gametes from the F1 generation
produces F2 offspring
PP
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?
 Simple “genetic bookkeeping” can predict genotypes
and phenotypes of offspring (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?
 Simple “genetic bookkeeping” can predict
genotypes and phenotypes of offspring (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?
 Simple “genetic bookkeeping” can predict
genotypes and phenotypes of offspring (continued)
7. Based on dominant and recessive rules, determine the
phenotypic fraction
– A genotypic ratio of 1 PP : 2 Pp : 1 pp yields 3 purpleflowered plants : 1 white-flowered plant
© 2014 Pearson Education, Inc.
Figure 10-8 Determining the outcome of a single-trait cross
Pp
self-fertilize
P
p
eggs
sperm
eggs
offspring
genotypes
genotypic
ratio
phenotypic
ratio
(1:2:1)
(3:1)
sperm
P
PP
P
P
PP
P
p
Pp
PP
Pp
purple
Pp
p
pP
pp
Punnett square of a single-trait cross
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p
P
pP
p
p
pp
pp
white
Using probabilities to determine the offspring of a
single-trait cross
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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Figure 10-9 Punnett square of a test cross
pollen
PP or Pp
sperm unknown
if PP
pp
all eggs
p
p
eggs
if Pp
p
all
sperm P
eggs
all Pp
sperm
P
Pp
p
© 2014 Pearson Education, Inc.
pp