Transcript Chapter 12

12
Inheritance, Genes, and
Chromosomes
12 Inheritance, Genes, and Chromosomes
12.1 What Are the Mendelian Laws of
Inheritance?
12.2 How Do Alleles Interact?
12.3 How Do Genes Interact?
12.4 What Is the Relationship between
Genes and Chromosomes?
12.5 What Are the Effects of Genes Outside
the Nucleus?
12.6 How Do Prokaryotes Transmit Genes?
12 Inheritance, Genes, and Chromosomes
The population of Tasmanian devils
was reduced by hunting and diseases,
and the remaining individuals are
closely related.
Now a type of cancer threatens the
population, spreading due to the
genetic relatedness.
Opening Question:
How can knowledge of genetics be used to
save the Tasmanian devil?
12.1 What Are the Mendelian Laws of Inheritance?
Humans have been deliberately
breeding plants and animals for
thousands of years.
Two theories emerged to explain
breeding experiments:
1. Blending inheritance—gametes
contain hereditary determinants that
blend in the zygote. Offspring
phenotypes are intermediate.
12.1 What Are the Mendelian Laws of Inheritance?
2. Particulate inheritance—hereditary
determinants are distinct and remain
intact at fertilization.
Experiments performed by the monk,
Gregor Mendel, supported the
particulate theory.
Figure 12.1 Gregor Mendel and His Garden
12.1 What Are the Mendelian Laws of Inheritance?
Mendel’s theory of inheritance was
published in 1866 but was largely
ignored until 1900.
By that time chromosomes had been
discovered and biologists realized that
genes (hereditary determinants) might
be carried on chromosomes.
12.1 What Are the Mendelian Laws of Inheritance?
Mendel worked with the garden pea,
which has both male and female sex
organs and normally self-fertilizes.
12.1 What Are the Mendelian Laws of Inheritance?
Mendel could control pollination and
fertilization by removing the male
organs and manually pollinating the
flowers.
12.1 What Are the Mendelian Laws of Inheritance?
Pea plants have many varieties with
easily recognized characteristics.
Character: observable physical feature
(e.g., seed shape)
Trait: form of a character (e.g., round
or wrinkled seeds)
Mendel worked with true-breeding
varieties.
12.1 What Are the Mendelian Laws of Inheritance?
Mendel developed hypotheses to
explain inheritance of different traits,
then designed crossing experiments
to test them.
• He transferred pollen from one plant
to another: the parental generation,
P
• The seeds and offspring were the first
filial generation, F1
12.1 What Are the Mendelian Laws of Inheritance?
• In some experiments the F1 plants
were allowed to self-pollinate and
produce a second filial generation,
F2
12.1 What Are the Mendelian Laws of Inheritance?
Mendel first performed monohybrid
crosses: crossing parental varieties with
contrasting traits for a single character.
• The F1 offspring were not a blend of the two
parental traits. Only one of the traits was
present (e.g., round seeds).
• Some F2 had wrinkled seeds. The trait had
not disappeared because of blending.
These results supported the particulate
theory.
Figure 12.2 Mendel’s Monohybrid Experiments (Part 1)
Figure 12.2 Mendel’s Monohybrid Experiments (Part 2)
12.1 What Are the Mendelian Laws of Inheritance?
Mendel made monohybrid crosses for
seven traits; all gave similar results.
The trait that occurred in the F1 and
was more abundant in the F2 was
called dominant, the other recessive.
In the F2 the ratio of dominant to
recessive traits was about 3:1.
Figure 12.2 Mendel’s Monohybrid Experiments (Part 1)
Figure 12.2 Mendel’s Monohybrid Experiments (Part 2)
12.1 What Are the Mendelian Laws of Inheritance?
Mendel proposed that hereditary
determinants (genes) occur in pairs
and segregate from one another
during formation of gametes.
He also proposed that each pea plant
has two genes for each character, one
inherited from each parent.
12.1 What Are the Mendelian Laws of Inheritance?
Diploid: the state of having two copies
of each gene
Haploid: having just a single copy
12.1 What Are the Mendelian Laws of Inheritance?
Different traits arise from different
forms of a gene (now called alleles).
• An organism that is homozygous for
a gene has two alleles that are the
same.
• An organism that is heterozygous for
a gene has two different alleles. One
may be dominant, (e.g., round [R]),
and the other recessive, (e.g.,
wrinkled [r]).
12.1 What Are the Mendelian Laws of Inheritance?
Phenotype is the physical appearance
of an organism.
Genotype is the genetic constitution of
the organism.
Mendel proposed that the phenotype is
the result of the genotype.
12.1 What Are the Mendelian Laws of Inheritance?
Mendel’s first law—
The law of segregation: the two
copies of a gene separate during
gamete formation; each gamete
receives only one copy.
Working with Data 12.1: Mendel’s Monohybrid Experiments
Mendel’s monohybrid crosses were key
to rejecting the blending theory of
inheritance.
Mendel calculated ratios in the F2
generation, but did not do statistical
analyses to determine whether the
observed patterns might be due to
chance alone.
Working with Data 12.1: Mendel’s Monohybrid Experiments
Mendel’s data from the F2 generation after
crossing green- and yellow-seeded plants:
Working with Data 12.1: Mendel’s Monohybrid Experiments
Use the hypothesis that the ratio of
yellow to green seeds in the F2
generation, 3:1, and perform a chisquare test to analyze the results for
each plant in the table.
Question 1:
What can you conclude about this
hypothesis from the individual plants?
How many crosses have P-values >
0.05?
Working with Data 12.1: Mendel’s Monohybrid Experiments
Now total the data from all the plants
and rerun the chi-square analysis.
Question 2:
What can you conclude?
What does your analysis indicate
about the need for using a large
number of organisms in studies of
genetics?
Figure 12.3 Mendel’s Explanation of Inheritance (Part 1)
12.1 What Are the Mendelian Laws of Inheritance?
In the F2 generation, half of the
gametes will have the R allele and the
other half will have the r allele.
Allele combinations can be predicted
using a Punnett square.
In-Text Art, Ch. 12, p. 236
Figure 12.3 Mendel’s Explanation of Inheritance (Part 2)
12.1 What Are the Mendelian Laws of Inheritance?
There are four possible combinations of
alleles in the F2 generation: RR, Rr,
rR, and rr.
If R is dominant, there are three ways
to get round seeds, and only one way
to get wrinkled seeds, resulting in the
3:1 phenotype ratio.
12.1 What Are the Mendelian Laws of Inheritance?
Genes are now known to be short
sequences of DNA; a DNA molecule
makes up a chromosome.
Alleles of a gene can separate during
meiosis I.
Figure 12.4 Meiosis Accounts for the Segregation of Alleles (Part 1)
Figure 12.4 Meiosis Accounts for the Segregation of Alleles (Part 2)
12.1 What Are the Mendelian Laws of Inheritance?
Genes determine phenotypes through
the proteins they encode.
Dominant genes are expressed;
recessive genes may be mutated and
no longer expressed, or encode nonfunctional proteins.
Wrinkled seed phenotype is due to
absence of starch branching enzyme
(SBE1).
12.1 What Are the Mendelian Laws of Inheritance?
One of Mendel’s hypotheses: there are
two possible allele combinations (RR
or Rr) for seeds with the round
phenotype.
He tested this hypothesis by doing test
crosses: F1 individuals are crossed
with homozygous recessive
individuals (rr).
His hypothesis accurately predicted the
results of his test crosses.
Figure 12.5 Homozygous or Heterozygous?
12.1 What Are the Mendelian Laws of Inheritance?
Mendel’s second law—
Independent assortment: copies of
different genes assort independently.
To test this he crossed true-breeding
peas that differed in 2 characteristics:
seed shape and color.
Round, yellow seeds (RRYY)
Wrinkled, green seeds (rryy)
12.1 What Are the Mendelian Laws of Inheritance?
F1 generation is RrYy—all round
yellow.
Crossing the F1 generation (double
heterozygotes) is a dihybrid cross.
Mendel asked whether, in the gametes
produced by RrYy, the traits would be
linked, or segregate independently.
12.1 What Are the Mendelian Laws of Inheritance?
• If linked, gametes would be RY or ry;
F2 would have three times more round
yellow than wrinkled green.
• If independent, gametes could be RY,
ry, Ry, or rY.
F2 would have nine different
genotypes; phenotypes would be in
9:3:3:1 ratio.
Figure 12.6 Independent Assortment (Part 1)
Figure 12.6 Independent Assortment (Part 2)
12.1 What Are the Mendelian Laws of Inheritance?
The experiments supported the hypothesis of
independent assortment.
It doesn’t always apply to genes located on
the same chromosome.
But it is correct to say that chromosomes
segregate independently during formation
of gametes, and so do any two genes
located on separate chromosome pairs.
Figure 12.7 Meiosis Accounts for Independent Assortment of Alleles (Part 1)
Figure 12.7 Meiosis Accounts for Independent Assortment of Alleles (Part 2)
12.1 What Are the Mendelian Laws of Inheritance?
One key to Mendel’s success was large
sample sizes.
By counting many progeny, he was
able to see clear patterns.
Later, geneticists began using
probability calculations to predict
ratios of genotypes and phenotypes,
and statistical techniques to determine
whether actual results matched
predictions.
12.1 What Are the Mendelian Laws of Inheritance?
Probability:
• If an event is certain to happen,
probability = 1.
• If an event cannot possibly happen,
probability = 0.
• All other events have a probability
between 0 and 1.
12.1 What Are the Mendelian Laws of Inheritance?
The multiplication rule—
Probability of two independent events
happening together: multiply the
probabilities of the individual events.
Tossing two coins: probability that both
will come up heads =
½×½=¼
Figure 12.8 Using Probability Calculations in Genetics
12.1 What Are the Mendelian Laws of Inheritance?
The multiplication rule can be applied
to a monohybrid cross:
F1 Rr plant self-pollinates; probability
that gamete will have either gene is
½.
Probabilities of F2 genotypes:
RR = ½ × ½ = ¼
rr = ½ × ½ = ¼
12.1 What Are the Mendelian Laws of Inheritance?
The addition rule—
The probability of an event that can
occur in two different ways is the sum
of the individual probabilities.
In F2 there are two ways to get Rr,
thus ¼ + ¼ = ½
Result: 1:2:1 ratio of genotypes
3:1 ratio of phenotypes
12.1 What Are the Mendelian Laws of Inheritance?
F2 in dihybrid crosses:
Probability of an F2 being round =
probability of heterozygote +
probability of homozygote or ½ + ¼ =
¾
Joint probability that a seed will be
round and yellow: ¾ × ¾ = 9/16
12.1 What Are the Mendelian Laws of Inheritance?
Human pedigrees can also show
Mendel’s laws.
A pedigree is a family tree showing the
occurrence of phenotypes and alleles.
Humans have small families, and so
pedigrees don’t show the clear
proportions that the pea plant
phenotypes did.
12.1 What Are the Mendelian Laws of Inheritance?
But pedigrees can be used to
determine whether a rare allele is
dominant or recessive.
For rare dominant alleles:
• Every affected person has an affected
parent.
• About half of the offspring of an
affected parent are also affected.
Figure 12.9 Pedigree Analysis and Inheritance (Part 1)
12.1 What Are the Mendelian Laws of Inheritance?
For rare recessive alleles:
• Affected people can have two parents
who are not affected.
• Only a small proportion of people are
affected: about ¼ of children whose
parents are both heterozygotes.
• There has usually been a marriage of
relatives
Figure 12.9 Pedigree Analysis and Inheritance (Part 2)
12.2 How Do Alleles Interact?
Mendel’s laws are still valid today, and
he laid the groundwork for future
genetic studies.
But we have learned that things are
often more complex:
• Over time genes accumulate
differences and new alleles arise.
• There may be more than two alleles
for one character.
12.2 How Do Alleles Interact?
• Alleles don’t always show simple
dominant-recessive relationships.
• A single allele may have several
phenotypic effects.
12.2 How Do Alleles Interact?
New alleles arise through mutations:
stable, inherited changes in the
genetic material.
The allele present in most of the
population is called the wild type.
Other alleles are mutant alleles.
Wild-type and mutant alleles reside at
the same locus (specific position on a
chromosome).
12.2 How Do Alleles Interact?
A genetic locus is polymorphic if the
wild-type allele is present less than
99% of the time.
Any one individual has 2 alleles at a
locus, but there may be many alleles
in a population.
Multiple alleles often show a hierarchy
of dominance.
12.2 How Do Alleles Interact?
Coat color in rabbits is determined by
multiple alleles of the C gene:
• C determines dark gray
• cchd determines chinchilla
• ch determines Himalayan (point
restricted)
• c determines albino
Figure 12.10 Multiple Alleles for Coat Color in Rabbits
12.2 How Do Alleles Interact?
Some alleles are neither dominant nor
recessive—a heterozygote has an
intermediate phenotype: incomplete
dominance.
In the F2, the original phenotypes
reappear, the alleles have not
“blended.”
Figure 12.11 Incomplete Dominance Follows Mendel’s Laws
12.2 How Do Alleles Interact?
Codominance: two alleles produce
phenotypes that are both present in
the heterozygote.
The ABO blood group system results
from three different alleles that
encode an enzyme that adds specific
groups to oligosaccharides on red
blood cell surfaces.
12.2 How Do Alleles Interact?
The three alleles, IA, IB, and IO produce
different versions of the enzyme.
12.2 How Do Alleles Interact?
The oligosaccharides act as antigens,
molecules that are recognized by
specific antibodies.
People make antibodies in the blood
serum which react with foreign
proteins—this protects the body from
invasion by “non-self” molecules or
organisms.
12.2 How Do Alleles Interact?
People in the A group make A antigen,
and anti-B antibodies.
People in the B group make B antigen
and anti-A antibodies.
People in the AB group make both A
and B antigens, and neither antibody.
The IA and IB alleles are codominant.
12.2 How Do Alleles Interact?
The enzyme in the O group is inactive,
so neither antigen is made; they have
both anti-A and anti-B antibodies.
Knowledge of blood groups is
extremely important in determining
compatibility of blood types for blood
transfusions.
Figure 12.12 ABO Blood Reactions Are Important in Transfusions
12.2 How Do Alleles Interact?
Pleiotropic: one allele has multiple
phenotypic effects.
The heritable human disease
phenylketonuria results from a
mutation in the gene for a liver
enzyme that converts the amino acid
phenylalanine to tyrosine.
12.2 How Do Alleles Interact?
Phenylalanine builds up to toxic levels,
and affects development.
The mutated allele is pleiotropic: it
results in mental retardation, and
reduced hair and skin pigmentation.
12.3 How Do Genes Interact?
Epistasis: phenotypic expression of one
gene is influenced by another gene.
Coat color in Labrador retrievers:
For alleles B (black) and b (brown) to be
expressed, allele E (pigment deposition)
must be expressed.
An ee dog is yellow regardless of which B
alleles are present. E is said to be epistatic
to B.
Figure 12.13 Genes May Interact Epistatically (Part 1)
Figure 12.13 Genes May Interact Epistatically (Part 2)
12.3 How Do Genes Interact?
Inbreeding: mating among close
relatives; can result in offspring of low
quality.
Close relatives tend to have the same
recessive alleles.
Inbreeding is a concern for very small
populations of endangered species.
12.3 How Do Genes Interact?
A cross between two different truebreeding homozygotes can result in
offspring with stronger, larger
phenotypes.
Called “hybrid vigor” or heterosis.
Hybridized corn and other crops and
animals have led to increased food
production.
Figure 12.14 Hybrid Vigor in Corn
12.3 How Do Genes Interact?
The mechanism of heterosis is
debated.
Dominance hypothesis: extra growth
can be explained by lack of inbreeding
depression; hybrids are unlikely to be
homozygous for deleterious recessive
alleles.
Overdominance: new allele
combinations result in superior traits.
12.3 How Do Genes Interact?
Environment also affects phenotype.
Light, temperature, nutrition, etc. can
affect expression of the genotype.
“Point restriction” coat patterns in
Siamese cats and rabbits: the enzyme
that produces dark fur is inactive at
higher temperatures. Nose, ears, etc.
are cooler, and thus darker in color.
Figure 12.15 The Environment Influences Gene Expression
12.3 How Do Genes Interact?
Effects of genes and environment on
phenotype:
• Penetrance: proportion of individuals
with a certain genotype that show the
phenotype
• Expressivity: degree to which
genotype is expressed in an individual
12.3 How Do Genes Interact?
The pea characters Mendel studies
were discrete and qualitative.
For more complex characters,
phenotypes vary continuously over a
range—quantitative, or continuous,
variation.
Quantitative variation is usually due to
both genes and environment.
Figure 12.16 Quantitative Variation
12.3 How Do Genes Interact?
Genes that determine these complex
characters: quantitative trait loci.
Identifying these loci can help improve
crop yields, understand disease
susceptibility and behavior.
12.4 What Is the Relationship between Genes and
Chromosomes?
In 1909, Thomas Hunt Morgan and
students at Columbia University
pioneered the study of the fruit fly
Drosophila melanogaster.
Much genetic research has been done
with Drosophila because of its size,
ease of breeding, and short
generation time.
12.4 What Is the Relationship between Genes and
Chromosomes?
Some crosses performed with
Drosophila did not yield expected
ratios according to the law of
independent assortment.
Some genes were inherited together;
the two loci were on the same
chromosome, or linked.
All of the loci on a chromosome form a
linkage group.
Figure 12.17 Some Alleles Do Not Assort Independently (Part 1)
Figure 12.17 Some Alleles Do Not Assort Independently (Part 2)
12.4 What Is the Relationship between Genes and
Chromosomes?
Absolute linkage is rare—genes on
the same chromosome do sometimes
separate.
Genes may recombine during prophase
I of meiosis by crossing over.
Chromosomes exchange
corresponding segments. The
exchange involves two chromatids in
the tetrad; both chromatids become
recombinant.
Working with Data 12.2: Some Alleles Do Not Sort Independently
Thomas Hunt Morgan studied linked
genes by doing F1 × homozygous
recessive test crosses.
They hypothesized that genes are
linked together on chromosomes and
that crossing over during meiosis
gives rise to less frequent
phenotypes.
Working with Data 12.2: Some Alleles Do Not Sort Independently
Morgan first performed a dihybrid cross
between black, normal-winged flies
(bbVgVg) and gray, vestigial-winged
flies (BBvgvg).
The F1 flies were interbred, yielding the
F2 phenotypes shown in the table
(Experiment 1).
Working with Data 12.2, Table 1
Working with Data 12.2: Some Alleles Do Not Sort Independently
Question 1:
Compare these data (Experiment 1)
with the expected data in a 9:3:3:1
ratio by using the chi-square test.
Are there differences, and are they
significant?
Working with Data 12.2: Some Alleles Do Not Sort Independently
To quantify linkage, Morgan crossed
homozygous black, normal-winged
females with homozygous gray,
vestigial-winged males.
The results of this test cross are shown
in the table (Experiment 2).
Working with Data 12.2: Some Alleles Do Not Sort Independently
Question 2:
Are these genes linked (Experiment
2)?
If they are linked, what is the map
distance between the genes?
Explain why these data are so
different from the data shown in
Figure 12.17.
Figure 12.17 Some Alleles Do Not Assort Independently
Working with Data 12.2: Some Alleles Do Not Sort Independently
In a third experiment, Morgan crossed
two strains of flies that were
homozygous for the body color and
wing genes.
The F1 flies were all gray and normalwinged, and these were interbred.
The results are shown in the table
(Experiment 3).
Working with Data 12.2: Some Alleles Do Not Sort Independently
Question 3:
What were the genotypes and
phenotypes of the original parents that
produced the F1? (Experiment 3)
Figure 12.18 Crossing Over Results in Genetic Recombination (Part 1)
Figure 12.18 Crossing Over Results in Genetic Recombination (Part 2)
12.4 What Is the Relationship between Genes and
Chromosomes?
Recombinant offspring phenotypes
appear in recombinant frequencies:
Divide number of recombinant
offspring by total number of offspring.
Recombinant frequencies are greater
for loci that are farther apart.
Figure 12.19 Recombinant Frequencies
12.4 What Is the Relationship between Genes and
Chromosomes?
Recombinant frequencies can be used
to infer the location of genes on a
chromosome, and make genetic
maps.
12.4 What Is the Relationship between Genes and
Chromosomes?
Gene sequencing has made mapping
less important, but it is still a way to
verify that a particular DNA sequence
corresponds with a particular
phenotype.
Linkage has allowed biologists to
identify genetic markers linked to
important genes: important in crop
breeding and identifying people
carrying medically significant
mutations.
12.4 What Is the Relationship between Genes and
Chromosomes?
In some cases, parental origin of an
allele is important, and thus an
understanding of sex determination.
Corn: each adult produces both male
and female gametes—monoecious.
Some plants and most animals are
dioecious—male and female
gametes are produced by different
individuals.
12.4 What Is the Relationship between Genes and
Chromosomes?
In most dioecious organisms, sex is
determined by differences in the
chromosomes.
In many animals, sex is determined by
a single pair of sex chromosomes
which differ from one another.
Both sexes have two copies of all other
chromosomes, called autosomes.
Table 12.2
12.4 What Is the Relationship between Genes and
Chromosomes?
Mammals:
• Female has two X chromosomes (XX)
• Male has one X and one Y (XY)
Male mammals produce two kinds of
gametes—half carry a Y and half
carry an X.
The sex of the offspring depends on
which gamete fertilizes the egg.
12.4 What Is the Relationship between Genes and
Chromosomes?
Sex chromosome abnormalities can
result from nondisjunction in meiosis:
• Pair of homologous chromosomes fail
to separate in meiosis I
• Pair of sister chromatids fail to
separate in meiosis II
Result is aneuploidy—abnormal
number of chromosomes.
12.4 What Is the Relationship between Genes and
Chromosomes?
In humans:
• XO—the individual has only one sex
chromosome; female, sterile, with
mental abnormalities (Turner
syndrome).
• XXY—Klinefelter syndrome, affects
males and results in sterility and
overlong limbs.
Suggests gene for maleness is on the
Y chromosome.
12.4 What Is the Relationship between Genes and
Chromosomes?
Other abnormalities helped pinpoint the
gene location:
• Some women are XY but lack a small
piece of the Y chromosome.
• Some men are XX but a small piece
of the Y chromosome is attached to
another chromosome.
12.4 What Is the Relationship between Genes and
Chromosomes?
The Y fragment in both cases contains
SRY (sex-determining region on the Y
chromosome).
Primary sex determination:
If SRY protein is present, the embryo
develops testes.
If there is no SRY, the embryo
develops ovaries.
12.4 What Is the Relationship between Genes and
Chromosomes?
A gene on the X chromosome, DAX1,
produces an anti-testis factor.
In males, SRY inhibits the DAX1 antitestis factor.
In females (who lack SRY), DAX1
functions to inhibit maleness.
12.4 What Is the Relationship between Genes and
Chromosomes?
Secondary sex characteristics: the
outward manifestations of sex.
The gonads produce hormones
(testosterone and estrogen) that
control the development of these
characteristics.
12.4 What Is the Relationship between Genes and
Chromosomes?
Fruit flies normally have three pairs of
autosomes and 1 pair of sex
chromosomes.
X and Y chromosomes are not true
homologs; many genes on X are not
present on Y.
In-Text Art, Ch. 12, p. 251
12.4 What Is the Relationship between Genes and
Chromosomes?
Sex-linked genes were discovered in
fruit flies:
A gene for eye color is on the X
chromosome, the Y doesn’t have it.
A single copy of a gene is called
hemizygous.
Figure 12.20 Eye Color Is a Sex-Linked Trait in Drosophila (Part 1)
Figure 12.20 Eye Color Is a Sex-Linked Trait in Drosophila (Part 2)
12.4 What Is the Relationship between Genes and
Chromosomes?
Sex-linked inheritance: inheritance of
a gene carried on a sex chromosome.
In mammals the X chromosome is
larger and carries more genes than
the Y, so sex-linked genes are usually
on the X chromosome.
12.4 What Is the Relationship between Genes and
Chromosomes?
Pedigrees of X-linked recessive
phenotypes show:
• Phenotype appears much more often
in males
• Daughters who are heterozygous are
carriers
• Mutant phenotype can skip a
generation if it passes from a male to
his daughter
Figure 12.21 Red-Green Color Blindness Is a Sex-Linked Trait in Humans
12.5 What Are the Effects of Genes Outside the Nucleus?
Mitochondria and plastids contain small
numbers of genes, remnants of the
genomes of endosymbiotic
prokaryotes.
Mitochondria and plastids are inherited
only from the mother.
Eggs contain cytoplasm and
organelles, but the only part of the
sperm to enter the egg is the nucleus.
12.5 What Are the Effects of Genes Outside the Nucleus?
There may be hundreds of
mitochondria or plastids in a cell, so
it’s not diploid for organelle genes.
Organelle genes tend to mutate faster
than nuclear genes and have multiple
alleles.
12.5 What Are the Effects of Genes Outside the Nucleus?
Organelle genes are important for their
assembly and function, mutations can
result in new phenotypes.
Some plastid gene mutations affect
chlorophyll synthesis, resulting in a
white phenotype.
Inheritance follows a non-Mendelian,
maternal pattern.
Figure 12.22 Cytoplasmic Inheritance
12.6 How Do Prokaryotes Transmit Genes?
Bacteria exchange genes by
conjugation; requires physical
contact between cells.
A sex pilus extends from one cell to
another, and brings them together.
Genetic material can pass through a
thin cytoplasmic bridge called the
conjugation tube.
Figure 12.23 Bacterial Conjugation and Recombination (A)
12.6 How Do Prokaryotes Transmit Genes?
DNA passes from a donor cell to a
recipient cell; there is no reciprocal
DNA transfer.
The donor DNA lines up with the
recipient’s DNA and crossing over can
occur, changing the recipient’s genetic
makeup.
Figure 12.23 Bacterial Conjugation and Recombination (B)
12.6 How Do Prokaryotes Transmit Genes?
Bacteria also have plasmids—small
circular chromosomes.
Plasmid genes fall into these
categories:
• Unusual metabolic functions—e.g.,
breaking down hydrocarbons
• Antibiotic resistance genes (R factors)
• Genes for making a sex pilus
12.6 How Do Prokaryotes Transmit Genes?
Plasmids can move between the cells
during conjugation.
Plasmids can replicate independently
of the main chromosome, or be
integrated into the main chromosome.
Figure 12.24 Gene Transfer by Plasmids
12 Answer to Opening Question
The Tasmanian Devil Genome Project
is dedicated to preserving the species.
Matings are planned between the most
genetically diverse individuals to
maximize heterozygosity.
A captive population from an area
where devils are cancer-free is being
developed, and are being genotyped.