Download Ch08 Inheritance Genes and Chromosomes

8
Inheritance, Genes, and
Chromosomes
Chapter 8 Inheritance, Genes, and Chromosomes
Key Concepts
8.1 Genes Are Particulate and Are Inherited
According to Mendel’s Laws
8.2 Alleles and Genes Interact to Produce
Phenotypes
8.3 Genes Are Carried on Chromosomes
8.4 Prokaryotes Can Exchange Genetic
Material
Chapter 8 Opening Question
How is hemophilia inherited, and why is it
most frequent in males?
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Breeding of plants and animals had yielded two
hypotheses by the mid-19th century:
•  Blending inheritance—gametes contained
determinants (genes) that blended when
gametes fused during fertilization
•  Particulate inheritance—each determinant
was physically distinct and remained intact
during fertilization
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
In the 1860s, the monk Gregor Mendel used
the scientific method in studies of garden
peas that clearly supported the particulate
hypothesis.
Garden peas were easy to grow and
manipulate: their flowers have both male and
female sex organs (pistils and stamens) that
produce gametes.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Peas normally self-fertilize, but male organs
can be removed to allow fertilization with
pollen from other flowers.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Characters—observable physical features
(e.g., flower color, seed shape)
Traits—forms of a character (e.g., purple
flowers, wrinkled seeds)
Mendel worked with true-breeding varieties—
when plants of the same variety are crossed,
all offspring have characters with the same
traits.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Parental generation = P
Resulting offspring = first filial generation, or F1
If F1 plants self-pollinate, they produce the
second filial generation, or F2.
“Hybrid” refers to offspring of crosses between
organisms differing in one or more characters.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
In the 1st experiment, Mendel crossed plants
differing in just one character, producing
monohybrids in the F1 generation.
The monohybrids were then allowed to selfpollinate to form the F2 generation—a
monohybrid cross.
Figure 8.1 Mendel’s Monohybrid Experiments (Part 1)
Figure 8.1 Mendel’s Monohybrid Experiments (Part 2)
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
One trait of each pair disappeared in the F1
generation and reappeared in F2 .
Mendel’s findings led to a rejection of the
blending hypothesis.
•  The F1 offspring were not a blend of the
parental traits, but only one trait was present.
•  The trait did not disappear in the F2
generation.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
All seven crosses between varieties with
contrasting traits gave the same kind of
data.
Mendel called the trait that appeared in the F1
and was more abundant in the F2 the
dominant trait, and the other trait
recessive.
The ratio of dominant to recessive in the F2
was about 3:1.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Mendel proposed that the genetic determinants
(genes) occur in pairs and are segregated in
the gametes.
•  Each plant has two genes for each
character, one from each parent.
We now use the term diploid to describe
having two copies of a gene, and haploid as
having one copy.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Mendel concluded that each gamete contains
one copy of each gene (haploid), the resulting
zygote contains two copies (diploid), because
it is produced by the fusion of two gametes.
Different traits arise because there can be
different forms of a gene—now called alleles
—for a particular character.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Phenotype—the physical appearance of an
organism (e.g., round seeds)
Genotype—the genetic makeup (e.g., Rr)
Capital letters designate the dominant allele,
lower case designate the recessive.
Round seeds can be the result of two different
genotypes—RR or Rr.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
True-breeding individuals have two copies of
the same allele—they are homozygous for
the allele (e.g., rr).
Heterozygous individuals have two different
alleles (e.g., Rr).
Figure 8.2 Mendel’s Explanation of Inheritance
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Mendel’s first law:
The law of segregation states that the two
copies of a gene separate when an
individual makes gametes.
•  Each gamete receives only one copy.
Gametes from an RR individual will all be R,
gametes from an rr individual will all be r,
and their offspring (F1) will all be Rr.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
In the F2 generation, gametes will be R or r.
Genotypes in the F2 generation can be
predicted using a Punnett square, which
considers all possible gamete combinations.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
The Punnett square predicts a 3:1 ratio of
phenotypes in the F2 generation.
Mendel’s experimental values were
remarkably close to this for all seven of the
character traits he compared.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Genes are now known to be relatively short
sequences of DNA found on the much
longer DNA molecules that make up
chromosomes.
Today, we can picture the different alleles of
a gene segregating as chromosomes
separate during meiosis I.
Figure 8.3 Meiosis Accounts for the Segregation of Alleles (Part 1)
Figure 8.3 Meiosis Accounts for the Segregation of Alleles (Part 2)
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Genes determine phenotypes mostly by
producing proteins with particular functions.
In many cases a dominant gene is expressed,
and the recessive gene is mutated so that it
is no longer expressed or produces a nonfunctional protein.
The wrinkled pea phenotype is due to a lack
of starch branching enzyme 1 (SBE1),
essential for starch synthesis.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Mendel verified his hypotheses by doing test
crosses:
•  Cross F1 with known homozygote which
has the recessive genotype (e.g., rr).
•  If the F1 individual was homozygous, all
offspring will show the dominant trait. If
the F1 was heterozygous, half of the
offspring will have the recessive trait.
Figure 8.4 Homozygous or Heterozygous? (Part 1)
Figure 8.4 Homozygous or Heterozygous? (Part 2)
Figure 8.4 Homozygous or Heterozygous? (Part 3)
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Mendel’s next experiments tested whether
genes are distributed independently in the
offspring.
•  He crossed peas that differed in two
characters—seed shape and color
•  True-breeding parents:
§  RRYY—round yellow seeds
§  rryy—wrinkled green seeds
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
F1 generation is RrYy—all round yellow.
Crossing the F1 generation (all identical
double heterozygotes) is a dihybrid cross.
Mendel asked whether, in the gametes
produced by RrYy, the traits would be linked
or segregate independently.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Two possible outcomes of the dihybrid cross:
•  If the alleles were linked, gametes would
be RY or ry; F2 would have three times
more round yellow than wrinkled green.
§ 
The ratio would be 3:1
•  If independent, gametes could be RY, ry,
Ry, or rY.
§ 
F2 generation would have nine different
genotypes, and the four phenotypes
would be in a 9:3:3:1 ratio.
Figure 8.5 Independent Assortment
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Mendel’s dihybrid crosses supported the
second prediction.
Mendel’s second law is the law of
independent assortment:
•  Alleles of different genes assort
independently during gamete formation.
This law does not always apply to genes near
each other on the same chromosome, but
chromosomes do segregate independently.
Figure 8.6 Meiosis Accounts for Independent Assortment of Alleles (Part 1)
Figure 8.6 Meiosis Accounts for Independent Assortment of Alleles (Part 2)
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
One reason Mendel was successful was his
use of large sample sizes.
By counting many progeny from each cross,
he was able to see clear patterns.
After his work became recognized, geneticists
began using probability calculations to
predict expected ratios of genotypes and
phenotypes and statistics to determine
whether actual results matched the
predictions.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Probability
•  If an event is certain to happen, its
probability = 1
•  If an event cannot possibly happen, its
probability = 0
•  All other events have a probability
between 0 and 1
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Two coin tosses are independent events;
each will come up heads ½ of the time.
The probability that both coins will come up
heads is:
½×½=¼
Multiplication rule: the probability of two
independent outcomes occurring together is
found by multiplying the individual
probabilities.
Figure 8.7 Using Probability Calculations in Genetics
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
In a monohybrid cross:
After self-pollination of an F1 Rr, the
probability that the F2 offspring will have the
genotype RR is
½×½=¼
Probability that offspring will have the rr
genotype is the same.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Probability that offspring will be heterozygous:
There are two ways to get a heterozygote—
Rr or rR.
Addition rule: probability of an event that can
occur in two or more different ways is the
sum of the individual probabilities of those
ways:
¼+¼=½
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Human pedigrees can show Mendel’s laws.
Humans have few offspring; pedigrees do not
show the clear proportions that the pea
plants showed.
But pedigrees can show patterns and can be
used to determine whether a rare allele is
dominant or recessive.
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Pattern of inheritance if a rare allele is
dominant:
•  Every person with the abnormal
phenotype has an affected parent.
•  Either all (if homozygous parent) or half
(if heterozygous parent) of offspring in an
affected family are affected.
Figure 8.8 Pedigree Analysis and Inheritance (Part 1)
Concept 8.1 Genes Are Particulate and Are Inherited According to
Mendel’s Laws
Pattern of inheritance if the rare allele is
recessive:
•  Affected people often have two
unaffected parents.
•  In an affected family, one-fourth of
children of unaffected parents are
affected. The parents are heterozygous
carriers.
Figure 8.8 Pedigree Analysis and Inheritance (Part 2)
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
New alleles arise by mutations—rare, stable,
inherited changes in the genetic material.
Wild type is the allele present in most of the
population. Other alleles of that gene are
mutant alleles.
A gene with a wild-type allele that is present
less than 99% of the time is called
polymorphic.
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
One gene may have more than two alleles.
Multiple alleles increase the number of
possible phenotypes and may show a
hierarchy of dominance in heterozygotes.
•  Example: coat color in rabbits
Figure 8.9 Multiple Alleles for Coat Color in Rabbits
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
Incomplete dominance
Some alleles are neither dominant nor
recessive, and heterozygotes have an
intermediate phenotype.
•  Example: flower color in snapdragons
Figure 8.10 Incomplete Dominance Follows Mendel’s Laws
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
Codominance
Two alleles of a gene produce both
phenotypes in the heterozygote.
•  Example:
§ 
ABO blood group system has three
alleles of the gene: IA, IB, and IO.
I and IB are codominant.
§  A
Figure 8.11 ABO Blood Groups Are Important in Transfusions
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
Epistasis
Phenotypic expression of one gene is
influenced by another gene.
•  Example: Coat color in Labrador retrievers
§  Allele B (black) is dominant to b
(brown)
§  Allele E (pigment deposition) is
dominant to e (no pigment deposition—
hair is yellow)
§  Gene E determines the phenotypic
expression of gene B.
Figure 8.12 Genes Interact Epistatically
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
Hybrid vigor (heterosis): hybrid offspring
grow larger, produce more seeds, etc, than
the parental varieties.
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
“Vigor” is a complex trait; most complex
phenotypes are determined by multiple
genes.
Most are quantitative traits: they must be
measured, rather than assessed
qualitatively (e.g., grain yields).
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
Genotype and environment often interact to
determine phenotype.
•  Example: point restriction coat patterns
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
These rabbits and cats have a mutant allele
for the coat color gene.
The enzyme encoded by the gene is inactive
at temperatures above about 35°C.
The extremities are cooler than the main body
(around 25°C), so the fur on these regions is
dark.
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
Two parameters describe the effects of genes
and environment on phenotype:
•  Penetrance: proportion of individuals
with a certain genotype that show the
expected phenotype
•  Expressivity: degree to which genotype
is expressed in an individual
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
Many people, but not all, who inherit a mutant
allele of the gene BRCA1 develop breast
cancer.
•  The mutation is said to be incompletely
penetrant.
A woman with the mutant allele may get both
breast and ovarian cancer, but another
woman may only get breast cancer.
•  The mutation is said to have variable
expressivity.
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
Heritability: relative contribution of genetic
versus environmental factors to variation in
a character in a population
•  Example: human height
Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
Heritability varies from 0 to 1.
For human height, heritability varies from
about 0.65 to 0.8.
If heritability is 0.65, 65% of the variation in
height is due to genetic factors, 35% is due
to environmental effects.
Heritability estimates apply only to variation
within populations, not individuals.
Concept 8.3 Genes Are Carried on Chromosomes
A gene is a sequence of DNA that resides at
a particular site on a chromosome—the
locus (plural loci).
Genetic linkage of genes on a single
chromosome can alter their pattern of
inheritance from those described by
Mendel’s law.
Concept 8.3 Genes Are Carried on Chromosomes
Genetic linkage was discovered by Thomas
Hunt Morgan, using the fruit fly Drosophila
melanogaster.
Much genetic research has been done with
Drosophila, which is considered a model
organism because of its small size, ease of
breeding, and short generation time.
Concept 8.3 Genes Are Carried on Chromosomes
Some crosses performed with Drosophila did
not yield expected ratios according to the
law of independent assortment.
Instead, some alleles for body color and wing
shape were inherited together.
Morgan hypothesized that the two loci were
linked on the same chromosome and could
not assort independently.
Figure 8.13 Some Alleles Do Not Assort Independently (Part 1)
Figure 8.13 Some Alleles Do Not Assort Independently (Part 2)
Concept 8.3 Genes Are Carried on Chromosomes
But if linkage were absolute, the F1 would
only have the two parental phenotypes in a
1:1 ratio.
Some of the offspring had recombinant
phenotypes, resulting from crossing over
during prophase I in meiosis.
•  Two of the four chromatids in the tetrad
exchange chromosome segments.
Figure 8.14 Crossing Over Results in Genetic Recombination
Concept 8.3 Genes Are Carried on Chromosomes
Recombinant offspring generally appear in
proportions related to the recombination
frequency between the two genes:
•  Calculated by dividing the number of
recombinant progeny by the total number
of progeny.
Recombinant frequencies are higher for loci
that are farther apart on the chromosome.
Figure 8.15 Recombination Frequencies
Concept 8.3 Genes Are Carried on Chromosomes
Recombinant frequencies can be used to infer
the locations of genes along a chromosome
and make a genetic map.
Concept 8.3 Genes Are Carried on Chromosomes
Fruit flies have one pair of sex chromosomes
(involved in sex determination) and 3 pairs of
autosomes (all the other chromosomes).
Concept 8.3 Genes Are Carried on Chromosomes
Males have X and Y sex chromosomes that
are different in size, and many genes on the
X chromosome are not present on the Y.
Thus, males have only one copy of some
genes.
A gene that is present as a single copy in a
diploid organism is called hemizygous.
Concept 8.3 Genes Are Carried on Chromosomes
Morgan’s experiments showed that the gene
for eye color was carried on the X
chromosome.
The wild-type is red (R), the mutant allele is
white (r).
Figure 8.16 A Gene for Eye Color Is Carried on the Drosophila X Chromosome
Concept 8.3 Genes Are Carried on Chromosomes
The term sex-linked inheritance refers to
inheritance of a gene that is carried on a sex
chromosome.
In mammals, the X chromosome is larger and
carries more genes that the Y; most sexlinked inheritance involves genes that are
carried on the X chromosome.
Figure 8.17 Red–Green Color Blindness Is Carried on the Human X Chromosome
Concept 8.3 Genes Are Carried on Chromosomes
Patterns in X-linked recessive phenotypes:
•  They appear much more often in males
than females.
•  A male with the mutation can pass it only
to daughters.
•  Daughters who receive one X-linked
mutation are heterozygous carriers.
•  Mutant phenotype can skip a generation if
it passes from a male to his daughter
(normal phenotype) and then to her son.
Concept 8.3 Genes Are Carried on Chromosomes
Mitochondria and plastids also have a small
chromosome.
Egg cells have abundant cytoplasm and
organelles, but the only part of the sperm
that takes part in fertilization is the nucleus.
So, mitochondria and plastids are inherited
only from the mother.
Inheritance of organelles and their genes is
thus non-Mendelian and is called maternal,
or cytoplasmic, inheritance.
Concept 8.3 Genes Are Carried on Chromosomes
Some organelle genes are important for
organelle assembly and function, and
mutations can have large effects.
In plants, a mutation that affects proteins that
assemble chlorophyll molecules into
photosystems results in all white plants.
Figure 8.18 Cytoplasmic Inheritance
Concept 8.4 Prokaryotes Can Exchange Genetic Material
Prokaryotes reproduce asexually by binary
fission, which gives rise to genetically
identical progeny.
But they can also exchange genetic material.
Transfer of genes from one individual to
another without sexual reproduction is called
horizontal or lateral gene transfer.
Along with mutation, this process generates
genetic diversity among prokaryotes.
Concept 8.4 Prokaryotes Can Exchange Genetic Material
Bacteria exchange genes by bacterial
conjugation.
A projection called the sex pilus initiates
contact between cells.
Genetic material then passes from the donor
to the recipient through a conjugation tube.
There is no reciprocal transfer of DNA.
Figure 8.19 Bacterial Conjugation and Recombination (Part 1)
Figure 8.19 Bacterial Conjugation and Recombination (Part 2)
Concept 8.4 Prokaryotes Can Exchange Genetic Material
In the recipient cell, the donor DNA lines up
with the recipient’s DNA and crossing over
occurs.
Genes from the donor are incorporated into
the recipient’s genome and are passed on
to its progeny.
Concept 8.4 Prokaryotes Can Exchange Genetic Material
Many bacteria also have plasmids—small
circular DNA molecules that replicate
independently.
Plasmids often have:
•  Genes for unusual metabolic tasks, such
as breaking down hydrocarbons
•  Genes for antibiotic resistance
Plasmids can move between cells during
conjugation.
Figure 8.20 Gene Transfer by Plasmids
Concept 8.4 Prokaryotes Can Exchange Genetic Material
Discovery of antibiotics such as penicillin
greatly reduced many lethal infections.
But over time, some bacteria acquired
mutations that made them resistant.
With a selective advantage, as penicillin use
spread, so did the resistant bacteria.
Scientists designed variants of penicillin; but
new strains of resistant bacteria also
develop.
This “arms race” still continues.
Answer to Opening Question
In hemophilia, the mutant gene for the clotting
factor is recessive, and is carried on the X
chromosome.
Affected males inherit their single X
chromosome from their mothers. If they
receive their mother’s recessive mutant
chromosome, they cannot produce the
clotting factor and have hemophilia.
Figure 8.21 Sex Linkage in Royal Families of Europe