Remington Grenier, Mercedes Cote, Tyana Nowlan,
and Rebecca Isaacs
Gregor Mendel found his theories of inheritance
through experiments with pea plants.
 It was Mendel’s idea that parents pass discrete genes onto
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their children that preserve the parents’ identities through
the generations.
He found that characters, or genetically inherited
characteristics that differs from person to person, were
what was passed on.
For each character, an organism inherits two alleles
(different versions of the same gene), one from each parent.
Alternative versions of genes account for variations in
inherited characters.
A trait is any variant of a character, such as blue or yellow
color for a flower
Law of Segregation
 One of Mendel’s major hereditary laws.
 States that:
 Every organism carries a pair of alleles for each trait
 The members of this pair separate during gamete formation.
 Example: If an individual is Bb for eye color, during gamete
formation, one gamete would receive a B, and the other would
receive a b.
 Mendel determined his law of segregation while
performing monohybrid crosses.
 Monohybrid Cross: a cross that involves a single character
in which both parents are heterozygous (BbxBb).
Dominant vs. Recessive
 If two alleles at a locus differ, then one, the dominant
allele (B), determines the organism's appearance.
 The other, the recessive allele (b), has no noticeable
effect on the organism's appearance.
 Also known as complete dominance.
 Example: A purple flower that has been crossed with a
white flower could either be BB or Bb because the
dominant allele (B) would overpower the recessive
allele (b).
Homozygous vs. Heterozygous
 Homozygous (pure): an individual is homozygous for a
gene if both of the given alleles are the same.
 Example: BB (homozygous dominant) or bb
(homozygous recessive).
 Heterozygous (hybrid): an individual is heterozygous
for a gene if the two alleles are different.
 Example: Bb
Phenotype vs. Genotype
 Genotype: an organism’s genetic makeup for a given
trait.
 Example: Considering fur color where B represents the
allele for brown and b represents the allele for black, the
possible genotypes include homozygous brown (BB),
heterozygous brown (Bb), and homozygous recessive
(bb).
 Phenotype: the physical expression of the trait
associated with a particular genotype.
 Example: Phenotypes for Mendel’s peas included round
or wrinkled, green or yellow, and purple or white flower.
Intermediate Inheritance
 Intermediate inheritance occurred when an individual heterozygous
for a trait showered characteristics not exactly like either parent.
 Two major types of inheritance include incomplete dominance and
codominance.
 Incomplete Dominance: the heterozygous genotype produces an
intermediate phenotype rather than the dominant phenotype; neither
allele dominates the other.
 Also known as “blending inheritance”
 Example: Crossing a snapdragon plant with red flowers with one that has
white flowers yields offspring with pink flowers.
 Codominance: both alleles express themselves fully in a heterozygous
organism.
 Example: Human Blood Groups
Other Forms of Inheritance
 Polygenice Inheritance: a single phenotypic character
is affected by two or more genes
 Example: Skin Color
 Multiple Alleles: in the whole population, some genes
have more than two alleles
 Example: ABO Blood Group Alleles
 Epistasis: one gene affects the expression of another
 Example: Coat Color of Mice
 Pleiotropy: One gene is able to affect multiple
phenotypic characters
 Example: Sickle-Cell Disease
 Trait-Any detectable variant in a genetic character.
 Hybridization- In genetics, the mating, or crossing,
of two true-breeding varieties.
 P Generation- The parent individuals from which
offspring are serived in studies of inheritance; P stands
for parental.
 F1 Generation- The first filial, or hybrid, offspring in a
series of genetic crosses.
 F2 Generation- Offspring resulting from
interbreeding of the hybrid F1 generation.
 Alleles- Any of the alternative versions of a gene that
produce distinguishable phenotypic effects.
 Dominant Allele- An allele that is fully expressed in
the phenotype of a heterozygote.
 Recessive Allele- An allele whose phenotypic effect is
not observed on a heterozygote.
 Homozygous- Having two identical alleles for a given
gene.
 Heterozygous- Having two different alleles for a given
gene.
 Phenotype- The physical and physiological traits of an
organism, which are determined by its genetic makeup.
 Genotype- The genetic makeup, or set of alleles, of an
organism.
 Mono hybrids -An organism that is heterozygous with
respect to a single gene of interest. All the offspring from a
cross between parents homozygous for different alleles are
monohybrids.
 Dihybrids- An organism that is heterozygous with respect
to two genes of interest. All the offspring from a cross
between parents doubly jomozygous for different alleles are
dihybrids.
 Complete Dominance- The situation in which the
phenotypes od the heterozygote and dominant
homozygote are indistinguishable.
 Incomplete Dominance- The situation in which the
phenotype of heterozygotes is intermediate between
the phenotypes of individuals homozygous for either
allele.
 Co-dominance- The situation in which the
phenotypes of both alleles are exhibited in the
heterozygote because both alleles affect the phenotype
in separate, distinguishable ways.
 Pleiotropy- The ability of a single gene to have multiple
effects.
 Epistasis - A type of gene interaction in which one gene
alters the phenotypic effects of another gene that is
independently inherited.
 Polygenic Inheritance- An additive effects of two or more
genes on a single phenotypic character.
 Carriers- In genetics, an individual who is heterozygous at
a given genetic locus, with one normal allele and one
recessive allele. The heterozygote is phenotypically
dominant for the character determined by the gene but can
pass on the recessive allele to offspring.
 Chorionic Villus Sampling (CVS)- A technique of
prenatal diagnosis in which a small sample of the fetal
portion of the placenta is removed and analyzed to
detect certain genetic and congenital defects in the
fetus.
Chapter 14 Graphic
 In his garden Mendel bred two different colored
flowers. While in the first generation the hybrids were
all the same color because they were heterozygous in
the F1 generation. His breeding of the plants also
shows that in the F2 generation they follow the same
guidelines that punnett squares do resulting in a 3:1
with heterozygote purple flowers and homozygous
recessive white flowers.
Mendelian Inheritance and
Chromosome Behavior
 Meiosis produces haploid gametes with Meiosis 1 including
the separation of homologous pairs and crossing over and
Meiosis 2 including the separation of sister chromatids.
 The behavior of chromosomes during meiosis accounts for
the law of segregation and independent assortment.
 This is also known as the chromosome theory of
inheritance, which states that Mendelian genes have
specific loci along chromosomes and these chromosomes
undergo segregation and independent assortment.
The Chromosomal Basis of Sex
 The sex of a organism is an inherited phenotypic
character usually determined by which sex
chromosomes are present.
 The sex chromosomes carry genes for some traits that
are unrelated to sex characteristics.
 Example: Recessive alleles causing color blindness are
carried on the X chromosome. Fathers transmit this and
other sex-linked alleles to all daughters but no sons. Any
male who inherits such an allele from his mother will
express the trait.
X-Inactivation
 X-inactivation: during the development of the female
embryo, one of two X chromosomes in each cell
remains coiled as a Barr body whose genes are not
expressed. A cell expresses the alleles of the active X
chromosome only.
 Not all cells inactivate the same X. As a result, different
cells will have different active X chromosomes.
Linkage
 Linked genes are genes along the same chromosome that
tend to be inherited together because the chromosome is
passed a long as a unit.
 Linked genes lie on the same chromosome and do not
follow Mendel’s law of independent assortment.
 Recombinant offspring exhibit new combinations of traits
inherited from two parents.
 Due to the law of independent assortment of chromosomes,
unlinked genes show a 50% frequency of recombination in the
gametes.
 Linked genes experience crossing over between nonsister
chromatids which accounts for the observed recombinants, which is
always less that 50% of the total.
Common Disorders
 Simple recessive disorders in which a person must be
homozygous recessive for the gene in question to have
the disease include:
 Tay-Sachs disease: a fatal genetic storage disease that
renders the body unable to break down a particular type
of lipid.
 Cystic Fibrosis: a recessive disorder that is the most
common lethal genetic disease in the United States. A
defective version of a gene on chromosome 7 results in
the excessive segregation of a thick mucus, which
accumulates in the lungs and digestive tract.
Common Disorders (cntd.)
 Sickle Cell Anemia: a recessive disease caused by the substitution of a
single amino acid in the hemoglobin protein of red blood cells, leaving
hemoglobin less able to carry oxygen and also causing the hemoglobin
to deform to a sickle shape when the oxygen content of the blood is low.
 Phenylketonuria: an autosomal recessive disease caused by a single
gene defect that leaves a person unable to break down phenylalanine,
which results in a by-product that can accumulate to toxic levels in the
blood and cause mental retardation.
 Huntington disease: an autosomal dominant degenerative disease of
the nervous system that shows itself when a person is in their 30s or
40s and is both irreversible and fatal.
Chromosomal Complications
 A change in the number of chromosomes in the
individual structure of chromosomes, such as
nondisjunction and aneuploidy, can affect the
phenotype.
 Examples: down syndrome (aneuploidy), trisomy 21
(nondisjunction), and turner syndrome
(nondisjunction).
 The breaking of chromosomes can result in deletions,
inversions, duplications, and translocations.
 Examples: cri-du-chat (deletion) and chronic
myelogenous leukemia (chromosomal translocation).
 Law of Segregation -Mendel’s first law, stating that
the two alleles in a pair segregate into different
gametes during gamete formation.
 Law of Independent Assortment - Mendel’s second
law, stating the each pair of alleles segregates, or
assorts, independently of each other pair during
gamete formation; applies when genes for two
characters are located on different pairs of
homologous chromosomes.
 Chromosome theory of inheritance- states that mendillian genes
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have specific loci along chromosomes and it’s the chromosomes that
undergo segregation and independent assortment
Wild type- the phenotype for the characteristic most commonly
observed in natural populations
Sex-linked gene- a gene located on either sex chromosome
Duchenne muscular dystrophy- a disease characterized by a
progressive weakening of the muscles and loss of coordination
Hemophilia- a sex linked disorder defined by the absence of one or
more of the protein required for the blood clotting factor
 Barr body- a dense object lying along the inside of the
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nuclear envelope in cells of female mammals, representing
a highly condensed, inactivated X chromosome
Linked genes- genes located close enough together on a
chromosome that they tend to be inherited together
Genetic recombination- general term for the production
of offspring with combinations of traits that differ from
those found in either parent
Parental types- an offspring with a phenotype that
matches on of the parental phenotypes
Recombinant types- an offspring whose phenotype
differs from that of the parents
 Crossing over-the reciprocal exchange of genetic
material between nonsister chromatids during
prophase I of meiosis
 Genetic map- an ordered list of genetic loci along a
chromosome
 Linked map- a genetic map based on the frequencies
of recombination between markers during crossing
over of homologous chromosomes
 Map units-a unit of measurement of the distance
between genes
 Nondisjunction- an error in meiosis or mitosis in which
members of a pair of homologous chromosomes or a pair of
sister chromatins fail to separate properly from each other
 Aneuploidy- a chromosomal aberration in which in which
one or more chromosomes are present in extra copies or are
deficient in number
 Monosomic- referring to a cell that has only one copy of a
particular chromosome instead of the normal two
 Trisomic-referring to a diploid cell that has three copies of
a particular chromosome instead of the normal two
 Polyploidy-a chromosome alteration in which the
organism possesses more than two complete
chromosomes sets
 Deletion- a mutational loss of one or more nucleotide
pairs of genes
 Duplication-An aberration in chromosome structure
due to fusion with a fragment from a homologous
chromosome, such that a portion of a chromosome is
duplicated.
 Inversion-An aberration in chromosome structure
resulting from reattachment of a chromosomal
fragment in a reverse orientation to the chromosome
from which it originated.
 Translocation- An aberration in chromosome
structure resulting from attachment of a chromosomal
fragment to a nonhomologous chromosome.
 Genomic imprinting-A phenomenon in which
expression of an allele in offspring depends on
whether the allele is inherited from the male or female
parent.
Chapter 15 Graphic
 The graphic shows Meiosis and how it demonstrates
Mendel’s laws exhibited during dihybrid crossing.
 On one side you see the Law of Segregation which
shows that the two alleles for each gene separate
during gamete formation. As an example, follow the
fate of the long chromosomes (carrying R and r).
 On the other you see the Law of Independent
Assortment which shows that alleles of genes on
nonhomologous chromosomes assort independently
during gamete formation.
DNA is the Genetic Material
 Experiments with bacteria and with phages provided
the first strong evidence that the genetic material is
DNA.
 Watson and Crick found that DNA is a double helix
with two sugar-phosphate backbones.
 The “rungs” on such a ladder would represent pairs of
nitrogenous bases
 Note that Adenine (A) always pairs with Thymine (T)
and Cytosine (C) with Guanine (G).
DNA Replication
 DNA occurs in the S-phase in a semi-conservative
fashion and in a 5’ to 3’ direction.
 Semi-Conservative: the replicated double helix
consists of one old strand and one new strand.
Steps of DNA Replication
 1. Helicase unwinds our double helix into two strands.
 2. Polymerase adds nucleotides to an existing strand.
 Since DNA polymerase can only add DNA in the 5’ to 3’
direction, a leading strand and lagging strand are
created.
 3. Ligase brings together the Okazaki fragments.
 4. Topoisomerase cuts and rejoins the helix.
 5. RNA primase catalyzes the synthesis of RNA
primers.
Proofreading and Repairing
DNA
 DNA polymerases proofread new DNA, replacing
incorrect nucleotides.
 Two repair mechanisms are mismatch repair and
nucleotide excision repair.
 Mismatch Repair: DNA polymerases replace an
incorrectly placed nucleotide with the proper
nucleotide.
 Nucleotide Excision Repair: enzymes cut out and replace
damaged stretches of DNA.
Replicating the Ends of DNA
Molecules
 The ends of eukaryotic chromosomal DNA get shorter
with each round of replication.
 Telomeres postpone the erosion of genes.
 Telomerase catalyzes the lengthening of telomeres in
germ cells.
A Chromosome Consists of DNA
Packed Together with Proteins
 Eukaryotic chromatin is composed mostly of DNA,
histones, and other proteins.
 Histones bind to each other and to the DNA to create
nucleosomes.
 Additional folding leads to highly condensed
chromatin.
 In interphase cells, most chromatin is less compacted
(euchromatin), but some remain highly condensed
(heterochromatin).
 Transformation- A change in genotype and
phenotype due to the assimilation of external DNA by
a cell.
 Semiconservative Model- Type of DNA replication
in which the replicated double helix consists of one old
strand, derived from the old molecule, and one newly
made strand.
 Origins of Replication-Site where the replication of a
DNA molecule begins, consisting of a specific
sequence of nucleotides.
 Replication Fork-A Y-shaped region on a replicating DNA
molecule where the parental strands are being unwound
and new strands are growing.
 Helicases-An enzyme that untwist the double helix of
DNA at the replication forks, separating the two strands
and making them available as template strands.
 Single-strand Binding Proteins- A protein that binds to
the unpaired DNA strands during DNA replication,
stabilizing them and holding them apart while they serve
as templates for the synthesis of complementary strands of
DNA.
 Topoisomerase- A protein that breaks, swivels, and
rejoins DNA strands. During DNA replication,
topoisomerase helps to relieve strain in the double
helix ahead of the replication fork.
 Primer/Primase- An enzyme that joins RNA
nucleotides to make the primer using the parental
DNA strand as a template. Primer is a short stretch of
RNA with a free 3’ end, bound by complementary base
pairing to the template strand, that is elongated with
DNA nucleotides during DNA replication.
 DNA Polymerases- An enzyme that catalyzes the elongation of new
DNA by the addition of nucleotides to the 3’ end of an existing chain.
There are several different DNA polymerases; DNA polymerase III and
DNA polymerase I play major roles in DNA replication in prokaryotes.
 Lagging Strand- A discontinuously synthesized DNA strand that
elongates by means of Okazaki fragments, each synthesized in a 5’-3’
direction away from the replication fork.
 Leading Strand-The new complementary DNA strand synthesized
continuously along the template strand toward the replication fork in
the mandatory 5’-3’ direction.
 Okazaki Fragments- A short segment of DNA
synthesized away from the replication fork on a
template strand during DNA replication, many of
which are joined together to make up the lagging
strand of newly synthesized DNA.
 Mismatch Repair-The cellular process that uses
specific enzymes to remove and replace in correctly
paired nucleotides.
 Nuclease- An enzyme that cuts DNA or RNA either
removing one or few bases or hydrolyzing the DNA or
RNA completely into its component nucleotides.
 Nucleotide Excision Repair- A repair system that
removes and then correctly replaces a damaged
segment of DNA using the undamaged strand as a
guide.
 Telomeres- The tandemly repetitive DNA at the end
of a eukaryotic chromosome’s DNA molecule that
protects the organism’s genes from being eroded
during successive rounds of replication.
 Telomerase- An enzyme that catalyzes the
lengthening of telomeres in eukaryotic germ cells.
 Heterochromatin-Eukaryotic chromatin that
remains highly compacted during interphase and is
generally not transcribed.
 Euchromatin- The less densed form of eukaryotic
chromatin that is available for transcription.
Chapter 16 graphic
 In DNA replication at the replication fork splits you’ll
see replication occurring simultaneously at two forks,
one at either end of a replication bubble. Viewing
each daughter strand in its entirety, you can see half of
it is made continuously as the leading strand, while the
other half is synthesized in fragments as the lagging
strand.
Evidence from the Study of
Metabolic Defects
 DNA controls metabolism by directing cells to make
specific enzymes and other proteins
 Beadle and Tatum did an experiment with mutant
strands showed that the one gene- one enzyme
hypothesis was true
 Genes code for polypeptide chains or for RNA
molecules
Basic Principles of Transcription
and Translation
 While translation is the informational transfer from
nucleotide sequence in RNA to amino acid sequence in
a polypeptide, transcription is the nucleotide-tonucleotide transfer of information from DNA and RNA
The Genetic Code
 Genetic information is encoded as a sequence or
codon.
 A codon is a three-nucleotide sequence of DNA or
mRNA that spacifies a particular amino acid or
termination signal; the basic unit of the genetic code.
 A codon must be read in the correct reading frame
 A reading frame is on an mRNA, the triplet grouping
of ribonucleotides used by the translation machinery
during polypeptide synthesis.
Molecular Components of
Transcription
 RNA synthesis follows the same base-pairing rules as
DNA replication, except that in RNA, uracil substitutes
for thymine. It is also catalyzed by RNA polymerase.
 RNA polymerase is an enzyme that links
ribonucleotides into a growing RNA chain during
transcription.
Synthesis of an RNA Transcript
 There are three stages of transcription.
 Initiation
 Elongation
 Termination
 Initiation is when promoters signal the initiation of
RNA synthesis
 Elongation is when the transcription factors help
eukaryotic RNA polymerase recognize promoter
sequences
 Termination is when the nucleotides signal an end to
RNA synthesis
Important RNA Transcription
Vocabulary
 A promoter is a specific nucleotide sequence in DNA that
binds RNA polymerase, positioning it to start transcribing
RNA at the appropriate place.
 A terminator in bacteria, a sequence of nucleotides in DNA
that marks the end of agene and signals RNA polymerase to
release the newly made RNA molecule and detach from the
DNA.
 Transcription Factors are regulatory proteins that bind to
DNA and affects transcription of specific genes.
 The transcription initiation complex is the completed
assembly of transcription factors and RNA polymerase
bound to a promoter.
Alteration of mRNA Ends
 Eukaryotic mRNA molecules are processed by
modification of their ends and by RNA splicing, before
leaving the nucleus. The 5’ end receives a modified
nucleotide cap, and the 3’ end receives a poly-A tail
 Poly-A tail is a sequence of 50 to 250 adenine
nucleotides added onto the 3’ end of a pre-mRNA
molecule.
 A 5’ cap is a modified form of guanine nucleotide
added onto the nucleotide at the 5’ end of a pre-mRNA
molecule
Split Genes and RNA Splicing
 In RNA splicing, introns are removed and exons
join. RNA splicing usually is carried out by
spliceosomes, but in some cases it is alone
catalyzes its own splicing. Ribozymes, the catalytic
ability of some RNA molecules, derives from the
inherent properties of RNA. Alternative RNA
splicing is allowed with the presence of introns.
Important RNA Splicing
Vocabulary
 Introns are noncoding, intervening sequences within
primary transcript that are removed from the transcript
during RNA processing; also refers to the region of DNA
from which this sequence was transcribed
 Exons are sequences within a primary transcript that
remain in the RNA after RNA processing; also refer to the
region of DNA from which this sequence was transcribed.
 Spliceosomes are a large complex made up of proteins and
RNA molecules that splices RNA by interacting with the
ends of an RNA intron, releasing the intron and joining the
two adjacent exons.
Molecular Components of
Translation
 Using transfer RNAs a cell translates an mRNA
message into protein. After binding, tRNAs line up via
their anticodons at complementary codons on mRNA.
Ribosomes help facilitate this coupling
 tRNA is an RNA molecule that functions as an
interpreter between the appropriate codons in the
mRNA.
 Anticodons are nucleotide triplets at one end of a
tRNA molecule that recognize a particular
complementary codon on an mRNA molecule.
Building a Polypeptide
 Ribosomes coordinate the tree stages of peptide bonds
between amino acids is catalyzed by rRNA. Ribosomes
can translate a single mRNA molecule simultaneously
forming a polyribosome
 rRNA is the most abundant type of RNA, which
together with proteins makes up ribosomes.
Completing and Targeting the
Functional Protein
 Modifications to protein after translation can affect
their three dimensional shape. Proteins destined from
the endomembrane system or for secretion are
transported into the ER. These proteins have a signal
peptide to which a signal-recognition particle binds,
enabling the translation ribosome to bind to the ER.
Types of Mutations
 There are several different types of mutations including
 Point mutations-A change in a gene at a single
nucleotide pair.
 Missense mutations- A base-pair substitution that
results in a codon that codes for a different amino acid.
 Nonsense mutations- A mutation that changes an amino
acid codon to one of the three stop codons, resulting in a
shorter and usually nonfunctional protein
 Frame shift mutation- A mutation occurring when the
number of nucleotides inserted or deleted is not a
multiple of three, resulting in the improper grouping of
the subsequent nucleotides into codons.
Important Mutation Vocabulary
 Insertion is a mutation involving the addition of one or
more nucleotide pairs to a gene.
 Deletion is a mutational loss of one or more nucleotide
pairs from a gene.
Mutagens
 Spontaneous mutations can occur during DNA
replication, recombination, or repair. DNA damage
that can alter a gene is caused by chemical and
physical mutagens
 A mutagen is a chemical or physical agent that
interacts with DNA and causes a mutation.
Comparing Gene Expression in
Bacteria, Archaea, and Eukarya
 Translation can begin while transcription is still in
progress because bacterial cells lack a nuclear
envelope.
 The nuclear envelope seperates transcription from
translation, and extensive RNA processing occurs in
the nucleus in a eukaryotic cell.
 Archaeal cells show similarities to both eukaryotic and
bacterial cells in their processes of gene expression
What Is a Gene?
 A gene is a region of DNA whose final functional
product is either a polypeptide or an RNA molecule
 Gene Expression- The process by which DNA directs
the synthesis of proteins or, in some cases, just RNAs.
 Transciption- The synthesis of RNA using a DNA
template.
 Messenger RNA (mRNA)- A type of RNA, sythesized
using adna template, that attaches to ribosomes in the
cytoplasm and specifies the primary structure of a
protein.
 Translation- The synthesis of a polypeptide using the
genetic information encoded in an mRNA molecule. There
is a change of “language” from nucleotides to amino acids.
 Primary Transcript- an initial RNA transcript; also called
pre-mRNA when transcribed from a protein-coding gene.
 Triplet Code- A set of three-nucleotide-long words that
specify the amino acids for polypeptide chains.
 Template Strand- The DNA strand that provides the
pattern, or template for ordering the sequence of
nucleotides in an RNA transcript.
 TATA Box- A DNA sequence in eukaryotic promoters
crucial in forming the transcription initiation
complex.
 RNA Processing- Modification of RNA transcripts,
including splicing out of introns, joining together of
exons, and alteration of the 5’ and 3’ ends.
 RNA Splicing- After synthesis of a eukaryotic primary
RNA transcript, the removal of portions of the
transcript that will not be included in the mRNA.
 Ribozymes- An RNA molecule that functions as an
enzyme, catalyzing reactions during RNA splicing.
 Alternative RNA Splicing- A type of eukaryotic gene
regulation at the RNA-processing level in which
different mRNA molecules are produced from the
same primary transcript, depending on which RNA
segments are treated as exons and which as introns.
 Wobble- Flexibility in the base-pairing rules in which
the nucleotide at the 5’ end of a tRNA anticodon can
form hydrogen bonds with more than one kind of base
in the third position of a codon.
 P Site- One of a ribosome’s three binding sites for
tRNA during translation. The P-site holds the tRNA
carrying the growing polypeptide chain.
 A Site- One of a ribosome’s three binding sites for
tRNA during translation. The A site holds the tRNA
carrying the next amino acid to be added to the
polypeptide chain.
 E Site- One of a ribosome’s three binding sites for
tRNA during translation. The E site is the place where
discharged tRNAs leave the ribosome.
 Signal Peptide- A sequence of about 20 amino acids
at or neat the leading end of a polypeptide that targets
it to the endoplasmic reticulum or other organelles in a
eukaryotic cell.
 Signal-Recognition Particle (SRP)- A protein-RNA
complex that recognizes a signal peptide as it emerges
from a ribosomes and helps direct the ribosome to the
endoplasmic reticulum by binding to a receptor
protein on the ER.
 Base-Pair Substitution- A type of point mutation;
the replacement of one nucleotide and its partner in
the complementary DNA strand by another pair of
nucleotides.
Chapter 17 Graphic
 The diagram show the path from one gene to one polypeptide. In the
first step of translation, RNA is copied from a DNA template. Next, in
RNA processing, the pre-mRNA goes through splicing, 5-capping, and
3-polyadenylation by which it is converted into mature mRNA.
 This mature mRNA then proceeds to exit the nucleus and attach itself
to a ribosome. In amino acid activation, amino acids are attached to
their corresponding tRNA with the assistance of aminoacyl-tRNA
synthetase (an enzyme) and ATP. In the last step (translation), a
sequence of tRNAs give their amino acids to the polypeptide chain
while mRNA moves through the ribosome. After the mRNA has
completely moved through the ribosome, the polypeptide is released.
Operons: The Basic Concept
 Cells control metabolism by regulating enzyme activity or the
expression of genes coding for enzymes. Genes are often
clustered into operons, with one promoter serving several
adjacent genes, in bacteria. An operator site on the DNA turn the
operon off and on.
 An operon is a unit of genetic function found in bacteria and
phages, ting of a promoter, an operator, and a coordinately
regulated cluster of genes whose products function in a common
pathway.
 An operator is a sequence of nucleotides near the start of an
operon to which an active repressor can attach. The binding of
the repressor prevents RNA polymerase from attaching to the
promoter and transcribing the genes of the operon.
Repressible and Inducible: Two
Types of Negative Gene Regulation
 In a repressible operon, the repressor is active when
bound to a corepressor, usually the end product of an
anabolic pathway. In an inducible operon, binding of
an inducer to an innately active repressor inactivates
the repressor and turns on transcription. Usually
functions in a catabolic pathway. Both types of operons
bind to a specific repressor protein to the operator
shuts off transcription
Negative Gene Regulation
Vocabulary
 A repressor is a protein that inhibits gene
transcription. In prokaryotes, repressors bind to the
DNA in or near the promoter. In eukaryotes, repressors
may bind to control elements within enhancers, to
activators, or to other proteins in a way that blocks
activators from binding to DNA.
 Corepressor is a small molecule that binds to a
bacterial repressor protein and changes its shape,
allowing it to switch an operon off
Positive Gene Regulation
 Some operons are subjected to positive control via
stimulatory activator protein
 A regulatory gene is a gene that codes for a protein,
such as a repressor, that controls the transcription of
another gene or group of genes
Eukaryotic Gene Expression can
be Regulated at any Stage
 Chromatin Modification
 Genes in highly compacted chromatin are generally not
transcribed
 Histone Acetylation seems to loosen chromatin
structure, enhancing transcription
 DNA methylation generally reduces transcription
 Transcription
 Regulation of transcription initiation; DNA control
elements bind specific transcription factors
 Coordinate regulation
Eukaryotic Gene Expression can
be Regulated at any Stage (cnt.)
 RNA processing
 Alternative RNA splicing
 mRNA degradation
 Each mRNA has a characteristic life span
 Translation
 Initiation of translation can be controlled via regulation
of initiation factors
 Protein processing and degradation
 Protein processing and degradation by proteasomes are
subject to regulation
Noncoding RNAs play multiple roles
in controlling gene expression
 Chromatin modification
 Small RNAs can promote the formation of
heterochromatin in certain regions, blocking
transcription
 Translation
 miRNA or siRNA can block the translation of specific
mRNAs
 mRNA degradation
 They can target specific mRNAs for destruction
A Genetic Program for
Embryonic Development
 Embryonic cells undergo differentiation, becoming
specialized in structure and function
Sequential Regulation of Gene Expression
During Cellular Differentiation
 Differentiation is heralded by the appearance of
tissue-specific proteins, which enable differentiation
cells to carry out their specialized roles
The Multistep Model of Cancer
Development
 Normal cells are converted to cancer cells by the
accumulation of mutations affecting proto-oncogenes
and tumor-suppressor genes
Inherited Predisposition and Other
Factors Contributing to Cancer
 Individuals who inherit a mutant oncogene or tumor-
suppressor allele have an increased risk of developing
cancer. Certain viruses promote cancer by integration
of viral DNA into a cells genome
 Inducer- A specific small molecule that binds to a
bacterial repressor protein and changes the repressor’s
shape so that it cannot bind to an operator, thus
switching an operon in.
 Cyclic AMP (cAMP)- A ring-shaped molecule made
from ATP that is a common intracellular signaling
molecule in eukaryotic cells.
 Activator- A protein that binds to DNA and
stimulates gene transcription. In prokaryotes,
activators bind in or near the promoter; in eukaryotes,
activators bind to control elements in enhancers.
 Histone Acetylation- The attachment of acetyl groups to
certain amino acids of histone proteins.
 Genomic Imprinting- A phenomenon in which
expression of an allele in offspring depends on whether the
allele is inherited from the male or female parent.
 Epigenetic Inheritance- Inheritance of traits transmitted
by mechanisms not directly involving the nucleotide
sequence of a genome.
 Control Elements- A segment of noncoding DNA that
helps regulate transcription of a gene by binding a
transcription factor. Multiple control elements are present
in a eukaryotic gene’s enhancer.
 Enhancers- A segment of eukaryotic DNA containing
multiple control elements, usually located far from the
gene whose transcription it regulates.
 Proteasomes- A giant protein complex that
recognizes and destroys proteins tagged for
elimination by the small protein ubiquitin.
 MicroRNAs (miRNAs)- A small, single-stranded RNA
molecule, generated from a hairpin structure on a
precursor RNA transcribed from a particular gene. The
miRNA associates with one or more proteins in a
complex that can degrade or prevent translation of an
mRNA with a complementary sequence.
 RNA Interference (RNAi)- A technique used to silence
the expression of selected genes. RNAi uses synthetic
double- stranded RNA molecules that match the sequence
of a particular gene to trigger the breakdown of the gene’s
messenger RNA.
 Small Interfering RNAs (siRNAs)- A small, singlestranded RNA molecule generated by cellular machinery
from a long, double-stranded RNA molecule. The siRNA
associates with one or more proteins in a complex that can
degrade or prevent translation of an mRNA with a
complementary sequence. In some cases, siRNA can also
block transcription by promoting chromatin modification.
 Cell Differentiation- The structural and functional
divergence of cells as they become specialized during a
multicellular organism’s development. Cell differentiation
depends on the control of gene expression.
 Cytoplasmic Determinants- A maternal substance, such
as a protein or RNA, placed into an egg that influences the
course of early development by regulating the expression of
genes that affect the development fate of cells.
 Induction- The process in which one group of embryonic
cells influences the development of another, usually by
causing changes in gene expression.
 Determination- The progressive restriction of
developmental potential in which the possible fate of each
cell becomes more limited as an embryo develops.
 Pattern Formation- The development of a multicellular
organism’s spatial organization, the arrangement of organs
and tissues in their characteristic places in threedimensional space.
 Positional Information- Molecular cues that control
pattern formation in an animal or plant embryonic
structure by indicating a cell’s location relative to the
organism’s body axes. These cues elicit a response by genes
that regulate development
 Homeotic Genes- Any of the master regulatory genes
that control placement and spatial organization of the
body parts in animals, plants, and fungi by controlling
the developmental fate of groups of cells.
 Embryonic Lethals- A mutation with a phenotype
leading to death of an embryos.
 Maternal Effect Gene- a gene that, when mutant in
the mother, results in a mutant phenotype in the
offspring, regardless of the offspring’s genotype.
 Egg-Polarity Genes A gene that helps control the
orientation of the egg.
 Bicoid- A maternal effect gene that codes for a protein
responsible for specifying the anterior end in Drosophila.
 Morphogens- A substance, such as Bicoid protein in
Drosophila, that provides positional information in the
form of a concentration gradient along an ambryonic axis.
 Oncogenes- A gene found in viral or cellular genomes that
is involved in triggering molecular events that can lead to
cancer.
 Proto-Oncogenes- A normal cellular gene that has the
potential to become an oncogene.
 Tumor-Suppressor Genes- A gene whose protein
product inhibits cell division, thereby preventing the
uncontrolled cell growth that contributes to cancer.
 Ras Gene- A gene that codes for Ras ultimately
resulting in stimulation of the cell cycle.
 P53 Gene- A tumor- suppressor gene that codes for a
specific transcription factor that promotes the
synthesis of cell cycle- inhibiting proteins.
Chapter 18 Graphic
 The diagram shows a simplified overview of gene
structure and expression. A protein-coding gene is
defined by the extent of the primary transcript. The
gene is first transcribed to yield a primary transcript,
which is processed to remove the introns. The mature
transcript (mRNA) is then translated into a sequence
of amino acids, which defines the protein.
The Discovery of Viruses
 Researchers discovered viruses in the late 1800s by
studying a plant disease
Structures of Viruses
 A virus is a small nucleic acid genome enclosed in a
protein capsid and sometimes a membranous envelope
containing viral proteins that help viruses enter cells
 A capsid is the protein shell that encloses a viral
genome. It may be rod- shaped, ployhedral, or more
complex in shape
General Features of Viral
Reproductive Cycles
 Viruses use enzymes, ribosomes, and small molecules
of host cells to synthesize progeny viruses. Each type
of virus has a characteristic host range
Reproductive Cycles of Phages
 Phages can reproduce by two alternative mechanisms
 Lytic cycle




Virulent or temperate phage
Destruction of host DNA
Production of new phages
Lysis of host cell causes release of progeny phages
 Lysogenic cycle


Temperate phage only
Genome integrates into bacterial chromosome as prophage,
which is replicated and passed on to daughter cells and can be
induced to leave the chromosome and initiate a lytic cycle
Reproductive Cycle of Animal
Viruses
 Many animal viruses have an envelope. Retroviruses
use the enzyme reverse transcriptase to copy their
RNA.
 The Viral Envelope is membrane that cloaks the capsid
that in turn encloses a viral genome
Evolution of Viruses
 Since viruses can reproduce only within cells, they
probably evolved after the first cells appeared. Perhaps
as packaged fragments of cellular nucleic acid. The
origin of viruses is still being debated
Viral Diseases in Animals
 Symptoms may be caused by direct viral harm to cells
or by the body’s immune response. Vaccines stimulate
the immune system to defend the host against specific
viruses
Emerging Viruses
 Outbreak of new viral diseases in humans is usually
caused by existing viruses that expand their host
territory
Viral Disease in Plants
 Viruses enter plant cells through damaged cell walls or
are inherited by the parents
 Bacteriophages (phages)- A virus that infects
bacteria; also called a phages
 Host range- The limited range of host cells that each
type of virus can infect.
 Virulent phage- A phage that reproduces only by a
lytic cycle
 Restriction enzymes- An endonuclease that
recognizes and cuts DNA molecules foreign to a
bacterium. The enzyme cuts at specific nucleotide
sequences.
 Temperate phages- A phage that is capable of
reproducing by either a lytic or lysogenic cycle.
 Lysogenic cycle- A type of phage reproductive cycle in
which the viral genome becomes incorporated into the
bacterial host chromosome as a prophage and does not
kill the host.
 Prophage- A phage genome that has been inserted
into a specific site on a bacterial chromosome.
 Retroviruses- An RNA virus that reproduces by certain
viruses that uses RNA as a template for DNA synthesis.
 Reverse transcriptase- An enzyme encoded by certain
viruses that uses RNA as a template for DNA synthesis.
 HIV- The infectious agent that causes Aids. HIV is a
retrovirus.
 AIDS- The symptoms and signs present during the late
stages of HIV infection, defined by a specified reduction in
the determines which competitor gains access to a
resource, such as food or mates.
 Provirus- A viral genome that is permanently inserted
into the host genome.
 Epidemic- A general outbreak of a disease
 Pandemic- A global epidemic.
 Viroids- Plant pathogen consisting of a molecule of
naked, circular RNA a few hundred nucleotides long.
 Prions- An infectious agent that is a misfolded version
of a normal cellular protein. Appear to increase in
number by converting correctly folded versions of the
protein to more prions.
Chapter 19 graphic
 In the lytic cycle, the phage attaches to the host cell and injects its
DNA. The host cell's enzymes and synthesis organelles make copies of
the viral DNA and viral proteins. The viral proteins and nucleic acids
then assemble themselves inside the host cell, making many copies of
the original infecting virus. The host cell then bursts open, releasing
hundreds of new viruses. These offspring infect new host cells and
repeat the cycle.

In the lysogenic cycle, a virus injects its genes into the host. The
viral DNA then adds itself directly to the host cell's DNA. Each time the
host cell reproduces, the viral DNA is copied along with the host's
DNA. Occasionally, the viral DNA separates from the host DNA and
starts a lytic cycle. New phages are then made and released.
Genetics of Plants Lab
 This laboratory involves breeding Wisconsin Fast Plants
(Brassica rapa) to apply the principles of genetics and
heredity you have learned. By growing and pollinating the
plants to produce offspring, you be able to determine what
kind of inheritance patterns certain genes displayed.
 Genetic Ratios:
 Monohybrid Cross: 3:1
 Dihybrid Cross: 9:3:3:1
 Epistasis: 9:4:3
 Linked Genes: 1:1
 Linked Genes, with some Crossover: 4:4:1:1
Genetics of Plants Lab
 For the AP exam, you must know how to do chi-square
analysis to evaluate the results of your genetic crosses.
A chi-square test is used to determine if your results
conform to the expected Mendelian frequencies.
 If your observed frequencies do not match your
expected frequencies, some nonrandom mating or
even crossover may be occurring.
 Formula for Chi-Square Test:
1. What does Mendel use for
his heredity experiments?
A. Lavender
B. Lilies
C. Pea plants
D. Mice
E. Cats
2. What were the laws that
Mendel created?
A. Law of Gravity and Law of Segregation
B. Law of Segregation and Law of Independent
Assortment
C. Law of Gravity and Law of Independent Assortment
D. Law of Heredity and Law of Genetics
3.When using a punnett square, there are letters
used to tell the genotype of the cross, what is the
combination that shows homozygous recessive?
A. PP
B. pp
C. pPp
D. Pp
E. pP
4. Some diseases cause multiple
symptoms, what is this called?
A. Genetics
B. Science
C. Hunger
D. Pleiotrophy
5. What is the best thing you can do to
be sure if your child has a chance of
having a genetic disorder?
A. Try and the find out
B. Get genetic counciling
C. Do not do anything
D. Ask a friend
6. Which is not a genetic
disorder?
A. Cystic fibrosis
B. The flu
C. Sickle-cell disease
D. Huntington’s disease
7. What does the heterozygous
genotype look like?
A. pp
B. PP
C. Pp
D. None of these
8. What is a dihybrid?
A. An individual that is heterozygous for two characters
B. The phenotype
C. A bird that dies
9. What did Morgan use in his
experiments?
A. Mice
B. Pea plants
C. Birds
D. Fruit flies
10. What chromosome(s) is considered
part of the sex-linked gene(s)
A. 12 and 22
B. X and 15
C. X and Y
D. 9 and 4
11. What is not a sex-linked
gene disorder?
A. Muscular dystrophy
B. The common cold
C. Hemophilia
D. All of the above
12. What is it called when the members of a
pair of homologous chromosomes do not
move apart properly during meiosis I?
A. Aneuploidy
B. Trisomic
C. Polyploidy
D. Nondisjunction
13. What causes Down
Syndrome?
A. An extra chromosome
B. Translocation
C. Birth
D. None of the above
14. Errors in meiosis or damaging
agents such as radiation can cause
___________ of a chromosome?
A. Creation
B. Entry
C. Breakage
D. Nothing
E. Lifting
15. Where are most imprinted
genes located at?
A. The brain
B. Chromosomes
C. Autosomes
D. In the ear
E. In the heart
16. _________ of the eukaryotic cell's
genes are located on nuclear
chromosomes. Or even in the nucleus.
A. All
B. None
C. 3
D. 10
E. Not all
17. Who proposed that DNA was
in the shape of a double helix
A. Watson and Crick
B. Mendel
C. Morgan
D. Franklin
18. Who showed that genes are
located along chromosomes?
A. Mendel
B. Crick
C. Watson
D. Franklin
E. Morgan
19. Griffith discovered what
phenomenon?
A. Gravity
B. Mice
C. Transformation
D. Heredity
E. Genes
20. How do viruses reproduce?
A. They just do
B. They take over a cell
C. Replicate like DNA
D. They don’t
21. What led to Watson and Crick’s
discovery that DNA is a double helix?
A. Viruses
B. Mendel
C. An x-ray
D. Mice
22. Who created the x-ray
crystallography of DNA
A. Mendel
B. Crick
C. Watson
D. Franklin
E. Morgan
23. In DNA there are 4 nitrogenous bases,
adenine (A), thymine (T), guanine (G), and
cytosine ( C), what pairs up with what?
A. A-C and T-G
B. C-T and A-G
C. A-T and C-G
D. A-U and G-C
24. Meselson and Stahl discovered
that DNA replication follows what
model?
A. Conservative
B. Semi-conservative
C. Dispersive model
D. M and S model
25. The special sites where DNA
replication begins are called_________.
A. Starting sites
B. Origins of replication
C. Replication
D. DNA site
26. What unwinds the parental
double helix at replication forks?
A. Primase
B. DNA ligase
C. Helicase
D. Topisomerase
E. Single-strand binding protein
27. ________ inherited by an organism leads to specific
traits by dictating the synthesis of proteins and RNA
molecules involved in protein synthesis.
A. DNA
B. RNA
C. Ligase
D. Genes
E. Cells
28. Who demonstrated the relationship
between genes and enzymes?
A. Beadle and Tatum
B. Mendel
C. Garrod
D. Weaver
E. Watson and Crick
29. _________ proteins are
enzymes.
A. None
B. Not all
C. All
D. None of the above
30. What type of RNA carries a genetic
message from the DNA to proteinsynthesizing machinery?
A. tRNA
B. rRNA
C. RNA
D. mRNA
E. DNA
31. What is an example of a
triplet code?
A. LMN
B. 123
C. KDES
D. F4D
E. AGT
32. When translating mRNA codons,
what is the starting codon?
A. GUG
B. ACA
C. AUG
D. GUU
E. CUG
33. In what direction can RNA
polymerase assemble a polynucleotide?
A. 3’ to 5’
B. 6’ to 2’
C. 5’ to 3’
D. Up
E. Right
34. What are the RNA molecules
that function as enzymes?
A. Ribosomes
B. Proteins
C. DNA
D. tRNA
E. rRNa
35. What is considered the
ultimate source of new genes?
A. DNA
B. RNA
C. Proteins
D. Mutations
E. None of the above
36. Which component is NOT
directly involved in translation?
A. mRNA
B. Ribosomes
C. DNA
D. GTP
E. tRNA
37. The operon can be switched
off by a protein called what?
A. Enzymes
B. RNA
C. DNA
D. Operator
E. Repressor
38. Inducible enzymes synthesis
induced by __________.
A. Natural signal
B. Chemical signal
C. RNA
D. Proteins
39. What hypothesis proposes that specific
combinations of modifications help
determine the chromatin configuration?
A. Epigenetic inheritance
B. Genomic imprinting
C. Histone acetylation
D. None of the above
40. To initiate transcription, eukaryotic
RNA polymerase requires the assistance
of what proteins?
A. Histones
B. Transcription factors
C. Enhancers
D. Inducers
41. To initiate transcription, eukaryotic
RNA polymerase requires the assistance
of what proteins?
A. Chemical signals
B. Viruses
C. mRNA
D. Histones
E. Inducers
42. Regulation by noncoding RNAs is
known to occur at what two points in
the pathway of gene expression?
A. mRNA translation
B. Chromation configuration
C. tRNA translation
D. Both A and B
E. Both B and C
43. The zygote gives rise to a large
number of cells through a succession of
what?
A. Mitosis
B. Transcription
C. Morphogenesis
D. Mitotic cell division
44. One important source of information in early
development is the egg's cytoplasm because it
contains both RNA and proteins encoded with
the mother's ____________.
A. RNA
B. DNA
C. Cells
D. miRNA
E. Ribosomes
45. Pattern formation in animals
begins in the ________ stage.
A. Fetus
B. Adult
C. Early embryo
D. Early child
E. Mid-life
46. ____________ contains double- stranded
DNA, single-stranded DNA, double-stranded RNA,
or single-stranded RNA.
A. Bacteria
B. Viruses
C. Histones
D. Prokaryotes
47. Bacteriophages have the most
complex capsids and infec what?
A. Bacteria
B. Liver
C. Other viruses
D. DNA
E. RNA
48. A phage reproductive cycle that
culminates in death of the host cell
is known as the what?
A. Reproductive cycle
B. Virus cycle
C. Host-death cycle
D. Life cycle
E. Lytic cycle
49. What are the RNA animal
viruses with the most complicated
reproductive cycle?
A. Bacteriophages
B. Tobacco mosaic virus
C. Retrovirus
D. Influenza viruses
50. A vaccine is a __________ variant or derivative of a
pathogen that stimulates the immune system to mount
defenses against the harmful pathogen.
A. All harmful
B. All harmless
C. Can be a bit harmful
D. Pleasant
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
C
B
B
D
B
B
C
A
D
C
11. B
12. D
13. A
14. C
15. C
16. E
17. A
18.E
19. C
20. B
21. C
22. D
23. C
24. B
25. B
26. C
27. A
28. A
29. B
30. D
31. E
32. C
33. C
34. A
35. D
36. C
37. E
38. B
39. C
40. B
41. A
42. D
43. D
44. B
45. C
46. B
47. A
48. E
49. C
50. B
Free Response #1
1.
In Order to perform DNA replication and Repair, many proteins
must work together
A. Explain the function of each of the following proteins in DNA
replication





B.
C.
Helicase
Primase
DNA Polymerase I
DNA Ligase
Single-strand Binding Protein
Nuclear exccision repair is a type of DNA repair system. List and
explain the steps that the cell must perform to complete this
repairing system.
When replicating the ends of DNA molecules, telomeres play a
large role. Discuss the importance of telomeres in DNA
replication.
Free Response #2
2. In humans, hemophilia is caused by an X-linked
recessive gene that prevents blood from correctly
clotting. Suppose a normal man reproduces with a
female whose father was a hemophiliac. With this
information, answer the following questions (draw
punnett squares to support your answer) questions on
next slide.
Free Response #2 (cntd.)
A. What are the possible genotypes of the female’s
mother
B. What are the chances that the male and female’s first
child will be a hemophiliac male?
C. Of the girls produced, what percentage will be
carriers?
D. If the couple has two sons and two daughters, what
are the chances that both sons will be hemophiliacs
and both daughters will be carriers
Free Response Answer #1
1. A.)
In DNA replication there are many different and important
proteins that play a role in the process. Helicase is an enzyme that
untwists the double helix of DNA at the replication forks, separating
them and making them available to be template strands. Primase is an
enzyme that joins RNA nucleotides to make the primer using the parental
DNA strand as a template. The DNA polymerase I is an enzyme that
catalyzes elongation of new DNA by adding nucleotides to the 3’ end of
an existing chain. DNA ligase is a linking enzyme that catalyzes the
covalent bonding if the 3’ end on one DNA fragment to the 5’ end of
another DNA fragment. The single-strand binding protein is a protein
that binds to the unpaired DNA stands during DNA replication
stabilizing them and holding them apart while they serve as templates for
the synthesis of complementary strands of DNA.
Free Response Answer #1 (cntd.)
B.)
1. A thymine dimmer distorts the DNA molecule.
2. A nuclease enzyme cuts the damaged DNA strand at
two points and the damaged section is removed
3. Repair synthesis by a DNA polymerase fills in the
missing nucleotides
4. DNA ligase seals the free end of the new DNA to the
old DNA, making the strand complete
C.)
The telomere keeps an organism’s genes from being
eroded while it goes through successive rounds of replication.
Free Response Answer #2
2.A.) The female could either have a mother who was
heterozygous for the trait of hemophilia or she could have a
mother who was homozygous dominant. You can determine
this because a female who are homozygous recessive for the
trait of hemophilia won’t survive to give birth.
Xh
XH Xh
Y
XH Y
Xh Xh Xh
Xh Y
B.)
XH
Because the female is a carrier for the trait there’s a 1 in 4
chance of having a male hemophiliac as a first child.
Free Response Answer #2 (cntd.)
C.) Of the two girls that could potentially be
produced one would be a carrier, 25% of the total
possibilities 50% of the possible females. The other
female would be a hemophiliac and, therefore, wouldn’t
survive past puberty.
D.) Because there’s a 25% chance the couple could
have a son with hemophilia and a 25% chance that they
have a daughter who’s a carrier of the trait. The chances
that they have two sons who are hemophiliacs and two
daughters who carry the trait is 1 in 256 or .4%.