Unit 3 – Genetic Continuity
Crosses with answers- http://www.gonzaga.k12.nf.ca/academics/science/biology/3201/genetics/genetics.html
Mendelian Genetics (patterns of inheritance)
Genetics is the study of patterns of inheritance. In humans, physical characteristics of a person (phenotype)
are due to heredity (genotype) and environmental influences.
Mendel studied patterns of inheritance using pea plants. He had two main laws aside from his Law of
Dominance.
 Law of Segregation – alleles split
during gamete formation (proved by
monohybrid cross 3:1 ratio)

Law of Independent Assortment alleles for different traits split
independent of each other during
gamete formation (proved by his
dihybrid cross 9:3:3:1 ratio)
In earlier times, scientists thought that inherited traits were "blended" in the offspring. Mendel discovered
traits were passed on for each characteristic with one copy of that trait coming from each parent. For many
characteristics there was more than one version of a trait (we now call these alleles). For each inherited
characteristic (gene) an organism has two alleles one inherited from each parent.
We now know more than Mendel did. Meiosis is the two-stage cell division use to produce monoploid
gametes (eg: sperm and egg) and the allele pairs are separated from each other (this is the real reason for
segregation). Fertilization reunites allele pairs to make the gene complete again (the fertilized egg would
inherit one allele from the father and one allele from the mother to restore the paired condition).
As Mendel stated Alleles are usually DOMINANT or RECESSIVE. A dominant allele is fully expressed
when present. A recessive allele is fully masked and is only expressed in homozygous form. Mendel was
able to come to this conclusion through performing monohybrid crosses (looking at a single trait and
following its expression through F1 and F2 generations). You should practice your crosses. Also use your
memory tricks.
Do some crosses: http://www.gonzaga.k12.nf.ca/academics/science/biology/3201/genetics/genetics.html
Terms

diploid cell: A somatic cell in human. In humans we have 23 maternal and 23 paternal chromsomes
giving us a diploid number of 46. (2n=46)

monoploid cell: Sex cell in humans. Also called haploid as these cell have only half the normal number
of chromosomes (like sperms and eggs). A monoploid human egg or sperm has 23 chromosomes.
(1n=23)
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gametes: Sex cells, either eggs or sperm, these cells are always monoploid.

Homozygous means having two identical alleles of a gene. TT or tt

Heterozygous is when you have two different alleles of a gene. Tt

A carrier is an individual that has a recessive allele of a gene that does not have an effect on their
phenotype.

A test cross is testing an unknown dominant individual by crossing it with a known homozygous
recessive. If ANY recessive offspring result the unknown was heterozygous.
The genotype is the genes possessed by an organism. The phenotype is the physical characteristics of an
organism. A dominant allele is an allele that has the same effect on the phenotype whether it is present in the
homozygous or heterozygous state (upper case letters). A recessive allele is an allele that only has an effect
on the phenotype when present in the homozygous state (lower case letters).
Incomplete dominant alleles: Alleles that are simultaneously expressed in an altered form – snap dragons
hybrids are pink not white or red. (RR – red, WW – white and RW – pink)
Codominant alleles are pairs of alleles that both affect the phenotype when present in the heterozygous
state. Alleles that are simultaneously fully expressed in the heterozygous condition (AB blood type). These
a a a o
can be symbolized by upper case letters. We use these symbols I I I I for type A blood. The ABO blood
a
groups are an example of multiple alleles of a single gene because this gene exists in three allelic forms: I
b
I
o
a
b
,
o
and I . The I and I is dominant to the I allele. Type O will only be expressed in the homozygous form;
when combined with A or B alleles it will not be expressed. For example, a person with both the A and B
alleles, carries AB type blood. Both blood group A and B are fully expressed. Some genes have more than
two alleles (multiple alleles). Blood groups are the best examples of multiple alleles and co-dominance –
know how to do these crosses!
The test cross is a way to set up a cross to figure out an unknown genotype. And the only unknown genotype
is if a phenotype is dominant. Basically, if someone expresses the dominant phenotype, you often will not
know whether that person's genotype is homozygous dominant or heterozygous. To do the test cross, you
cross the individual with the dominant phenotype (and unknown genotype) with a homozygous recessive
individual. If any of the offspring appear with the recessive phenotype, you know that your plant with the
dominant phenotype that you crossed is actually heterozygous.
Genes are on Chromosomes! (Sutton suggested this theory)
The chromosome theory states that chromosomes are linear sequences of genes. The unifying theory states
that inheritance patterns may be generally explained by assuming that genes are located in specific sites on
chromosomes (recall that each gene makes a specific protein). Sutton was the first to point out that
chromosomes obey Mendel's rules—the first clear argument for the chromosome theory of heredity. Sutton
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worked with grasshopper chromosomes, and it was in this paper that he showed that chromosomes occur in
distinct pairs, which segregate at meiosis.
Linked genes are on the same chromosome – they do not follow the Law of Independent Assortment. The
discovery that genes were on chromosomes made the Law of Independent Assortment invalid for genes on
the same chromosome. Refers to meiosis – tetrad separation is the reason for independent assortment – but it
only works if the genes that are assorting are on different chromosomes
Sex in humans is determined by two chromosomes, called X and Y (X is bigger than Y in karyotypes). All
males (♂) have one X chromosome and one Y chromosome. Females (♀) have two X chromosomes. In
meiosis, therefore, females can only produce gametes with an X chromosome, while males can produce
gametes with either an X or a Y chromosome. The male's gametes, then, are those that decide gender: the
child can have XX (female) or XY (male) chromosomes depending on what it receives from its father. This
is another example of segregation.
Color-blindness and hemophilia are probably the most common examples of sex-linked traits in humans.
Both are due to a recessive sex-linked allele on the X chromosome. They are more common in males than
females. Obviously a recessive X-linked gene will only be expressed in the homozygous form, as this is part
of the definition of recessive genes. Therefore, if an X-linked recessive alleles is present in a male, it will
always be expressed, as this is the only X gene the male possesses. However, females have two X genes,
only one of which is actually expressed. The other is bound up in an inactive structure known as a Barr body.
Therefore if the X chromosome is the one bound in the Barr body, its recessive alleles are not expressed, and
the female may be a carrier without displaying any effects. Morgan suggests sex linkage in fruit fly eye
colour – cross.
Polygenic traits have two or more genes. As opposed to monogenic such as blood type – you have a distinct
blood type. Skin color and height are polygenic. It is by determined by a number of genes (so we have a
range of height in humans not tall and short people depending on how many alleles you have for “tallness”).
You must be able to analyze and interpret models of human karyotypes (remember the lab on Karyotypes). If
you can’t perform crosses for dominant, sexlinked, codominant and incomplete dominant traits you will have
great difficulty with the remainder of this course!
Molecular Genetics (the DNA molecule)
Levene analyzed the components of the DNA molecule. He found it contained four nitrogenous bases:
cytosine, thymine, adenine, and guanine; deoxyribose sugar; and a phosphate group.
Frederick Griffith was able to induce a non-pathogenic (harmless) strain of the bacterium Streptococcus
pneumoniae to become pathogenic (deadly). Griffith referred to a transforming factor (we know it as DNA)
that caused the non-pathogenic bacteria to become pathogenic.
Oswald Avery, Colin MacLeod, and Maclyn McCarty revisited Griffith's experiment and concluded the
transforming factor was DNA. Their evidence was strong but not totally conclusive. The then-current
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favourite for the hereditary material was protein; DNA was not considered by many scientists to be a strong
candidate.
Hershey and Chase sought an answer to the question, “Is it the viral DNA or viral protein coat (capsid) that is
the viral genetic code material which gets injected into a host bacterium cell? With the blender experiment
they conducted show that DNA was the molecule of heredity.
Watson and Crick gathered all available data in an attempt to develop a model of DNA structure. Franklin
took X-ray diffraction photomicrographs of crystalline DNA extract, the key to the puzzle. The data known
at the time was that DNA was a long molecule, proteins were helically coiled (as determined by the work of
Linus Pauling), Chargaff's base data (A to T and G to C ratios), and the x-ray diffraction data of Franklin
and Wilkins.
Barbara McClintock was the first scientist to predict that transposable elements, mobile pieces of the genetic
material (DNA), were present in eukaryotic genomes.
The sides of the ladder of DNA consist of alternating phosphate groups and deoxyribose
(a sugar). The two sides are antiparrallel, meaning that the sugar and phosphates are
running in opposite directions (anti parallel). DNA is the primary molecule of heredity
and controls the production of proteins in all organisms. RNA is a molecule that assists
DNA conduct its primary function – making proteins or polypepetides.
DNA replication: There are 6 steps and several molecules involved.
 Helicase – opens DNA strands for replication
 RNA primer – a starting point for DNA growth (initiation)
 DNA polymerase – grows a new DNA strand by adding nucleotide to the open DNA molecule
(elongation)
 DNA ligase – joins growing the growing strands
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New strands of DNA have to be laid in the 5'---3' direction, DNA polymerase can lay continuously on one
strand but not on the other strand. Instead, RNA primase lays short segments of RNA primer nucleotides at
many points along the strand. When one segment of primer comes in contact with another, DNA polymerase
I attaches and replaces the primer with DNA. These segments of DNA are called Okazaki fragments. Once
these fragments have been laid, they are joined by yet another enzyme known as DNA ligase, which attaches
DNA into the gaps between fragments and completes the new strand. The 3'--->5' strand with Okazaki
fragments is called the lagging strand, while the leading strand is the continuously replicating one.
Protein Synthesis (Transcription – Translation)
Transcription (is……
- Initiation - RNA polymerase attaches to DNA and
separates the two strands of the DNA
– Elongation - RNA polymerase adds RNA nucleotides to
the DNA template.
- Termination - Transcription ends when RNA polymerase
reaches a termination site on the DNA. When it reaches the
terminator, the RNA polymerase releases the RNA strand.
Initiation – Elongation - Termination
Translation (is ……
Initiation: once the RNA reaches the
cytoplasm, it attaches the ribosome. AUG is
called the start codon (a codon is three
bases on the mRNA) because it initiates the
translation process. A tRNA with its
anticodon attaches to the mRNA and start
building the aminoacid chain with
Methionine. The ribosome has two tRNA
binding sites and tRNA molecules will
move into these sites attach to the mRNA
"conveyor belt".
Elongation - another tRNA carrying an
amino acid attaches itself to the next codon.
The two amino acids (methionin and the other) bond with each other. The initiator tRNA breaks off and
tRNA, then another tRNA molecule attaches to the codon. The amino acid that is attached to tRNA makes
another peptide bond with the amino acid ……and so on and so on…….
The mRNA keeps moving through the ribosome with tRNAs coming in and adding new amino acids –
elongating the chain. Codons from the codon table can be used to determine the amino acid in the sequence
(page 590)
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Termination
The stop codon is one that does not code for an amino acid and that terminates the translation process. The
polypeptide is released and is a protein.
Gene expression
There are hormonal and environmental factors on gene expression. The example of sex influenced and sex
limited genes – balding is dominant in males and recessive in females, while bearding is limited to females.
The environment can have a dramatic effect on gene expression. Use the example of twins – clones but
slightly different in all cases due to slight differences in the environment. Also rabbits in the winter vs the
summer; the environment influences the expression of hair colour for camouflage.
Mutations - A mutation to genetic material is usually not beneficial. Mutagens are things that cause
mutations, they include: 1. Physical Mutagens (radiation), 2. Toxic Chemicals (pesticides, etc).
There is a difference between somatic and germ mutations. Some people may have mutations in their skin
cells or hair. Such mutations are termed somatic or are found in body cells. Germ mutations occur only in the
sex cells. These mutations are more threatening because they can be passed to offspring (involved in every
cell of the offspring and effecting protein synthesis). Transcription of mutated DNA will produce faulty
mRNA leading to the production of a bad protein.
Meiosis is a prime time for mutations to occur.
The germ mutations that occur during meiosis
could be passed on during a fertilization.
Chromosomal mutations involve whole or a
part of a chromosome and gene mutations
involve changes to the bases in the DNA of
one gene. Chromosomal mutations involve
many genes and thus will affect many proteins.
Nondisjunction occurs during meiosis; tetrads
may not segregate or in meiosis II, sister
chromatids may stick together.
Human genetic disorders caused by non disjunction mutations – syndromes
Down syndrome
Turner syndrome
Klinefelter Syndrome
Jacob’s Syndrome
Both sexes
Females (♀)
males (♂)
males (♂)
3 - 21st chromosomes
X chromosome only (no genitals, stocky
XXY chromosomes (male with female characteristics)
XYY male with speech and hearing problems
Gene Mutations changes to the DNA molecule bases



Point mutation - when a base is replaced with a different base.
Insertion - when a base is added
Deletion - the loss of a base
A frame shift mutation results from a base deletion or insertion. Each of these changes the triplets that follow
the mutation. Frame shift mutations have greater effects than a point mutation because they involve more
triplets (recall how important triplets are to protein synthesis). All the triplets are changed, this in turn
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changes the amino acids of the protein! Protein shape determines how a protein will function. A change in
one amino acid may change the shape enough to distort the protein (as in sickle cell disease). Thus, change in
one base could potentially distort a whole protein.



Silent Mutation – when a mutation has no effect on the individual
Mis-sense Mutation – when a mutation changes a protein – sickle cell
Non-sense Mutation – when a mutation codons to starts or stops and prevents the protein productionl
Genetics Diseases
Tay-Sachs is an inherited disorder caused by the absence of a vital enzyme, resulting in the destruction of the
nervous system. It is always fatal; to date there is no cure. A Tay-Sachs baby develops normally for the first
few months. Then a relentless deterioration of mental and physical abilities begins.
Phenylketonuria (PKU) is the absence or deficiency of an enzyme that is responsible for processing the
essential amino acid phenylalanine characterizes PKU. When the phenylalanine hydroxylase enzyme is
absent or deficient, phenylalanine abnormally accumulates in the blood and is toxic to brain tissue. The cure
is a diet free of phenylalanine.
Sickle Cell Disease is an autosomal recessive disease caused by a point mutation in the hemoglobin beta
gene (HBB) . A mutation in HBB results in the production of a structurally abnormal haemoglobin. Under
certain conditions the RBCs distort into sickled shapes. These deformed and rigid RBCs become trapped
within small blood vessels and block them, producing pain and eventually damaging organs.
Progeria - Accelerated aging syndrome in which most of the characteristic stages of human senescence are
compressed into less than a decade. Defect probably in DNA repair. Not sure if it is dominant
Huntington's disease is caused by a dominant gene and leads to damage of the nerve cells in areas of the
brain, including the the basal ganglia and cerebral cortex. Each person whose parent has Huntington's disease
is born with a 50-50 chance of inheriting the faulty gene. Anyone who inherits the faulty gene will, at some
stage, develop the disease (usually after age 30).
Genetics – Technology
The use of bacteria in gene technology is well documented. Most of its DNA is in one circular chromosome
but it also has plasmids (smaller circles of DNA helix). These plasmids can be removed and cleaved by
restriction enzymes at target sequences. Restriction enzymes cut DNA only at specific sequences, allowing
two different DNA strands to be cut with the same restriction enzyme and reattached. DNA fragments from
another organism are then cleaved by the same restriction enzyme as described previously and these pieces
can be added to the open plasmid and spliced together – making Recombinant DNA. The recombinant
plasmids formed can be inserted into new host cells, typically a bacterium due to their rapid reproduction
rate, and copied by the host.
Describe the various methods of detecting genetic disorders.
Amniocentesis
CVS
fetoscopy
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
Genetic screening is the testing of an individual for the presence or absence of a gene.
Genetic markers are a known piece of DNA that may lie near a gene of interest – like the gene for a
disease
Gene probe is a fragment of DNA used to detect in DNA or RNA samples the presence of nucleotide
sequences that are complementary to the sequence in the probe – such as those for genetic disorders.
Gene therapy involves replacement of defective genes. One method involves the removal of white blood
cells or bone marrow cells and, by means of a vector such as a virus, bacteria, the introduction and
insertion of the normal gene into the chromosome. The cells are replaced in the patient so that the normal
gene can be expressed.
PCR (polymerase chain reaction) copies and amplifies minute quantities of DNA. The DNA is heated
and cooled, primers are added and polymerase grows DNA, this cycle is repeated making many copies of
DNA. The DNA sample can be used in a gel electrophoresis to identify the DNA.
Gel electrophoresis involves the separation of fragmented pieces of DNA according to their charge and size.
Restriction enzymes must be used to cut the DNA – these enzymes will cut DNA at specific sites unique to
the DNA making unique sized fragments. Cutting with many enzymes will make many unique fragments –
like a fingerprint. See the above gel – who has the victim’s blood on their clothes, suspect 1, 2 or 3?
Why?
What caused the fragments?
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The Human Genome Project is an international cooperative venture established to sequence the complete
human genome, it should lead to an understanding of many genetic diseases, the development of genome
libraries and the production of gene probes to detect sufferers and carriers of genetic diseases (eg Duchenne
muscular dystrophy – sex linked don’t forget!). It may also lead to production of pharmaceuticals based on
DNA sequences. The project found 20 500 genes, fewer than they expected because we make 100 000
proteins. (that put and end to the one gene – one protein idea!)
What is a GMO? A genetically modified organism (GMO) is a plant, animal or microorganism whose
genetic code has been altered, subtracted, or added (either from the same species or a different species) in
order to give it characteristics that it does not have naturally.
Scientists can now transfer genes between species that otherwise would be incapable of mating, for example,
a goat and a spider. Some see GMOs as the way to the future, others believe that scientists have gone too far,
tinkering with the essence of life. Examples include salt tolerance in tomato plants, which allow them to
grow in overly irrigated farmlands, delayed ripening in tomatoes, herbicide resistance in crop plant, factor IX
(human blood clotting) in sheep milk. Agriculture - Agricultural products with a higher yield; insect and
herbicide resistant. Fruits and vegetables that grow in dry environments and are cold resistant.
Some gene transfers are regarded as potentially harmful. A possible problem exists with the release of
genetically engineered organisms in the environment. These can spread and compete with the naturally
occurring varieties. Some of the engineered genes could also cross species barriers, and many genetically
modified organisms display surprising and unforeseen side effects due to their modification. An excellent
example of this is a corn variety modified to be more resistant to several types of disease. While the plant did
indeed become more resistant, in the process the modification had affected the chemical composition of their
pollen coat. The pollen was now toxic to the Monarch butterfly, and thousands of them died during their
migration through the Midwest, where the corn was planted. The result of all this could be massive
disruption of the ecosystem. Benefits include more specific (less random) breeding than with traditional
methods.
Clone - a group of genetically identical organisms or a group of cells artificially derived from a single parent
cell. How to Clone the safe method! 1 .The 8-cell stage embryo resulting from invitro fertilization is
divided into separate cells. 2. Each cell is grown into an embryo again and then transferred to surrogate
mothers such as cattle and sheep. 3. The process can be repeated many times to produce a line of offspring
that are all genetically identical, they are clones of the original embryo. Dolly the sheep was cloned from
udder cells and the nucleus was electrically stimulated into cleavage. This was then placed into a surrogate
mother sheep. Dolly aged quickly and died early so this method of cloning is far from perfect.
Stem Cells - Similarities and differences between embryonic and adult stem cells (from Unit 2)
Human embryonic and adult stem cells each have advantages and disadvantages regarding potential use for
cell-based regenerative therapies. Of course, adult and embryonic stem cells differ in the number and type of
differentiated cells types they can become. Embryonic stem cells can become all cell types of the body
because they are pluripotent. Adult stem cells are generally limited to differentiating into different cell types
of their tissue of origin. However, some evidence suggests that adult stem cell plasticity may exist,
increasing the number of cell types a given adult stem cell can become.
Large numbers of embryonic stem cells can be relatively easily grown in culture, while adult stem cells are
rare in mature tissues and methods for expanding their numbers in cell culture have not yet been worked out.
This is an important distinction, as large numbers of cells are needed for stem cell replacement therapies.
STSE – Genetics research in Newfoundland and Labrador – we have many companies conducting research
in our province – looking for useful genes such as the flounder anti-freeze gene. Human populations in our
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province were very small an isolated at one time, making rare genetic diseases common in some places. The
inheritance of these diseases is being investigated.
Can you use the table on page 590? Try this:
DNA
TAC
GGG
TTT
GGG
▼Transcription▼
CTC
make the message
mRNA
▼Translation ▼use the message
Amino acid chain
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AAA
ATT