Mr. Ramos
Genetics Study Guide
Students, here is a study guide for the Genetics Test. Read it and study it carefully in conjunction with
your class & book notes. The Genetics test has 50 Multiple Choice questions worth 50% of your total
class grade. The test is curved. Also, take advantage of this generous study guide. I will NOT always do
these nice things for you all.
Cell Growth
SC.912.L.16.8
Explain the relationship between mutation, cell cycle, and uncontrolled cell growth potentially resulting in cancer.
Cell Cycle
-Cells grow and divide. This is what we call the cell cycle.
-The cell cycle is divided in two parts called interphase and
mitosis.
-Interphase deals with growth, and mitosis deals with cell
division.
-Interphase is further divided into 3 parts: G1, S, and G2
G1 = cell grows
S = DNA is replicated (aka copied)
G2 = more cell growth & organelle replication
Keep in mind that not all cells grow and divide. Neurons and muscle cells do not grow or divide. We
refer to their special state as G0.
Liver cells are also in G0 just like neurons and muscles, but liver cells have the ability to re-enter the cell
cycle and divide when they are needed.
Check-Points
-Cells undergo check points throughout several stages of the cell
cycle. If the cell does not pass the check-point, it is programmed to
kill itself.
-The cell suicide process is called apoptosis (this kind of suicide is
good…otherwise you get cancer)
-Check-points prevent DNA errors that may have serious consequences on
individuals.
Cancer
-Cancer is uncontrolled cell growth that spreads to other parts of the body (metastasis).
-If the cell’s DNA becomes damaged and changed, we say that there is a DNA mutation.
-These mutations, or genetic mistakes, begin to accumulate. If the problem is not solved, the cells may
become cancerous.
-These mutations may cause cells to grow
uncontrollably, disregarding check-points and the
apoptotic (cell suicide) process.
-As cells continue to grow and divide, they begin to
clump up and form masses of cells called tumors.
-If the cells within the tumor stay in place and do not
spread to other parts of the body, the tumor is called
a benign tumor. Benign tumors are not cancerous.
- If the cells within the tumor spread to other parts of
the body, the tumor is said to be malignant (evil).
Malignant tumors are cancerous.
-Fun fact: a fancy word for “spread” is “metastasis.”
Malignant tumors have the ability to metastasize to
other parts of the body.
Fun fact: watch out for the suffix “-oma,” which is
indicative of a tumor (examples: carcinoma,
melanoma, sarcoma, glioma, etc)
Cancer Treatment
Surgery is used if the tumor is localized and easy to remove (benign
tumors). Sometimes doctors avoid surgery, because there is a risk
that this may cause the cancer cells to spread during the operation.
Radiation therapy uses high energy radiation to kill cancer cells. Sometimes
a laser beam is fired directly at a tumor to kill or shrink cancerous cells.
Chemotherapy involves injecting patients intravenously with chemicals
that target cells in the body that multiply quickly. Since cancer cells
multiply quickly, they will most likely die. Keep in mind that hair cells also
multiply quickly, so they will die too. This is why people’s hair fall off
during chemotherapy.
Mitosis
SC.912.L.16.14
Describe the cell cycle, including the process of mitosis. Explain the role of mitosis in the formation of new cells
and its importance in maintaining chromosome number during asexual reproduction.
SC.912.L.16.15:
Compare and contrast binary fission and mitotic cell division.
Mitosis
Mitosis is nuclear division, by definition; however, most people think of mitosis as cell division.
-The goal of mitosis is to produce two identical daughter cells.
-Mitosis produces diploid cells.
-Mitosis only produces somatic cells (body cells). It does NOT produce gametes (sex cells).
-Mitosis can be divided into 4 phases: prophase, metaphase, anaphase, and telophase.
These stages occur continuously, but we separate them into phases to identify
them during a test and to better explain what is occurring. *Remember PMAT.
In prophase, the DNA prepares itself for the mitotic journey by coiling up into
chromosomes. The centrosomes begin to move to opposite poles of the cells.
*Remember that when you move to a new home, you pack things into boxes so
that nothing gets lost & so it is easier for you to carry. The cells do the same
thing. They pack their messy DNA into organized chromosomes.
In metaphase, the sister chromatids meet in the middle of the cell. The nucleus is
no longer present at this point and the Centrosomes are on opposite ends of the
cell
In anaphase, the sister chromatids are torn apart. They Centrosomes release
spindle fibers that pull the chromatids apart and to opposite poles of the cell.
In telophase, everything comes back together. The nucleus and nucleolus
re-forms, and the chromosomes become loose DNA again. At this point the
two daughter cells are about to split in half.
During Telophase of mitosis, cytokinesis occurs. Cytokinesis is the process
by which the cell’s cytoplasm splits and two identical daughter cells are
formed.
In animal cells, a cleavage furrow is formed during cytokinesis. This
cleavage will eventually cause the two cells to split and become two
independent cells.
In plant cells, a cell plate forms between the two plant cells. This cell plate
eventually becomes the cell wall. Notice how the plant cells are stuck to
each other.
Importance of Mitosis
Mitosis is important because it allows somatic cells to regenerate. Somatic means body, so all cells in
your body are somatic, with the exception of sperm and egg cells, which are called sex cells.
Human somatic cells contain 46 chromosomes. In order to create two identical cells, these 46
chromosomes must duplicate during the S-phase of Interphase to become 92 chromosomes. Then, the
cell can divide in half so that each new cell acquires 46 chromosomes.
46
Chromosome
s
92
Parent Cell
S – phase (Interphase)
Chromosome
s
Parent Cell
46
Mitosis
46
It is important to maintain the chromosome number. If the chromosome number changes, some disease
or abnormality may result. In humans, the chromosome number is 46. Therefore, all somatic cells MUST
have 46 chromosomes.
Binary Fission vs. Mitosis
-Remember that mitosis by definition is nuclear
division. Bacteria, however, do not have nuclei.
Therefore, bacteria do not undergo mitosis.
-Bacteria replicate through a process called
binary fission (literally: to cut in half).
During binary fission, bacteria replicate their
circular DNA, send them to opposite poles of
the cell, and the bacteria cut themselves in half.
-There is no prophase, metaphase, anaphase, or
telophase in binary fission because these
are phases of mitosis.
Meiosis
SC.912.L.16.16
Describe the process of meiosis, including independent assortment and crossing over. Explain how reduction
division results in the formation of haploid gametes or spores.
Meiosis
-Meiosis produces 4 non-identical sex cells known as gametes.
-Meiosis occurs in two divisions, which are called Meiosis I and Meiosis II.
-Meiosis produces haploid cells.
-In males, meiosis only occurs in the testes to produce sperm cells. The sperm cell is the male gamete.
-In females, meiosis only occurs in the ovaries to produce egg cells. The egg cell or ovum is the female
gamete.
*Before we continue into meiosis, it is important to understand some chromosome terminology.
Human Chromosomes are pieces of linear DNA that have been wrapped and condensed to make the
process of cell division occur smoothly without losing any information.
Remember that humans have a total of 46 chromosomes (23 from dad and 23 from mom). At the end of
meiosis, each sperm or egg cell will be left with only 23 chromosomes.
When DNA undergoes the S-phase of Interphase, the 46 human chromosomes make exact copies that
result in 92 chromosomes. Each identical copy of the chromosome
is called a sister chromatid. The sister chromatids will be joined
together like Siamese twins at a center location called the
centromere.
Moreover, for every chromosome you receive from your mom, you
will receive a similar chromosome from your dad. These similar
chromosomes are called homologous chromosomes.
At the end of mitosis or meiosis, the sister chromatids split to become an individual
chromatid or chromosome.
Meiosis I
-Meiosis I is divided into prophase I, metaphase I, anaphase I, and telophase I.
-2 non-identical daughter cells will be produced in meiosis I.
Prophase I is probably the most important and complicated phase of meiosis I. It is
highly tested on exams. In prophase I, a mom chromosome will pair with a dad
chromosome. The pairing only occurs for homologous chromosomes, which are those
chromosomes from mom and dad that are similar (same size, same genes, etc). The
pairing of these homologous chromosomes is called synapsis. Furthermore, because
you can see a total of 4 sister chromatids in this homologous pairing, we call them
tetrads.
Now, the most important thing will take happen. A piece of the mom chromosome will
flip with a piece of the dad chromosome. They exchange genetic information. As a
result, this is one reason why everyone on this planet looks different. Imagine all the
combination of genes that take place during this random crossing over of genetic
information.
In Metaphase I, the homologous chromosomes align themselves randomly in the
middle of the cell, known as the metaphase plate. They will then be randomly
separated during anaphase. This random separation of chromosomes and genes is
known as independent assortment, and it is the second reason why everyone in the
world is different.
*Pay attention to this. In metaphase I, the tetrads align in the middle, while in
metaphase II the sister chromatids align in the middle.
In anaphase I, the homologous chromosomes, or tetrads, are randomly pulled apart to
opposite poles of the cell.
*Pay attention to this. In anaphase I, the tetrads are separated, while in anaphase II the
sister chromatids are separated.
In Telophase I and cytokinesis, two daughter cells are produced. Each daughter
cell has now 46 chromosomes (instead of 92), and they will each undergo a second
division in meiosis II to produce a total of 4 daughter cells with 23 chromosomes
each.
Meiosis II
-Meiosis II is divided into prophase II, metaphase II, anaphase II, and telophase II.
-A total of 4 non-identical cells will be seen at the end of meiosis II.
Prophase II is not as interesting as prophase I. Prophase II is actually quite simple and boring.
The only thing that happens in prophase II is that the nucleus breaks down and the DNA
becomes chromosomes.
In metaphase II, the sister chromatids align in the metaphase plate.
*Remember that in metaphase I the tetrads, or homologous
chromosomes, aligned in the middle.
In anaphase II, the sister chromatids are pulled to opposite poles of the cell.
*Remember that in anaphase I the tetrads, or homologous chromosomes, were pulled apart.
In Telophase II and cytokinesis, these two non-identical daughter cells split to
become 4 non-identical daughter cells with 23 chromosomes each. These are called
sex cells, or gametes.
*Remember that in Telophase I we produced 2 non-identical daughter cells with
46 chromosomes each.
Mitosis vs. Meiosis
SC.912.L.16.17
Compare and contrast mitosis and meiosis and relate to the processes of sexual and asexual reproduction and their
consequences for genetic variation.
Comparison Graph
# of cells produced
Takes place
Diploid vs. Haploid
Mitosis
2
Everywhere except testes & ovaries
Diploid
Meiosis
4
Testes and Ovaries
Haploid
Reproduction
Chromosome Count
# of divisions
Type of cell produced
Examples of cells
Asexual
46
1
Somatic cells (body cells)
Skin cells, hair cells, liver cells, etc
Sexual
23
2
Sex cells (gametes)
Sperm and egg cells only
-A diploid cell contains both sets of chromosomes. One set is from mom and the other set is from dad. The human
diploid count is 46.
-A haploid cell contains half the set of chromosomes. The human haploid count is 23. Sperm and egg are haploid
cells because they each contain 23 chromosomes.
Human Karyotype
-A human karyotype is a picture taken of the human chromosomes to
determine the sex: male or female.
-The homologous chromosomes are paired and placed in order from longest
to shortest.
- To determine the sex, look at the very last pair of chromosomes (pair #23).
If you see two chromosomes that are the same size, then we refer to the sex
as female. If you notice that one chromosome on the 23 rd pair is much longer
than the other one, we refer to the sex as male.
- The female’s 23rd pair is XX, while the male is XY.
Many genetic disorders occur on the X chromosome. These diseases that
affect the sex chromosomes are known as sex-linked diseases.
Fortunately for females, they are usually not affected by these sex-linked
diseases because they have a back-up X chromosome in case one fails.
Remember women are XX.
Unfortunately for males, they are more prone to suffer from sex-linked
diseases since they only have one X chromosome and one Y. If the X chromosome is damaged, then genetic
diseases, such as Hemophilia, ALD, color blindness, and more are seen in males. These diseases may also occur in
females, but are less common.
Reproductive Systems
SC.912.L.16.13
Describe the basic anatomy and physiology of the human reproductive system. Describe the process of human
development from fertilization to birth and major changes that occur in each trimester of pregnancy.
Purpose of Reproduction
-The purpose of reproduction is to populate the world and to pass on one’s genetic information to the
next generation.
- Sexual reproduction leads to genetic variation.
-The main function of the male reproductive system is to make sperm cells.
-The main function of the female reproductive system is to make egg cells, or ova.
Male Reproductive System
The journey of sperm cells:
1. Sperm cells are made in the testes. (The male hormone testosterone is also produced in the testes)
2. Sperm cells travel to the epididymis to grow and mature.
3. Sperm cells then travel up and around the vas deferens.
4. The vas deferens becomes the urethra and the sperm is bathed in a milky fluid known as seminal
fluid.
5. The prostate gland and seminal vesicle release the seminal fluid onto the sperm cells. This
combination of sperm and fluid is called semen.
6. The semen continues to travel down the urethra until it is released from the penis.
*It is important to note that the bladder, an organ that contains urine, is not part of the reproductive
system. It is actually part of the urinary system. Both, urine and semen travel through the urethra and
out of the penis.
The testes are located outside of the body in a sac called the scrotum. Sperm cells die when it gets too
hot, so the testes are outside the body where it tends to be a bit cooler for sperm cells to survive.
Female Reproductive System
The journey of egg cells:
1. Egg cells are made in the ovaries. (The female hormone Estrogen is also made in the ovaries)
2. Usually one egg cell is released every month from one of the ovaries. This process is called ovulation.
3. The egg cell will travel down the entire length of the fallopian tube (some books refer to the fallopian
tube as the oviduct or uterine tube)
4. The egg cell must be fertilized by a sperm cell while it is in the fallopian tube. If fertilization does not
take place during this time, the egg cell will die. This is a common test question: Where does fertilization
occur? In the fallopian tube
5. If the egg cell is fertilized, it will ultimately become a blastocyst. The blastocyst will implant itself on
the wall of the uterus to become an embryo and then a fetus.
6. After about 9 months, the fetus is ready to be delivered. The cervix, which is the neck of the uterus,
will dilate so the fetus can move into the vagina, or birth canal.
7. The fetus then exits the female’s body through the vagina, or birth canal.
*It is important to note that babies DO NOT grow in the stomach!! Otherwise you would have a
barbecued baby. The baby grows and develops in the uterus.
Stomach
Uterus
DNA and RNA
Introduction to DNA and RNA
-DNA stands for deoxyribonucleic acid.
-RNA stands for ribonucleic acid.
-All nucleic acids (DNA and RNA) are composed of basic building blocks called nucleotides.
Nucleotide
-DNA is double stranded (double helix/twisted ladder).
-RNA is single stranded.
-A nucleotide is composed of a sugar, a phosphate, and a nitrogenous base.
-The sugar in DNA is called deoxyribose.
-The sugar in RNA is called ribose.
-DNA contains 4 nitrogenous bases: A, T, C, G
-RNA contains 4 nitrogenous bases : A, U, C, G
Notice above that DNA and RNA share the bases A, C, and G. However, the base “T”
is only found in DNA while the base “U” is only found in RNA.
Nitrogenous bases
A = adenine
Purine
G = guanine
C = cytosine
U = uracil
Pyrimidine
T = thymine
-The purines are adenine and guanine. These are bases that have two rings.
-The pyrimidines are cytosine, uracil, and thymine. These bases have one ring.
*Mnemonic: pyramids cut. (Pyrimadines: cytosine, uracil, thymine)
-Purines will always attach themselves to pyrimidines.
- Adenine will always pair with thymine in DNA. A double hydrogen bond holds
them together.
-Guanine will always pair with cytosine in DNA. A triple hydrogen bond holds
them together.
In RNA, adenine will pair with uracil, since there is no thymine in RNA.
*So what is the difference between deoxyribose and ribose? Take a look below
Deoxyribose literally means, “a ribose without oxygen.”
Comparing DNA and RNA
Nucleic Acid
Sugar
Bases
Strands
DNA
Deoxyribose
A, T, C, G
2 strands
RNA
Ribose
A, U, C, G
1 strand
The structure of DNA is anti-parallel. Notice above (picture on the right) that the pointy tip of the DNA
pentagon is pointing up on the left DNA strand, but on the right strand the pointy tip of the pentagon is
pointing down.
*Important: the sugars and phosphates make up the backbone of the DNA ladder, while the bases make
up the rungs (aka steps) of the ladder.
DNA History
British scientist Rosalind Franklin took an X-ray picture of DNA in 1952.
(X-ray Crystallography)
American biologist James Watson and British physicist Francis Crick used the X-ray picture from Rosalind
Franklin to finish their 3-dimensional model of DNA. Watson and Crick later received the Nobel Prize,
while Rosalind Franklin died and never won anything.
DNA Replication
SC.912.L.16.3
Describe the basic process of
DNA replication and how it
relates to the transmission and
conservation of the genetic information.
SC.912.L.15.15
Describe how mutation and genetic recombination increase genetic variation.
SC.912.L.16.4
Explain how mutations in the DNA sequence may or may not result in phenotypic change. Explain how mutations in
gametes may result in phenotypic changes in offspring.
Introduction to DNA Replication
Cells must replicate their DNA before they divide. This occurs in the S-phase of Interphase.
DNA strands are complementary. This means that if you know one strand of DNA, you can draw its
second strand.
DNA strand: 3’– A T T A G C T C – 5’
Complementary Strands
DNA strand: 5’ – T A A T C G A G – 3’
*Notice that the strands are also anti-parallel. The 3’-end is
complementary to the 5’-end.
DNA replication is semi-conservative. This means that when
DNA replicates, it will contain one old strand and one new
strand.
The Process of DNA Replication
1. The enzyme helicase unwinds the double stranded DNA.
2. Single-Strand Binding Proteins (SSBs) attach to the DNA strands to prevent reannealing.
SSBs
3. Primse adds RNA primers to both strands of DNA.
4. DNA polymerase searches for the RNA primers that were created by primase. DNA polymerase then
adds DNA nucleotides in a 5’  3’ direction. Because DNA polymerase can only add nucleotides in a 5’
 3’ direction, one strand will grow continously (leading strand) while the other strand will have to be
made in discontinously (lagging strand) in fragments called Okazaki fragments.
Lagging Strand
Leading Strand
5. The pieces of RNA must be removed from DNA. RNase H will remove the RNA primers from the DNA.
RNase H
RNA primer
DNA polymerase
6. Another DNA polymerase returns and fills in the Okazaki fragments from the removal of the RNA
primers.
7. Finally, Ligase must link short strands of DNA. This completes the process of DNA replication.
Mutations in DNA
If I asked you to copy a book by hand, what are the chances that you will not make a mistake during the
copying process? Do think you can copy a book with extreme precision and detail, such that you will not
miss a comma, capital letter, or period?
The enzymes in charge of DNA replication sometimes make mistakes when they copy DNA. These
mistakes are called mutations.
Just one tiny mistake may ruin your life. Look below how one tiny change in a DNA base causes an
individual to develop a disease of the blood called sickle-cell anemia. That one tiny change caused the
amino acid valine to form instead of glutamic acid. The mutation shown below is an example of a
point mutation.
*Relax, although a lot of mutations are occuring in your body every single day, not all of them are
dangerous. In order for your future child to inherit one of these diseases, the mutation must occur in the
sex cells (sperm and egg) during meiosis.
Types of Mutations
-As you saw in the picture above, sickle-cell anemia is an example of a point mutation.
-There are three types of point mutations: silent mutation, missense mutation, and nonsense mutation.
Point Mutations:
1. Silent mutation: This is a point mutation that
does not change the amino acid sequence in the
protein. It is silent because nothing really
happens.
2. Missense mutation: This point mutation
creates a brand new amino acid sequence in the
protein. Sickle cell anemia is an example of a
missense point mutation.
3. Nonsense mutation: This point mutation does
not code for any amino acid. It causes the amino
acid chain to stop.
*Notice in the picture above how the normal amino acid should be lysine (Lys). With a silent mutation,
the same amino acid (Lys) is still made even though there was a change in the DNA. With a nonsense
mutation, no amino acid is placed and the protein STOPS. Finally, with the missense mutation, a
different amino acid results.
The most dangerous types of mutations are those that cause the DNA frame to shift. These are called
frameshift mutations. Any addition or deletion of a base may cause the DNA frame to shift. Therefore,
additions and deletions may lead to frameshift mutations.
Point mutations are base-substitution mutations; they are NOT frameshift
mutations.
Notice the picture to the right. The deletion of the base “G” from “GAA”
causes a shift in the entire DNA sequence. This shift now contains the new
DNA information “AGG,” “CAC,” and “GT,” which will now create the
amino acids Lys and His, instead of the normal code of Glu, Ala, and Gly.
This is how a frameshift mutation looks like in an english sentence.
Normal: THE CAT AND THE MAN ARE FAT
Deletion of “T” in “THE”: HEC ATA NDT HEM ANA REF AT
Insertion of “A” (random letter): ATH ECA TAN DTH EMA NAR EFA T
Do you see why frameshift mutations are so bad!?
Protein Synthesis
SC.912.L.16.5
Explain the basic processes of transcription and translation, and how they result in the expression of genes.
SC.912.L.16.9
Explain how and why the genetic code is universal and is common to almost all organisms.
Introduction to Protein Synthesis
Cells carry out life functions by making proteins. Everything in your body is protein with some
exceptions, such as your skeleton and teeth.
Central Dogma of Molecular Biology: DNA  RNA  Protein
Image all of us in a cooking school. DNA is our cook book and the chefs are called ribosomes.. It contains
all the recipes to make different kinds of foods, or proteins. (Ribosomes make proteins)
If we have 1 cook book called DNA, and we have 30 chefs, we need to do something so that each chef
has a copy of the recipe they are going to cook. Therefore, we make plenty of photocopies of the recipe,
which we call RNA.
There are 3 common types of RNA: mRNA, rRNA, and tRNA.
The photocopies that we just made with our “recipe” is called mRNA (messenger RNA) because it carries
the message, or the recipe. In eukaryotes, this message (mRNA) is carried out of the nucleus into the
cytoplasm.
The chefs, or ribosomes, need to grab that message (mRNA). The ribosome will read the message and
try to make a protein. Unfortunately, the ribosome doesn’t have any supplies to cook this recipe, so a
tRNA (transfer RNA) must transfer those supplies to the ribosome. Each tRNA will transfer 1 amino acid
to the ribosome. Remember that lots of amino acids add up to make a protein.
rRNA or ribosomal RNA is simply the RNA found within the ribosome. There is nothing special about this
kind of RNA. Think of it as the “uniform of the chef.”
When DNA is turned into RNA, the process is called
transcription.
When RNA is turned into protein, the process is called
translation.
Transcription
In eukaryotes, transcription takes place in the nucleus of
the cell. In transcription, the DNA will be “photocopied”
or “transcribed” into RNA (specifically mRNA).
*It is extremely important for the test that you know how to transcribe a piece of RNA from DNA. I will
provide you with a piece of DNA, and I am going to show you how to transcribe it. Look below
DNA strand: 3’– A T T A G C T C – 5’
DNA strand: 5’ – T A A T C G A G – 3’ (let’s use this DNA strand to create our mRNA template)
mRNA: 3’ – A U U A G C U C – 5’ (Notice how we use “U” in RNA instead of “T”)
Finally, the mRNA that we just created needs to be edited. To finalize our mRNA, we need to remove a
piece of the mRNA called the intron. Remove all the introns. We are now done with transcription.
Translation
Translation is the process by which RNA is translated to protein, and it occurs in the cytoplasm for both
eukaryotes and prokaryotes.
In translation, a ribosome grabs on to the mRNA and begins to
read its message. Several tRNA, each carrying one amino acid, will
head toward the ribosome. The ribosome will grab on to the tRNA
based on the instructions from the mRNA.
The mRNA is read 3 letters at a time. These three letters is called a
codon. The first three letters, or first codon, in all proteins is the
start codon AUG. AUG codes for the amino acid methionine.
The ribosome must attach a tRNA with the anti-codon UAC to the
codon AUG on the mRNA. Look at the picture to the right 
Once the tRNA leaves its amino acid, the tRNA is discarded, or
thrown away. Remember, every tRNA brings only 1 amino acid.
*On the exam, I will give you an mRNA strand and ask you to place the right amino acids according to
the strand. For example, notice the RNA Ipicture
GUG codes for Val.Eh,
CACrelax
codes
for His. CUG codes
doc.
have toabove.
memorize
for Leu. Every three letters (codon) codes for 1 amino acid.
You don’t.
every amino acid and
every 3 letter
I will give you a
code????
chart.
Use this chart to create your protein. Every three letters is
one amino acid.
mRNA: AUG – UAU – CGU – GUU – CUA – GGU – UAA
Protein: Met – Tyr – Arg – Val – Leu – Gly – Stop
You do not need to memorize the names of the amino
acids. You do not need to memorize that Met stands for
methionine, or Tyr stands for tyrosine, etc…
Mendelian Genetics
SC.912.L.16.1
Use Mendel’s laws of segregation and independent assortment to analyze patterns of inheritance.
SC.912.L.16.2
Discuss observed inheritance patterns caused by various modes of inheritance, including dominant, recessive,
codominant, sex-linked, polygenic, and multiple alleles.
SC.912.L.14.6
Explain the significance of genetic factors, environmental factors, and pathogenic agents to health from the
perspectives of both individual and public health.
Introduction to Heredity
Gregor Mendel is the father of heredity. He did hundreds of experiments on
pea plants. Unfortunately, he was not recognized for his work until years
after his death.
*Before we continue, let us go over the concept of a gene and an allele.
Let’s say we want to study eye color. Let “B” represent brown eyes and “b”
represent blue eyes.
Remember that you have two parents (a mommy and a daddy), so you will
get one letter from each parent. You have three different mathematical
possibilites: BB or Bb or bb
If you get BB, then you have brown eyes.
If you get bb, then you have blue eyes.
But what if you get Bb? Does this mean you have one brown and one blue eye? Absolutely NOT.
If you get Bb, then you have brown eyes. (I will explain why in a moment)
The two letters combined “BB”, “Bb”, and “bb” represent genes. Genes are segments of DNA that code
for a specific protein.
Each individual letter “B” or “b” is called an allele. Therefore, your mom gives you one allele and your
dad gives you the other allele.
BB= Gene
Bb = Gene
bb= Gene
B = allele
b =allele
Mendel’s Laws
1. Law of Dominance: The dominant allele will be expressed.
* Remember Bb = brown eyes. This occurs because brown is dominant to blue, so brown is physically
expressed while blue is hidden.
2. Law of Segregation: In meiosis (back when sperms and eggs were made), the alleles were segregated,
or separated. As a result, every sperm and egg is different. Then, each time a baby is formed, the baby
looks slightly different from his or her mom, dad, brother, sister, etc…
3. Law of Independent Assortment: Independent traits, such as eye color and hair color, assort
themselves indepdently. In other words, just because you have blue eyes does not mean you will
automatically have blue hair.
How did Mendel figure this out?
1. Gregor Mendel got a purple pea plant and self-fertilized it to create a pure breed (PP = purple).
2. Gregor Mendel got a white pea plant and self-fertilized it to create a pure breed (pp = white)
*Notice how we use “P” for purple and “p” for white. We keep the letters the same, but one letter is big
and one is small.
3. Gregor Mendel then did an experiment. He got his pure purple pea plant
and crossed fertilized it with his pure white flower. To his surprise, 100% of
the pea plants were purple! What happened to the white!? Did it dissapear?
4. Mendel then got one purple pea plant that was produced from the purple
and white parent pea plants. He calls these purple pea plants the
F1 Generation because they are the offspring of the parent generation above.
5. When Mendel self-fertilized a purple pea plant from the F1 generation,
he got the following results: 75% purple and 25% white pea plants. The
white did not dissapear! It reappeared in this new generation, which he
called the F2 generation.
The Punnett Square (Monohybrid Cross)
Scientists today use punnett squares to predict outcomes. A punnet square is a box (literally a square)
that we will used to determine probabilities.
Let’s use the following assumptions to create a punnet square:
Example 1:
1. Assume your mom has brown eyes and has the genotype BB. (the genotype, or genes are the two
letters)
2. Assume your dad has blue eyes and has the genotype bb.
3. Let’s create a punnett square to determine the probabilities that you or your siblings will come out
with brown or blue eyes.
b
b
B
B
If you notice from the punnett square above, all of the children of mom and dad have the genotype
“Bb.” This means that they all have
brown eyes, since brown is
Bb
Bb
dominant over blue. However since
they have a little “b,” they have the
possibility of passing that “b” allele
to the next generation.
Bb
Bb
Example 2:
1. Assume your mom has brown
eyes and has the genotype Bb.
2. Assume your dad has brown eyes
and has the genotype Bb.
3. Let’s create a punnett square to determine the probabilities that you or your siblings will come out
with brown or blue eyes.
B
b
BB
Bb
Bb
bb
B
b
Notice how ¾, or 75%, of the children will have brown eyes. Notice how ¼ of the children, or 25%, will
have blue eyes.
*Since we are only studying one gene in this punnett square (eye color), we call this a monohybrid cross.
Genotype vs. Phenotype
As stated above, a genotype is the gene, or genetic make-up, of
an organism. In the examples above, the three genotypes for eye
color were “BB”, “Bb”, and “bb.” These genotypes were used to
describe brown and blue eyes color.
Phenotype is the physical manifestation of an organisms
genotype. For example, the physical manifestation of an
organism with the genotype “BB” is brown eyes. Therefore,
brown is the phenotype. The physical manifestation of an
organism with the genotype “bb” is blue eyes. Therefore, blue is
the phenotype.
Try this:
1. Look at your eyes in the mirror and tell me your genotype…………you can’t!
2. Now, look at your eyes in the mirror and tell me your phenotype…..do you have blue, brown green,
etc?
As you probably inferred, you can tell a person’s phenotype by simply looking at them, but you cannot
determine their genotype by simply looking at them.
Homozygous and Heterozygous Genotypes
The genotype BB is homozygous dominant. (Also known as purebred)
The genotype bb is homozygous recessive. (Also known as purebred)
The genotype Bb is heterozygous. (Also known as hybrid)
*This is extremely important for the test. The math problems that you will solve on the test will use
these fancy words.
Test Example Question:
1. Alberto and Jennifer are thinking of having children and would like to know the probabilitie of having
a child with brown or blue eyes. Alberto has brown eyes and Jennifer has blue eyes. They go see a
geneticist to discuss these possibilities. The genetecist informs Alberto that he is heterozygous while
Jennifer is homozygous recessive. Figure out the possibilites that their children will have brown or blue
eyes by creating a punnett square.
2. Since Alberto has brown eyes, his genotype is either BB or Bb. If you continue reading, it says that
Alberto is heterozygous. This means that he must be Bb.
3. Jennifer has blue eyes. The passage above states that Jennifer is homozygous recessive. Jennifer must
then have the genotype bb. Even if the passage would not have said that Jennifer was homozygous
recessive, we still know that the genotype is bb because there is only one genotype for blue eyes, since
it is a recessive trait.
4. Create a punnett square and solve
B
b
BB
bb
Bb
bb
b
b
Notice that 2/4, or 50%, of the children will have brown eyes, and 2/4, or 50%, of the children will have
blue eyes.
The Punnett Square (dihybrid Cross)
Sometimes we want to study two traits at the same time. In this example, the two traits we will study
are shape and color of seeds.
In terms of shape, round is dominant to wrinkled.
R = round
r = wrinkled
In terms of color, yellow is dominant to green.
Y = yellow
y = green
In this example we are going to make the following cross between two plants: RrYy x RrYy
Remember that “R & r” represent shape and “Y & y” represent color.
The first plant’s genotype is RrYy, so this plant has round seeds and yellow color. The second plant’s
genoptype is identical to the first plant. To solve this problem, we need to make a bigger punnett square
with 16 boxes, but how?
Let’s use a concept from math called FOIL:
RrYy  RY
Ry
rY
ry
(both plants in this example have the same genotype)
Test Cross
A test cross is a test used to determine if a dominant individual is homozygous or
heterozygous.
Let’s use the example of widow’s peak, which is a dominant trait. If an individual has a
widow’s peak, he or she may be WW or Ww. So if you look at the picture to the right, is
Eddie from “The Munsters” homozygous (WW) or heterozygous (Ww)? You don’t really
know.
So the question is, how do we determine if Eddie Munster is homozygous (WW) or
heterozygous (Ww). The answer is, we need to have him marry and mate with a female
who does not have a widow’s peak (ww).
Let’s use Elena from “The Vampire Diaries” since she does not have a widow’s peak
and is therefore homozygous recessive (ww).
*Keep in mind that this is a probability, or a chance event.
-If all of the children have a widow’s peak, then Eddie Munster is more than likely WW.
-If at least one of the children does NOT have a widow’s peak, then Eddie Munster is Ww.
- If you are in doubt, do a punnett square and you will see.
Single-Gene Traits and Polygenic-geneTraits
What do the following four features below have in common? They are all single-gene traits, which
means that they are all affected by only one gene. Single-gene traits are easy because you either have
them or you don’t. This allows us to create punnett squares. Single-gene traits can also determine if a
person is adopted or not.
On the other hand, polygenic-gene traits, are affected by two or more genes. Therefore, they are much
harder to be used to determine important information like whether someone is adopted or not.
Beyond Mendelian Genetics: Incomplete Dominance, Codominance, & Sex-Linked Traits
Some traits do not follow the laws that we learned from Gregor Mendel, especially the law of
dominance.
Incomplete dominance occurs when neither trait is dominant over the other. A
blend of the traits appears.
Codominance is when both alleles are expressed. Look at the Dalmatian to the right.
The Dalmation is NOT a blend of white and black. The Dalmation IS both white and
black.
The examples that will be used on test for codominance are based on blood types, so
learn this.
For codominant traits we use superscripts:
Blood Type A: IAIA or IAi
Blood Type B: IBIB or IBi
Blood Type AB: IAIB (blood type AB is the only blood type that is codominant)
Blood Type O: ii (notice that O-blood is recessive, and we use ii)
Blood types can also be used to determine if a person is adopted or not. Read at
the example below.
The Charlie Chaplin case: In 1941, Charlie Chaplin became romantically involved with a young actress
named Joan Barry. In 1943, twenty months after the relationship ended, she had a child and claimed
Charlie Chaplin as the father. A paternity suit ensued. Chaplin had type O blood. Barry had type A
blood. The baby had type B blood. Was the baby Chaplin’s? (Yes or No) Provide written explanation
and/or charts to support your answer.
You are still
going to pay
child support!
LOSER =P
That’s
not my
baby!
Blood Genotype: ii
Blood Genotype: IAIA or IAi
Draw a Punnett square and find out if Chaplin really is the baby’s daddy. Since Chaplin has blood type O,
which is recessive, use the alleles ii. Since Joan Barry is blood type A, she may have the genotype IAIA or
IAi. On the other hand, the baby is blood type B, which means that the baby’s genotype must be IBi, it
cannot be IBIB because we know the mother, Joan Barry, does not carry “B” alleles.
The conclusion:
The baby could not have been Chaplin’s, since the B allele carried from the baby did not come from its
mother and could not have come from Chaplin either. Three pathologists testified to this effect.
However, the jury was undeterred by the “scientific evidence” and ruled that Chaplin was the father,
ordering him to pay child support.
Sex-Linked Traits
Sex-linked traits are traits that are passed down to the child by the sex chromosomes (XX or XY).
Most sex-linked traits are carried on the X chromosome and most are
recessive traits. This means that for a female to have the disease, she
would need two defective X chromosomes. The male, however, only
needs one defective X chromosome to inherit the condition.
Hemophilia is a blood-clotting disorder carried on the X chromosome. It is
referred to as an X-linked recessive trait, since a female needs two
defective X chromosomes to inherit the disease.
Look at the example to the right, the mother is a carrier for hemophilia,
which means that she does not have the disease (XXh). The father does
have hemophilia since his only X chromosome is defective (XhY).
Pedigrees
HE.912.C.1.4
Analyze how heredity and family history can impact personal health.
A pedigree is a picture representation of a family history.
Pedigrees help us determine the genotype of an organism.
Use the key below to understand how a pedigree works.
Most Important Pedigree Rules:
-Male = Square
-Female = Circle
-Shaded in = affected individual
-Roman Numeral = Generation
Autosomal Dominant Trait Characteristics:
1.
2.
3.
4.
Does not skip generation
Appears in both sexes with equal frequency
Both sexes can transmit the trait to the offspring
Affected persons have at least one affected parent
Autosomal Recessive Trait Characteristics:
1. Skips generations
2. Appears with equal frequency in both sexes
3. Affected offspring are usually born to unaffected parents.
4. Appears more frequent in children of consanguine marriages
X-Linked Recessive Trait Characteristics:
(Highly tested on exams)
1. Appears more often in males
2. Not passed from father to son
3. Passed from mother to son
4. Women may have the disease, but the majority of women are
carriers
5. Hemophilia & color blindness are examples
X-Linked Dominant Trait Characteristics:
1. Appears in males and females, but they often appear more
in females. In fact, all females of affected parents will have it.
2. Does not skip generation
3. Affected men pass the trait to all their daughters but not
their sons
4. Affected women (if heterozygous) pass the trait on to about
½ of their sons and daughters.
Y-Linked Trait Characteristics:
(Extremely unlikely to be tested on exams)
1. Only men have it
2. Trait is passed from father to son
3. Do not skip generations
4.Extremely rare
Biotechnology
SC.912.L.16.12:
Describe how basic DNA technology (restriction digestion by endonucleases, gel electrophoresis,
polymerase chain reaction, ligation, and transformation) is used to construct recombinant DNA
molecules (DNA cloning).
SC.912.L.16.10
Evaluate the impact of biotechnology on the individual, society and the environment, including
medical and ethical issues.
Introduction to Biotechnology
Biotechnology is the manipulation of living organisms or their parts to produce useful products.
With biotechnology, we can do the following things and much more?
1. Paternity testing
2. Cloning of organisms
3. Crime investigation (Fingerprints/DNA analysis)
4. Genetically modifying fruits and vegetables to produce larger amounts of foods that are also resistant
to pests
5. Using bacteria to produce human hormones, like insulin, to treat diabetes
6. Finding of the cure of hundreds of diseases in the future like diabetes, cancer,
Dolly: First cloned
and HIV
animal in the year 1996
7. Being able to modify a human genetically (imagine instead of plastic surgery,
genetic surgery; being able to change your eye color from brown to blue)
Gel electrophoresis
-Gel electrophoresis is a technique that separes DNA, RNA, or proteins fragments
according to their size.
- Gel electrophoresis is commonly used in crime scenes to analyze the evidence
with the suspects’s DNA. This procedure may also be used to determine paternity.
For the test, you need to be able to read a gel electrophoresis.
Gel electrophoresis is very simple to read.
Compare the “Evidence” with all four suspects
shown on the picture to the left.
Notice that the “Evidence” matches with “Suspect 3”
This indicates that “Suspect 3” is probably guilty.
Cloning
Cloning is the
artificial
production of a cell or organism that is
genetically identical to the parent cell or
organism.
Follow the steps in the picture on the right 
Recombinant DNA
Recombinant DNA is literally combining DNA from different
organisms.
Combining DNA is done in a field of science called genetic
engineering.
Viruses are used as vectors to insert the new DNA.
The hormone insulin is now made with the help of bacteria.
Therefore, we are able to manufacture insulin very efficiently
and at a low cost. (Highly tested on exams)
Restriction enzymes are like scissors. In this example, we use a
restriction enzyme to cut out the gene that codes for insulin in
humans. We then combine that human gene with a bacterial
circular chromosome called a plasmid. The plasmid is then
inserted in the bacterium. The bacterium is now capable of
producing human insulin.
In addition, we can use a technique called polymerase chain reaction (PCR) to make hundreds of copies of the
gene, in this case the insulin gene. These copies can then be transferred to many bacteria, and manufacturing insulin
has become easy and cheap in today’s world.