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Before you start, you should pay attention to these details:First, the answer must be in unique words (own words)
Second, Just use the resources that are in the instructions (do not use external sources).
Third, I will attach some files and links that you may need it.
Human Mendelian Traits | Ask A Biologist (asu.edu)
From DNA to Protein – YouTube
Human Mendelian Traits | Ask A Biologist (asu.edu)
Home – OMIM – NCBI (nih.gov)
Instructions
This assignment is an opportunity for you to explore human genetics, particularly in areas that
might interest you personally. These include genetic diseases, variations in chromsome number,
and the heritability of human traits such as handedness and athletic performance.
This assignment is formatted as a three-question quiz. In Question 1 you are asked to select and
read about a particular genetic condition, then answer 4 questions about it. In Questions 2 and 3
you will do the same for a particular human trait, and for a particular human chromosome.
You will be accessing various sections of this website:
https://medlineplus.gov/genetics/understanding/ (Links to an external site.)
There will be links provided in the two questions to the relevant portions of the site, but I wanted
to include the general link here, in the event you wish to explore other sections on your own. It
contains clear, useful information about various aspects of genetics and may help you answer
questions you had about your own health or genetic risk.
Question 1:
Navigate to this list of genetic conditions: https://medlineplus.gov/genetics/condition/ (Links to
an external site.)
The site contains link to over 1300 genetic conditions organized alphabetically–some common,
some rare. Choose ONE genetic condition to read about. Then answer the following questions
about it. You do not need to rewrite the questions, but please number your answers.
NOTE–Before choosing a condition, read about it and make sure you understand and can
explain in your own words how it affects the body.
1. What is the name of the disease you selected?
2. Describe in your own words how the body is affected by this disease. You are welcome to
use terms and language contained in the description, but do not simply copy and paste text from
the article.
3. Is the condition associated with a single gene or multiple genes? How commonly does it
occur? Are particular sexes or ethnic groups affected more commonly? Does it onset at a
particular age?
4. If the condition is associated with a specific gene or genes, summarize the pattern of
inheritance–dominant/recessive/X-linked, etc. Or if the condition relates to chromosomal
variation, specify the abnormality.
Question 2:
Navigate to this webpage Genetics and Human Traits:
https://medlineplus.gov/genetics/understanding/traits/ (Links to an external site.)
Choose ONE of the 13 traits listed. Examples include fingerprints, eye color, intelligence, etc.
Click on the link to read about your selected trait. Then answer the following questions. You do
not need to rewrite the questions, but please number your answers.
1. What trait did you select?
2. Before reading the article, what were your previous beliefs about the trait you chose? Did you
believe it was totally determined by genetics, partially by genetics, or not determined by
genetics?
3. According to the article, what is the role of single or multiple genes in determining the trait? If
genetics plays a role, is a single gene or multiple genes responsible? Summarize what is known
about these genes.
4. If factors other than genes help determine the expression of the trait, what are those factors?
Question 3:
Navigate to this webpage https://medlineplus.gov/genetics/chromosome/ (Links to an external
site.)
Choose ONE of the 24 chromosomes listed (choose from Numbers 1-22 or X or y). Click on the
link to read about the chromosome. then answer the following questions. You do not need to
rewrite the questions, but please number your answers.
1. What chromosome number (or letter) did you select?
2. Approximately how many genes does the chromosome contain?
3. Each chromosome has several chromosome disorders listed. Name one disorder associated
with your chromosome.
4. What is the difference between a chromosome disorder and a gene variant?
Last lecture, we talked about how heredity had been such a puzzle to biologists of the 1
9th century. They knew that biological traits passed from one generation to the next, but
the mechanism by which this occurred was a mystery. During the 20th century, scientis
ts began to unravel the mystery of heredity. Gregor Mendel’s pioneering work created a
model for genetic inheritance. Biologists identified processes such as mutation, and loc
ated the genetic material in the cell’s chromosomes. The discovery and decoding of DN
A revealed how evolution happens at the molecular level.
This section examines heredity in light of modern genetics. We will begin with a discussi
on of DNA and how DNA builds an organism—
in other words, how genes direct the construction of an organism’s structure. DNA (or d
eoxyribonucleic acid) is the molecule that contains the basic information for replicating,
building and maintaining living organisms. As we look at DNA, we’re going to take a sim
plified (rather than highly technical) approach that helps us understand its basic structur
e and function.
DNA structure
DNA is a double stranded molecule that looks like a ladder twisted in opposite directions
. This shape is referred to as a double helix. The sides of the ladder are for structural st
ability. They are made up of a sugar and phosphate molecule.
The rungs of the ladder carry the code. They are made up of a family of bases (chemica
l components derived from nitrogen). There are 4 possible types of bases: adenine, thy
mine, cytocine and guanine (A,T,C and G are the abbreviations). You can think of thes
e bases as the alphabet for carrying out the code.
These bases always pair with each other in the same way. A always pairs with T and G
always pairs with C. These base pairs are the essential structure of the genetic code. Si
nce these pairings are so predictable, if we know the sequence of bases on one side of
the DNA helix, we can predict the sequence on the other side. This predictability means
that the DNA molecule can make an exact copy of itself very accurately.
DNA is bundled together in chromosomes
Chromosomes
In general conversation we tend to talk about genetics in terms of chromosomes and ge
nes, as something apart from DNA, so it’s good to emphasize that chromosomes and g
enes are DNA. More specifically, DNA are contained within chromosomes in the nucleu
s. All a chromosome is, is a single very long DNA molecule (it can be as long as 7 feet),
that is held together by proteins that fold it up into a dense strand. It is these strands whi
ch are visible under a microscope in the cell nucleus.
Each human somatic (body) cell contains 23 pairs of chromosomes or 46 chromosomes
total. 23 you inherit from your mother and the other 23 you inherit from your father. Chr
omosomes are homologous, which means that they’re paired. 22 of those human chrom
osome pairs are autosomes. 1 pair are the sex chromosomes, the X and Y. Females ha
ve two X-chromosomes, XX. Males have one X and one Y.
Genes and genomes
In molecular terms, a gene is a segment of DNA that contains the instructions for the pr
oduction of a protein or part of a protein. Since proteins are what builds the organism, g
enes are recipes for building the organism. A genome is all the DNA in a species.
Non-coding DNA
It has been known for some time that over 95 percent of our DNA doesn’t actually seem
to do anything. Sequences of DNA, known as introns, occur between the coding regions
and are spliced out of the sequence during protein synthesis. Other vast stretches of D
NA never play any role in protein synthesis. In the past, these noncoding regions were dismissed as “junk DNA.” Researchers studying DNA suspect that
some of our noncoding DNA may play a role in gene regulation or may serve as future raw material for n
atural selection to act upon.
DNA Function
DNA has two primary functions: First, to replicate or make copies of itself, and second, t
o provide the cell for instructions for building proteins (protein synthesis).
Replication
During cell division the DNA molecule needs to copy itself so each new cell has its own
complete DNA. This is quite simple, because DNA has a very basic but efficient replicati
ng process.
There are two types of cell division.
1. Mitosis is the process for somatic cell division—
it’s basically one cell splitting into two identical cells.
2. Meiosis, which occurs for the gametes (sperm and egg) is similar but it has an additi
onal step that reduces the number of chromosomes in the gamete.
When DNA replicates, the two intertwining strands are unzipped by enzymes that break
the bonds between them. Each strand of the original molecule then acts as a template f
or the formation of a new complementary DNA molecule. The two separate strands eac
h attract complementary bases to produce two identical double strands. Where does it f
ind these bases? They are freefloating in the nucleus of the cell. All the raw material for the replication of DNA is readily
available in the cell.
Protein Synthesis
The essential function of DNA is to control the production of proteins. This is called prot
ein synthesis.
Proteins are large chain-
like molecules that are twisted and folded back on themselves in complex patterns. The
y serve as structural material for the body, gas transporters, hormones, antibodies, neur
otransmitters, and enzymes. Since proteins are so essential, it’s crucial that protein synt
hesis occur accurately—that the right messages are sent to carry out the work correctly.
Proteins at their most basic level are made up of linear chains of small molecules called
amino acids. The string of amino acids is known as a polypeptide chain. These chains
are then folded and coiled into the specific protein structure. DNA contains the code for t
he amino acid sequence. There are a total of 20 amino acids. Each AA is coded for by t
hree of the four chemical bases on the DNA molecule. You can think of these as 3letter words which are called triplets.
In other words, each triplet specifies a particular AA. All the possible three letter combos
of 4 letters add up to 64 possible combinations. Since there are only 20 AA’s, some tripl
ets code for the same AA’s. Some triplets translate into stop, which means cut off the se
quence.
So, how are the DNA code instructions transferred to the rest of the cell to start the man
ufacture of proteins? This takes place in two steps: Transcription and translation. Transc
ription takes place in the nucleus and translation takes place in the cytoplasm.
Transcription
Transcription is simply making a copy of the DNA code. You might ask, why is it necess
ary to make a copy? Why can’t you just use the actual DNA molecule when it’s replicati
ng? The answer is that the DNA molecule is not equipped to leave the nucleus. In fact i
t would be risky for the DNA molecule to travel outside the nucleus because the cytopla
sm, where protein synthesis takes place is a rougher neighborhood. All sorts of cellular f
unctions take place there and if the DNA molecule got damaged, it would be irreparable.
It’s safer to just make a copy.
During transcription, the two DNA strands separate, like when they’re replicating, but onl
y partially, and this time only one of the strands attracts the free floating bases to form a
single strand. It doesn’t copy the whole strand, just the portion it needs. This strand of
RNA that’s created in the nucleus is called messenger RNA or mRNA because it will lea
ve the nucleus and migrate out into the cytoplasm to deliver the DNA message.
RNA is the molecule that carries out instructions for protein synthesis. RNA is very simil
ar to DNA, but it has a slightly different chemical composition. Instead of the base letter
T for thinine, it has U for uracil. Even though its chemical composition is slightly different
, it’s still transmitting the same information.
After the strand of mRNA is created from the DNA template, it undergoes some editing
where the noncoding regions of DNA are spliced out. After this editing occurs, the messenger RNA lea
ves the cell nucleus and travels to the ribosomes.
An overview of protein synthesis, with transcription taking place in the nucleus and trans
lation at the site of the ribosomes.
Translation
The next step is called translation because it involves translating or decoding the mRNA
message into the actual amino acids. These will eventually chained together to produce
proteins. Translation takes place in the cytoplasm within a structure called the ribosome
s.
A second type of RNA are involved in the translation. These are transfer RNA or tRNA.
The purpose of tRNA is to bind to specific amino acids and transport them to the riboso
mes, and deposit them in order. In order to do this, the tRNA molecule has two things. It
has one end that is attracted to a specific mRNA codon. It also has an attachment site f
or a particular amino acid.
When the mRNA strand arrives at the ribosomes, each tRNA is going to look for the mat
ching codon on the mRNA. It will line up with the appropriate codon and deposit its amin
o acid. As each tRNA molecule drops off its amino acid, the AA’s are bonded together r
esulting in a polypeptide chain.
Advances in Genetics–Epigenetics
The recent research into the regulatory function of DNA reveals a complex web of intera
ctions between genes, gene expression, development, and the environment. Expect to
see much more about this exciting area of genetics in the near future.
Scientists are learning more about genes interact with the environment through an additi
onal mechanism called the epigenome. The epigenome is made up of cellular material t
hat covers the DNA molecule and has the ability to switch genes on and off without cha
nging the genetic code itself. Environmental factors such as diet, stress and exposure to
toxins can alter the epigenome and the altered epigenome can then apparently be pass
ed down to one’s offspring.
Does this remind you of anything we recently studied? It’s a stretch to say that JeanBaptiste Lamarck’s ideas about acquired characteristics have been successfully resurre
cted, but the new research in epigenetics certainly suggests that genetics is much more
complex than most people even 20 years ago realized.
Additional Resources (Optional)
Online exercise from Learn Genetics Tutorials produced by the University of Utah where
you Transcribe and Translate a Gene.
Youtube video of a realistic simulation of protein synthesis in “real time.”
Video clip and accompanying material from PBS Nova Science Now on Epigenetics. Pa
y special attention to the study done with the mice and the chemical BPA, and also to th
e potential applications of this research for cancer, Alzheimer’s and other diseases.
This week we will explore the basic principles of heredity developed first by Gregor Men
del and refined by later scientists. Mendel’s experiments were instrumental in demonstr
ating how genetic traits are transmitted from parent to offspring. We will also look at mor
e complex relationships between genes and their expression, such as the genetics of be
havior.
Early ideas about Heredity
As mentioned previously, the weakness in Darwin’s work was that he didn’t have a clear
idea for how traits were passed on from parent to offspring. In medieval times, heredity
was seen as something mysterious.
When the invention of the microscope allowed egg and sperms cells to be glimpsed, pe
ople came up with the notion that egg or sperm had a copy of a tiny preformed human
inside.
Based upon their observations with ornamental plant and animal breeding, scientists in t
he nineteenth century realized that both parents contribute to the characteristics of offsp
ring. One predominant theory, one that Darwin preferred was called Pangenesis: the ble
nding of fluids. It was thought that a substance from each parent formed an inseparable
mixture in their offspring and determined the inherited characters.
But what’s wrong with this theory? If it were true, this would mean that evolutionary cha
nges would be diluted and washed out, like a drop of red paint in the ocean. Any existin
g variation would disappear over time, and we would all resemble the average—
one height, one skin color, one hair color. The blenders couldn’t explain why population
s don’t not reach a uniform appearance.
Gregor Mendel
To finally unlock the mystery of inheritance, we have to turn to Gregor Mendel, a monk
who lived in what is now the Czech Republic. Mendel had studied biology and mathema
tics in Vienna and developed an interest in heredity. While he was working at his monas
tery, he began conducting a series of plant breeding experiments. He bred various pure
strains of pea plants together and kept track of the results through generations. He chos
e seven different traits to keep track of, which were characteristics that had only two for
ms of expression—such as green vs. yellow peas or wrinkled vs. smooth.
He observed in his experiments that the traits followed a predictable pattern. For exampl
e with the green and yellow pea plants. He noted that in the 1st generation cross of a pu
rebred green and purebred yellow seed plant that all the offspring were yellow.
However, when these yellow seed offspring were crossed with each other, some green
seed plants reappeared. And the approximate ratio of yellow to green seed plants was a
lways about 3:1. This proved the same with the other six traits that he studied. During th
e initial cross, all the offspring would look the same. But during the next generation, you
would see the traits of both original parents in a 3:1 ratio.
It was obvious that some regular mechanism was operating here. And it was not pange
nesis or blending. If the blending theories were right, the outcome would be all greenishyellow seeds, and the original color would not be preserved.
Blending vs Particulate inheritance
Blending Inheritence
Particulate Inheritance
In Mendel’s conception of inheritance, traits remain as particles and get passed down in
pairs, one from each parent.
Mendel showed that inheritance didn’t involve the blending of substances but was partic
ulate. He showed that traits are passed on by individual particles according to very spec
ific principles. Mendel called these particles “factors.” We call them genes. He publishe
d a paper on these results and sent it out to scientists, however, no one paid much atte
ntion to it. It wasn’t until the 20th century that Mendel’s work was recognized for being a
pioneering step in genetics research.
Basic Genetics Terminology
Gene and allele
A gene is a section of DNA that specifies the construction of a protein. Each gene has i
ts locus or its particular location on a chromosome. Remember genes occur in pairs bec
ause chromosomes occur in pairs.
Although the two genes of a pair deal with the same trait, they may vary in their informat
ion about it. This happens when they differ in form. For instance, one form of the gene
may specify green peas, another yellow peas. The various forms (or alternatives) of a g
ene are called alleles. In this way, alleles and genes are somewhat synonymous. All of
the pea plant traits Mendel studied had two alleles. Other genes have only one allele, a
nd some have many alternate forms.
Genotype and phenotype
To keep the distinction clear between genes and the traits they specify, we use the word
genotype for the genetic makeup of the individual (that can be for the whole genetic m
akeup, or for a single gene pair). We call the observable appearance of a trait the phen
otype.
Dominant and recessive
The traits Mendel studied were each controlled by a single gene with two different allele
s. In the case of pea color, the two different alleles are yellow and green pea color. The
yellow allele is dominant over the recessive green allele, meaning that its effect masks
the effect of the green allele that it’s paired with. We normally use capital letters to indic
ate the dominant allele and lowercase to indicate recessive.
Homozygous and heterozygous
Therefore, we can say that an individual with a homozygous dominant genotype has tw
o dominant alleles for the trait being studied. A homozygous recessive individual has tw
o recessive alleles. A heterozygous individual has two different alleles.
Independent assortment
Mendel also made crosses where two traits were sorted at the same time to see whethe
r there was a relationship between them. He was able to demonstrate that that traits ar
e inherited independently of one another. One trait is not influenced by expression of an
other trait. However, this rule of independent assortment only holds true if the traits are l
ocated on different chromosomes.
Punnett Squares and Calculating Probability of Inheritance
A Punnett square is a simple way of calculating the ratios of genotypes and phenotypes
in the offspring, if the parent genotypes are known. For the purposes of this course, you
are only required to complete a simple 4box Punnett square for a single trait with two alleles (see below).
Directions for Punnett squares:
1. If you are diagramming this on paper, simply draw a square and then divide it int
o four smaller squares.
2. The problem will usually specify the letter representing the trait. In the case belo
w, the trait is flower color, represented by the dominant allele (B) for pink, and the
recessive allele (b) for white.
3. The problem will specify the parent genotypes (in the case below, they are both
Bb). Place the letters of one parent across the top of the square. Place the letters
of the other parent on the left side of the square. It does not matter which parent
goes where because you are multiplying them together.
4. Fill in the four squares by combining the parent allele “letters” for each one. Tho
se are the potential genotypes. Convert these to percentages (25% BB, 50% Bb,
25% bb).
5. Based on the genotype percentages, list the corresponding phenotype percentag
es (75% pink; 25% white).
For another look, check out the instructional video Punnett Square Practice Problems, w
hich guides the viewer through solving two simple problems.
Simple patterns of inheritance in humans
Simple Mendelian traits refer to traits controlled for by a single gene and inherited in a si
mple dominant/recessive fashion, much like Mendel’s peas, and the flower color exampl
e above. Some examples of these in humans are minor, fairly neutral variations such as
whether you can roll your tongue, have a Hitchhiker thumb, or can taste a substance ca
lled PTC. The ABO blood group system (pp. 8385) also follows a straightforward Mendelian pattern of inheritance, though there are 3 al
leles for the gene instead of two.
Just a few of the human genetic diseases and disorders inherited in this simple fashion i
nclude cystic fibrosis, Huntington’s disease, Tay Sachs disease and achondroplasia (a t
ype of dwarfism). Figure 3.31 in Chap. 3 in your textbook has a list of some human dise
ases that follow a Mendelian pattern of inheritance.
Patterns of complex inheritance
Although there are thousands of traits inherited in the above simple fashion, it is actually
rather uncommon for a trait to be determined by only a single gene. More commonly, m
ultiple genes interact with each other and also with the environment to produce very co
mplex outcomes.
Polygenic traits are when more than one gene acts to control a trait. Skin color, hair an
d eye color, and height are all examples of more complex inheritance. One or more gen
es may contribute to the phenotype. This produces continuous variation. Polygenic traits
like height and body form are likely to be affected by the environment, rather than solel
y under genetic control.
Pleiotropic traits are when a single gene can have multiple effects on an organism. Th
ey may seem like they are unrelated, but they are actually caused by a single gene. For
example, the genetic disease sickle cell anemia is controlled by only one type of allele,
but the expression of the allele can have multiple effects on different organ systems incl
uding the skeletal system, the heart the lungs and spleen and kidneys.
Traits can be both polygenic and pleiotropic with multiple genes interacting in exceeding
ly complex ways. And of course, gene expression is influenced by a myriad of environm
ental factors.
For example, relationships between genes and human behavior are some of the most c
omplex of all. Behavior undoubtedly has some genetic component. However, due to th
e tremendous influence of the environment, it is exceedingly difficult to study the genetic
basis of a behavior in isolation.
For Further Reading (Optional)
Follow this link for some examples of simple Mendelian traits.
For a fascinating peek at a vast encyclopedia of human genetic disorders, check out thi
s link to the site Online Mendelian Inheritance in Man. Try typing one of the above disor
ders into the site’s search function. You will get masses of information about the disease
‘s clinical features and inheritance. There is also a Gene Map features on the left sideba
r that shows the gene’s locus (location) on its chromosome. This is highly technical, but
demonstrates the vast quantity of data collected and compiled by geneticists to date.
