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Week 9
Genetics and Biotechnology
Between 1856 and 1863 or so, in what is now Austria, lived a man
named Gregor Mendel. Mendel was very, very sharp at
Mathematics and Science and had studied them in great detail.
Mendel wanted to be a teacher but he could not pass his teachers
exams (although he tried several times). Mendel gave up and
decided to become a monk. As a monk, Mendel did some teaching
(about the equivalent of teaching high school) and in a garden in
the monastery, he grew pea plants (Figure 9.1a).
Figure 9.1a
Mendel grew his pea plants which came in all sorts of colors and
variations. Mendel even would cross certain variations and
produce other varieties.
Now there are a couple of really neat things about working with
pea plants. First of all, they have both and male and female parts.
Second, using a paint brush or a stick, you can control which
flower breeds with which flower because you can take the male
sperm from one and transfer it the female parts of another plant.
This allows you to control which plants reproduce with which so
that when you look at the offspring, you know where they came
from.
Many people at the time had done similar things. But Mendel’s
incredible mathematic skills served him well and allowed him to
see things that others had overlooked. Mendel found that if he took
pure breeding purple flowers and pure breeding white flowers and
crossed them, he got babies that were all purple (we call this first
generation the F1 which stands for filial). But then if you took
those purple F1’s and crossed them with each other, you will get a
ratio of 3 purple flowers to 1 white flower). Now what was so
difficult was that you just didn’t get three purples ones and one
white one. You got something like 303 purple ones and 101 white
ones. So Mendel was able to look at these results (which were
always different) and say “hey, that’s a 3:1 ratio”. Here is a picture
of what I’m talking about (Figure 9.2a):
Figure 9.2a
So how does that work? Well, Mendel didn’t know anything about
chromosomes or DNA or genes or any of that stuff. But Mendel
came up with this: He reasoned that for each trait, such as flower
color, seed color, or seed shape, there are two factors that control
it. Each plant has a copy of 2 factors. The factors do not interfere
or erase each other; but sometimes one is dominant and will hide
the effects of a more recessive one. So in this case, there is what
we call an allele for purple flowers and an allele for what we call
white flowers. An allele is an alternate form of a gene. The purple
allele is dominant over the white allele and so, if you have one
copy of a purple allele in a plant and one copy of a white allele in a
plant your plant will always be purple.
Now let’s look at this a little closer. This time, we will look at
seeds that are yellow or green. Yellow seeds are dominant to green
seeds which are recessive.
First, we take pure breeding yellow seed plants and pure breeding
green seed plants. Pure breeding means that when you cross them
with themselves they also produce that same color. So, yellow seed
plants will always produce yellow seed plants and green seed
plants will always produce green seed plants. When we cross
yellow seed plants and green seed plants we get....all YELLOW
SEED PLANTS. (This is called a monohybrid cross because you
are crossing ONE type of trait). Here is how that happens (Figure
9.3a):
Figure 9.3a
Notice in the picture above the square with four boxes. This is
called a Punnet Square. In the part labeled 1, notice that we use the
capital letters YY in each yellow seed and yy in each green seed.
Each of these letters represents a gene on a chromosome. More
specifically, each letter represents an allele on a chromosome. For
example, lets say that the gene that codes for seed color is on
chromosome 4. The yellow seeded plants have two chromosome
4’s in each cell. On each chromosome #4 is a gene that codes for
yellow seed color. In contrast, the green seeded plants have the
gene codes for green seeds. Now when I say they have a gene that
codes for green or yellow seeds, that means it has a section of
DNA that makes a protein that makes the seeds yellow or green.
When the plants in 1 go through meiosis, they can only donate one
chromosome from each set (remember, you only get one set of
chromosomes from your Mom and one from Dad). If you are the
yellow plant, you can only donate yellow seed genes. If you are the
green plant, you can only donate green seed genes. That’s all they
have. So when you do the crosses, you see that all the babies (in
the part labeled 4) are Yy and are yellow. They have one yellow
allele and one green allele. But remember that yellow is dominant
and green is recessive so they will all look yellow. Now let’s cross
a Yy and a Yy and see what you get Figure 9.4a:
Figure 9.4a
When you cross Yy with Yy and do the punnet square notice that
you get a ratio of 3 yellow seeded plants to one green seeded
plants.
Let’s look at this. Smooth seeds are dominant over wrinkled seeds
which are recessive. Now lets take plants with seeds that are
yellow AND smooth and cross them with plants that have green
AND wrinkled seeds. In turns out that the gene for yellow or green
seeds is on a different chromosome than the gene for wrinkled or
smooth seeds.
So what do you get? You get all smooth and yellow seeded plants.
Now when you cross those, you get this ratio 9:3:3:1. This is called
a dihybrid cross. Here is the punnet square showing you all of the
possible gametes and how you get to that ratio (Figure 9.5a):
Figure 9.5a
Rule of Addition and Rule of Multiplication:
One thing you might be asked is, what are the chances of having
something that is green and wrinkled? Or, you might be asked,
what are the chances of having something that is green or
wrinkled? “And” questions require you use the rule of
multiplication. Meaning that you multiply the odds of each event
happening. So, what are the chances of making something green?
(1 in 4) What are the chances of making something wrinkled? (1 in
4) Multiplying these together you get 1 in 16. “Or” questions
require you use the rule of addition. In this case, you add them up.
What are the chances of being green or wrinkled? Well, (1⁄4 + 1⁄4
= 1⁄2 ). If you don’t believe me, count how many seeds in the big
punnet square at the top are either green or wrinkled!
So far, we have looked at some simple dominant/recessive traits.
But in the real world, things aren’t that simple. There are all kinds
of scenarios that make things more complex.
Incomplete Dominance.
Ok, let’s take a look at another type of plant. These are snap
dragons. Red snapdragons are dominant over white snapdragons.
But when you cross pure breeding red snap dragons with pure
breeding white snapdragons you get this (look at the F1) (Figure
9.6a):
Figure 9.6a
This is called incomplete dominance. In this case, it turns out if
you don’t have both copies making the protein to make the flowers
red then they end up being an intermediate of Red and White and
you get Pink.
Polygenic Inheritance
Sometimes more than one gene is responsible for a phenotype (a
phenotype is the physical appearance of something whereas the
genotype is the genetic makeup that codes for that phenotype).
Human skin color is a good example. There are at least 3 genes
that are responsible for human skin color. Each has a dominant
recessive property. So a person with AABBCC has very dark skin
and a person with aabbcc is an albino (has very light skin). Dark
skin genes are dominant over light skin genes.
Pleiotropy
The opposite of Polygenic Inheritance is pleiotropy. Pleiotropy
means that one gene controls or effects lots of phenotypes. For
example, there is a gene that codes for a protein called
hemoglobin. One allele for hemoglobin causes abnormal red blood
cells. This leads to anemia, poor circulation, possible brain damage
and a whole bunch of other characteristics.
Epistasis
There are times when one gene affects the expression of another
gene. Certain types of mice, for example, have a regular dominant
vs. recessive gene that codes for hair color. Black hair is dominant
over brown hair. However, there is another completely separate
gene that determines whether there will be any color in the hair at
all. This gene, when the recessive trait is expressed, causes the
mice to not have color in the hair so they turn out white. The
dominant version allows for the brown vs. black gene to be
expressed.
Multiple Allele Systems.
Even though you only have two copies of a gene (one on each
chromosome), sometimes there are more than 2 alleles you can
have. For example, there are three basic alleles for blood type: A,
B and O. But you can only have a maximum of two. (You might be
wondering about the + and -. That is called the Rh factor and + is
dominant over – which is recessive.)
Codominance
Sometimes two or more alleles are dominant at the same time.
Blood type is a good example of this codominance. A and B alleles
are both dominant and O is recessive. So if your blood type is A,
then your genotype could be AA or AO. If you have one A allele
and one B allele then your blood type will be AB.
Sex-Linked Traits
So far, we have looked at genes that might be on chromosome 1 or
7 or 13 or whatever. But some genes are located on the X and Y
chromosome. Since the X is so much bigger, it has more genes.
Color blindness in humans is an allele that is located on the X
chromosome. Males are more likely to be color blind than females
because they only have one X chromosome. Females can be color
blind but they must have had a dad that was colorblind and their
mom must have been a carrier. Punnet Squares for sex-linked traits
look a little different because you have to put the X’s and Y’s in
the squares with the alleles attached to them (Figure 9.6b)
Figure 9.6b
In this case, the male is shown on the right and female is on top.
The “B” represents a normal “non-colorblind” gene and the “b”
represents a copy of the color blind gene. The male in the bottom
right hand corner would be a possible color-blind child resulting
from these two people because he would have an X chromosome
from mom that carried the “b” allele. Since he is a male, he has no
other “X” chromosome and therefore no “B” allele to rely upon.
Most alleles make proteins that serve some positive function.
Some, however, are harmful to people. There are several alleles
that result in human genetic disorders. Here are a few examples:
Recessive disorders:
Cystic fibrosis – This is the most common lethal genetic disorder
in the United States. 1 out of 25 Caucasians is a carrier for cystic
fibrosis. The allele responsible for cystic fibrosis is supposed to
make a Chloride ion protein but it does not make it properly. As a
result, the movement of Chloride ions is restricted and people with
cystic fibrosis end up with a build up of mucus in their lungs and in
their digestive tract.
Sickle-cell Anemia- This recessive disorder is most common in
African-Americans (in the U.S.). The allele codes for an abnormal
form of hemoglobin which cause people with sickle-cell anemia to
have abnormal red blood cells (they are sickle-cell
shaped). Because of the sickle-cell shaped red blood cells, people
with this disorder have poor circulation. The allele that causes
Sickle-Cell Anemia might have evolved because it is advantageous
in some parts of the world. In parts of Africa, people who are
carriers of sickle-cell anemia seem to be more resistant to Malaria.
Malaria also infects red blood cells and kills many, many people in
places like Africa. Having one allele with sickle- cell (but not both)
causes slight changes in the red blood cell shape which leads to
resistance to Malaria (Figure 9.7a).
Figure 9.7a (1 is a picture of normal red blood cells and 2
represents those that are “sickled”).
Tay-Sachs – This disorder is most common in a specific group of
Jewish people that live in central Europe. In Tay-Sachs, an enzyme
that is important for breaking down a brain lipid is defective in
people that carry two copies of the allele. This results in seizures,
blindness and brain damage.
Dominant Disorders:
Achondroplasia – This disorder is also known as dwarfism. It is a
dominant disorder and therefore only requires one allele to cause
the phenotype. In fact, homozygous dominant individuals do not
survive long enough to even be born. This disorder causes a lack of
growth in the long bones (such as your femur and humerus).
Huntington Disease – Huntington disease is a dominant disorder
that causes the destruction of the nervous system. The symptoms
of Huntington disease do not even appear until about middle age.
Therefore, many people pass on the allele to Huntington disease
before they even know they had the disease.
This is also a good place to mention that just because something is
dominant or recessive has nothing to do with how common it is.
Many people assume that genes that are dominant are more
common than those that are recessive. This is not necessarily true.
The gene for Huntington’s, for instance, is actually more rare than
the recessive form of the gene.
Polydactyly – Is a dominant genetic disorder which causes a person
to have extra toes or fingers (Figure 9.8a).
Figure 9.8a
Abnormal Chromosome Numbers or Chromosome Structure:
Some genetic disorders are not caused specifically by a gene that
makes a protein. Some are caused because there is too much DNA
or not enough. Recall our discussion when we talked about
Meiosis and the fact that you start out with a diploid cell and you
make 4 haploid cell and each has one of each chromosome in it (1
chromosome 1 and 1 chromosome 2 and so forth). Well,
sometimes a mistake happens and you might get two chromosome
2’s in one cell and none in another. Here are some specific
examples of these cases:
Down Syndrome - Down Syndrome occurs when you get 3
chromosome 21’s in a cell. This normally happens when a normal
sperm fertilizes an egg that has 2 chromosome 21’s ( remember, an
egg should have only 1 chromosome 21). The effects of down
syndrome are highly varying. Some children have very few effects
while other have a wide range of developmental and neurological
problems. As a woman gets older, the chance of her having a child
with down syndrome increases:
Mother’s Age Chance of having child with Down’s Syndrome
20 30 40 45
1 in 1925 1 in 1205 1 in 110 1 in 32
Klinefelter - A variety of chromosome number disorders occur
with the sex chromosomes. For example, a person might end up
with two X’s and one Y. Are they male or female? They turn out to
be males with a condition called Klinefelters Syndrome. These
males may develop breasts, lack facial hair, have a tall stature and
have dysfunctional testes.
Cri-du-Chat – Sometimes part of a chromosome will break off.
This is called a deletion. An example of a deletion is Cri-du-chat
which means “cry of the cat” in French. People
with this disorder often have a small head, low set ears and they
often make a crying sound that sounds like a distressed cat.
Biotechnology:
The discovery of DNA and genetics has led us into a new field of
technology. Modern biotechnology allows us to grow insect
resistant food, solve crimes, clean-up oil spills, treat diabetes and
heart disease, just to name a few examples. We will now go into
some detail about how we can accomplish some of these.
One of the most important discoveries came in the 1970’s with the
discovery of restriction enzymes. Restriction enzymes are found in
bacteria and they basically make cuts in DNA. Doesn’t sound like
much does it? But they don’t just cut the DNA anywhere. They
make cuts in specific sequences. And there are lots of different
restriction enzymes and each recognizes and makes cuts in specific
places. So, one thing we can do is this. Let’s say you have a
protein you want to make, like insulin. Insulin is needed by
diabetics to control blood sugar. Well, you take one of my cells
that has a gene for making insulin. You use a restriction enzyme to
cut the gene out of one of my cells. You then use the restriction
enzyme to make a cut in a bacteria’s DNA. You then mix the
insulin DNA gene from my cell with the bacteria. The bacteria will
pick it up and insert it into the section of DNA that was cut out by
the restriction enzyme and will start making that protein. It’s really
a brilliant invention!
We can use this same technology to basically insert a gene in
almost anything. I leave the other examples up to your book but
they include inserting growth genes in salmon (so they grow faster)
and inserting insect resistant genes from bacteria into plants (such
as cotton plants) so that insects cannot eat them as easily!
Coming Up....(but you need to know at
least this part for this coming quiz!)...
One of the basic characteristics of life that we discussed way back
in the beginning of the semester was that all living things are
capable of evolving. We have also spent some time taking about
how evolution works and how small genetic changes can occur in a
population over time. This was called microevolution. In the
upcoming weeks, we will be learning more about larger genetic
changes over time (or macroevolution).
Unlike microevolution which can often be easily measured over a
persons lifetime, macroevolution may take millions (or billions) of
years. So, we cannot gather the same type of evidence to study
macroevolution like we can with microevolution. When studying
macroevolution, we often examine the bones or fossilized remains
of organisms and compare them to similar organisms. We must be
very careful, however, and clearly distinguish between similarities
that are homologous vs. those that are analogous. When two
structures are homologous, we say that they develop from the same
basic tissues or
they share a common ancestry. When two things are analogous, we
say that they share a similar function. For example, the rear leg of
a horse and the rear leg of a cat are clearly homologous. They have
the same basic bones located in the same basic parts and they grow
from the same basic sets of developmental cells. A birds wing and
an insects wing, on the other hand, are clearly the same in function,
but they come from totally different body parts. The wings of a
bird are modified front arms whereas the wings of an insect grow
out of its back. So, these have the same function but they are not
homologous. You would be incorrect, for example, to say that
bird’s and insects are very closely related to one another in an
evolutionary sense because they can both fly.
ASSIGNMENT #9 – Print this sheet off and turn it in with your
lab next week. This sheet of paper goes on top (then the lab).
1) Where do restriction enzymes come from?
A) birds B) the soil C) bacteria D) humans E) Insulin
2) Which of the following is an example of a dominant disorder?
A) Polydactyl B) Cri du Chat C) Down Syndrome D) Klinefelters
Syndrome E) Tay Sachs
3) Which of the following is an example of a recessive disorder?
A) Polydactyl B) Cri du Chat C) Down Syndrome D) Klinefelters
Syndrome E) Tay Sachs
4) Cross-over occurs during:
A) Anaphase I of meiosis B) Anaphase II of mitosis C) Prophase I
of meiosis D) Metaphase I of meiosis E) Telophase II of mitosis
5) If a man and woman are heterozygous for both tay sachs and
cystic fibrosis, what are the chance of them having a child with
both tay sachs and cystic fibrosis?
A) 1⁄2 B) 1⁄4 C) 1/8 D) 1/9 E) 1/16
6) What is Marfan’s syndrome? 7) What is PCR and why is it
useful (book or internet)? 8) What are stem cells and what can they
be used for (book or internet)? 9) Explain what a STR is how it is
useful in crime scene investigations (book or internet). Words that
you may be asked to define or use in fill-in-the blank types of
questions:
Meiosis, Prophase I, Metaphase I, Anaphase I, Telophase I,
Prophase II, Metaphase II, Anaphase II, Telophase II, Cross-over,
Tetrad, Centromere, Sister Chromatids, Daughter Chromosomes,
Daughter Cells, Mendel, P, F1, F2, Homozygous, Heterozygous,
Dominant, Recessive, Sex-Linked, Codominate, Punnet Square,
Monohybrid Cross, Dihybrid Cross, Multiple Allele System,
Hemophilia, Colorblindness, Marfan’s Syndrome, Down
Syndrome, Cri du Chat, Klinefelter, Tay Sachs, Cystic Fibrosis,
Restriction Enzymes, PCR, STR, Homology, Analogy.