Download Basic Mendellian Genetic

Basic Mendelian Genetics
Complete and Incomplete Dominance
In the field of "genetics" or the study of inheritance, scientists study how certain characteristics or traits are
inherited through sexual reproduction. This involves looking at how gametes are made, how alleles are carried
by gametes and what effect the combined gametes of an egg and sperm will have on the life functions of an
organism.
The father of genetics is Gregor Mendel, who studied the inheritance of various traits and characteristics in
ordinary pea plants in the 1860's in Europe. Even though Mendel uncovered the basic mechanisms of how
traits are inherited, no one read the paper he published and understood what he said until the process of
meiosis was finally understood in the early 1900's. Mendel, at the time of his studies, did not know that
chromosomes even existed, much less the genes found on them, and yet he was able to describe the way that
genes are carried on chromosomes, and how they combine to create whole organisms.
One of the most important discoveries Mendel made was that every organism has two alleles for each gene,
(except in a few cases involving sex chromosomes that we will discuss later). The two alleles each carry a
message for making proteins that somehow affect the organism. How these alleles interact with each other is
one of the fundamental principles of genetics. Mendel discovered two ways that this can work and called them:
"complete dominance and incomplete dominance."
Before we begin the discussion of Complete and Incomplete Dominance we need to understand some basic
genetic terminology.
Phenotype: The phenotype refers to the appearance of the organism. This could refer to some obvious trait
such as purple flower color, or to a biochemical trait such as the particular form of an enzyme.
Genotype: The genotype refers to the particular combinations of genes that give rise to the phenotype. These
are represented by pairs of letters which represent the genes involved in the genotype.
Allele: An allele is an alternate form of a gene. For example while a diploid individual might carry two copies
of a gene the copies may not be identical. Perhaps each gene codes for a slightly different form of an enzyme.
Homozygous: Homozygous refers to an individual having both alleles of the gene pair be the same allele. For
example both AA and aa individuals are Homozygous.
Heterozygous: Heterozygous refers to an individual having different (non- identical) alleles for each gene in
the gene pair. For instance the Aa individuals produced from the cross AA x aa are heterozygous.
F1 generation: F1 stands for first filial generation. These are the offspring from a particular set of parents
during a monohybrid or other sort of genetic cross.
F2 generation: The F2 or second filial generation refers to the offspring of the F1 generation when F1's are
crossed.
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Complete Dominance
In some genes there are alleles that if present, will express themselves in the organism no matter what the other
allele is. For example, if you have the allele for unattached ears, that's what you will have, NO MATTER
WHAT THE OTHER ALLELE IS!!
We have talked already the reasons for this. Alleles are sometimes mutated, or their codes changed. When this
happens, they often lose their ability to produce the protein the way it should be made. Because the protein no
longer functions, this allele has no impact on the organism. However, if the other allele on the homologous
pair chromosome is not mutated and still functions correctly, it will be able to make the protein needed by the
organism. This allele, (the non-mutated one), takes over and completely "masks" or hides the affect of the
mutation on the other allele.
Since organisms have two alleles for each gene, we generally write two letters to represent the two alleles in
their genetic makeup. Usually, geneticists use letters, like the letter "A" to represent an allele. Alleles that are
dominant are written as an upper case letter, (i.e. A, B or C). Alleles that are recessive are written as a lower
case letter, (i.e. a, b or c).
An example of a completely dominant relationship between two alleles is the ability to taste PTC paper. If you
were able to taste it, that is because of a dominant allele, which we will call "P." If you couldn't taste it, it was
because of a recessive allele, which we will call "p." Everyone has two alleles for the PTC gene. You can have
one of three possible pairings of those alleles, PP, or Pp, or pp. In order for you not to be able to taste PTC
paper, you have to have two recessive alleles or pp as your allele combination. If you had either PP or Pp, you
would be able to taste PTC paper. In the case of two dominant allele as a pair (PP), it is common sense that
you would taste the paper. If the allele pair you have is Pp, then you still have one "good" allele that is
masking or covering up the mutated allele, p.
Incomplete Dominance
In cases of incomplete dominance, when an organism is heterozygous, or possesses one of each kind of allele,
each allele is expressed. In other words, the proteins produced by both alleles of a gene have an affect on the
organism. What often results is a combination of the phenotypes produced by each of the alleles.
An example is the coloration of flowers in petunias. Some petunias are red. This is produced by an "R" allele.
Red petunias have the genotype "RR." White alleles are produced by the "r" allele and have the genotype of
"rr." There are also pink petunias, and they have the genotype "Rr" Neither allele dominates the other. They
both produce their own effect.
Actually, what is really occurring here is somewhat more complex than what it might seem. In each cell, one
of the alleles seems to dominate and it shuts the other allele down. If you were to look at a pink petunia under
the microscope, you would see cells that are both red and white in color. Because our eyes can't see this
closely, our eyes combine the two colors and we think we see pink.
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No pigment in
the cells of the
white flower.
Red pigments are
found in some
cells in the PINK
flower.
Red pigments are
found in every cell
of the flower.
You can see this same effect in the hair of many people who have light colored hair but not blond hair. You
will actually see that their hair is made up of several different colors of hair. Individual strands of hair may be
brown or blond and when placed together, they create what is sometimes referred to as a "dishwater blond
effect.
Another example of incomplete dominance is the coloration on some cows. You've seen cows that are spotted
in color, right? How do they get their spots? When they were small embryos, their cells were dividing up into
different regions of skin. In certain areas, one allele takes over and produces black coloration. In another area,
the other allele takes over and produces no color at all. Hence, black and white spots.
When working incomplete dominance problems, geneticists continued to use upper case letter and lower case
letters to represent two alleles (i.e. B = Black Cow, b = white cow). This is confusing because you think that
one allele is going to dominate the other when you write them side by side, like "Bb." In reality, neither allele
is dominating the other. Keep this in mind. Also, the name: "Incomplete Dominance" seems inappropriate,
since no dominance is involved. Some scientists therefore use the term "Co-Dominance" to refer to this
relationship between alleles.
Work on the problems relating to incomplete dominance on the worksheet "Complete and Incomplete
Dominance" to make sure you are familiar with how this concept works, but first make sure you know how to
set up genetics problems and calculate probabilities and percentages.
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Steps to setting up and solving genetics problems
1. After reading the problem, try to give symbols to the alleles, if this isn't already done for you.
Often, when describing a condition, a problem will tell you that an allele has a certain symbol. For
example, premature balding is dominant over normal head of hair. Premature balding = B and normal head
of hair = b. However, sometimes it won't and you will have to give them names. Dominant alleles are
given capital letters, such as "A, B or C." Recessive alleles are given small case letters, such as "a, b or c."
If the problem involves multiple alleles, the best way to name them is to use a single letter and then
smaller superscript letters for each allele. For example: Blood types: IA, IB, 10. Sometime you might also
have to name the phenotypes, as well. For example: Type A, Type B or Type 0 blood. Note that dominant
alleles are always written first and recessive allele second. For example, Bb, Hh, IAIO.
2. Remember these terms: Genotype and Phenotype
Genotype refers to the pairing of alleles that an organism has. Each organism has two alleles for each
gene. One allele is on the chromosome an organism received from its mother; the other allele is on the
chromosome the organism got from its father. All the normal body cells of the organism have two alleles
for each gene. (Sex cells have only one allele). An example of an allele pairing would be PP, or Pp or pp
for two alleles called P and p.
3. Identify the Dominant and Recessive alleles.
Dominant alleles mask the presence of a recessive allele. If P is the allele for the ability to taste PTC paper
and p is the allele that causes people to not be able to taste PTC paper, P is dominant to p. A person who
has at least one P allele in its genotype will be able to taste PTC paper. That means that both a PP person
and a Pp person can taste KTC. Only a pp person will not be able to taste.
4. Know these terms: Heterozygous, Homozygous Dominant and Homozygous Recessive.
A homozygous condition is when both alleles are the same kind. The allele pairing of PP is homozygous.
The allele pairing of pp is also homozygous. The Pp pairing is referred to as " heterozygous." A person
who is homozygous dominant has the allele pairing of PP. A person who is homozygous recessive is pp.
5. Have you identified what is known from reading the problem?
Solving genetics problems can be a lot like algebra word problems. The first thing you have to do to
successfully work the problem is start identifying what you know ...
If you are dealing with a cross of two organisms or people, which is the usual case, try to figure out what
the phenotypes are, and if possible what their genotypes are. Sometimes the problem won't tell you both,
but it will give you enough information to figure out what you need to know.
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For example. Straight hair line is dominant over widow's peak. A woman who has a widow's peak marries
a man who has a straight hair line. They produce a child with a widow’s peak. What were the genotypes of
the parents?
Let's start by giving a name to each allele. S = straight hair line. s = widows peak. This problem didn't give
you the genotypes of the parents directly, but rather indirectly. Since straight hair Line is dominant over
widow's peak, a person with widow's peak can only be homozygous recessive. Knowing this, the rest of
the problem is pretty straight forward.
6. Use a Punnet square where possible.
Punnet squares make your life easier. Invented by a scientist named "Punnet," they help you to figure out
the probabilities of a cross. You should use them in almost every problem.
Start by first figuring out what possible sex cells an organism can make from the information known, and
put the alleles or allele combination alongside each box. Females go on top. Males are on the side. Each
letter represents a single sex cell. (egg or sperm).
Using the widow's peak problem from above, we know that the female's genotype is "ss." A female with
this genotype will produce egg cells that each has a single "s" allele. The male's gametes are unknown but
we know that he has to contribute at least one recessive allele. Yet, he has a straight hairline, which is a
dominant trait. Therefore, he must heterozygous.
These are each egg cells
s
These are each a
sperm cell with
one allele each as
a result of
meiosis.
s
S
Each box represents a
25% chance of an allele
pairing occurrence.
s
s
S Ss
s
Combine the alleles, one
at a time. Always write
the dominant allele first.
s
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s
S Ss
s
Ss
s ss
ss
After combining all the
alleles your Punnet
square should look like
this.
7. Know what the parts of the Punnet square mean.
Each small square inside the larger square represents a predicted probability of 25% if you are looking at a
single cross of one allele pair. If the results have two or more boxes that are the same, (i.e. two boxes
above have Ss), then you can add those percentages together. For the above cross, the chance of producing
a Ss individual is 50%.
8. Follow standard conventions when answering problems.
In many problems, you will be asked to give the genotypes, phenotypes and their ratios. As far as I know,
I'm the only one who requires this, but I invented it when I was a pup like you and have used it ever since.
It gives you a full-proof way of making sure you don't make silly mistakes. Using the data from the
hairline problem above, write your answer as follows:
Genotype (G): Ss ss
Genotype Ratio (GR): 1 : 1
Phenotype (P): Straight Widow
Phenotype Ratio (PR): 1 : 1
9. 9 . Make your Big letters BIG and your little letters small.
One way to get yourself confused and make mistakes is to confuse letters are supposed to mean dominant
and recessive. An example would be the letter "C." If you use the letter C to represent a dominant allele,
and the letter "c" to represent a recessive one, they often, in the haste of writing them on paper, don't look
all that different. You might see "cc" if you were not paying careful attention and think it means "CC."
Find a way to distinguish letters from one another when you write them. This type of mistake is easy with
some letters, whose lower case letters look different from their larger case letters, (A and a, B and b, or D
and d.) It is not so easy with letters when the upper case looks very similar to the lower case, (0 and o, C
and c, P and p.) One way that I found works is to put a line under the upper case letter of a symbol for a
dominant allele if I think there is a chance it might get confused. For example, if I were doing the straight
hairline problem above, I might have written my dominant S with a line underneath it like this: S
10. Always write the dominant allele first.
Whenever you write the genotype of an organism that is heterozygous for a dominant and recessive trait,
always write the dominant allele first. Here are examples of the correct way to write heterozygous
genotypes: Aa, Bb, Cc. This is the wrong way: aA, bB, cB.
Do the problems on the sheet: "Basic Inheritance Problems"
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Basic Inheritance Problems
Naming Skills – Make sure you are comfortable naming alleles.
1. Premature baldness is dominant in humans over a normal head of hair, There are two alleles. Come up
with a letter to represent each allele and then show what the possible allele pairings are:
Allele for premature baldness: ___________________________
Allele for normal head of hair: ___________________________
What are the possible allele pairings? ___________________________________________________
2. If brown eyes in humans is dominant over blue eyes, and if the brown eye allele is represented as "B"
and blue as "b", then what color eyes will the following persons have?
BB ______________
Bb ______________
bb _______________
3. The term "homozygous" means that you have two alleles that are the same. "Homozygous dominant
means that both alleles are dominant alleles, while "homozygous recessive" means that both alleles are
recessive. "Heterozygous" means that you have one of each kind of allele. Write the term(s) that
correctly identify each of the following allele pairings:
BB _______________
Aa ______________
cc _______________
dd _______________
DD ______________
Dd _______________
4. If “G” stands for an allele that creates grey fur in grey squirrels and "g" stands for an allele that
produces no color, or white squirrels, then write down the correct allele pairing for the following
squirrels:
A white squirrel ______________
a homozygous dominant squirrel ______________
A squirrel that is
Heterozygous _____________
a squirrel that is grey _____________
5. Set up a Punnet square to show the cross of a white squirrel with a heterozygous squirrel.
6. What do the letters stand for outside the square? __________________
7. What do the paired letters stand for inside the four boxes in the square? __________________
8. What does each box represent inside the square? ____________________
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9. Each box represents what percentage? ____________________
10. If unattached earlobes (E) is dominant in humans to attached earlobes (e), then write down the
genotypes of the following individuals:
a. A person with attached earlobes: ___________________
b. A person with unattached earlobes: ____________________
c.
The “product rule” is a rule in math that states to calculate the probability of the occurrence of two random
events; you multiply the probability of each of the event’s individual chance of occurrence. For example,
if the chance of being struck by lightning is 1/10,000 and the chance that you will be in a car accident is
1/5, than what is the chance that both will occur? 1/10,000 * 1/5 = 1/50,000
11. If you are heterozygous for a gene, Aa, what are the chances that you will produce a homozygous
dominant (AA) child if you marry someone that is:
a. Also heterozygous: ____________________
b. Homozygous recessive: ___________________
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Phenotypes and Genotypes and Ratios
There are two simple words that we will use in the future to describe some concepts. I don't know why, but
students always have trouble remembering which one is which because they sound alike. You need to take
care to learn what they mean and if need be, memorize it.
A "genotype" refers to the pair of alleles that an organism has for a particular gene. For example, if we were
talking about the gene for attached or unattached earlobes, we know that the unattached earlobe allele is
dominant over the attached earlobe. So let's use "E" to refer to the unattached earlobe allele. We will use "e" to
refer to the attached earlobe allele. The possible allele pairings for this gene are: EE or Ee or ee. Each of these
is a genotype, or pairing of alleles.
"Phenotype" refers to the outward expression of the combination of a pair of alleles. The outward expression
of Ee would be a person with an unattached earlobe. The phenotype of this person is therefore: unattached
earlobe. The phenotype of an ee person would be a person with attached earlobes.
When you are given genetic problems, you will be asked to give both the possible genotypes and phenotypes
of the given cross. You will need to write them out in a clear and understandable way that everyone can
understand.
A person who is heterozygous (Ee) for the earlobe gene marries a person who is homozygous recessive (ee).
What will be the possible genotypes and phenotypes for this cross?
E
e
e
Genotypes: ___________________
e
Phenotypes: __________________
Another thing you will often be asked to do is give a ratio for the genotype and phenotypes produced as a
result of a cross. You are probably familiar with ratios. A ratio is simply a comparison between numbers. The
most common comparison is between two numbers. For example, what is the ratio of 2 to 4? The answer
should be 2:4, (the ":" means ratio).
However, the problem isn't quite done. You still have to reduce these numbers to the lowest common
denominator. That is, you have to divide them by any numbers that are capable of dividing either the 2 or 4
and producing a whole number. Both numbers are divisible by 2, so dividing each number by 2 we get l: 2 as
the ratio.
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Some other examples:
Compare 3 to 9 as a ratio. 3:9 but each number is divisible by 3, so the ratio is 1:3.
Compare 12 to 16 as a ratio. 12:16 but each number is divisible by 4, so the ratio is 3:4
Compare 36 to 72 as a ratio. 36:72, but both numbers are divisible by 4, so dividing each, we get
9:18. We are not yet done, yet. 9 and 18 are both divisible by 9, so the answer is 1:2.
Remember, you have to reduce the answer until no number (except 1) can be divided into both. It is possible
to compare more than two numbers. For example, the following numbers 2, 4 and10, expressed as a ratio are
2:4:10 which can be reduced by dividing by 2 to become 1:2:5.
Try these problems to see if you can understand the concept of ratios:
Express 9 and 12 as a ratio: __________________
Express 21 and 42 as a ratio: __________________
Express 42 and 64 as a ratio: ____________________
Express 25, 75 and 150 as a ratio: _____________________
The answers to these problems are: (3:4, 1:2, 21:32, 1:3:6)
Look at the following sample problem which will show you how to set up the answer to easily show all
genotypes, phenotypes and ratios.
The allele B produces brown eyes. The allele b produces blue eyes. B is dominant over b. In the cross of a
brown eyed person who is heterozygous for eye color with another heterozygous person, what would be the
genotypes and phenotypes produced and what would be the ratio of each?
B
b
B BB Bb
Genotypes: BB, Bb, and bb
Genotype Ratio: 1 : 2 : 1
b Bb bb
Phenotypes: Brown eyes and Blue eyes
Phenotype Ratio: 3 : 1
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Note that it is not possible to compare one number to zero. A ratio is, by definition, a comparison of two
numbers. 0 is not a number. This problem always comes up when a cross results in 100% or all of one thing
resulting. In these cases, you don't use a ratio. You just report the answer as 100%. Complete the following
problem:
The allele B produces brown eyes. The allele b produces blue eyes. B is dominant over b. A homozygous
brown eyed person marries a blue eyed person. Complete the cross and report the genotypes, phenotypes and
their ratios?
B
B
Genotypes: ___________________
b
b
Genotype Ratio: ______________
Phenotypes: _________________
Phenotype Ratio: _____________
Are you confused? If yes, ask some questions.
If no, then complete the Complete and Incomplete Allele Problems.
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Complete and Incomplete Allele Problems
1. A particular type of butterfly has a characteristic wing shape with several types of traits. One butterfly
has the wing shape shown below which is called "oval." The allele that creates this type of condition
appears to be dominant over another allele which produces the "V" shape wings shown below.
Geneticists call the oval wing producing allele "0" and the "V" shape producing allele "o".
A butterfly that is Oo is mated to a butterfly that is oo. What possible genotypes and phenotypes will
be produced by this cross and what are their ratios to each other? Remember: These problems are
easier to do if you make the different letters representing the allele types markedly different from each
other. A big "0" in your handwriting may look like a little "o".
Genotypes: ___________________
Genotype Ratio: ______________
Phenotypes: _________________
Phenotype Ratio: _____________
2. In the same type of butterflies, a scientist crosses an Oo butterfly with an Oo butterfly. What are the
genotypes and phenotypes produced and what is their ratios?
Genotypes: ___________________
Genotype Ratio: ______________
Phenotypes: _________________
Phenotype Ratio: _____________
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3. A particular type of mouse is known to have two types of tails. In one type of mouse, the tails are very
long. In another type of mouse, the tails are comparatively short in size. Long tail appears to be
caused by an allele “T”, that is dominant to another allele “t”, which creates the shorter tail.
A mouse which has a long tail is mated with a mother mouse that has a long tail. The female mouse
gives birth to 9 baby mice 6 of the mice have long tails and 3 of them have short tails. What were the
genotypes of the parents of the baby mice?
Prove your answer by showing a likely way that these alleles were inherited using a Punnet square,
and by showing what the predicted genotypes and phenotypes should have been as well as their ratios
to each other.
Genotypes: ___________________
Genotype Ratio: ______________
Phenotypes: _________________
Phenotype Ratio: _____________
Explain your rationale for choosing the above parental genotypes.
Did the actual cross produce the ratios you predicted? Why or why not? {Explain your answer in terms
of meiosis and gamete pairing.)
4. Two mice are mated that both have short tails. What would you predict the genotypes, phenotypes and
ratios of this cross to be? Can you make your prediction without using a Punnet square?
5. Suppose the cross in "4 produced 12 baby mice, 11 of which had short tails and 1 of which had a long
tail. What possible explanations might there be for the one long tailed mouse’s' condition?
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6. Sickle cell anemia is a disease in which some people's blood cells will suddenly take on an oblong
"sickle" shape, rather than the usual rounded shape of normal blood cells. The gene for the sickle cell
condition contains a mutation which changes only one amino acid in the hemoglobin protein, the
chemical that transports oxygen in the blood cells. The sickle cells will cause clotting in veins and
arteries and the unusual shape slows blood flow, with resulting damage to tissues and painful episodes
in affected persons. The normal allele is “S”. The mutated allele is “s”, and is recessive to “S”.
A person who is homozygous recessive marries a person who is heterozygous for this condition. What
are the probabilities they will have children affected by the disease? Show the likely genotypic
results, phenotypic results and the ratios of both.
7. PKU or "phenylketonuria" is a disease in which a person, who is homozygous recessive, lacks a
certain enzyme that converts the amino acid phenylalanine into another amino acid, tyrosine. Instead,
phenylalanine is converted into toxic chemicals that build up in cells and damage the central nervous
system. If left untreated, a child will gradually become mentally handicapped.
Two parents who are not affected by the disease have a child who tests positive at birth for PKU.
How would YOU explain to them the manner in which their child inherited this disease?
What are the chances that their next child might also have the disease (remember product rule)?
A person who carries a recessive allele that is masked by a dominant allele is said to be a "carrier."
What are the chances that this couple’s next child will be a carrier?
Was either parenting a carrier of the disease? Explain your answer.
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8. In Petunias, R is the allele for red color, while r is the allele for white color. Petunias that are
heterozygous, or Rr, are pink in color, displaying incomplete dominance. Cross an Rr petunia with an
rr petunia. Show the Punnet square, all genotypes and phenotypes, and all ratios produced from the
cross.
9. A homozygous dominant petunia is crossed with a heterozygous petunia. Show the Punnet square, the
genotype and phenotype produced and all ratios.
10. If a farmer mates a black cow with a white cow, he ends up with spotted cows. Can you explain how
this occurs? Show a Punnet square and the predicted genotypes and phenotypes to explain your
answer, as well as their ratios.
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11. Some Imups have recently been discovered that have a strange coloration. While most Imups are
grayish in color, these b p s are banded, with alternating segments in their thorax (middle section)
being white. No one is quite sure what causes this condition, since it has not been seen before. The
usual Imup coloration alleles are G and g, with G producing grey color and g producing white color.
See below.
Having a strain of "pure" grey Imups on hand (GG), a scientist crosses one of the banded Imups with a
grey one. ("Pure" means homozygous). The results the scientist got were as follows:
56 Grey and 53 Striped. Next, the scientist mated two of the striped Imups and got the following:
23 Grey, 52 Striped and 27 White.
Please explain what is going on here. Use Punnet squares to back up your explanation.
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Multiple Alleles, Pleiotropy, and Epistasis
So far, we have looked at the situation in which only a pair of alleles exists for a given gene. However, nature
is rarely so simple. In many cases, more than two alleles may code for the production of a protein at a single
gene. A good example of this is blood types. There is a protein which coats the outside of blood cells and is
responsible for blood types. An allele called IA is responsible for producing the protein A, while IB produces
the protein B. A is co-dominant with B. A homozygous IAIA individual produces blood type A. A person
with the genotype IBIB produces blood type B. However, a person who is IAIB creates the blood type AB.
There is another allele, called IO which does not produce any protein at all. Both IA and IB are dominant to IO.
Therefore, a person who is IAIO will have blood type A and a person who is IBIO will have blood type B. Only
if a person is IOIO will they have a blood type 0.
The table below shows all the possible combinations of alleles and the resulting phenotypes.
Genotype
Phenotype
IAIA
IAIO
IBIB
IBIO
IAIB
IOIO
Type A
Type A
Type B
Type B
Type AB
Type O
Genetic problems involving multiple alleles, like the blood type alleles, are worked the same way as problems
involving two alleles; the only difference is that there are more possible outcomes. In the case of blood types,
we end up with four different phenotypes, A, B, AB and 0. Here is an example of how a multiple allele
problem might be worked.
A person with the genotype IAIO marries a person with the genotype IAIB. What are the possible genotypes and
phenotypes of children resulting from this marriage and what are their ratios?
A
IA
IO
A A
A O
I I I I I
Genotypes: IAIA, IAIO, IAIB, AND IBIO
Genotype Ratio: 1 : 1 : 1 : 1
IB IAIB IBIO
Phenotypes: Type A, Type AB, Type B
Phenotype Ratio: 2 : 1 : 1
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There are many other types of multiple allele situations which get even more complex. Take the case of rabbit
coloration. Some rabbits have a brown coat that is caused by the allele CB. Other rabbits have a white coat,
caused by the allele Cb. Still others have a condition called the Himalayan color, in which the tips of their
nose, ears, legs and tail are colored but the body is white. This is caused by the Ch allele. Another allele, CCH,
gives rabbits a "chinchilla" coloration, or grey color. The CB allele is dominant to the Cb allele. The Cb allele
is, in turn, dominant over the Ch allele. Finally, this allele is dominant over CCH allele. The order of
domination can be written as:
CB > Cb > Ch > CCH
Pleiotropy and Epistasis
The conditions we have been examining so far involve single characteristics or traits produced by just one
gene. For example, in our rabbit example above, even though there are multiple alleles, they all produce just
one trait: fur color. However, occasionally, alleles have more far reaching affects and may cause differences
in more than one characteristic. An example of this is sickle cell anemia, previously described. Persons who
are homozygous recessive, ss, for sickle cell have a number of disabilities, such as painful swelling, and sore
joints, a feeling of coldness and shivers, and sometimes other extreme types of illness.
People who are albinos have a condition in which a recessive allele causes no pigment to be produced by a
number of cells in very different locations. Albinos often will have very white skin, no coloration in their hair
and no pigment in their eyes. These are multiple traits which are all affected by a single gene, and this type of
effect is called "pleiotropy."
Another type of interaction, called "epistasis," occurs when an allele at one gene site determines the behavior
of alleles at a completely different gene site. An example of this occurs in the fleshy red combs of chickens,
which top their heads. A gene in one area has the affect of changing the size of the combs, even though it has
nothing to do with the overall shape of the combs which is controlled by another gene. See if you can come
with an analogy that will help you remember the difference between pleiotropy and epistasis. One idea might
be to think of human behavior, and its intended and sometimes unintended consequences.
Now it is time to put your multiple allele knowledge to work.
Complete the Multiple Allele Problems.
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Multiple Allele Problems
1. A person with an AB blood phenotype marries a person with a type O blood phenotype. Show the
Punnet square, and all possible genotypes and phenotypes and their ratios.
2. A person with a genotype of IAIO marries a person with a phenotype of IBIO. Show the Punnet square,
and all possible genotypes and phenotypes and their ratios.
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3. In Imups, a trait has been observed in which some Imups seem to have fewer thorax segments that
others. Segments are repeating sections of the Imup, immediately following the head region. In most
insects and other invertebrate organisms, this portion of the body is referred to as the "thorax" region.
The tail section is referred to as the "abdomen." Even though Imups are not insects, we will use this
terminology in describing them.
Some Imups have 5 segments, while others have 4. A few are occasionally seen with 3 segments.
Scientists suspect that multiple alleles might be involved, but they aren't really sure.
The assumption is made that we are seeing the effects of three alleles, although the fact that it could be
that there are actually two alleles which are co-dominant is kept in mind. The alleles are named S3, S4,
and S5.
A pure breeding 5 segmented Imup is now mated to a pure breeding 4 segmented Imup. Here are the
results: 89 5-segmented Imups, no 4-segmented Imups.
What does this tell you about the type of inheritance between these two alleles, is it dominantrecessive or co-dominance? Why?
A "pure" 4 segmented Imup is now mated to a "pure" 3 segmented Imup. The results of this cross are:
93 4 segmented Imups, and no 3 segmented Imups.
What does this tell you about the relationship between the hypothesized S4 allele and the S3 allele?
At this point, can we rule out that co-dominance is involved between two alleles? Explain your
answer. Also explain whether there are any other types of crosses that could be done to confirm this.
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At this point in the investigation, what do you think the number of alleles are that are creating this
condition and what is the relationship between them?
An Imup with an unknown genotype but which possesses the 5 segment phenotype is crossed with an
Imup that has the 3 segment condition. Here are the results of the cross:
49 Imups with 5 segments
53 Imups with 4 segments
What is the actual genotype of each organism? Show what you think happened in this cross using a
Punnet square.
4. A particular type of spider is discovered that has red, black, blue or white abdomen coloration. If a
pure red is crossed with a pure blue, the resulting offspring are all black. If a pure red is crossed with a
white, however, the offspring are all red. If a pure blue is crossed with a white, the offspring is all
blue. How many alleles are involved in this condition and what is their relationship to each other?
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Di-Hybrid Crosses
The problems we have worked so far have been fairly simple ones involving just one pair of alleles. However,
we have to keep in mind that when organisms are undergoing meiosis to create gametes, there are thousands of
alleles being sorted out.
The complexity of this process and the variety that sexual reproduction produces can be illustrated by doing a
di-hybrid cross; that is, by following the inheritance of two different genes.
Basically, the process of solving a di-hybrid cross is the same as that with a single hybrid or set of alleles.
There are only two tricks to successfully working these problems: first, set up the problem correctly, and
second, keep track of everything.
Let's say I've got a female who can taste PTC, a dominant trait, and a male who can't taste, and they are
married. The female is heterozygous, Pp, for this condition, where as the male is homozygous recessive
(P=taste allele, and p = can't taste allele). At the same time, the male is heterozygous for balding. The allele
for going bald is dominant, B, over the allele for non-balding, b. The female is homozygous recessive for this
trait, (bb). A problem like this would ask you to determine all genotypic and phenotypic results and the ratio of
each. How would you work this problem?
First, write down what the genotypes of the parents are.
The male is ppBb and the female is Ppbb.
Next, determine the possible gametes that each individual can produce. Recall that so long as these allele pairs
are on different chromosomes, they will be sorted independently from each other, according to Mendel's Law
of Independent Assortment.
The male will have the following gametes: pB and pb
The female will have the gametes: Pb and pb.
If you have difficulty figuring this out, an allele Punnet Square will help you figure it out:
If you use the above method of a Punnet square to figure out what the possible alleles might be, don't get this
confused with the actual Punnet Square. In the actual Punnet Square we are about to use, each of the grouping
of letters represents a gamete. In the squares above, each of the letters represents a chromosome. Basically, the
squares above will tell you all the possible ways the chromosomes can "flip" during Metaphase 1 of Meiosis.
Completing two Punnet squares to determine the gametes is optional, but it does help prevent mistakes.
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To work the cross we need to set up another Punnet Square, like the one below:
Now fill in the blanks as follows.
Genotypes: PpBb, ppBb, Ppbb, and ppbb
Genotype Ratio: 1 : 1 : 1 : 1
Phenotypes: PTC/bald, non-PTC/bald, PTC/not bald,
and non-PTC/not bald
Phenotype Ratio: 1 : 1 : 1 : 1
NOTE: The allele pairs need to be written together
(Don't write them like: PbpB).
Seems pretty simple, just a few more letters, right? Well, in some case that is correct. You would use the same
size Punnet square and complete the problem like we just did. However, in many cases di-hybrid inheritance
problems can become much larger. Take for instance the following problem:
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Recall that Imups have several characteristics that have been closely studied by scientists. One of these
characteristics is straight or bent antennas. The other one is dark or white body color. Straight antennae is
caused by an allele, A, that is dominant over the bent antennae allele, a. Dark color, D is dominant over white,
d. Assume that these two different genes are found on different chromosomes so that independent assortment
occurs. Two Imups are crossed that are both heterozygous for both conditions. Predict all the possible
genotypes and phenotypes and their ratios.
So both the male and female Imups of this cross are AaDd (heterozygous for both traits). So we need to
determine what the possible genotypes of the sperm and egg would be so we do an initial cross using the
parent’s genotype.
A a
D
AD aD
d
Ad ad
Genotypes: AD, aD, Ad, ad
We use these genotypes as the letters to represent the sperm and egg in the cross between the parents below.
AD
aD
Ad
ad
AD
aD
Ad
ad
Then like in the smaller crosses you combine the rows and columns and then determine the genotypes,
phenotypes and ratios. Take this time to complete the cross above and fill in the following:
Genotypes: _______________________________________________________________________________
Genotype Ratio: __________________________________________________________________________
Phenotypes: ______________________________________________________________________________
Phenotype Ratio: __________________________________________________________________________
If you are ready, work the Di-Hybrid Problems.
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Di-hybrid Problems
1. Two Imups are being studied by researchers. One Imup is heterozygous for both antennae shape and
body color, while the other is homozygous recessive for antennae shape and homozygous dominant
for body color. Recall that straight antennae is caused by an allele, A, that is dominant over the bent
antennae allele, a. Dark color, D is dominant over white, d. Assume that these two different genes are
found on different chromosomes so that independent assortment occurs. Predict all the possible
genotypes and phenotypes and their ratios.
X
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2. In a certain type of flower, blue coloration is co-dominant with white coloration. The intermediate
color appears somewhat blue-green. Furthermore, thick stems are dominant to thin stems. Assume
these genes are on different chromosomes so that independent assortment occurs. Assign names to
each of the alleles, and then predict the genotypes and phenotypes, as well as the ratios of all for the
following cross: A plant that is heterozygous for both color and stem thickness with a plant that is
heterozygous for color, but homozygous recessive for thickness.
3. What percent of the flowers will be blue? _______________
4. What percent of the flowers will be thick? _______________
5. What percent of the flowers will be both white and thin? _________________
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Polygenic Inheritance
As long as you are now an expert in figuring out di-hybrid crosses, we will look at one additional way that
traits are inherited; it's called "polygenic inheritance".
Essentially, this is very similar to epistasis. Virtually all of the situations we have looked at so far have been
one gene-one trait type situations. In other words, one set of alleles at a particular location on a chromosome
was responsible for a given characteristic, like baldness (the B and b alleles) or ability to taste PTC, (P and p).
However, in some cases, some biological processes are more complex and rely upon more than one gene. For
example, hair color is polygenic. The enormous range of colors that occur in hair is due to the fact that more
than one gene is at work. In fact, no one knows exactly how many genes are at work in determining hair color.
Some estimate there may be as many as 10 or more.
Another characteristic that is polygenic is height. This makes sense, if you think about it. If height were
determined by just two alleles working at a single gene site, people would come in one of two heights! You are
either 6'1" or 5'4". That isn't what happens, obviously. The determination of height in humans is not well
understood, as are many of the polygenic characteristics. Some of the multiple genes can affect one another
and have dominant and recessive characteristics, which makes the whole process terribly difficult to figure out.
On the other hand, there are some characteristics that are polygenic which are fairly easy to understand. In
these cases, the number of Dominant alleles seems to determine how much you have of a given characteristic.
For example, in some plants, height of the plant seems to work this way. Let's work a sample problem.
A sunflower plant is polygenic for height. It has three genes, each of which has two alleles that are involved in
height determination, A and a, B and b, and C and c. The more dominant alleles in total a plant has, the taller it
will be. The tallest plant is AABBCC, while the shortest is aabbcc. A plant of medium height would be
AaBbCc.
Height is not the only characteristic determined by polygenic inheritance. Virtually any characteristic in which
there appears to be a range of variation will be a result of polygenic inheritance.
Complete the Polygenic Inheritance Problems.
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Polygenic Inheritance Problems
1. Dandelions show a wide variety of heights. A graph of the heights produces a "normal curve as shown
below. Scientists have isolated three genes that seem to control height in a polygenic manner. The
more dominant alleles a plant has, the taller it will be. Each allele seems to contribute the same in
terms of height, so that a plant with the genotype: AABbCc, with four dominant alleles, will be the
same height as a plant that has the genotype aaBBCC. With all recessive alleles, aabbcc, a dandelion
will grow to be 6 cm tall. Each dominant allele adds another 1 cm to the growth of the plant. What is
the type of inheritance involved here. Explain your answer.
2. Assume that everyone in the classroom is roughly the same age. Notice the variety of heights within
the class. For your study group, choose either males or females and create a table to record the survey
you will make of height. First, create categories for height. Each category should be two inches of
height starting from the shortest person to the tallest. Then ask each person their height,
(measurements aren't necessary, since most people will know their own height).
Record how many people fall into each category. For example, how many girls fall into the category
of height of 5'0 to 5'2. The next category would be 5'3" to 5'4". How many girls fall into that category?
Use the space below for your table.
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3. Create a bar graph of height (x axis) versus number of students in each category (y axis).
4. Notice the shape of your graph. It should resemble something like a bell. The most number of students
will fall into the middle of the range of heights. What is the type of inheritance involved in human
height? What is the basis for your answer? Explain.
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Sex Determination and Linkage
It appears that the genes that orchestrate the development of sexual characteristics are found on pair of
specialized chromosomes, called the sex chromosomes. (All other chromosomes are referred to as
"autonomies”). The sex chromosomes come in two varieties. One is an "X” chromosome, which appears fairly
normal. The other chromosome, called the “Y” chromosome, is quite different. It appears to be missing
information or rather it is just often smaller than the "X" chromosome. Unlike other chromosomes, the X and
Y sex chromosomes don't always have the same alleles. In some cases, there may be an allele on either one of
the chromosomes that is not found on the other one. Some of these alleles may have to do with sexual
characteristics, such as the development of certain sexual characteristics.
You probably know that in humans, males have an X and Y pair of sex chromosomes, while females have two
X chromosomes. The X-Y pairing that creates a male occurs in many organisms, but not all. In some cases, an
X-X pairing is a male. Furthermore, in some species, sex appears to be something the organism can control.
Some fish, for example, can change their sex, from female to male or vice versa. Recent discoveries have also
shown that in some species, the mother of the offspring also can play a role in determining the sex of her
offspring, after her eggs have been fertilized. So, don't think that in every case, XY = male, and XX = female;
it just happens to be the way it works in humans, which is what we will focus on here.
As a result of meiosis, a human will produce sex cells that contain either one X chromosome or one Y
chromosome. Half of a male's sperm will be one or the other, (see diagram below.) Females will produce eggs
that always contain an X chromosome. Thus, in a sense, in humans, it is the male's sex cells that determine the
sex of the offspring.
We can use a Punnet square to illustrate the possible combinations that will result when a male's sperm
fertilize a female's eggs. Fill in the square below:
X
X
X
Y
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What percent of the children produced, based upon the Punnet square above, will have the combination of
XY? ____________________ What percent will have the combination of XX? ____________________
As previously stated, some genes can exist on one of the sex chromosomes and not be found on the other. This
happens most often with a gene on the X chromosome that is not present on the Y. A good example of this is
hemophilia, a disease in which a certain protein that is involved in helping blood to clot is not manufactured by
an allele "h” which is found on the X chromosome. The normal allele, “H”, also found on the X chromosome,
produces the protein correctly, so that a person's blood will clot properly if they are cut or wounded. The Y
chromosome carries neither the H nor h allele. Look at the table below and notice the how the different allele
combinations give rise either to the normal condition or the disease condition.
Alleles
Normal
XHXH
√
XHXh
√
√
XhXh
XHY
XhY
Has the
Disease
√
√
One of the advantages of having two alleles for every gene now becomes apparent. A female who has one H
and one h allele can escape having the disease, because one good allele will "mask or make up for the
defective allele. A female who has this heterozygous condition of Hh is called a "carrier," however, because
she carries the disease but doesn't show it. Males are at a disadvantage because the defective allele is not able
to be masked in their chromosome pair. Since the gene only appears on the X chromosome and not on the Y,
the male will have the disease if he inherits a single X chromosome carrying the defective allele. In fact, men
are more at risk than women for a number of diseases because of this very same situation which may explain
the fact that in most large populations, females out number males by 5 1% to 49%, whereas we would expect
equal numbers of each.
When a particular gene is determined to be on a given chromosome, we say that it is "Linked" to that
chromosome. Thus the phrase "sex-linked" may be heard in connection with diseases or other conditions
inherited like hemophilia. The idea of linkage is not confined to sex chromosomes, however. We often talk
about genes that are "linked" together if they both appear on the same chromosome pair. There are perhaps as
many as 2,000 genes in humans that appear on the same chromosome pair. All of these genes are "linked"
together. During meiosis, the alleles for all of these genes will "travel" together since they are on the same
chromosome, instead of being independently assorted (according to Mendel's rule.) What this means is that the
frequency of finding various combinations of any two traits will be reduced. Scientists use this reduction of
frequency to determine which genes are on which chromosomes.
Do the problems on the work sheet entitled: Sex Determination and Linkage.
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Sex Determination and Linkage
NOTE: when you do these problems involving sex linkage, write the alleles the following way: XB, Xb and Y.
You set up the problem in a Punnet square the same way as shown below, with the male's gametes on the left
and the female's on the top. For males, always write the X chromosome before the Y. Always write the
dominant allele before the recessive allele.
Example Punnet square: A "carrier" female is married to a male with hemophilia.
1. A male who is normal for hemophilia marries a woman who is a hemophiliac. Show the Punnet
square, the genotypes, phenotypes and ratios of each for the cross.
2. The gene for pre-mature baldness appears to be sex linked. The allele of B causes a person to begin
loosing their hair at an early age (late twenties.). It is dominant over the allele b, which gives a person
a normal head of hair. Both alleles are found only on the X chromosome in humans. A woman who is
not losing her hair at age 45, married a man who at age 47 has a healthy head of hair. However, they
have two boys who are losing their hair at age 27. How is this possible? In addition to answering the
question, show the Punnet square, all genotypes and phenotypes and their ratios
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3. For the above cross, what would you predict to be the percentage of their children that could expect to
have the condition?
PREDICTION: ____________________
4. For the above cross, what would you predict is the chance that a male offspring would be bald?
PREDICTION: ____________________
5. For the above cross, what would you predict is the chance that a female would be bald?
PREDICTION: ____________________
6. A male who is normal for hemophilia marries a woman who is a heterozygous for hemophilia. Show
the Punnet square, the genotypes, phenotypes and ratios of each for the cross.
7. A male who is a hemophiliac marries a woman who is normal. They have a girl who is a hemophiliac.
What would the genotype have to be for each spouse? Show the Punnet square, the genotypes,
phenotypes and ratios of each for the cross that will provide evidence for your answer to the question.
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