1. 1. Kelp, a large, multicellular type of algae, has air bladders in the blades to help them float near the
surface of the ocean where they can get more sunlight. Assume that having many air bladders (M) is a
dominant trait. What gametes would be produced by a heterozygous individual?
2. 2. In humans, sickle cell anemia is a disease caused by a mutation of a single locus gene which codes for
an important blood protein. The allele for the normal protein (S) is dominant to that for the one causing
sickle cell anemia. What gametes would be produced by an individual who suffers from sickle cell
anemia?
3. 3. In areas of the very deep and damp southeastern United States lives what is, for all intents and purposes,
a giant flying cockroach known as a Palmetto bug. Assume that long antennae (L) are dominant to short
antennae and that normal wings (N) are dominant to bent ones. What gametes will be produced by an
individual that is homozygous dominant for antennae and heterozygous for wings?
4. What gametes will be produced by an individual that is heterozygous for antennae and homozygous
recessive for wings?
5. 4. In horses, the color black is due to a dominant allele (B) and chestnut color is due to the recessive allele
(b). The trotting gait is due to a dominant allele (T) and the pacing gait to the recessive (t). What will be
the gametes produced by a homozygous black pacer? Suppose we cross the black pacer above with a
homozygous chestnut trotter. What will be the gametes produced by the F1 generation?
6. 5. Suppose you are designing a new Jurassic Park and you would like to have the crowd drawing presence
of Velociraptor without the danger of having your crowds eaten. You have managed to come up with two
varieties, very aggressive (A), which is apparently a dominant trait in Velociraptors, and non-aggressive.
They also come in two colors, red (R), which is dominant, and green. At the moment you have a green
female that is homozygous dominant for aggression and a non-aggressive male that is homozygous
dominant for color. What gametes will be produced by the male and female? What gametes will be
produced by the F1 generation of these two Velociraptors (providing of course that the female doesn't
dismember the male before mating)?
7. In dogs, wire hair (S) is dominant to smooth (s). In a cross of a homozygous wire-haired dog with a
smooth-haired dog, what will be the phenotype of the F1 generation? The genotype? What would be the
ratio of wire-haired to smooth-haired dogs in the F2 generation ?
8. Woodrats are medium sized rodents with lots of interesting behaviors. You may know of them as packrats.
Let's assume that the trait of bringing home shiny objects (H) is controlled by a single locus gene and is
dominant to the trait of carrying home only dull objects (h). Suppose two heterozygous individuals are
crossed. How many of each genotype would be expected if only 4 offspring were produced? How many
of each phenotype?
9. Saguaro cacti are very tall cylindrical plants that usually have two L-shaped arms, one on each side.
Suppose you lived in southern Arizona where the Saguaro cactus is common and you happen to have one
growing in your yard. Your Saguaro has two arms but one is longer than the other. Now, assume that arm
length in these cacti are controlled by a single gene with arms of the same length (A) being dominant to
arms of different lengths. What is the genotype of your cactus? Could one of the parents of your cactus
have had a phenotype with arms the same length? If so, what would have been the genotype of that
parent? Suppose you cross your cactus with that of your neighbor which has arms of the same length.
10.
Your great grandchildren (it takes a Saguaro cactus a long time to mature) find that half of the
resulting offspring have arms the same length and half have arms of different lengths. What was the
genotype of your neighbor's cactus?
11.
The common grackle is a species of robin-sized blackbirds that are fairly common (hence the
name) over most of the United States. Suppose that long tails (L) were dominant to short tails in these
birds. A female short-tailed grackle mates with a male long-tailed grackle who had one parent with a long
tail and one parent with a short tail. What is the male's genotype? How many of each genotype will be
found in the F1 generation (assume 4 offspring)? How many of each phenotype will be found in the F1
generation?
12.
The ability to curl your tongue up on the sides (T, tongue rolling) is dominant to not being able to
roll your tongue. A woman who can roll her tongue marries a man who cannot. Their first child has his
father's phenotype. What are the genotypes of the mother, father, and child? What is the probability that a
second child won't be a tongue roller?
13.
In northeast Kansas there is a creature know as a wildcat. It comes in three colors, blue, red, and
purple. This trait is controlled by a single locus gene with incomplete dominance. A homozygous (BB)
individual is blue, a homozygous (bb) individual is red, and a heterozygous (Bb) individual is purple.
What would be the genotypes and phenotypes of the offspring if a blue wildcat were crossed with a red
one? What are the genotypic and phenotypic ratios of the F2generation?
14.
2. The lubber grasshopper is a very large grasshopper, and is black with red and yellow stripes.
Assume that red stripes are expressed from the homozygous RR genotype, yellow stripes from the
homozygous rr genotype, and both from the heterozygous genotype. What will be the phenotypic ratio of
the F1 generation resulting from a cross of two grasshoppers, both with red and yellow stripes (red : both :
yellow)? What would be the genotypic ratio of the F1 generation (RR : Rr : rr)? What genotypes would
be produced by crossing a grasshopper with both color stripes and one with yellow stripes (choose all that
apply)? What phenotypes would be produced by crossing a grasshopper with both color stripes and one
with yellow stripes (choose all that apply)?
15.
Suppose you have two rose plants, both with pink flowers. You cross the two plants and are
surprised to find that, while most of the offspring are pink, some are red and some are white. You decide
that you like the red flowers and would like to make more. What cross would you perform to produce the
most red flowered plants? Your mother decides she would like some of the pink flowered roses. Which
cross would give you the most pink flowered plants?
16.
4. A naturalist visiting an island in the middle of a large lake observes a species of small bird with
three distinct types of beaks. Those with short, crushing beaks (BB) consume hard shelled nuts, those with
long, delicate beaks (bb) pick the seeds from pine cones, and those with intermediate beaks (Bb), consume
both types of seeds though they are not as good at either. Assume that this difference in beak morphology
is the result of incomplete dominance in a single locus gene. Which of the mated pairs below will have the
best adapted offspring in a year in which most of the food available is in the form of hard shelled nuts?
What would be the phenotypic ratio of the F1 generation resulting from a cross of Bb x bb
(Short:Intermediate:Long)?
5. Racoons have rings around their tails and a habit of washing their food in water before eating it.
Suppose that both of these traits are controlled via incomplete dominance so that wide bands on the tail
are BB, medium sized bands are Bb, and narrow bands are bb and that washing all their food is WW,
washing some of their food is Ww, and washing no food is ww. How many of each genotype will be in the
F1 generation resulting from a cross of two racoons, both with medium sized tail bands and that wash
some of their food (assume 16)? How many of the F1 generation will have wide tail bands and won't wash
any of their food?
1. About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC). The
ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the result
of having two recessive alleles (t). Albinism is also a single locus trait with normal pigment being
dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who cannot taste
PTC has a father who is an albino taster. She marries a homozygous, normally pigmented man who is a
taster but who has a mother that does not taste PTC. What are the genotypes of the possible children
(choose all that apply)? What percentage of the children will be albinos? What percentage of the children
will be non-tasters of PTC?
2. Wolves are sometimes observed to have black coats and blue eyes. Assume that these traits are
controlled by single locus genes and are located on different chromosomes. Assume further that normal
coat color (N) is dominant to black (n) and brown eyes (B) are dominant to blue (b). Suppose the alpha
male and alpha female of a pack (these are the dominant individuals who do most of the breeding) are
black with blue eyes and normal colored with brown eyes, respectively. The female is also heterozygous
for both traits. How many of the offspring (assume 16) living in the pack will have each of the following
genotypes? What percent of the offspring will be normal colored with blue eyes?
3. In the breeding season, male Anole lizards court females by bobbing their heads up and down while
displaying a colorful throat patch. Assume for this question that both males and females bob their heads
and have throat patches. Assume also, that both traits are controlled by single locus genes on separate
chromosomes. Now, suppose that anoles prefer to mate with lizards who bob their heads fast (F) and have
red throat patches (R) and that these two alleles are dominant to their counterparts, slow bobbing and
yellow throats. A male lizard heterozygous for head bobbing and homozygous dominant for the red throat
patch mates with a female that is also heterozygous for head bobbing but is homozygous recessive for
yellow throat patches. How many of the F1 offspring have the preferred fast bobbing / red throat
phenotype (assume 16 young)? What percentage of the offspring will lack mates because they have both
slow head bobbing and yellow throats? What percentage of the offspring will have trouble finding mates
because because they lack one of the dominant traits?
4. Carrion beetles lay their eggs in dead animals and then bury them in the ground until they hatch.
Assume that the preference for fresh meat (F) is dominant to the preference for rotted meat and that the
tendency to bury the meat shallow (S) is dominant to the tendency to bury the meat deep. Suppose a
female carrion beetle homozygous dominant for both traits mates with a male homozygous recessive for
both traits. What will be the genotype of the F1 generation? What will be the phenotype of the F1
generation?
5. Suppose in a strain of soybeans, high oil (H) content in the seeds is dominant to low oil content and four
seeds (E) in a pod is dominant to two seeds in a pod. A farmer crosses two soybean plants, both with high
oil content and four seeds per pod. The resulting F1 offspring have a phenotypic ratio of 9:3:3:1 (High oil /
four seeds : High oil / two seeds : Low oil / four seeds : Low oil / two seeds). What genotype were the
parent plants? Suppose the farmer chooses two of the high oil / four seed plants and crosses them. The F2
generation have all high oil / four seed phenotypes. What were the genotypes of the plants chosen by the
farmer to cross? Which known genotypes might the farmer cross her high oil / four seed plants with to
determine their genotype?
1. A boy, whose parents and grandparents had normal vision, is color-blind. What are the genotypes for
his mother and his maternal grandparents. Use XB for the dominant normal condition and Xb for the
recessive, color-blind phenotype.
2. The bison herd on Konza Prairie has begun to show a genetic defect. Some of the males have a
condition known as "rabbit hock" in which the knee of the back leg is malformed slightly. We do not yet
know the genes controlling this trait but for the sake of our question, we shall assume it is a sex-linked
gene and that it is recessive. Now, suppose that the herd bull (the dominant one which does most of the
breeding) who is normal (XN) mates with a cow that is a carrier for rabbit hock. What are his chances of
producing a normal son? If he mates with this cow every year, what percentage of their daughters have
normal knees? What percentage of their daughters will be carriers of rabbit hock?
3. A woman with red-green color-blindness has a mother with normal vision. Knowing that colorblindness is a sex-linked recessive gene, can you determine what her father's phenotype is? If so, what is
it? The woman marries a man with normal vision. What is the probability they will have sons who are
red-green color-blind? What is the probability they will have daughters who are red-green color-blind?
5. Clouded leopards are a medium sized, endangered species of cat, living in the very wet cloud forests of
Central America. Assume that the normal spots (XN, pictured here) are a dominant, sex-linked trait and
that dark spots are the recessive counterpart. Suppose as a Conservation Biologist, you are involved in a
clouded leopard breeding program. One year you cross a male with dark spots and a female with normal
spots. She has four cubs and, conveniently, two are male and two female. One each of the male and female
cubs have normal spots and one each have dark spots. What is the genotype of the mother? Suppose a few
years later, you cross the female cub that has normal spots with a male that also has normal spots. How
many of each genotype will be found in the cubs (assume 4)? Will any of the cubs from this latest cross
have dark spots? If so, how many and of what sex will they be?
Some genes have more than two alleles. One of the best-known examples is the gene that is referred to as
the “ABO Blood Group,” which actually has quite a number of alleles. However, we will discuss/consider
only the three most-common of these. This gene codes for the structure of a certain antigen on the surface
of our RBCs. The three alleles we will work with are symbolized by IA, IB, and i. However, keep in mind
that a person can only have two alleles, two copies of a gene. Thus, the possible genotypes are IAIA, IAi,
IBIB, IBi, IAIB, or ii. (Sometimes, you will see these simplified as AA, AO, BB, BO, AB, and OO, but that
does make it harder to remember that these are all alleles for the same gene.) The allele, IA, codes for,
“make type A antigen,” the allele IB codes for, “make type B antigen,” and (to simplify things somewhat)
the i allele codes for, “I don’t know how to make either A or B.” Thus, both IAIA and IAi individuals
receive instructions to “make type A antigen,” and both IBIB and IBi individuals receive instructions to
“make type B antigen.” Individuals who are IAIB receive two sets of instructions: “make type A” and
“make type B,” so they have both the A and B forms of that antigen on the surface of their RBCs. People
who are ii don’t have any instructions to make either A or B, so by “default” they make what we refer to
as type O antigens. Since both IA and IB code for “make something” whereas i codes for, “I don’t know
how, therefore, both IA and IB are dominant over i. However, since both IA and IA code for “make
something,” neither of them is dominant over the other. Thus we say that IA and IB are codominant over i.
Suppose a person with type A blood and a person with type B blood get married. What are the
possible genotypes their children could have?
¼ of their children would have type A blood, ¼ would have type B blood, ¼
would have type AB blood, and ¼ would have type O blood.
There is another gene that codes for another, different antigen that also occurs on the surface of our RBCs,
and technically, that gene also has multiple alleles. However, most people either have or do not have one
particular allele called the “d” allele. This gene codes for an antigen that is called “Rh factor” because it
was first discovered in Rhesus monkeys. People who have instructions to “make d antigen” are referred to
as Rh+ (the allele is often symbolized by the letter “R”), while those who have “I don’t know how to make
d antigen” instructions are called Rh– (the allele can be symbolized by “r”). Since this is a totally separate
gene than the ABO blood group, if you’re doing a genetic cross that involves both ABO and Rh, that
would be a dihybrid cross. Ms. Johnston, Ms. Johnson, and Ms. Johnstone all entered the same
hospital and gave birth to baby girls on the same day, and all three babies were taken to the nursery to
receive care, there. Someone later claimed that the hospital mixed up the babies. As a hospital
administrator, it is your job to make sure that each pair of parents has the correct baby, so you order blood
typing to be done on all the parents and all the babies. Here are the results:
In humans, there’s another X-linked gene that codes for androgen (testosterone) receptors in our cells. The
dominant allele (XA) codes for “make functioning receptors that can correctly receive and bind onto
testosterone,” but there is a recessive allele (Xa) that either codes for “I don’t know how to make
testosterone receptors” or else “make ‘broken’ receptors that can’t receive and bind onto testosterone.” In
humans, there is also a Y-linked gene that codes for “make testes,” and when present, they, in turn, make
testosterone, and the testosterone, in turn, goes to other cells in the body, and when received by the
receptors, triggers other events within those cells. During embryonic development, one such event is
growth and development of male genitalia. Consider a person who is genotype XAY. Because this
person has a Y chromosome including a normally-functioning gene to make testes, at the appropriate time
during embryonic development, testes will form and will start to secrete testosterone. Because this person
also has the correctly-functioning allele for the androgen (testosterone) receptor gene, those receptors will
form and will begin functioning. As they receive the testosterone made by the testes, this will stimulate
development of male genitalia, and (assuming all other genes are working “normally”), this baby will be a
boy. Consider a person who is genotype XAXA. Because this person does not have a Y chromosome,
there is no gene to provide instructions to make testes, therefore no big prenatal surge of testosterone,
therefore no stimulus to make male genitalia (in spite of properly-functioning testosterone receptors), so
by “default,” female genitalia develop, and (assuming all other genes are working “normally”), this baby
will be a girl. ½ of the children will be females, and ½ will be males Consider a person who is
genotype XAXa. Because this person does not have a Y chromosome, there is no gene to provide
instructions to make testes, therefore no big prenatal surge of testosterone, therefore no stimulus to make
male genitalia (in spite of properly-functioning testosterone receptors in many/most cells), so by “default,”
female genitalia develop, and (assuming all other genes are working “normally”), this baby will also be a
girl. However, she is a carrier for the non-functioning androgen-receptor gene. Because of inactivation of
X chromosomes to form Barr bodies, some of the cells in her body will not be able to receive testosterone
messages, which might show up in things like having somewhat less pubic and/or armpit hair. ¾ of the
children will be females, and ¼ will be males No, that’s not a typo! The phenotype ratio is 75:25, not
50:50. There’s a “new” genotype in there that hasn’t been mentioned, yet: XaY, and the corresponding
phenotype is female. Let’s look at that person’s embryonic development. There is a Y chromosome, and
assuming its genes are functioning normally, testes do develop and begin to secrete testosterone. However,
all that testosterone has nowhere to go; the testosterone receptors don’t work, so as far as everything else
in her body is concerned, its as though all that testosterone isn’t even there. Thus, just exactly like
anyone/everyone else whose body doesn’t get a message/stimulus to make male genitalia, by “default”
female genitalia develop, and (assuming all other genes are working “normally”), this baby will be a girl.
This is called Androgen Insensitivity Syndrome (AIS). See the Sex-Linked Genes Web page for a more
detailed explanation.