Heredity
1. Sarah is doing an experiment on pea plants. She is studying the color of the pea plants. Sarah
has noticed that many pea plants have purple flowers and many have white flowers.
Sarah crosses a homozygous white flower and a homozygous purple flower. The cross results in
all purple flowers.
What is true of the color of pea plants?
A. Purple flowers and white flowers are recessive to red.
B. Purple flowers are dominant to white flowers.
C. White flowers are dominant to purple flowers.
D. White flowers and purple flowers are codominant.
2. A cross between two squash plants that produce yellow squash results in 124 offspring: 93
produce yellow squash and 31 produce green squash. What are the likely genotypes of the plants
that were crossed?
A. one YY, one Yy
B. both YY
C. both yy
D. both Yy
3. A cross between two squash plants that produce yellow squash results in 124 offspring: 93
produce yellow squash and 31 produce green squash. What are the likely genotypes of the plants
that were crossed?
A. one YY, one Yy
B. both yy
C. both YY
D. both Yy
4. A student crosses two true-breeding pea plants, one with green pods and the other with yellow
pods. If yellow is dominant over green, what phenotypic results will the student find in the F1
generation?
A. 75% green, 25% yellow
B. 75% yellow, 25% green
C. 100% yellow
D. 100% green
5. An organism's genotype can best be defined as its
A. number of recessive genes.
B. inherited physical appearance.
C. inherited combination of alleles.
D. number of chromosomal pairs.
6. One possible form of a gene that codes for a particular trait is known as _______.
A. a phenotype
B. a genotype
C. an allele
D. a chromosome
7. It is possible for an organism to inherit a gene with two dominant alleles. What is a gene with
two dominant alleles that are expressed at the same time?
A. codominant
B. polygenic inheritance
C. incompletely dominant
D. heterozygous
8. What is the term used to describe the heritable, physical characteristics of a living organism?
A. phenotype
B. pedigree
C. genotype
D. allele
9. Which hereditary rule explains why a self-fertilizing parent that is heterozygous for the A
locus (Aa) can produce offspring that are AA or aa?
A. dominance
B. codominance
C. principle of independent assortment
D. principle of segregation
10. An allele is one of the alternative forms of a gene that governs a trait. If a fern plant is
heterozygous for a particular trait, how can the alleles that control that trait be described?
A. two dominant
B. two recessive
C. three recessive, one dominant
D. one dominant, one recessive
11. A(n) _______ is a characteristic arising from genes located on chromosomes that are not
gender-determining.
A. autosomal trait
B. genotype
C. allele
D. sex-linked trait
12. Human height is a polygenic trait. This means that the
A. trait is controlled by more than one pair of genes.
B. trait is controlled by the genes inherited from the mother only.
C. trait is completely controlled by only one pair of genes.
D. trait is controlled by the genes inherited from the father only.
13. Carla receives an allele for blue eyes from her mother, and an allele for brown eyes from her
father. If brown eye color is a dominant trait and blue eye color is a recessive trait, what can be
determined about the color of Carla's eyes?
A. Carla's eye color can not be determined.
B. Carla has brown eyes.
C. Carla has green eyes.
D. Carla has blue eyes.
14. Fifty percent of the offspring produced by a cross between pea plants have seeds with a
wrinkled (r) appearance caused by the presence of a homozygous recessive gene. What were the
genotypes of the parents?
A. RR × Rr
B. Rr × Rr
C. Rr × rr
D. RR × rr
15. Guinea pig coat color is determined by a single gene. The allele for black coat color is
dominant to brown. In a cross between two black-haired guinea pigs, 20 offspring are born. If
both parents were heterozygous, probability would predict that approximately how many of the
20 offspring would have brown hair?
A. 10
B. 15
C. 5
D. 0
16. Trey goes to a rabbit farm to look at a group of newborn rabbits. The newborns are all
different colors. They are gray, black, white, light brown, and dark brown.
What type of inheritance pattern are these rabbits likely displaying?
A. incomplete dominance
B. codominance
C. multiple alleles
D. dominant-recessive
17. Punnett squares depict the genotypes of two parents and are used to predict the inherited
traits of offspring. Which of the following would be the missing predicted trait in the table
below?
A. AO
B. AA
C. OA
D. OO
18. In pea plants, tall (T) plants are dominant over short (t) plants. If a heterozygous (Tt) pea
plant is crossed with a homozygous dominant (TT) pea plant, all of the resulting pea plants
should be tall (TT or Tt). Each plant will receive a dominant allele from the homozygous
dominant plant, while they could receive either a dominant or recessive allele from the
heterozygous plant.
The fact that each plant gets only one allele from each parent plant is detailed in the Law
of _______.
A. Independent Assortment
B. Multiple Alleles
C. Genetic Inheritance
D. Segregation
19. Mrs. Smith has blood type A. Her father has blood type A, and her mother has blood type B.
If Mr. Smith has blood type AB, what is the probability that they will have a child with blood
type AB?
A. 0%
B. 25%
C. 50%
D. 100%
20. A cross between two squash plants that produce yellow squash results in 124 offspring: 93
produce yellow squash and 31 produce green squash. What are the likely genotypes of the plants
that were crossed?
A. both YY
B. one YY, one Yy
C. both yy
D. both Yy
21. Lupe grows pea plants in her garden. The pea plants have flowers that can be either purple or
white, with purple color being dominant to white color. The peas produced by Lupe's pea plants
can also be either round or wrinkled, with round peas being dominant to wrinkled peas.
Lupe crosses two pea plants that are heterozygous for both traits. If a gamete from this cross
receives a dominant allele for flower color, how does this influence the probability of the gamete
receiving a dominant allele for pea shape?
(Assume that the genes for flower color and pea shape follow the law of independent
assortment.)
A.
B.
C.
D.
It increases the probability that the gamete will receive a dominant allele for pea shape.
It decreases the probability that the gamete will receive a dominant allele for pea shape.
It has no effect on the probability of the gamete receiving a dominant allele for pea shape.
It causes the gamete to be unable to receive a dominant allele for pea shape.
22. A student crosses two pea plants. One is homozygous dominant for axial flowers, and the
other is heterozygous for axial flowers. If the student examines 200 offspring pea plants, which
of the following is a reasonable result?
A. 98 with axial flowers, 102 with terminal flowers
B. 149 with axial flowers, 51 with terminal flowers
C. 200 with axial flowers, 0 with terminal flowers
D. 47 with axial flowers, 153 with terminal flowers
23. Which of the following describes an allele whose characteristic phenotype is masked by the
presence of a second, different allele?
A. codominant
B. polygenic
C. recessive
D. dominant
24. Gregor Mendel developed several laws of heredity over the course of his genetic research.
What does the first law of heredity, the law of segregation, state about genes?
A. Alleles of different genes separate independently of one another during gamete formation.
B. Dominant alleles are always more likely to be inherited.
C. Mutations can only occur in heterozygous organisms.
D. Two alleles for a trait separate when gametes are formed.
25. Fifty percent of the offspring produced by a cross between pea plants have seeds with a
wrinkled (r) appearance caused by the presence of a homozygous recessive gene. What were the
genotypes of the parents?
A. RR × Rr
B. Rr × Rr
C. Rr × rr
D. RR × rr
26. According to Mendel's Law of Segregation, meiosis involves the separation of a parent
organism's alleles in order to form gametes. Since the alleles separate into different gametes,
only one allele passes from each parent on to an offspring. This segregation of alleles during
meiosis
A.
B.
C.
D.
increases the chance that an offspring will receive a dominant allele.
decreases the chance that an offspring will receive a dominant allele.
increases the genetic variability of the offspring.
decreases the genetic variability of the offspring.
27. AB blood type is an example of __________.
A. independent assortment
B. incomplete dominance
C. codominance
D. polygenic inheritance
28. Lisa breeds shetland sheepdogs. Both her male and female dogs are blue merles (genotype
Mm). About one fourth of the puppies have black and white fur (genotype mm). Half of the
puppies are blue merles like the parents (genotype Mm). However, one fourth of the puppies are
completely white with blue eyes and are completely deaf (genotype MM).
What type of inheritance pattern does this most likely display?
A. dominant-recessive
B. incomplete dominance
C. sex-linked
D. codominance
29. A certain type of flower has two alleles for color (blue, purple), and two alleles for stem
height (tall, short). A tall blue flower and a short purple flower are crossed, resulting in tall blue
flowers, short blue flowers, tall purple flowers, and short purple flowers.
What law does this example help to prove?
A. Law of Segregation
B. Law of Multiple Alleles
C. Law of Genetic Inheritance
D. Law of Independent Assortment
30. A recessive gene on the X chromosome codes for color blindness. Males are more likely to
be colorblind than females because
A. males have two copies of the X chromosome.
B. females have no copies of the X chromosome.
C. males have only one copy of the X chromosome.
D. color blindness is associated with high testosterone levels.
Answers
1. B
2. D
3. D
4. C
5. C
6. C
7. A
8. A
9. D
10. D
11. A
12. A
13. B
14. C
15. C
16. C
17. D
18. D
19. B
20. D
21. C
22. C
23. C
24. D
25. C
26. C
27. C
28. B
29. D
30. C
Explanations
1. If all of the offspring of a homozygous white flowered pea plant and a homozygous purple
flowered pea plant have purple flowers, this means that purple flowers are dominant to white
flowers. If white flowers were dominant, all of the offspring would have white flowers.
2. The genotypes of the plants that were crossed must both have been Yy. A cross between two
Yy parents would result in approximately 1/2 Yy offspring, 1/4 YY offspring, & 1/4 yy
offspring. Of those offspring, approximately 3/4 (YY and Yy) would express the dominant
phenotype. The other 1/4 (yy) would express the recessive phenotype (yy). This is consistent
with the results given for the cross.
A cross between two YY parents would result in 100% YY offspring that expressed the
dominant phenotype. Likewise, a cross between a YY parent and a yy parent would result in
100% Yy offspring that also expressed the dominant phenotype. A cross between two yy parents
would result in 100% yy offspring that expressed the recessive phenotype.
3. The genotypes of the plants that were crossed must both have been Yy. A cross between two
Yy parents would result in approximately 1/2 Yy offspring, 1/4 YY offspring, & 1/4 yy
offspring. Of those offspring, approximately 3/4 (YY and Yy) would express the dominant
phenotype. The other 1/4 (yy) would express the recessive phenotype (yy). This is consistent
with the results given for the cross.
A cross between two YY parents would result in 100% YY offspring that expressed the
dominant phenotype. Likewise, a cross between a YY parent and a yy parent would result in
100% Yy offspring that also expressed the dominant phenotype. A cross between two yy parents
would result in 100% yy offspring that expressed the recessive phenotype.
4. True-breeding plants are those that produce offspring with the same phenotype as the parents
when self-fertilized. This means that the parents must have been homozygous for the trait under
study. Whether the green versus yellow pods are controlled by one gene locus or multiple ones,
the F1 generation of a cross between homozygous dominant and homozygous recessive parents
will always resemble the dominant parent trait. Thus, the F1 generation of this cross will result in
100% yellow offspring.
5. The genotype of an organism can be defined as its inherited combination of alleles. The
phenotype of an organism, on the other hand, is its inherited physical appearance.
6. An allele is one possible form of a gene that codes for a particular trait. Humans have two
alleles for most traits. These alleles can be dominant or recessive. Dominant alleles are always
expressed in the phenotype of an organism that has either one or two copies of the allele.
Recessive alleles are expressed only if an organism has two copies of the recessive allele.
7. Codominance occurs when two dominant alleles are expressed at the same time. Incomplete
dominance occurs when an individual displays a phenotype that is intermediate between the two
parents. Polygenic inheritance occurs when several genes influence a trait. Heterozygous is the
term describing an individual with two different alleles present for the same trait.
8. The phenotype of an organism refers to the heritable traits, or characteristics, that are exhibited
by that organism. Each specific phenotypic trait can be controlled by one gene, multiple genes,
or a combination of genes and environmental influences.
9. The principle of segregation states that the two alleles present in the reproductive organs of an
organism will be separated, or segregated, into different cells as the gametes (sex cells) form.
As a result, each sex cell will only contain one allele for each gene locus. These segregated
alleles can then combine with a gamete of the opposite sex type, allowing one allele from each
parental gamete to combine to form the offspring.
So, if this heterozygous parent (Aa) combines with a homozygous dominant parent (AA), there is
a 50% chance that a homozygous dominant offspring (AA) is produced. If it combines with a
homozygous recessive parent (aa), there is a 50% chance that a homozygous recessive offspring
is produced (aa). If the heterozygous parent combines with another heterozygous parent, there is
a 25% chance of producing a homozygous dominant offspring, and a 25% chance of producing a
homozygous recessive offspring.
10. If the two alleles of a particular gene present in an individual are the same, the individual is
said to be homozygous. If the alleles of a particular gene present in an individual are different,
the individual is heterozygous for that trait.
11. An autosomal trait is a physical or biochemical characteristic arising from genes located on
chromosomes that are not gender-determining.
Traits coded for by genes on all human chromosomes other than X and Y chromosomes are
autosomal traits.
12. Polygenic traits are traits that are controlled by more than one pair of genes, so multiple
independent pairs of genes have similar effects on the same trait. Human height, body form, and
skin color are examples of polygenic traits.
13. Carla is heterozygous for the trait of eye color. She has one gene for blue eyes, and one for
brown eyes. Since the gene that codes for brown eye color is dominant, the trait for this eye color
will be expressed, and Carla will have brown eyes.
14. To produce 50% homozygous recessive plants, one of the parents must have contributed
100% of the recessive alleles to the cross; one parent must have the genotype rr. The other parent
must have had one recessive allele and one dominant allele to produce 50% of each characteristic
when paired with the recessive alleles from the homozygous recessive parent; this parent must
have the genotype Rr.
15. A cross between heterozygous parents would produce ratios of approximately 1/4
homozygous dominant (black hair), 1/2 heterozygous (black hair) and 1/4 homozygous recessive
(brown hair). This predicts about 3/4 of the offspring would have black hair and 1/4 would have
brown hair. It is important to remember that probability is not certain, so it would not be unusual
to find actual results that vary wildly from predicted numbers, especially in small offspring sizes.
16. Since the rabbits display more than two colors (and more than just variations of a color), it is
likely that the rabbits' pattern of inheritance involves multiple alleles. Rabbits actually have four
alleles, of which dark gray (black) is dominant to all other alleles and solid white is recessive.
17. Each square represents a segregated outcome. The missing square is a cross between O and O
and could only produce OO, which would be known as simply O.
18. The fact that each plant gets only one allele from each parent plant is detailed in the Law of
Segregation.
In normal body cells there are two copies of each chromosome. Each chromosome has its own
copy of an allele, so in a body cell, two different alleles can be present for one trait. However, in
gametes, there is only one copy of each chromosome and one allele present. Thus, each new
organism receives only one allele from each parent.
19. Typically, there are two alleles for a trait—a dominant trait and a recessive trait. The blood
type trait, however, has three alleles, which correspond to the blood types A, B, and O. The A
and B alleles are dominant to the O allele, but they do not show dominance over each other. The
result is that if a person has an A allele and a B allele, he or she will have type AB blood.
Mrs. Smith's parents have blood types A and B. Since Mrs. Smith has blood type A, it logically
follows that she must carry one A allele and one recessive antigen-free O allele obtained from
her mother, who has type B blood. (If her parents had both been homozygous for their blood
types, Mrs. Smith could only have had AB blood. Her mom must have been heterozygous (BO)
for her blood type in order for Mrs. Smith to have blood type A.)
So, if Mrs. Smith's blood type (AO) is crossed with Mr. Smith's blood type (AB), their offspring
will have a 50% chance of being blood type A (includes AA and AO), a 25% chance of being
blood type AB, and a 25% chance of being blood type B (BO only).
20. The genotypes of the plants that were crossed must both have been Yy. A cross between two
Yy parents would result in approximately 1/2 Yy offspring, 1/4 YY offspring, & 1/4 yy
offspring. Of those offspring, approximately 3/4 (YY and Yy) would express the dominant
phenotype. The other 1/4 (yy) would express the recessive phenotype (yy). This is consistent
with the results given for the cross.
A cross between two YY parents would result in 100% YY offspring that expressed the
dominant phenotype. Likewise, a cross between a YY parent and a yy parent would result in
100% Yy offspring that also expressed the dominant phenotype. A cross between two yy parents
would result in 100% yy offspring that expressed the recessive phenotype.
21. The law of independent assortment states that the inheritance of one trait will not affect the
inheritance of another trait. In other words, alleles assort independently of one another during
gamete formation.
If genes follow the law of independent assortment, this means that receiving a dominant allele
for one trait does not have any influence on the probability of the gamete receiving a dominant
allele for another trait. In Lupe's cross, if a gamete receives a dominant allele for flower color,
this has no effect on the probability of the gamete receiving a dominant allele for pea shape.
22. A Punnett square is a diagram used by biologists to determine the probability of an offspring
having a particular genotype. In a Punnett square, the parental genotypes are written on the
outside of the squares and the products are written inside the squares (much like a multiplication
table). The Punnett square for a cross between a homozygous dominant pea plant and a
heterozygous pea plant appears below.
Since all of the offspring possess at least one dominant allele (A), the expected result would be
100% axial flowers (50% homozygous dominant (AA), 50% heterozygous (Aa)).
23. An allele whose phenotype cannot be seen when an alternate allele is present on the other
chromosome is said to be recessive, because its phenotype is hidden, or dominated, by the other
allele. Recessive alleles are usually designated by lower case letters to indicate that they recede
in the presence of a dominant allele.
24. The law of segregation describes the behavior of chromosomes during meiosis. Specifically,
the law of segregation says that two alleles representing the same trait will always separate
during meiosis so that each gamete will only have one copy of the allele for that trait.
In another law, the law of independent assortment, Mendel states that the inheritance of one trait
will not affect the inheritance of another trait. In other words, the genes of an organism sort
independent of one another in the gametes (e.g., there is no relationship in inheritance between
the color of a pea plant and its' height).
25. To produce 50% homozygous recessive plants, one of the parents must have contributed
100% of the recessive alleles to the cross; one parent must have the genotype rr. The other parent
must have had one recessive allele and one dominant allele to produce 50% of each characteristic
when paired with the recessive alleles from the homozygous recessive parent; this parent must
have the genotype Rr.
26. The Law of Segregation states that different alleles for the same trait separate when gametes
are formed. Since the alleles separate into different gametes, an offspring could receive a gamete
with one allele or the other. Thus, a mother that is heterozygous for brown eyes (Bb) could pass
either a dominant brown allele (B) or a recessive blue allele (b) for eye color to her offspring.
One allele is no more likely than the other to be passed from parent to offspring.
The segregation of alleles during meiosis increases the genetic variability of the offspring.
27. AB blood type is an example of codominance.
Codominance is the condition in which a heterozygous individual expresses the phenotype of
two alleles. This happens when neither allele is recessive.
28. The merle gene in dogs displays the inheritance pattern of incomplete dominance.
Merle dogs usually have a solid base color (black, brown, etc.) and lighter gray or blue patches,
with a mottled effect to the fur color and some white markings.
The merle gene (M) is a dominant mutation, while the normal gene (m) produces normal colors
for shetland sheepdogs. When two merle dogs are bred, it results in some double merles (MM).
These double merles do not display the merle color, but are white and can have severe defects,
such as eye problems and deafness. Thus, the homozygous recessive, heterozygous, and
homozygous dominant genotypes all lead to different results, which makes this an example of
incomplete dominance.
29. The fact that the cross of a tall blue flower and a short purple flower leads to four varieties of
flowers is an example of the Law of Independent Assortment.
Since each allele and trait are sorted independently of each other, any combination of traits and
alleles is possible.
30. There are many genetic disorders—such as color blindness, hemophilia, and muscular
dystrophy—that are associated with recessive alleles on the X chromosome.
While many females may carry the defective genes, males have the disorders much more often
than females. This is because males have only one copy of the X chromosome and females
have two copies.
If a female is heterozygous (Rr) for hemophilia, she will be a carrier, but will not have the
disease because the defective gene is recessive. If a male has the gene for hemophilia, however,
he will have the disease because there is no duplicate copy of the gene.