Name:
Date:
Period:
Punnett Squares and Genetics
I. Intro: To do the following problems you will need to remember the following list of terms.
Test your memory of the terms or description in the column on the right with the
meaning or example given in the column on the left. Write the number of each answer
in the space provided.
1.
2.
3.
4.
5.
6.
7.
genotype
homozygous dominant
homozygous recessive
recessive
heterozygous (hybrid)
phenotype
dominant
_____ RR
_____ gene pair an individual carries, letters
_____ physical appearance from gene pairs
_____ phenotype always shown when present
_____ containing two different forms of a gene
_____ rr
_____ masked when dominant trait present
II. Assigning Symbols: For each of the following problems describe the genotype’s phenotype
as given as either homozygous dominant, homozygous recessive, or heterozygous.
8. DD ______________________________
9. Dd ______________________________
10. dd ______________________________
11. ss ______________________________
12. Yy ______________________________
13. WW ______________________________
In humans, brown eye color (B), is dominant over blue eye color (b). what are the phenotypes of
the following genotypes?
14. BB ______________________________
15. bb ______________________________
16. Bb ______________________________
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III. Crosses as Predictors: For each of the sets of problems below do the following:
a) assign letter symbols b) show the P cross c) draw a Punnett square and plot the gametes from
each parent d) list the genotypic ratios of the offspring as a fraction
e) list the phenotypic ratios of the offspring as a fraction
17. A heterozygous, smooth pea pod, plant is crossed with a wrinkled pea pod plant. Smooth is
dominant over wrinkled.
a)
c)
d)
b)
e)
18. In humans, acondroplasia “dwarfism” (D) is dominant over normal (d). A homozygous
dominant (DD) person dies before the age of one. A heterozygous (Dd) person is dwarfed.
A homozygous recessive individual is normal. A heterozygous dwarf man mates with a
heterozygous dwarf woman.
a)
c)
d)
b)
e)
19. In humans, free earlobes (F) is dominant over attached earlobes (f). if one parent is
homozygous dominant for free earlobes, while the other has attached earlobes can they
produce any children with attached earlobes?
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20. In humans widow’s peak is dominant over straight hairline. A heterozygous man for this
trait has children with a woman who is also heterozygous. Complete the Punnett for
possible offspring.
a)
c)
d)
b)
a)
e)
21. In pea plants, axial is dominant over terminal flower position. Cross two hybrid plants.
c)
d)
b)
e)
Working Backwards: Sometimes we only know about the offspring and we want to learn about the
parents. Patterns emerge in Punnett squares. For example, when both parents are heterozygous
the phenotypic ratio always comes out 3 to 1. If one parent is homozygous recessive and the other
is heterozygous, the phenotypic ratio always comes out 1 to 1. Keeping this in mind see if you can
solve the next two problems.
22. In pea plants, yellow seeds are dominant and green seeds are recessive. A pea plant with
yellow seeds is crossed with a pea plant with green seeds. The resulting offspring have
about equal numbers of yellow and green seeded plants. What are the genotypes of the
parents?
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23. In another cross, a yellow seeded plant was crossed with another yellow seeded plant and
it produced offspring of which about 25% were green seeded plants. What are the
genotypes of both parents?
IV. More Mendel: For the following questions, this cross will be used: Aa x aa
A represents axial flowers, a represents terminal flowers.
24. What is the dominant allele? ______________ recessive allele? ______________
25. What is the phenotype of Aa? ______________ aa? ______________
26. What fraction of the gametes from Aa will be recessive? ______________
27. What fraction of the gametes from aa will be recessive? ______________
28. Create a Punnett square. Write in the gamete possibilities for the Aa parent outside the
left side of the square. Write in the gamete possibilities for the aa parents above the top of
the square.
29. What fraction of offspring (inside the square) would be homozygous? ______________
30. What fraction of the offspring would be heterozygous? ______________
31. What fraction of the offspring would have axial flowers? ______________
32. What fraction of the offspring would have terminal flowers? ______________
33. In pea plants, tall is dominant and short is recessive. Cross a heterozygous tall with a
heterozygous tall plant. After the cross, list the genotypic and phenotypic ratios predicted.
(Remember to assign letters, figure the cross, draw a Punnett)
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V. A Dog Called Spot (?): Imagine this microscopic drama. A sperm cell from a male
dog fuses with an egg cell from a female dog. Each dog’s gamete carries 39
chromosomes. The zygote that results from the fusion of the gametes contains
78 chromosomes – one set of 39 chromosomes from each parent. One pair of the
zygote’s chromosomes are shown below.
Each chromosome of the homologous pair contains alleles for the same traits. But one
chromosome may have a dominant allele and the other a recessive allele. Use the drawings
and the table to answer the questions.
Trait
Hair Length
Hair Texture
Hair Curliness
Coat Pattern
Dominant Gene
Short (L)
Wiry (T)
Curly (H)
Spotted (A)
34. Will the new puppy have a spotted coat? Explain.
35. Does the female dog have a spotted coat? Explain.
36. Does the male dog have a spotted coat? Explain.
37. What will be the texture of the puppy’s coat?
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Recessive Gene
Long(t)
Silky (t)
Straight (h)
Solid (a)
38. Will the texture of the puppy’s coat resemble that of either of its parents? Explain.
39. Explain why you cannot completely describe the puppy’s parents even though you can
accurately describe the puppy.
IV. Dihybrid Cross: Two factor crosses involve analyzing two traits at the same time, such as
pea pod color and pod shape. Predicting the outcome of two-factor crosses require
basically the same procedure as that for crosses involving one trait. Keep in mind that
in two-factor crosses the genes controlling the two different traits are located on
nonhomologous chromosomes. During meiosis, nonhomologous chromosomes assort
independently. This means that each of the chromosomes of any pair of homologous
chromosomes has an equal probability of ending up in a gamete with either
chromosome from any other pair of homologous chromosomes. The genes that are
located on nonhomologous chromosomes also assort independently.
Because of independent assortment, a plant that is heterozygous for two traits
(genotype example AaBb) will produce equal numbers of four types of gametes – AB,
Ab, aB, and ab. What does all that mean!?!?
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Let’s look at some examples. Two-Trait Punnett square steps:
a) assign letter symbols
b) show the P cross
c) draw a Punnett square and plot the gametes from each parent
d) list the phenotypic ratios of the offspring as a fraction (we’re not going to list genotypes
for two-traits)
We’re crossing two hybrids for two traits, pea pod color and shape. In this species of peas, Yellow
is dominant over Green and Round is dominant over Wrinkled.
a) Y – yellow y - green
R - round
r – wrinkled
b) RrYy x RrYy
c)
d)
Yellow Round 9/16
Yellow Wrinkled
Green Round 3/16
Green Wrinkled
3/16
1/16
I wonder if that ratio means anything, looks
kind of familiar…
40. Alleles segregate during: _______________________________________
41. If RRYY was crossed with rryy: (R = round, r = wrinkled, Y = yellow, y = green)
a. What alleles would be in the gametes from parent RRYY? _________________
b. What alleles would be in the gametes from parent rryy? _________________
c. When the two types of gametes fuse, what is the genotype of the offspring? ________
d. What is the phenotype of the offspring? ______________________________________
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42. RrYy was crossed with RrYy.
a. List the 4 possible gametes from parent RrYy: _________________________________
b. What fraction of their offspring will be:
Round & yellow _________________
round & green _________________
Wrinkled & yellow _________________ wrinkled & green __________________
43. In horses, black coats (B) and walking gaits (W) are dominant to white coats (b) and pacing
gaits (w).
a. If the male horse is homozygous for both dominant traits, what is his genotype? ______
b. What alleles would be found in the horse’s sperm? _________________
c. The female horse is homozygous for both recessive traits, what is her genotype? _____
d. What alleles are found in the horse’s eggs? _________________
e. Construct a Punnett square chart to show the possible offspring from the mating of
these two horse types.
44. Now examine the results of mating two horses from the F1 generation.
a. What is the genotype of the male horse? _________________
b. What alleles would be contained in his gametes? ______________________________
c. What is the genotype of the female horse? _________________
d. What alleles would be contained in her gametes? ______________________________
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e. Construct a Punnett square chart to show the possible offspring from the mating of
these two horse types.
f. List the genotypic fractions of the possible offspring in the F2 generation.
___ BBWW
___BbWW
___bbWW
___BBWw
___BbWw
___bbWw
___BBww
___Bbww
___bbww
g. List the phenotypic fractions of the possible offspring in the F2 generation.
___ black walkers
___ black pacers
___ white walkers
___ white pacers
h. Conclusion: Which horse type(s) would be most likely to occur during the F2
generation? ____________________________________________________________
i.
Why would this be helpful to horse breeders?
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V. Analyzing Inheritance: Offspring resemble their parents by inheriting their genes for
characteristics. To learn about inheritance, scientists have experimented with breeding
various plants and animals. In each experiment shown in the table, two pea plants with
different characteristics were bred. Then, the offspring produced were self-pollinated to
produce a second generation of offspring. Consider the data and answer the questions that
follow. (Parents = P, First Gen = F1, Second Gen = F2)
45. In the first generation of each experiment, what generalization can you make about the
characteristics of the offspring compared to the parents’ characteristics? (tell me more
than, “they’re different”)
46. What generalizations can you make about the characteristics of the second generation
compared to the characteristics of the first generation? (Were ALL the characteristics
shown in the second generation shown in the first generation? Explain why you think this
occurred.)
47. Compare the second generation characteristics to the parents’ characteristics.
48. How would you explain why the second generation could express all the characteristics of
the parents even though the first generation did not.
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VI. Trihybrid Cross: Use the following information on fruit flies to complete a trihybrid cross.
49. A = wings, a = wingless, E = red eyes, e = white eyes, H = hairy body, h = hairless
P1 fruitflies are heterozygous for wings, red eyes, and hairy bodies.
a. What are the genotypes of the P generation?
_________________________________ X _________________________________
b. List the possible gametes for both parents.
Female: _________________________________
Male: _________________________________
c. Plot the possible gametes in the Punnett square below. Keep like alleles together or
it will get confusing…write small so genotypes of possible offspring for in each
square.
d. What are the possible phenotypes of the offspring?
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VII.
Gregor Mendel was an Austrian monk who studied genetics during the mid 1800’s. His
experiments with pea plants laid foundations for modern genetics and gained him the
title “Father of Genetics”. The traits Mendel examined in pea plants included: seed
shape, seed color, seed coat color, pod shape, pod color, flower position, and plant
height. Each trait is controlled by a gene. In these pea plant characteristics, each trait
had two different variations, or alleles. For example, genes on a chromosome contain
the instructions for plant height. Height is the trait. The alleles for height are short or
tall. Specific symbolism is used in genetics. A dominant allele, an allele always
expressed in a trait when present, is represented by a capital letter. For height, the
dominant allele is represented by “T”. A recessive allele, an allele only expressed in a
trait when the dominant allele is not present, is represented by the lower case letter of
the dominant trait. For height, the recessive allele is represented by “t”. For example,
a plant with genotype Tt will have a phenotype of tall, a plant with genotype tt will
have a short phenotype, and a plant with genotype TT will have a tall phenotype. If you
noticed, a genotype is the genetic make-up of a trait represented by letters while the
phenotype is the physical appearance of that trait. Below is a table showing the traits
Mendel observed (with dominant trait listed on top of picture).
Complete the following chart of the traits Mendel studied knowing the rules of genetic symbolism.
(Remember, dominant allele receives capital letter describing that trait while the recessive allele
receives lower case of same letter. In table above, dominant traits are listed above the recessive traits.)
Seed
Shape
Seed
Color
Seed
Coat
Pod
Shape
Dominant
Recessive
50. Define Dominant, Recessive, and Allele.
51. Define Genotype and Phenotype.
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Pod Color
Flower
placement
Plant
Height
52. Give an example of a trait. Also, list the two alleles for that trait, label which trait is
dominant and which trait is recessive.
VIII.
NonMendelian Genetics: Incomplete, Codominant, Multiple Allele, and X-Linked Traits
will be discussed in the following section. Remember, the “rules” for these types of
crosses are slightly different than those of traditional Mendelian genetics.
Incomplete Dominance: Remember during incomplete dominance there is no dominant allele and
the heterozygous condition makes a third different phenotype. When completing the questions
below assign letters to genotypes ensuring the third phenotype (intermediate) is a heterozygous
genotype. Use the same 5 steps from Mendelian genetics to complete the crosses below.
53. Spongebob breeds his red jellyfish with Patrick's blue jellyfish. The resulting 7 baby jellies
are purple.
a. If two of the purple jellyfish bred, what would be the predicted genotypes and
phenotypes of those jellies?
b. If one purple jellyfish and one red jellyfish bred, what would be the phenotypes of the
offspring?
c. Is it possible to produce more red jellyfish is you cross bred a blue jelly and a purple jelly?
Use a Punnett square to show your answer.
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54. When red Japanese 4 o’clock flowers are crossed with white flowers they produce offspring
that are pink.
a. What color of flowers would you get if you crossed two pink flowers?
b. What color flowers would you get if you crossed a red with a pink flower?
c. What type of flowers could you cross to obtain white flowers?
55. Andalusian fowl (a type of chicken) either have black, white or blue feathers.
a. If a blue feathered rooster is crossed with a black feathered hen what will the baby
chicks look like?
b. How could you tell from the description of the chicken this problem was incomplete
dominance?
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Codominance: Remember during codominance both conditions are expressed in a third
phenotype. When completing the questions below assign letters to genotypes ensuring the third
phenotype (intermediate) is a heterozygous genotype.
56. Mario and Luigi are growing some mushrooms. Mario has a red mushroom and Luigi has a
white mushroom. They breed their mushrooms to produce a mushroom with red and
white spots. Now, show the phenotypes if you bred a red and white spotted mushroom
with a solid white mushroom.
a. Is it possible to get a red and white spotted mushroom by breeding 2 white mushrooms?
Show a Punnett square to support your answer.
b. What are the possible phenotypes if you bred two red and white spotted mushrooms?
c. If Mario wants to raise a patch of mushrooms with both red and white spotted mushrooms
and solid red mushrooms, is this patch possible? If so, what are the parent mushrooms’
phenotype?
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57. In cattle, red coat color is codominant to white color. The intermediate color is called roan
(red and white splotches).
a. If a roan bull is mated to a white cow, what are the possible genotypes and phenotypes
of the offspring?
b. Old Man Whithers, a farmer, wants to have a pure herd of roan cattle that breeds true.
Is this possible? Why or why not?
Multiple Allele: so far we have studied traits or genes that are coded for by just two alleles.
However, some traits are coded for by more than two alleles. One of these is blood type in
humans. This is a violation of Mendel’s Principle of unit characteristics.
In humans, there are four types of blood; A, B, AB, and O. the alleles A and B are codominant to
each other and the O allele is recessive to both A and B alleles. So a person with the genotype AA
or AO will have A type of blood. See the tables below for other examples. Note the special
notation used when assigning allele symbolism to the different blood types.
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58. What are the possible genotypes that will produce B type blood? _____________________
59. What is the only genotype that will produce O type blood? __________________________
60. What is the only genotype that will produce AB type blood? _________________________
61. Let’s say, you are blood type O and you mate with a person who has AB blood.
a. Complete a Punnett square for this cross.
b. List the possible blood types of your offspring __________________________________
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62. In the 1950’s, a young woman sued film star/director Charlie Chaplin for parental support
of her illegitimate child. Charlie Chaplin’s blood type was already on record as type AB. The
mother of the child had type A and her son had type O blood.
a. Complete a Punnett square for the possible cross of Charlie and the mother.
b. The judge ruled in favor of the mother and ordered Charlie to pay child support
costs of the child. Was the judge correct in his decision based on bloody typing
evidence? Explain.
X-Linked Traits: As many of you know boys are different than girls. In human sex is determined by
the twenty third pair of chromosomes known as “sex chromosomes.” If you have two x-shaped
(XX) chromosomes you are destined to be a female. If you have an x and a Y-shaped (XY)
chromosome you are destined to be a male, genetically speaking. Since the X and y chromosomes
carry different information, any genes found on the X chromosomes are referred to as X-linked
genes (also sometimes referenced as Sex Linked genes). Therefore, women will have two alleles
for these genes because they have two (XX) chromosomes. On the other hand, men have only one
allele for each of these genes because they have only one X chromosome (XY). This is clearly a
violation of Mendel’s Principles of Unit Characteristics, which implies that you receive one set of
alleles form each parent.
Example: In fruit flies, the gene for eye color is carried on the X chromosome which is a sex
chromosome (X-Linked). The allele for red eyes is dominant over the allele for white eyes. If a
white eyed female fruit fly is mated with a red eye male, predict the possible offspring.
Step 1: Since the female has white eyes, she must be XrXr. The male is red eyed because he has
only one X chromosome, he has only one allele for eye color. His eyes are red so he must be X RY.
Since the allele R is present on the X chromosome only, and there is no other allele for eye color
because the male other sex chromosome is a Y chromosome.
Step 2: For X-Linked traits we need to list the genotype in a different fashion. We must identify the
individual as being male or female according to their sex chromosomes. Females are XX, and males
are XY. X Linked traits are found only on the X chromosome, therefore the letters are placed as
superscripts (above) the X chromosome. Therefore the genotype for the female fly is XrXr and the
male is XRY.
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The Punnett square for the parent flies are shown below.
The individual XRXr will be a female because she has two X chromosomes. She will have red eyes
because she has at least one dominant allele. The individual X rY will be a male because he has the
X and Y chromosomes. He will have white eyes because he has only one allele and it is recessive.
So from this cross you would expect all of the females to have red eyes and all of the males to
have white eyes.
63. Hemophilia is an X-Linked trait. A person with hemophilia is lacking certain proteins that
are necessary for normal blood clotting. Hemophilia is caused by a recessive allele so use
“N” for normal and “n” for hemophilia. Since hemophilia is X linked, remember a woman
will have two alleles (NN or Nn or nn) but a man will have only one allele (N or n). a woman
who is heterozygous (a carrier) for hemophilia mates with a normal man.
a. What are the genotypes of the parents?
b. Make a Punnett square for the above cross.
c. What is the probability that a male offspring will have hemophilia? _____________
d. What is the probability of having a hemophiliac female? ______________________
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64. Can a color blind female have a son that has normal vision? Color blindness is caused by an
X-Linked recessive allele. Explain your answer.
65. Baldness is an X-Linked trait. What parental genotypes could produce a bald woman?
Explain your answer.
66. In cats, the gene for calico cats is codominant. Females that receive a B and an R gene have
black and orange splotches on white coats. Males can only be black or orange, but never
calico. A calico female’s genotype: XBXR.
a. Show the cross of a female calico cat with a black male.
b. What is the probability the kittens will be black male? ________________________
c. What is the probability the kittens will be calico male? ________________________
d. What is the probability the kittens will be calico female? ______________________
e. Show the cross of a female black cat, with a male orange cat.
f. What is the probability the kittens will be calico female? ______________________
g. Describe the color of the male kittens.
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