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PREDICTING INHERITED TRAITS & PUNNETT SQUARE ANALYSIS
GENETICS TERMS AND VOCABULARY
DNA- Deoxyribonucleic acid. It is the molecule that codes for our traits.
CHROMOSOME - A structure found in the nucleus of a cell. It consists of DNA and
proteins. A chromosome contains smaller segments called GENES.
GENE- A segment of a chromosome that determines a particular trait of an organism by
coding for specific proteins.
GAMETE- Egg and sperm cells (sex cells). They have half the chromosomes compared do
other cells in the body (23 individual chromosomes for humans).
SOMATIC CELLS- Cells in the body other than egg or sperm. They have a full set of DNA.
For humans this is 23 pairs of chromosomes for a total of 46 chromosomes.
ZYGOTE- A fertilized egg. When a sperm and egg unite a zygote is formed that eventually
develops into a mature organism. Zygotes will have a two sets of chromosomes (in humans
this is 23 pairs or 46 total), one set from the mother and one from the father.
ALLELES- Different versions of the same gene. We have two alleles for each gene, one is
inherited from the mother and the other is inherited from the father.
PHENOTYPE- a person or organism’s characteristics or traits. A person’s phenotype can be
influenced by their genetic make-up (genotype) as well as the environment.
GENOTYPE- this is a person’s or organism’s genetic make-up. The genotype is indicated by
the set of alleles. Knowing what versions of the gene one has or ‘carries’ will help you
determine their traits (phenotype).
HETEROZYGOUS- having two different alleles for a given trait (the allele inherited from
the mother is different from the allele inherited from the father). Note: in the above
example Joe is heterozygous for tongue rolling (Rr), because the versions of the gene
(alleles) are NOT identical)
HOMOZYGOUS- having identical alleles for a trait (the allele inherited from the mother is
identical to the allele inherited from the father).
DOMINANT TRAIT- only one allele for the trait needs to be present for the trait to show
up (be expressed)
RECESSIVE TRAIT- both alleles for the trait must be present for the trait to show up (be
expressed)
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A Dog Called Spot
Imagine this microscopic drama: a sperm cell from a male dog
fertilizes an egg cell from the female dog. Each dog’s gamete contain 39
chromosomes and the zygote (which will eventually develop into a puppy) will
have a total of 78 chromosomes, one set from the mother and one from the
father.
Chromosome
From Mother
Chromosome
From Father
L
h
a
T
l
H
a
t
Two of the puppy’s chromosomes are shown above. It is a homologous pair
because each chromosomes contains alleles (versions of a gene) that code for
the same traits. One of the chromosomes in the pair came from the mother and
one came from the father.
The only difference between the two is that one may have a dominant allele
(capital letter) and the other a recessive allele (lower case) for a given trait.
To have a dominant trait the puppy only needs to have one copy of the dominant
allele. However, to have a recessive trait, they puppy must have both copies of
the recessive allele.
Using the chromosomes above, the chart below, and your vocabulary list to
answer the following questions.
TRAIT
Hair Length
Hair Texture
Hair Curliness
Coat Pattern
DOMINANT
Short = L
Wiry = T
Curly = H
Spotted = A
RECESSIVE
Long = l
Silky = t
Straight = h
Solid = a
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1. What is the texture of the puppy’s coat? How do you know? Explain.
2. What is the texture of the father’s coat? How do you know? Explain.
3. What is the texture of the mother’s coat? How do you know? Explain.
4. What is the pattern of the puppy’s coat? How do you know? Explain.
5. Is the pattern of the puppy’s coat the same as pattern of the parent’s
coat? How do you know? Explain.
6. Does either parent have curly hair? Which one(s)? How do you know?
Explain.
7. List the traits for which the puppy is homozygous. Tell the genotype
(letters) and the phenotype (the trait/appearance).
8. List the trait for which the puppy is heterozygous. Tell the genotype
(letters) and the phenotype (trait).
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9. Based on the information provided in this scenario, why can’t you
completely describe the puppy’s parents even though you can accurately
describe the puppy?
10. What are the chances that a puppy will have a spotted coat, if his
mother is solid and his father is heterozygous for a spotted coat?
Mother’s Genotype:
Father’s
Genotype:
Eggs --Sperm
Offspring/Puppy Possibilities:
Genotype
Probability
(fraction)
Probability (%)
Phenotype
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Practice with Punnett Squares
Punnett squares will help you predict the chances or probability that a child (or
offspring of any organism) will be born with a particular trait IF you know the
genetic make-up of the parents.
Example Punnet Square: What are the chances that the children of Joe
(who is heterozygous for a widow’s peak) and Sally (who does not have a
widow’s peak) will have a widow’s peak hairline?
1. The first step in a punnett-square analysis is to determine what traits
are associated with what genes. For this example you will look at the presence
of a ‘widow’s peak’ hairline which is ‘dominant’ over a straight hairline.
You will need to assign allele codes for the alleles (if not already provided to
you). Capital letters represent a dominant trait. For this example we can use:
o W = allele for widow’s peak hairline (dominant - only one allele needs to be
present for trait to be expressed)
o w = allele for straight hairline (recessive - both alleles must be present
for trait to be expressed)
Genotype (alleles present) Phenotype (trait that is present or ‘expressed’)
WW
Widow’s Peak Hairline (homozygous)
Ww
Widow’s Peak Hairline (heterozygous)
ww
Straight Hairline
(homozygous)
The mother has a straight hairline so what would her ‘genotype’ be: ____ ____
The father has a widow’s peak and is heterozygous for the trait, therefore the
father’s ‘genotype’ would be: _____ _____
2. Parents pass on their traits to offspring (children) through their sex cells or
gametes (sperm or egg). Because sex cells (gametes) will have half the genetic
material of other cells, your next step will be to determine the genetic
possibilities for sex cells.
Joe’s sperm:
OR
Sally’s Egg’s:
OR
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3. Next you would create and complete the punnett-square. The top row
represents the gamete (sex cell) possibilities for one parent, while the first
column represents the gamete (sex cell) possibilities for the other parent.
The interior of the punnett-squares show the genotype possibilities when the
two sex cells unite and offspring or children are produced.
Mother: ww
Egg
Egg
Sperm
Father: Ww
ww
ww
Ww
Ww
Sperm
4. Once the punnett-square is completed
you will need to determine the frequency of
each trait (phenotype) based on the genotypes
presented in the square. Create a table to do this.
Offspring Genotype
Ww
ww
Phenotype (trait)
widow’s peak hairline
straight hairline
Frequency
2/4
2/4
Percent
50%
50%
5. Finish each problem by answering the original question.
“For any child that Joe and Sally have together, there is a 50% chance that the
child will have a widow’s peak and a 50% chance that the child will have a
straight hairline”
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Practice Punnett-Square Problems
1. Cystic fibrosis (CF) is an inherited chronic disease that affects the lungs
and digestive systems of about 30,000 children and adults in the United States.
A defective gene causes the body to produce unusually thick, sticky mucus that
clogs the lungs and leads to life-threatening lung infections and obstructs the
pancreas. 50 years ago few children with cystic fibrosis lived to attend
elementary school. Today many people with CF live into their 30’s and 40’s and
beyond due to advances in treating the disease. Unfortunately however there is
no cure for the disease.
Cystic fibrosis caused by a single gene and is a recessive trait, therefore both
copies of the gene must be present for the person to be affected.
Jennifer and Tim are a young married couple planning a family. Tim’s younger
sister had CF and died before she finished high school. It is possible Tim
carries the gene for CF. They have decided to have genetic tests before trying
to conceive in order to determine whether they could have a child affected by
the diseases. The couple received their test results and was devastated to find
out that they are both carriers. Carriers are people who have one cystic
fibrosis allele and one normal allele. They are not affected by the disease.
What are the chances that a child born to Tim and Jennifer will have
cystic fibrosis?
Jennifer:
Note: Use ‘C’ for the healthy allele
since it is dominant over ‘c’ the
cystic fibrosis allele.
Egg
Egg
Sperm
Tim:
Sperm
Offspring Genotype
Phenotype (trait)
Frequency
Percent
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2. Huntington's Disease is a devastating, degenerative brain disorder for
which there is, at present, no effective treatment or cure. Huntington’s
slowly diminishes the affected individual's ability to walk, think, talk and
reason. Eventually, the person with the disease becomes totally dependent
upon others for his or her care. Signs of the disease don’t usually show up
until age 30 or 40. However, Huntington’s disease is a dominant traitwhich means you only need to inherit one copy of the allele to have the
disease.
Sheila does not show any signs of having Huntington’s disease right now.
Sheila’s father does have the disease (and is ‘heterozygous’). Sheila’s mother
is healthy and does not ‘carry’ the gene for Huntington’s. What is Sheila’s
probability of having the gene Huntington’s and therefore developing the
disease later in life?
Note: use ‘H’ for the Huntington’s allele &
‘h’ = healthy allele.
Mother:
Egg
Egg
Sperm
Father:
Sperm
Complete the chart below and finish with a one sentence written response to
the original question.
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3. Hypocholesterolemia is a disorder that causes excess cholesterol in the
blood and heart disease. This disorder is caused by a dominant allele. If each
of your parents were heterozygous for this condition, what are your chances of
being healthy (NOT having hypocholesterolemia)?
Mother:
Egg
Egg
Sperm
Father:
Sperm
4. Ability to taste PTC is dominant over the inability to taste PTC. If your
mother cannot taste PTC and your father is heterozygous for tasting. What
are the chances that you WILL be able to taste PTC?
Mother:
Egg
Egg
Sperm
Father:
Sperm
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Gregor Mendel was an Austrian monk who worked on inheritance during the
1800’s. Although at the time nothing was known of DNA or genes, Mendel’s
work on garden peas laid the foundation for understanding patterns of inheritance. In fact
the type of genetics we are studying in this packet is called ‘Mendelian Genetics’. Please
read chapter 11-1 pages 263-266 before and answer the following questions.
1. ‘True-breeding’ plants were the basis of Mendel’s experiments. (a) Please define‘truebreeding’ plants? (b) What are hybrid plants?
2. Why weren’t the hybrid plants simply a blend of the characteristics of the parent
plants?
3. Please understand the concepts of: P-generation plants, F1 generation plants, and F2
generation plants.
4. When a ‘true bred tall plant’ was crossed with a ‘true bred short plant’ the offspring
(F1) were all tall. Explain why these offspring were all tall rather than a blend?
5. (a) When the F1 plants (all tall) were crossed with each other what were the results in
the F2 generation? (b) Please explain these results in terms of segregation of alleles
and gamete formation?
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Read pages 267-269 (11-2) and the material presented earlier in this packet to answer
each of the following questions (use the chart below and construct a punnett-square for
each question)
1. What would the offspring phenotype and genotype probabilities be when a plant with
constricted seed pods crossed with a plant that was heterozygous for inflated seed
pods?
Parent 1:
Gamete
Gamete
Gamete
Parent 2:
Gamete
2. What would the offspring genotype
and phenotype probabilities be when a
white flowered plant crossed with a
plant that was homozygous for purple
flowers.
Parent 1:
Gamete
P
p
A
a
Y
y
R
r
I
i
G
g
T
t
Gamete
Gamete
Parent 2:
Gamete
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Two-trait Crosses
Predicting traits can get more complicated when you are examining more than one trait at a
time. Please read pages 270-272 and answer the following questions in preparation to
perform punnett-square analysis for more than one trait at a time.
1. Explain what happened when Mendel crossed ‘true-bred’ round & yellow seeded plants
(genotype: RRYY) with ‘true-bred’ wrinkled & green seeded plants (genotype: rryy)?
2. What did the F2 generation look like when Mendel crossed the F1 generation (genotype:
RrYy) with one another?
3. Explain the principle of independent assortment.
Punnett-Squares: practice with two-trait crosses
4. What happens when you cross a plant that has green & wrinkled seeds with a plant that is
heterozygous for yellow seeds and homozygous for round seeds? What are the genotype and
phenotype possibilities for the offspring?
a) What is the genotype of a plant that has green and
wrinkled seeds?
b) What is the genotype of a plant that is
heterozygous for yellow seeds and homozygous for
round seeds?
c) For the plant described in (a), what are the game
possibilities for that plant?
d) For the plant described in (b), what are the
gamete possibilities?
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Parent 1:
#4 continued
Complete the punnett-square.
Gamete
Gamete
Gamete
Gamete
Gamete
Parent 2:
Gamete
Gamete
Gamete
What are the offspring genotype and phenotype probabilities? (Construct a chart below).
5. Describe the offspring genotype and phenotype possibilities when you cross a plant that
has white flowers and is heterozygous for a tall stem (parent 1) with a heterozygous purple
plant that is heterozygous for a tall stem (parent 2).
Parent 1 GENOTYPE:__________
Parent 2 GENOTYPE:__________
Parent 1 GAMETE Possbilities:
Parent 2 GAMETE Possibilities: _________________
Parent 1:
________________________
Gamete
Gamete
Gamete
Gamete
Gamete
Parent 2:
Gamete
Gamete
Gamete
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A new example.
6. Suppose that the gene that determines one’s ability to roll thier tongue is on one
chromosome nd the ability to taste a certain bitter chemical called PTC is on another
chromosome. Both tasting and rolling are dominant. Determine the probability of the
different genotypes and phenotypes for the offspring when the mom is Ttrr and the dad is
ttRr?
T = allele for _______________
R = allele for ________________
t = allele for _______________
r = allele for ________________
MOM’s genotype:
DAD’s genotype:
MOM’s phenotype:
DAD’s phenotype:
MOM’s gamete (egg) possiblities:
DAD’s gamete (sperm) possibilities:
Mother:
Egg
Egg
Egg
Egg
Sperm
Father:
Sperm
Sperm
Sperm
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