Download Practice with Punnett Squares

PREDICTING INHERITED TRAITS &
PUNNETT SQUARE ANALYSIS
Learning Objectives:
1. Students will know and apply the following vocabulary terms: DNA, chromosome, gene,
trait, allele, dominant, recessive, genotype, phenotype, homologous chromosomes,
homozygous, heterozygous, gamete, zygote, somatic cell, carrier.
2. Students will predict the outcome of genetic crosses involving one characteristic (single trait crosses).
3. Students will predict the outcome of genetic crosses involving two characteristics (2trait crosses).
Dog Called Spot
Imagine this microscopic drama: a sperm cell from a male dog
fertilizes an egg cell from the female dog. Each parent 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 chromosome 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.
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?
____ ____
2. What is the texture of the mother’s coat? How do you know? ____ ____
3. We cannot determine the texture of the father’s coat? Why not? ____ ____
4. What is the pattern of the puppy’s coat? How do you know?
____ ____
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 trait for which the puppy is has matching alleles. Give the genotype
(letters) and the phenotype (the trait/appearance). HINT: there is only
one.
When the alleles match the genotype is considered ________________.
8. List the traits for which the puppy’s alleles do not match. In other words, the
allele inherited from the mother is different from the allele inherited from
the father. (HINT: there are three). Give the genotype (letters) and the
phenotype (trait).
When the alleles do NOT match the genotype is considered _______________.
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IMPORTANT BACKGROUND INFORMATION
It is important to keep in mind that when gametes (i.e. sperm or egg) are formed
the chromosome number is reduced down to one set (compared with the 2
complete sets that are in all other cells). Therefore, when fertilization occurs
two sets of chromosomes are again restored. With guidance in class, draw a
series of pictures that compare chromosome numbers in a) somatic cells; b)
gametes; c) zygotes (fertilized eggs).
Predicting Inheritance--Practice with Punnett Squares
Example 1: Tay Sachs is a rare and devastating condition (featured in the video
clip ‘Cracking the Code’). Tay Sach’s is a recessive trait which means that only
people who inherit both copies of the recessive allele will have the disease. What
are the chances that a child born to two parents who are both heterozygous will
have a child with Tay Sachs?
STEP 1: assign codes for the alleles (if not already provided to you). Capital
letters represent a dominant trait. For this example we can use:
o H = Healthy
o h = Tay Sach’s disease
Genotype (alleles present)
HH
Hh
hh
HINT: when choosing your
own codes use letters that
look different as capitals and
lowercase.
Phenotype (trait that is present or ‘expressed’)
Healthy (homozygous)
Healthy (heterozygous)
Tay Sach’s disease (homozygous)
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STEP 2: determine genotypes of parents.
a) Mother: _____ _____
b) Father: _____ _____
STEP 3: determine the genetic possibilities for the gametes (sex cells).
Parents pass on their traits to offspring through their sex cells or gametes
(sperm or egg). Sex cells (gametes) will have half the genetic material of other
cells.
Male Gametes (sperm):
OR
Female Gametes (eggs):
OR
STEP 4: set-up 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 for the
children/offspring (when the two sex-cells unite).
Mother: __ __
Egg
Egg
Sperm
Father: __ __
Pollen
Step 5: create a chart providing the genotypes and phenotypes possibilities and
probabilities for offspring (as a frequency and as a percentage)
Genotypes
Phenotype (trait)
Frequency
Percent
Step 6: answer the original question in a complete sentence.
“For two parents who are healthy but carry a Tay Sach’s allele (both parents are
heterozygous), there is a ________ chance their child will be healthy and a
__________ chance their child will have Tay Sach’s.
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Practice Punnett-Square Problems
1. Though earwax tends to get a bad rap, it actually helps to keep your ear canal clean by
trapping dirt and dust that gets into your ear and moving it out. Interestingly, earwax
actually comes in two types.
Jill: ____ ____
Jill has the dry the wax and Jack is heterozygous for
Egg
Egg
sticky earwax. What are the possibilities and
probabilities for their children?
Sperm
Earwax Alleles
Wet/Sticky/Yellow = dominant = _____
Jack: ___ ___
Sperm
Dry/flaky/grey-yellow = recessive = _____
OFFSPRING POSSIBILITIES and PROBABILITIES
Genotype
Phenotype (trait)
Frequency
2.
Percent
In certain rabbit breeds, black fur is dominant to white fur. If two heterozygous rabbits
mate, what are the possible genotypes and phenotypes for the baby rabbits. What are the
chances they will have a baby with white fur?
Mother:
Egg
Egg
Sperm
Father:
Sperm
OFFSPRING POSSIBILITIES and PROBABILITIES
Genotype
Phenotype
Frequency
Percent
There is a ____ % chance that two heterozygous parents will have a baby rabbit with white fur.
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Practice Problems Continued- Single Trait Crosses
Complete each of the following problems in your journal! Be sure to follow all six
steps and show your work.
 Step 1: Assign codes to alleles (unless already provided for you).
 Step 2: Determine the genotype of the parents.
 Step 3: Determine the gamete possibilities.
 Step 4: Set-up and complete the punnett-square.
 Step 5: Complete genotype & phenotype possibilities & probabilities chart.
 Step 6: Answer the original question in a complete sentence
3. Ability to taste PTC is dominant over the inability to taste PTC. What are the chances that
a child WILL be able to taste PTC, if her mother cannot taste PTC and her father is
heterozygous for tasting? Complete all 6 steps in the punnett-square analysis.
4. A widow’s peak hairline is a dominant trait. What are the chances that two parents who
have a widow’s peak (both are heterozygous gentoypes) will have a child without a widow’s
peak hairline? Complete all 6 steps in the punnett-square analysis.
5. In mice the ability to run normally is a dominant trait. The recessive trait causes mice to
run in circles (geneticists call these ‘waltzing’ mice). Determine the probability of each
genotype and each phenotype of the potential offspring when a male heterozygous normal
mouse mates with a female homozygous normal mouse. Complete all 6 steps in the punnettsquare analysis.
6. Cystic fibrosis (CF) is an inherited chronic disease that causes the body to produce
unusually thick, sticky mucus that clogs the lungs and leads to life-threatening lung
infections and obstructs the pancreas. 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.
Jennie and Tom are a young married couple planning a family, and decide to have genetic
tests prior to having children. The couple received their results and found out that they are
both carriers for CF. Carriers are people who have one cystic fibrosis allele and one
normal allele, so they do not have the disease but they could pass down the gene to their
children. What are the chances that a child born to Tim and Jennifer will have cystic
fibrosis? Complete all 6 steps in the punnett-square analysis.
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7. 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. Huntington’s disease is a dominant
trait- which 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? Complete all 6 steps in the punnett-square
analysis.
Note: use ‘H’ for the Huntington’s allele & ‘h’ = healthy allele.
8.
Sickle-cell anemia is a recessive inherited disease. It causes a problem with the protein
hemoglobin resulting in blood cells that are distorted or sickle-shaped. This in turn can
cause symptoms ranging from pain and fatigue to breathlessness and heart issues. What are
all the genotype and phenotype possibilities and probabilities that can result from
heterozygous parents? Complete all 6 steps in the punnett-square analysis.
9. In dogs a certain form of deafness is inherited and is a recessive trait. What are the
chances that two heterozygous parents will have a deaf puppy? Complete all 6 steps in the
punnett-square analysis.
10. What would the offspring phenotype and genotype probabilities be when a pea plant with
constricted seed pods crossed with a plant that is heterozygous for inflated seed pods?
You will need the pea plant chart on the next page to complete this problem.
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Gregor Mendel ‘s Work
Who was Gregor Mendel? (see page 263 and video clip):
Mendel’s principles:

Genes are passed from parents to
offspring (we now know this occurs
through genes on chromosomes).

Some alleles are dominant, some are
recessive.

In sexually reproducing organisms
(including humans), somatic cells
each have 2 copies of a gene, one
gene from each parent.


Genes segregate from each other
during formation of a gamete (eg.
sperm and egg).
The alleles for different genes
usually segregate independently of
each other
L
l
A
a
Y
y
R
r
I
i
G
g
T
t
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Two-trait Crosses- predicting two characteristics at one time.
1. What happens when you cross a plant that has green & wrinkled seeds with a plant that is
heterozygous for yellow seeds and heterozygous for round seeds? What are the genotype
and phenotype possibilities and probabilities for the offspring?
STEP 1: Assign alleles (the chart on page 5 provides allele codes).
STEP 2: Determine genotype of parents
-and-
STEP 3: Determine gamete possibilities.
a) What is the genotype of a plant that has green and wrinkled seeds (parent 1)?
_____ _____ , _____ _____
What are the gamete possibilities for the plant with green and wrinkled seeds?
b) What is the genotype of a plant that is heterozygous for yellow seeds and heterozygous for
round seeds (parent 2)?
_____ _____, _____ _____
What are the gamete possibilities for this plant?
STEP 4: Set-up and complete the punnett-square
Parent 1:
Gamete
Gamete
Gamete
Gamete
Gamete
Parent 2:
Gamete
Gamete
Gamete
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STEP 5: Complete the chart—“OFFSPRING POSSIBILITIES and PROBABILITIES”
Genotype
Phenotype (trait)
Frequency
Percent
2. 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 2:
Gamete
Gamete
Gamete
Gamete
Gamete
Parent 1:
Gamete
Gamete
Gamete
OFFSPRING POSSIBILITIES and PROBABILITIES
Genotype
Phenotype (trait)
Frequency
Percent
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3. Suppose that a gene that determines one’s ability to roll their tongue is on one chromosome
and the ability to taste the 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 ________________
DAD’s genotype: ____ ____, ____ ____
MOM’s genotype: ____ ____, ____ ____
DAD’s phenotype: _____________________
MOM’s phenotype: _________________
Mother:
Egg
Egg
Egg
Egg
Sperm
Father:
Sperm
Sperm
Sperm
OFFSPRING POSSIBILITIES and PROBABILITIES
Genotype
Phenotype (trait)
Frequency
Percent
4. What are the offspring genotype and phenotype possibilities and probabilities, when a
mother and father who are both heterozygous for a widow’s peak and both are heterozygous
for sticky ear wax? Remember (widow’s peak is dominant and sticky ear wax is dominant)
___ = allele for widow’s peak hairline
____ = allele for sticky earwax
___ = allele for straight hairline
____ = allele for dry earwax
DAD’s genotype: ____ ____, ____ ____
MOM’s genotype: ____ ____, ____ ____
DAD’s gamete (sperm) possibilities:
MOM’s gamete (egg) possibilities:
Set-up complete the Punnett-Square, then complete a chart of the results in your journal.
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