Download Practice with Punnett Squares

Name:____________________________
Date:______________
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). Ex: AA or aa
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)
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
l
h
a
T
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
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.
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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).
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?
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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
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.
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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 lifethreatening 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?
Note: Use ‘C’ for the healthy allele
since it is dominant over ‘c’ the
cystic fibrosis allele.
Jennifer:
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 above 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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