How to Solve Genetics Problems
Punnett Squares
• A tool used to
PREDICT the
POSSIBLE
OUTCOMES of a
genetic cross.
• Does NOT tell you
what traits the
offspring WILL have.
Punnett Squares show POSSIBLE
Outcomes
Allows you to
determine
the
LIKELIHOOD
of the
offspring
inheriting a
particular
phenotype.
Steps to solving a Punnett square:
1. Determine the phenotypes
and given in the problem.
Then assign letters to
represent alleles for the
genes.
2. Determine the Genotypes
of the parents (as much as
possible)
3. Set up the Punnett Square.
4. Solve the problem /
Answer the question. Give
ratios, percents.
Problem:
In pea plants, tall plants are
dominant to short plants.
Show the cross between a
purebreeding (homozygous)
tall plant and a short plant.
Steps to solving a Punnett square:
1. Determine the
phenotypes and given
in the problem. Then
assign letters to
represent alleles for the
genes.
Phenotypes:
Tall (dominant) = T
Short (recessive) = t
In pea plants, tall plants
are dominant to short
plants. Show the cross
between a
purebreeding
(homozygous) tall plant
and a short plant.
Steps to solving a Punnett square:
2. Determine the
Genotypes of the
parents (as much as
possible)
In pea plants, tall plants
are dominant to short
plants. Show the cross
between a
purebreeding
(homozygous) tall plant
and a short plant.
Genotypes:
Parent 1: Purebreeding/Homozygous Tall =
Parent 2: Short (homozygous recessive) = tt
TT
Steps to solving a Punnett square:
3. Set up the Punnett
Square.
Genotypes:
Purebreeding/Homozygous Tall = TT
Short (homozygous recessive) = tt
T
t
t
T
In pea plants, tall plants
are dominant to short
plants. Show the cross
between a
purebreeding
(homozygous) tall plant
and a short plant.
Steps to solving a Punnett square:
4. Solve the problem /
Answer the question.
Give ratios, percents.
Genotypes:
Purebreeding/Homozygous Tall = TT
Short (homozygous recessive) = tt
T
t
t
T
Tt
Tt
Tt
Tt
In pea plants, tall plants
are dominant to short
plants. Show the cross
between a
purebreeding
(homozygous) tall plant
and a short plant.
100% Tt
100% Tall
Punnett Square Patterns
1. Cross a HOMOZYGOUS
DOMINANT
(purebreeding) and a
HOMOZGYOUS RECESSIVE.
1. Result is always 100%
dominant phenotype.
Why???
Example: TT x tt
2. Cross 2
HYBRIDS/HETEROZGYOUS
genotypes.
Example: Tt x Tt
2. Genotype Ratio is
always 1:2:1
Phenotype Ratio is
always 3:1
Happy Hump Day!
1. Check your HW answers at the
SSS.
2. Complete the Independent
Practice Worksheet from
yesterday…then check those
answers at the SSS
3. Take out your warm-up ½ sheet
and notes (blue sheet) from
yesterday.
Independent Practice Time
• Work through the problems.
1. Read the information.
2. Make a key.
3. Do the cross.
4. Answer the question!
Answers are at the SSS.
NO PENS/PENCILS.
Quiz
#1
#2
Human Genetic Diseases
• Diseases caused by DOMINANT ALLELES:
– Huntington’s Disease – disease that deteriorates
the brain.
• Diseases caused by RECESSIVE ALLELES:
– Cystic Fibrosis
– Sickle Cell Anemia
Cystic Fibrosis
• An inherited disease that causes thick sticky
mucus to build up in the lungs and digestive
track.
• Caused by a recessive allele
Sickle Cell Anemia
• An inherited disease that causes a mutation in
the hemoglobin protein. This mutation
causes the shape of red blood cells to “sickle”
resulting in pain, fatigue, and an inability of
cells to receive oxygen and nutrients.
• Heterozygous genotype does provide some
protection against Malaria!
• Caused by a recessive allele
Example #1: Watch Me!
Cystic fibrosis is a recessive disease. Being
normal is dominant. If Sarah and Ben are
both heterozygous normal, what is the
likelihood they will have a child with the CF
disease?
#2) With a Partner!
• Cystic fibrosis is a recessive disease. Being normal is
dominant. Manuel and Luisa just learned their baby
boy has CF disease. Although Manuel is normal,
Luisa has cystic fibrosis. What must be Manuel’s
genotype?
#3) On Your Own!
Sickle cell anemia is a recessive disease. Being
normal is dominant. Niqim and Shea just
learned their baby girl has sickle cell disease.
Both parents are phenotypically normal. What
must be each parent’s genotype?