Download Monohybrid and Dihybrid Punnett Squares

10/19/2013
Predicting Traits
Monohybrid and Dihybrid Punnett
Squares
Date _____________
Punnett Squares
• Monohybrid crosses
–Examines the
inheritance of one trait
–Can be homozygous-homozygous,
heterozygous-heterozygous, or
heterozygous-homozygous
• Dihybrid crosses
–Examines the inheritance of two traits
Monohybrid Punnett Squares
Parent
Offspring’s
alleles go
alleles go
here
here
• The principles of Mendelian genetics
can be used to predict the inherited
traits of offspring
• A punnett square can be used to
predict genetic combinations in the
offspring that result from different
parental
allele combinations
Crossing Individuals
• When you are tracing the probability of
traits being passed down through
families, there are three terms to refer to
these generations.
– P generation = parental generation; first in
the line
– F1 generation = first filial generation; comes
from the P generation
– F2 generation = second filial generation;
comes from the F1 generation
• Could be considered the “grand children”
Interpreting Results T = tall plant
t = short plant
• 1:4 ratio (25%) that an offspring
will carry two dominant alleles
– Homozygous dominant
– What will this offspring look like?
• 1:4 ratio (25%) that an offspring
will carry two recessive alleles
– Homozygous recessive
– What will this offspring look like?
T
t
T t
TT Tt
Tt tt
• 2:4 ratio OR 1:2 ratio (50%) that an offspring will
carry one dominant allele and one recessive allele
– Heterozygous
– What will this offspring look like?
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Dihybrid Punnett Squares
Allele Combinations
• The setup is the hardest part!
• You have to ensure that all allele combinations
are calculated.
• Let’s talk about pea traits:
– R = round shape / r = wrinkled shape
– Y = yellow color / y = green color
• Calculate the possible traits of two
parents that are heterozygous for
both traits:
– RrYy x RrYy
• RrYy x RrYy
RrYy
RY
Ry
ry
rY
Interpreting Results
RY
RY
RRYY
Ry
RRYy
rY
RrYY
ry
RrYy
Ry
RRYy
Rryy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Basic Monohybrid Crosses
1. Aa x aa
2. BB x bb
3. Ee x Ee
4. Homozygous dominant x
heterozygous
5. Homozygous recessive x
homozygous recessive
6. Heterozygous x heterozygous
• This is the hardest
dihybrid cross to
calculate!
• All ratios will be out
of 16 instead of 4.
–
–
–
–
–
–
–
–
–
1:16 or 1/16 RRYY
2:16 or 2/16 RRYy
1:16 or 1/16 RRyy
2:16 or 2/16 RrYY
4:16 or 4/16 RrYy
2:16 or 2/16 Rryy
1:16 or 1/16 rrYY
2:16 or 2/16 rrYy
1:16 or 1/16 rryy
RY
Ry
rY
ry
RY
RRYY RRYy
RrYY
RrYy
Ry
RRYy RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
What will these
genotypes look
like?
Word Problems – Mono. Crosses
1. A heterozygous round seeded plant is crossed with a
homozygous round seeded plant. What are the possible
genotypes and phenotypes of the offspring?
2. In pea plants, purple flowers are dominant to white flowers. If
two white flowered plants are crossed, what percentage of
their offspring will be white flowered?
3. Sickle-cell anemia is a recessive disorder in which the red
blood cells are sickle-shaped and very rigid and fragile. They
do not transport oxygen as well as normal red blood cells.
Two individuals that carry the sickle-cell trait, but do not have
it, want to have a child. They have come to you for genetic
advice. What would you tell them about their child
potentially having sickle-cell anemia?
4. In dragons, the allele for fire breathing is dominant. Dragons
can be fire breathers or non-fire breathers. If a heterozygous
fire-breathing dragon is crossed with one that does not
breathe fire, what is the possibility that an offspring will be
born a fire-breather?
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Focus Questions
• What are Punnett squares used
for?
• What is the difference between a
monohybrid and dihybrid cross?
• Know how to set up a Punnett
square and calculate
percentages/ratios!
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