Transcript 6-8 Punnett Square

Punnett Square
Middle School Science
Teacher Notes
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Punnett Square
In 1905, English scientist Reginald Punnett, devised a method for finding the
probabilities of possible genotypes of offspring of a cross or breeding
experiment. This diagram method is known as the Punnett Square.
• The alleles, or alternative gene forms, from each parent are listed on the
top and the left side of the square.
• Each square represents 25% of the offspring.
• A heterozygous parent has two different alleles for a single trait (Tt).
• A homozygous parent has the same two alleles for a single trait (TT or tt).
t
t
T
Tt
Tt
T
Tt
Tt
Dominant and Recessive
A dominant gene takes precedent over a recessive gene.
• Dominant genes are denoted by capital letters in the Punnett square.
• Recessive genes are denoted by lowercase letters in the Punnett square.
In a laboratory, a scientist is crossing a plant with yellow berries and a plant
with red berries. Both plants are homozygous. The red trait is dominant.
c
c
C
Cc
Cc
C
Cc
Cc
The offspring in this experiment will all appear to be red. However, 100% of
the offspring will be heterozygous since they will contain an allele for red and
one for yellow.
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Punnett Square Activity
A scientist working in a lab is crossing a heterozygous tall pea plant with a
homozygous short pea plant. Tall is the dominant trait.
• Draw a Punnett square to model this experiment.
• Describe the probabilities of the resulting population.
Punnett Square Activity
A scientist working in a lab is crossing a heterozygous tall pea plant with a
homozygous short pea plant. Tall is the dominant trait.
• Draw a Punnett square to model this experiment.
• Describe the probabilities of the resulting population.
t
t
T
Tt
Tt
t
tt
tt
50% of the offspring will be heterozygous (Tt). These offspring will be tall
since tall is the dominant trait.
50% of the offspring will be homozygous (tt). These offspring will be short
since they contain only the allele for the short trait.
A scientist is doing crosses on silkworms. These silkworms are known to
produce yellow (dominant) or white (recessive) cocoons. She crosses two
silkworms that both have yellow cocoons. In the offspring about 25% of them
appear to have white cocoons. The most reasonable explanation is the
silkworms she crossed were
a. both heterozygous.
b. both homozygous.
c. a combination of one heterozygous and one homozygous.
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