Transcript Punnett Squares

PUNNETT
SQUARE
VOCABULARY
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Punnett Squares were invented
by Reginald Punnett to help
predict the genetic outcomes
of crosses between parents
carrying specific traits.
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A Punnett Square is made up
of four smaller squares.
Each square is worth 25%.
T = tall (dominant)
t= short
(recessive)
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The different forms of the same
trait or gene are called alleles.
For example, the gene that
controls height in pea plants has
one allele for tall height and one
for short height.
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An offspring inherits two
alleles for each trait. An
offspring may inherit two
alleles for the same form of a
trait or two different alleles.
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When two alleles are present,
the form of the dominant allele
appears. A dominant allele is the
one that occurs most often. It is
represented with a capital letter.
It only takes one gene for it to
show up.
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The allele that is hidden when
a dominant allele is present is
called the recessive allele. The
form of the trait determined
by It occurs least often. The
recessive allele appears only
when
two recessive alleles
are inherited.
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Dominant and recessive alleles
can be used to explain Mendel’s
observations. An organism that
has two identical (same) alleles
for a trait is called a purebred.
Miss Ellie, from
Tennessee, was
voted the ugliest
purebred in the U.S.8
Another name for purebred is
homozygous. The prefix homo
means same. A purebred has
the same alleles, either
two dominant (TT) or
two recessive (tt).
This horse is a
purebred
Arabian
stallion.
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An organism that has two
different alleles for a trait is
called a hybrid. In a hybrid,
the dominant allele appears, so
all the F1 generation
plants were tall.
These hybrid Labradoodle
puppies had purebred
Poodle and Labrador
Retriever parents.
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Another name for hybrid
is heterozygous. The
prefix hetero means
different. A hybrid has
two different alleles (Tt),
but the dominant trait is
the one that shows.
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(?)Some interesting hybrids (?)
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When plants from the F1
generation produced offspring,
some of the plants received
two dominant alleles, some
received one dominant and one
recessive allele, and some
received two recessive alleles.
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The plants that received
two recessive alleles
were short. All
others were tall.
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The genotype is the
code for the trait like
BB or bb.
Remember:
genotype = genes
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The phenotype is the
physical characteristic
that is expressed like
blue eyes or brown eyes.
Remember:
phenotype = physical
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In the above example, the alleles
for the father are written
across the top of the square
(TT).
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The alleles for the mother
are down the side (tt).
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T
T
t
Tt
Tt
t
Tt
Tt
This Punnett Square shows purebred dominant
(TT) and purebred recessive (tt) parents. 21
T
t
T
t
TT
Tt
Tt
tt
This Punnett Square shows parents that are
both hybrids. They have one dominant allele (T)
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and one recessive (t).
Punnett squares can be used to
calculate the probability or
likelihood that a certain
combination will occur among the
offspring. The higher the
probability, the more likely
it is to happen.
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In the above Punnett square, the
probability that the flower will
be purple is 3 out of 4 or ¾
which is .75 or 75%.
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Look again at the chart for
plant height. What is the
probability that the
offspring will be tall?
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Test Practice
1. Genes are carried by
structures called _________.
A. chromosomes
B. alleles
C. hybrids
D. dominants
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2. A scientist crossed a tall pea
plant with a short pea plant. Out
of the offspring, 42 plants were
tall and 14 were short. What
percentage were short?
A. 10%
B. 13%
C. 25%
D. 50%
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3. A scientist crossed a purebred
long-winged fruit fly with a
purebred short-winged fly.
Long wings are dominant over
short wings. What is the
probability that the offspring
will have long wings?
A) 25% B) 50% C) 75% D) 100%
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Fill in your own Punnett square
to find out the percentage of
long-winged flies.
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4. In humans, the allele for a
cleft chin (C) is dominant. A
woman with the genotype cc has
children with a man with the
genotype Cc. If the couple has
six children, how many do you
think would have a cleft chin?
A) 0
B) 1
C) 3
D) 6
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Fill in your own Punnett square
to find out how many children
will have a cleft chin.
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