Transcript Working Genetics Problems

Working Genetics Problems
solving problems with Punnett squares
© 2006 Plano ISD, Plano, TX
Always follow these 3 steps when
working genetics problems.
© 2006 Plano ISD, Plano, TX
1
Write the key. Information needed to
write the key will either be stated in
the problem or given.
2
Determine the genotypes of both
parents.
3
Solve the problem using a Punnett
square.
© 2006 Plano ISD, Plano, TX
Possible
Each
reproductive
cell contributes
combinations
of half
of theare
genes
parent’s
shown in
genes
a
Punnett
to its
square.
offspring.
Each parent
Purple
Both
White
One
parent
color
is dominant
parents
flower
contributes
color a
a purple
over
contribute
is
purple
recessive
white.
flower
a
flower
white
to
gene;
purple;
one
gene.
flower
therefore,
parent
gene.
the
contributes
a
offspring
white
flower
is
white.
gene.
© 2006 Plano ISD, Plano, TX
Possible
Each
reproductive
cell contributes
combinations
of
half ofare
genes
its shown
genes
to aitsPunnett
in
offspring.
square.
Yellow seed
color is
dominant
over green.
© 2006 Plano ISD, Plano, TX
Substitute
letters for
genes.
Purple
flower color
is dominant
over white.
P-
purple
p - white
© 2006 Plano ISD, Plano, TX
P
p
PP
Pp
Pp
pp
P
p
Substitute
letters for
genes.
Yellow seed
color is
dominant
over green.
Y - yellow
y - green
© 2006 Plano ISD, Plano, TX
Y
Y
y
Yy
Yy
y
Yy
Yy
How are these flowers different?
PP
Pp
Both flowers have the same
physical appearance or phenotype:
they are both purple.
The flowers have different genetic
makeup or genotype.
© 2006 Plano ISD, Plano, TX
Describe the phenotype of these peas.
© 2006 Plano ISD, Plano, TX
A purple flower can be pure
(homozygous)
PP
or
hybrid (heterozygous).
Pp
The phenotype is the same: both flowers
are purple.
The genotype is different: one flower is
PP, one is Pp.
© 2006 Plano ISD, Plano, TX
A white flower can be only
be pure (homozygous).
Why?
© 2006 Plano ISD, Plano, TX
pp
Follow these steps for every genetics problem.
1. Write the key.
1
upper case = dominant trait P = purple
lower case = recessive trait p = white
2. Determine both genotypes (parents).
2
Cross a pure purple flower with a white
flower. PP x pp
3. Work the problem with Punnett square.
3
© 2006 Plano ISD, Plano, TX