Download Mendels P generation had the genotypes FF (for purple) and ff (for

The Genetics of Mendel s Experiments
  Mendel
s P generation had the
genotypes FF (for purple) and ff (for
white)
–  True breeding is also homozygous
 FF is homozygous dominant
 ff is homozygous recessive
The Genetics of Mendel s Experiments 
We can show the results Mendel observed
using a Punnett Square:
–  A Punnett Square shows possible genetic
combinations in the zygotes
–  Mendel crossed his true breeding purple
and white flower pea plants
 We write this as FF x ff
FF x ff
• During Meiosis, alleles separate (when homologous
chromosomes separate), to show this, we can
separate the letters
• The offspring from this mating can only get one
letter (allele) from each parent
FF x ff
If we put the
gametes into a
Punnett
Square, then
we can show
the possible
combinations
FF x ff
• Other Info:
• Capital Letters
always go first
• It doesn t matter
which side you put
each parent on
• All offspring are Ff,
which is
heterozygous and
will be purple
Ff
Ff
Ff
Ff
Each square
represents a possible
zygote (offspring)
Predicting Genetics
 
Sometimes worksheets will ask for the
percentages
–  It will be 0, 25, 50, 75, or 100%
–  0 out of 4 = 0%
–  1 out of 4 = 25%
–  2 out of 4 = 50%
–  3 out of 4 = 75%
–  4 out of 4 = 100%
The Genetics of Mendel s Experiments 
What Mendel did not know:
–  All of F1 pea plant flowers heterozygous
(two different alleles), or Ff
 That is why they were all purple
–  Remember dominant alleles mask
recessive alleles
 So with one purple allele present and
one white, only purple would show
because it is dominant
The Genetics of Mendel s Experiments
 
Punnett Square for the
F2 generation:
–  Ff x Ff (from F1 gen)
–  3 of the 4 squares
would have purple
flowers, while1 would
have white flowers
–  That s 75% purple,
25% white flowers
FF
Ff
Ff
ff
F2 generation
 
Phenotypic ratio:
–  This means how
many dominant to
how many recessive
–  In this example, it
would be 3:1
–  Note- the : means
to
FF
Ff
Ff
ff
F2 generation
 
Genotypic ratio:
–  XX : Xx : xx (or,
homozygous dominant
to heterozygous to
homozygous recessive)
– 
In this example, it would
be 1:2:1
FF
Ff
Ff
ff
Punnett Square Examples
 
Let s do a Punnett square for BB x Bb
–  B= black fur in bunnies
–  b= white fur in bunnies
–  So black fur is dominant
Punnett Square Examples
 
Phenotypic ratio:
 
Genotypic ratio:
 
What are the chances
of a white bunny?
Punnett Square Examples
 
 
 
Phenotypic ratio:
–  100% black bunnies
(4:0)
Genotypic ratio:
–  2BB:2Bb:0bb = 1:1:0
What are the chances
of a white bunny?
–  0%
BB
Bb
BB
Bb
Punnett Square Examples
 Let
s look at a heterozygous
cross
–  Bb
x Bb
Punnett Square Examples
 Bb
x Bb
– 
Phenotypic ratio:
– 
Genotypic ratio:
– 
What are the chances of
a white bunny?
– 
What are the chances of
a black bunny?
Punnett Square Examples
 
 
 
 
Phenotypic ratio:
–  3 black :1 white = 3:1
Genotypic ratio:
–  1BB:2Bb:1bb = 1:2:1
What are the chances of
a white bunny?
–  25% (1 in 4)
What are the chances of
a black bunny?
–  75% (3 in 4)
BB
Bb
Bb
bb
Predicting the Results of Heredity
 
What do these ratios and percents mean?
–  If we flip a coin, there is a 50% chance that it will
land on heads. But it is still possible to get 5 tails in
a row (although it is highly UNLIKELY!)
–  The more times you flip it, the more likely your
results will be 50:50
–  If Bb and Bb bunnies mate, there is a 1:4 chance
the offspring will be white (this does NOT mean
that they will or will not have white bunnies)
–  If they have LOTS of children, about 25% of them
will be white
Predicting Dihybrid Crosses
 
When 2 traits are being looked at…
 
Let s do a cross between two
heterozygous tall, heterozygous purple
flowered pea plants
–  So, TtFf x TtFf
Predicting Dihybrid Crosses
 
 
Instead of 2 possible gametes, there will be 4
So, the Punnett Square will be 4 x 4
TtFf
Predicting Dihybrid Crosses
 
 
 
Phenotypic Ratios
–  Tall, purple : tall,
white : short,
purple : short,
white
Keep same letters
together, capitals 1st
You will not be asked
for genotypic ratios
for dihybrid crosses
Predicting Dihybrid Crosses
  Phenotypic
Ratio
–  9:3:3:1
  This
is always
the ratio in a
hybrid x hybrid
dihybrid cross
TTFF TTFf TtFF
TtFf
TTFf
TTff
TtFf
Ttff
TtFF
TtFf
ttFF
ttFf
TtFf
Ttff
ttFf
ttff