Download Using Punnett Squares

Using Punnett Squares
Test Crosses and Incomplete or
Co-dominance
March 3, 2010
You can use a Testcross to
determine Genotypes
• How could you find out if a parent (P
generation) is TT or Tt? Because they are
both tall.
• You perform a test cross. If you cross the
parent with a homozygous recessive (tt)
then you can find out.
HOW, YOU ASK?
You can use a Testcross to
determine Genotypes
T
t
t
T
T
t
t
t
Lets try a problem!
• Your pet guinea pig has black hair. This
trait is dominant and can be represented
by a B allele. Your neighbor has a white
guinea pig. This trait is recessive and can
be represented by a b allele. You want to
breed the two guinea pigs but want all of
the offspring from the mating to be black.
You are not sure, however, of the
genotype of your guinea pig.
Work it out . . .
• Infer: What may be the possible
genotypes of your black guinea pig?
• Infer: What is the genotype of the white
guinea pig and how do you know?
• How could you determine the genotype of
your guinea pig? Outline a procedure.
Complex Patterns of Inheritance
• Up to this point we have been looking at
simple Mendelian inheritance. This is
inheritance controlled by dominant and
recessive paired alleles. Some patterns
aren’t this simple.
• Examples: Incomplete or Co-dominance
Incomplete Dominance
• When inheritance follows a pattern of
dominance, heterozygous and homozygous
dominant individuals both have the same
phenotype. Example: TT and Tt are both tall.
• When traits are inherited in an incomplete
dominance pattern, however, the phenotype of
heterozygous individuals is intermediate or in
the middle of the two homozygotes. Example:
TT=tall, Tt=medium, and tt=short.
Incomplete dominance: Appearance
of a third phenotype
• For example, if a homozygous red-flowered
snapdragon plant (RR) is crossed with a
homozygous white-flowered snapdragon plant
(R′ R′), all of the F1 offspring will have pink
flowers.
Incomplete
dominance:
Appearance
of a third
phenotype
Red
White
All
pink
Red
(RR)
White
(R’R’)
Pink
(RR’)
Pink
(RR’)
All pink flowers
1 red: 2 pink: 1 white
Incomplete dominance: Appearance
of a third phenotype
• The new phenotype occurs because the
flowers contain enzymes that control pigment
production.
• The R allele codes for a red pigment. The R’
allele codes for a broken enzyme that makes
no pigment.
Incomplete dominance: Appearance
of a third phenotype
• Because the heterozygote has only one copy
of the R allele, its flowers appear pink because
they produce only half the amount of red
pigment that red homozygote flowers produce.
Codominance: Expression of both alleles
• Codominant alleles cause the phenotypes of
both homozygotes to be produced in
heterozygous individuals. In codominance,
both alleles are expressed equally.