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

TT TT 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 phenotypeIncomplete 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

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 phenotypeIncomplete 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 phenotypeIncomplete 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 allelesCodominance: 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.