Download Co-dominance and Incomplete dominance

Co-dominance and
Incomplete dominance
Other Patterns of Inheritance
• Not all genes show a simple pattern of
inheritance…
• Many genes have more than two alleles
• Many traits are controlled by more than one gene
Complete Dominance
• Definition: A situation in which the phenotypic effect of one
allele ‘masks’(or suppresses) the expression of the other
allele in a heterozygous combination.
• Description:
•
If two alleles have a complete dominance relationship, the phenotype of the
heterozygote will look the same as the phenotype of the homozygous
dominant.
Complete Dominance
• Example: Gerbil Fur Color
• The black (B) allele is completely
dominant to the (b) white allele of the
same gene.
• BB gerbils are black, bb gerbils are white,
and Bb gerbils are black. You can’t tell the
genotype of a black gerbil.
B
• Punnett Square:
b
b
B
Incomplete Dominance
• Definition: a situation in which one allele is not completely
dominant over another allele
• Description:
• There is no dominant allele.
• The phenotype of the heterozygote will be an intermediate
between the two alleles. It looks like two traits have blended
together. (Note: this does not mean the alleles actually blend).
Incomplete Dominance
• Example: Snapdragons
• One gene controls flower color and has two alleles,
one for red flowers and one for white flowers.
• When two homozygous plants for red and white
flowers mate, the heterozygote they produce will be
pink.
• Punnett Square:
R
W
W
R
Codominance
• Definition: A situation in which the phenotypes produced
by both alleles are completely expressed.
• Description:
• There is no dominant allele.
• In the phenotype of the heterozygote, both alleles will be completely
expressed.
• Example: Snapdragons Codominance
• One gene controls flower color and has two
alleles, one for red flowers and one for white.
• When two homozygous plants for red and
white flowers mate, the heterozygote will be
spotted red and white.
•
Punnett Square:
R
W
W
R
Practice Problems
• At the pet store, there are two kinds of fish. One fish has
orange scales, and the other fish has white scales. When an
orange fish is mated with a white fish, the result is an orange
and white stripped fish.
•
•
What type of dominance is this?
Show the punnett square for this parental cross & include the genotypes and
phenotypes of the offspring.
Practice Problems
• A homozygous black bird is crossed with a homozygous
white bird. The offspring are all bluish-grey.
• What type of dominance is this?
• Show the cross as well as the genotypes and phenotypes of the
offspring.
Practice Problems
• What results if a black bird is crossed with a bluish-gray bird?
• What would result if two bluish-grey birds are crossed, what are
the genotype and phenotype percentages of the offspring?
Practice
Problem:
In irises (a type of flower), the flowers can be
purple, pale lavender and white in color. What
type of dominance does this flower show?
What would be the genotypic and phenotypic
results of a cross if both parents were pale
lavender?