Other Methods of Inheritance:
Codominance and Incomplete
Dominance
The Different Forms of Inheritance
 Mendel’s Inheritance (Monohybrid +
Dihybrid) = Last lesson
 Incomplete dominance = This lesson
 Codominance= This lesson
 Sex linked = Next Lesson
Incomplete Dominance
 Incomplete dominance is another example of how some
traits are inherited different from Mendel predicted.
 In incomplete dominance neither of the two alleles for the
same gene can dominate the other.
 The result is that the phenotype is a blend of both parents
 This is similar to the old theory of blended inheritance.
Incomplete dominance
Incomplete Dominance:
 a condition in which neither of the two
alleles for the same gene can completely
conceal the presence of the other.
 As a result, the offspring or heterozygote
exhibits a phenotype that is a mixture or
somewhere between the a dominant and
recessive phenotype.
Example: Flower colour of Snapdragons
Red Flower x White Flower  Pink Flower
Practice Incomplete Dominance
What would be the result if we crossed two pink
snapdragon flowers (CRCW) together. What
percentage of the offsrping would be pink? Red?
White?
Codominance Inheritance
 a condition in which both alleles for a trait are equally
expressed in a heterozygote: both alleles are dominant
 For example: Roan animal
Red Bull x White Cow  Roan Calf (has red and white hair)
E.g. a Roan
animal
E.g. 2 Codominance
E.g. 3 Codominance
Rules
 Choose ONE letter to represent the trait and always use
CAPITALS. (for example: colour of snapdragons, use capital C
 The two alleles are represented as superscripts (example: R for
Red (CR ) and W for White (CW )
 Complete the hybrids using the same rules as monohybrids and
dihybrids. Heterozygotes (CRCW) will display the intermediate
trait (pink flowers)
Example
 The four o’clock flower shows incomplete dominance. A
pure red flower is crossed with a pure white flower. Use a
punnett square to find the phenotypes and genotypes of the
F1 generation and F2 generation
Multiple Allele Genes
 Most genes in the human body are actually controlled by more
than 2 alleles = multiple allele genes
 One common example is human blood type
 It is controlled by 3 alleles: iA and iB and i
Example:
Homework
 Entire class: Examine sample problem on page 246
 H.W. # 1-4, 8, 9 (page 247)