1. Incomplete Dominance & Co-dominance

2. Review of Mendelian Genetics
Law of Independent Assortment
Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.
Law of Segregation
The two alleles for each gene separate during gamete formation.

3. Mendelian Genetics: Dominant & Recessive Review
Mendel’s experiments demonstrated complete dominance - offspring always resembled one of the two parents
One allele is DOMINANT over the other (because the dominant allele can “mask” the recessive allele)
Purple Purple White

4. Mendel continued
All Mendel’s plants displayed dominant or recessive alleles. In other words, only one trait showed up.

5. There are some crosses where both alleles of a pair are expressed in a heterozygote
Snapdragons, an example of incomplete dominance
HUH?

6. Non-Mendelian Genetics
Incomplete Dominance
Codominance
Multiple Alleles
Polygenic Traits
Sex-Linked Traits

7. Incomplete Dominance
a third (new) phenotype appears in the heterozygous condition as a BLEND of the dominant and recessive phenotypes.
Neither allele is completely dominant over the other allele
Result: A heterozygous phenotype
Example:
Red (FR) x White (FW) = Hybrid Pink (FRFW)

8. Neither Red (FR) or White (FW) is dominant over the other.
example: when a homozygous red flower (FRFR) is crossed with a homozygous white flower (FWFW), the alleles blend to make a hybrid (FRFW) with pink flowers
FRFR = red
FWFW= white
FRFW = pink

9. = Blue- Feathered (CBCW)
Andalusian Chickens
Incomplete dominance
Neither Black (CBCB) nor
White (CWCW) are dominant
black-feathered (CBCB) x white-feathered (CWCW)
= Blue- Feathered (CBCW)

10. Incomplete Dominance Example Questions:
In snapdragons, colour has incomplete dominance. If you cross a purebred red snapdragon (FRFR) with a purebred white (FWFW) snapdragon what are the phenotypic and genotypic ratios? What are the phenotypic and genotypic ratios for the F2 generation of the example?
Indicate the ratios that would be expected if the following crosses are made:
a) red x pink
b) white x white
c) pink x pink

11. Codominance
Two equally dominant alleles are expressed at the same time.
Heterozygous phenotype will have both phenotypes visible

12. Codominance Example #1: Speckled Chickens
CBCB= black feathers
CwCw = white feathers
CBCw = black & white speckled feathers
Notice –
NO GRAY!
NO BLEND!
Each feather is
either black or white

13. Codominance Example #2: Rhodedendron
RRRR = allele for red flowers
RwRw= allele for white flowers
Cross a homozygous red flower with a homozygous white flower.

14. Codominance Example #3: Roan cattle
Cattle can be:
red (CRCR – all red hairs)
white (CwCw – all white hairs)
roan (CRCw – red and white hairs together)

15. Codominance Example #4: Appaloosa horses
Gray horses (HGHG) are codominant to white horses (HwHw).
The heterozygous horse (HGHw) is an Appaloosa (a white horse with gray spots).
Question: Cross a white horse with an appaloosa horse.

16. Codominance Example Questions:
A red cow (CRCR) was crossed with a roan cow (CRCw). What proportion of the offspring would be roan?
If you cross 2 roan cows (CRCw), what proportion of the offspring will be white cows?