1. 17.5 Co-dominance & Multiple Alleles
Learning objectives
Understand how co-dominance affects the inheritance of characteristics
Understand how multiple alleles affect inheritance
Understand how human blood groups are inherited.

2. In a nutshell… Codominance = both alleles equally dominant
Multiple alleles = more than 2 alleles exist but each individual can only have 2 (on their 2 loci on homologous chromosomes)

3. Co-dominance Both alleles expressed in phenotype
Example: flower colour in snapdragons
One allele codes for an enzyme involved in production of red pigment, another allele codes for a non-functioning enzyme.
If this was normal monohybrid cross you would have Rr = red RR = red rr = white
Red and white alleles equally dominant so if heterozygous – pink flowers
Use different letters superscript from letter representing gene
CWCW = white CWCR = pink CRCR = red

4. Co-dominance – Drawing Crosses

5. Multiple Alleles Sometimes a gene has more than 2 alleles
Example: human blood groups. There are 3 alleles for the immunoglobulin gene (antibody which leads to the production of antigens on surface of RBCs)
Allele IA leads to the production of antigen A
Allele IB leads to the production of antigen B
Allele IO does not lead to the production of any antigen
One human can only have 2 of the alleles as there are only 2 loci
IA and IB are co-dominant, IO is recessive to both

6. Possible genotypes for human blood groupsAB O

7. Practise crosses
A male with blood genotype IAIB and a female with blood genotype IOIO
A male with blood genotype IAIO and a female with blood genotype IBIO

8. Multiple dominance and dominance hierarchy
Sometimes multiple alleles all have different levels of dominance.
Example: coat colour gene in rabbits (C )
In order of dominance:
Agouti coat CA
Chinchilla coat CCh
Himalayan coat CH
Albino coat Ca
Possible genotypes?