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.

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)

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

Co-dominance – Drawing Crosses

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

Possible genotypes for human blood groups AB O

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

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?