2018 Genetic Variation confusion

Hi there
I have a question from 2018 Genetic Variation Question 2

My understanding is co-dominance occurs when there is more than one dominant allele expressed. In this question co-dominance is described to occur when a heterozygous induvial expresses the dominant and recessive allele Rr. This can be seen in 2b.
I am confused how co-dominance is occurring when a dominant and recessive allele is expressed together because I though it was specific to when two dominant allele where expressed together.

I am really confused so any guidance would be extremely appreciated.


I think you might be referring to Question 3?
(Q2 is on Hector’s dolphins)

In reality, Complete Dominance is very rare.

To understand Co-dominance better, get rid of the word dominant and recessive from your vocab for a while.
Co-dominance is when BOTH alleles (of a gene) in heterozygotes are fully expressed. So:
RR individuals will have (100%) NORMAL red blood cells.
rr individuals will have (100%) sickle shaped red blood cells.
Rr individuals will have 50% normal and 50% sickle shaped red blood cells

Try to slowly move away from the terms dominant and recessive allele, or at least use these terms only for Complete Dominance (and maybe Multiple alleles).
These two terms don’t quite work for Incomplete Dominance and Co-dominance examples.

Hope that helps.

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