The difference is due to the inheritance pattern. Blood type needs two recessive alleles to express Rh-. Color-blindness needs only one.
⇒ Blood type is coded by a diallelic Rh gene that expresses complete dominance.
The allele Rh+ dominates over the allele Rh-.
Genotyopes and Phenotypes are as follows,
To express Rh-, a person must inherit two recessive alleles from their parents.
⇒ Color-blindness is an X-linked condition coded by a recessive allele.
As most sex-linked genes, it is transmitted together with the X chromosome.
The phenotype depends on the number of recessive alleles and the sex of the person.
Genotyopes and Phenotypes are as follows,
In this case, only one recessive allele is enough to express the affection.
In conclusion, Blood type needs two recessive alleles to express Rh-. Color-blindness needs only one recessive allele.
You can learn more about the inheritance pattern at
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