Basically, a recessive allele leads to formation of a product that has a low activity or no activity, which is complemented in the presence of the dominant allele (there can be effects related to dosage, in some cases). This can be easily understood in terms of an enzyme; a dominant allele would code for an enzyme with full activity whereas the recessive allele codes for one with a reduced activity. There are further complexities but this is a simple example.
A null allele however results in zero activity of the gene. This may be because of many reasons:
- The allele may produce a non-functional protein (for e.g. an enzyme
with mutated active site such that it cannot carry out the catalysis)
- The geneic region of the allele may be transcribed but not translated (loss of ORF)
- There may be no transcription at all (deletion of DNA sequences
critical for transcription)
In cases where there is no dosage effect, a null allele is always recessive but the converse is not true.
R _ would produce the same as
Rr, correct or not?
r _ would produce the same as
rr, correct or not?
_ _ would produce the same as
rr, correct or not?
The first two these assumptions are true for a simplistic case (no dosage effects). Since,
r is a hypomorphic allele (by assumption) i.e. it has a reduced activity whereas
_ has zero activity,
_ _ may not have the same phenotype as
rr. Only in cases where there is a threshold on the activity leading to phenotype, can a low activity allele/genotype (activity below threshold) lead to same phenotype as the null allele.
If we add further complexities,
R _ or
r _ may not be functionally equivalent to
rr respectively. Even
RR may behave functionally different than
Rr. This would happen because of altered gene dosage (expression level) and reduced expression level of the functional product can cause a change in the net activity. (Also see: Can difference in the expression potential of alleles lead to dominance?)