In seed propagated plants a mutation-related phenotype is only expressed when it is heritable – passed on from one sexual generation to the next. Most of these heritable mutations arise in meiosis during the generation of male (pollen) or female (ovule) gametes – often referred to as germ cells. There are numerous mutations arising from mitosis in somatic cells which are not heritable, unless a flower bud arises directly from a “sport” tracing to such somatic mutation. Although these are rare, as many have pointed out here, they do happen.
If there is a mutation-related phenotype in fruit tracing to a somatic mutation, the mutation is by definition a dominant mutation, i.e. a single copy of a mutant allele is sufficient to express the mutant phenotype. It is exceedingly rare to get simultaneous and identical mutations on both chromosomes. Dominant mutations are due to a mutant hyperactive allele masking the wild type allele – typically, but not always, gain of function mutants. As pointed out in this thread, these will segregate 3:1 in the following generation. A recessive mutation almost always results in a loss of function phenotype, and will not be expressed directly in fruit tracing to a somatic mutation, or in the first generation after a meiotic mutation. Two copies of a recessive mutant allele (homozygous state) are required for expression of the mutant phenotype. The comment here that mutations sometimes “skip a generation” is due to fact that a mutant homozygous recessive phenotype is only possible after a sexual generation of recombination.
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