Mutations are changes in the genetic material of a living organism that can be passed on to its offspring. In eukaryotic organisms, most mutations are recessive, meaning they do not show up in the phenotype of the organism until it has two copies of the mutation, one from each parent. In order for a mutation to be expressed, the offspring must receive two copies of the mutated gene.
The reason for this is that most genes in eukaryotes are found in pairs, and each gene in a pair can have a different version or variant. When an offspring only has one copy of the mutation, it has one normal functioning gene and one non-functioning gene. In this case, the normal functioning gene masks the effects of the mutated gene so that the mutation is not expressed.
If the offspring has two copies of the mutated gene, however, both genes will be non-functioning and the effects of the mutation will be visible. This is why most mutations are recessive, as the mutated gene must be inherited from both parents in order to be expressed.
In some cases, a mutation can be dominant, meaning that it can be expressed even when present in only one copy. This occurs in rare cases, however, and most mutations in eukaryotes are recessive. Mutations that are expressed only when present in both copies are known as compound heterozygous mutations.
In conclusion, most mutations in eukaryotes are recessive because they require two copies of the mutated gene in order to be expressed. While there are some dominant mutations, they are relatively rare. Compound heterozygous mutations can also occur, where different versions of the same gene must be inherited from each parent in order for the mutation to be expressed.