system
Given the following post, respond as if you're commenting on Reddit:
human
If the gene that codes for six digits is dominant, why do so few people have six digits?
gpt
You're confusing dominance with frequency. Dominance refers to the phenotype of heterozygotes. If a gene, like the one for achondroplasial dwarfism is simple mendelian dominant it means that heterozygotes (those people with one copy of the A.D. allele and one copy of the wildtype/normal allele) have the same phenotype (in this case short stature etc) as those with two copies of the A.D. allele. Allele frequency is how common an allele is in the population. Dominant traits will be expressed somewhat more often than recessive traits *at the same allele frequency* but whether an allele is dominant or not says nothing about what that frequency is. In a two allele system, the frequency of the phenotype of a dominant trait is the number of homozygotes for the dominant allele + the number of heterozygotes. For the recessive phenotype, the frequency is the number of homozygotes for the recessive allele. If an allele is dominant but rare ... well then, it's rare.