I think you might be getting hung up on the difference between the expression of a gene and its inheritance. Dominant alleles of a gene express their phenotype over recessive alleles of a gene but both alleles have an equal chance of being passed on to offspring.
There’s a 25% chance that any single progeny from two parents who are heterozygous (Aa) will be homozygous recessive (aa) but a 75% chance that any single progeny will have at least one recessive allele (Aa or aa). This means even for rare recessive phenotypes the recessive allele may be fairly common in the gene pool.
The last thing to keep in mind is that the concept of alleles is slightly different than the concept of genes. Two people may both have recessive alleles but they may still have small differences in the exact sequence of their genes, so while they both express the same phenotype and have the same allele genotype their genes aren’t exactly the same.
I think you might be getting hung up on the difference between the expression of a gene and its inheritance. Dominant alleles of a gene express their phenotype over recessive alleles of a gene but both alleles have an equal chance of being passed on to offspring.
There’s a 25% chance that any single progeny from two parents who are heterozygous (Aa) will be homozygous recessive (aa) but a 75% chance that any single progeny will have at least one recessive allele (Aa or aa). This means even for rare recessive phenotypes the recessive allele may be fairly common in the gene pool.
The last thing to keep in mind is that the concept of alleles is slightly different than the concept of genes. Two people may both have recessive alleles but they may still have small differences in the exact sequence of their genes, so while they both express the same phenotype and have the same allele genotype their genes aren’t exactly the same.