It is a common misconception that dominant alleles must be more common in the gene pool than recessive alleles. The truth is that dominance and recessiveness have little to do with how common an allele is.

For example, type O is the most common ABO blood type in North America, but it is caused by the recessive allele i. Blood type AB, caused by the two dominant ABO alleles, is the rarest.

the presence of extra fingers or toes, is a dominant trait, yet rare in the population (1 in every 700 to 1,000 live births).

It's one thing for any given cell to have all the usual genes in its nucleus, but it's a different matter whether those genes are turned on or off-whether the cell shows any detectable effects from them.

For a gene to be turned on and have an effect on the individual is called gene expression. Even if an allele is dominant, it isn't always expressed in the phenotype.

If a dominant allele is silenced and has no effect on some people who carry it, it is said to show incomplete penetrance in that population

—that is, the genotype doesn't "penetrate to" and affect everyone who has it.

One reason the connection between genotype and phenotype isn't inevitable is that environmental factors play an important role in the expression of all genes. At the very least, all gene expression depends on nutrition.

Brown eyes, for example, require not only genes for the enzymes that synthesize the pigment melanin, but also the dietary raw material, phenylalanine, from which the melanin is made (fig. 4.21).

No gene can produce a phenotypic effect without nutritional and other environmental input, and no nutrients can produce a body or specific phenotype without genetic instructions that tell cells what to do with them.

Just as you need both a recipe and ingredients to make a cake, it takes both heredity and environment to make a phenotype.

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Question	Answer
It is a common misconception that dominant alleles must be more common in the gene pool than recessive alleles. The truth is that dominance and recessiveness have little to do with how common an allele is.	For example, type O is the most common ABO blood type in North America, but it is caused by the recessive allele i. Blood type AB, caused by the two dominant ABO alleles, is the rarest.
Polydactyly	the presence of extra fingers or toes, is a dominant trait, yet rare in the population (1 in every 700 to 1,000 live births).
It's one thing for any given cell to have all the usual genes in its nucleus, but it's a different matter whether those genes are turned on or off-whether the cell shows any detectable effects from them.	For a gene to be turned on and have an effect on the individual is called gene expression. Even if an allele is dominant, it isn't always expressed in the phenotype.
If a dominant allele is silenced and has no effect on some people who carry it, it is said to show incomplete penetrance in that population	—that is, the genotype doesn't "penetrate to" and affect everyone who has it.
One reason the connection between genotype and phenotype isn't inevitable is that environmental factors play an important role in the expression of all genes. At the very least, all gene expression depends on nutrition.	Brown eyes, for example, require not only genes for the enzymes that synthesize the pigment melanin, but also the dietary raw material, phenylalanine, from which the melanin is made (fig. 4.21).
No gene can produce a phenotypic effect without nutritional and other environmental input, and no nutrients can produce a body or specific phenotype without genetic instructions that tell cells what to do with them.	Just as you need both a recipe and ingredients to make a cake, it takes both heredity and environment to make a phenotype.

Created by: Russells3709

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