Codominance Allele

codominance allele - Often, one allele in the pair expresses itself more than the other. A dominant allele masks the effect of other alleles for the trait. For example, if a person has one allele for free earlobes and one allele for attached earlobes, that person has a phenotype of free earlobes. We say the allele for free earlobes is dominant. A recessive allele is one that, when present with another allele, has its actions overshadowed by the other; it is masked by the effect of the other allele. 

Codominance Allele

Having attached earlobes is the result of having a combination of two recessive characteristics. A person with one allele for free earlobes and one allele for attached earlobes has a phenotype of free earlobes. The expression of recessive alleles is only noted when the organism is homozygous for the recessive alleles. If you have attached earlobes, you have two alleles for that trait. Don’t think that recessive alleles are necessarily bad. The term recessive has nothing to do with the significance or value of the allele—it simply describes how it can be expressed. Recessive alleles are not less likely to be inherited but must be present in a homozygous condition to express themselves. Also, recessive alleles are not necessarily less frequent in the population.

Sometimes the physical environment determines whether or not dominant or recessive genes function. For example, in humans genes for freckles do not show themselves fully unless a person’s skin is exposed to sunlight

Mutations Produce New Genes

Mutations Produce New Genes - Spontaneous mutations are changes in DNA that cannot be tied to a particular causative agent. It is suspected that cosmic radiation or naturally occurring mutagenic chemicals might be the cause of many of these mutations. It is known that subjecting organisms to high levels of radiation or to certain chemicals increases the rate at which mutations occur. It is for this reason that people who work with radioactive materials or other mutagenic agents take special safety precautions.

Mutation

Naturally occurring mutation rates are low (perhaps 1 chance in 100,000 that a gene will be altered), and mutations usually result in an allele that is harmful. However, in populations of millions of individuals, each of whom has thousands of genes, over thousands of generations it is quite possible that a new beneficial piece of genetic information could come about as a result of mutation. When we look at the various alleles that exist in humans or in any othe organism, we should remember that every allele originated as a modification of a previously existing gene. For example, the allele for blue eyes may be a mutated brown-eye allele, or blond hair may have originated as a mutated brown-hair allele. When we look at a species such as corn (Zea mays), we can see that there are many different alleles for seed color. Each probably originated as a mutation (figure 12.1). Thus, mutations have been very important for introducing new genetic material into species over time.

In order for mutations to be important in the evolution of organisms, they must be in cells that will become gametes. Mutations to the cells of the skin or liver will only affect those specific cells and will not be passed on to the next generation.