From their molecular configuration, why is a certain allele dominant and the other recessive?

Answer 1

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Answer 2

Because genes (and their alleles) function through the proteins they encode, it is really the protein made by the dominant allele that is responsible for these effects.
Take for example the flowers on the pea plant, where the C allele is dominant to the c allele. These alleles for the petal color gene code for a protein that makes a purple pigment. Imagine that the C allele has a mutation in its DNA that makes the protein it encodes unable to produce the purple pigment. The c allele, however, makes a normal protein that is fully functional. So, why isn't the heterozygous plant (which has the two different alleles C and c) light purple instead of white? It has half the amount of normal purple pigment-making protein, right? In genetic terms, this is given the fancy name of "haploinsufficiency", which basically means that having half the
amount of normal protein is not enough, and is the same as not having any at all. Let's imagine that you go to the store and want to buy a loaf of bread that costs $2, but you only have $1. In terms of buying the bread, the fact that you have $1 really doesn't matter-you still don't have enough to buy it. In this sense, having $1 isn't really any different from having $1.99 or no money at all. Unless you can meet the cost of $2, you cannot buy any bread. In the same way, unless the pea plant has two c alleles, it cannot make a purple flower.

Answer 3

The molecular configuration of a DNA strand or RNA strand in a gene is, as the other answers show, not the important part of why a gene is 'dominant' or 'recessive' but what matters is the effectiveness of the coded protein. However, once you know that a particular allele (form of a gene, sequence of DNA) such as allele X acts recessive to another form such as allele W, any time you find that sequence pair together, you can predict that the X effects will be hidden by the W effects. After their dominance has been established in another experiment, you don't need to analyze the proteins to find out which is going to be dominant.

Answer 4

matahari is close, but not quite there. An allele C is dominant over c usually beacuse c makes a non-functional protein; whereas C is a function protein. In nearly all situations functional is dominant over non-functional. Often recessive alleles have nonsense mutations that cause a premature stop codon. Other times there are missense mutations in a codon that encode an essential amino acids for the protein to function. Rarely you get complete deletion of the DNA encoding the protein. In the purple flower example with C and c. The resulting C/c plant is light purple. The bread analogy is wrong. Haploinsufficiency is like being only able to buy half the loaf of bread. By matahari's example C/C would be dark purple c/c would be white. Most traits are not haplo-insufficient and the heterozygote has the wild-type (C/C) phenotype.

Answer 5

Think of it this way, the recessive gene commonly codes for a "broken" protein. Generally, the broken proten just does nothing, usually the remaining dominant gene can code for enough functional protein to compensate (though there are exceptions), unless you are homozygous recessive. Every once in a while, the broken protein codes for a protein that is functional and therefore results in a different phenotype.