If one is dominant, the other one must be not dominant. In that case, we call it recessive. A dominant gene, or a dominant version of a gene, is a particular variant of a gene, which for a variety of reasons, expresses itself more strongly all by itself than any other version of the gene which the person is carrying, and, in this case, the recessive.
Now, it usually refers to inheritance patterns frequently used in conjunction with a Punnett square where, if an individual has two versions of a gene, and one is observed to frequently be transferred from one generation to another, then it is called dominant.
Biochemically, what is going on in this case is that the genetic variation, for a variety of reasons, can either induce a function in a cell, which is either very advantageous or very detrimental, which the other version of the gene can't cover up or compensate for.
People with one sickle-cell allele and one normal allele have a small number of sickled cells, and their cells sickle more easily under certain conditions. So we could say that red blood cell shape has a co-dominant inheritance pattern. That is, individuals with one copy of each allele have an in-between phenotype. So is the sickle cell allele dominant, recessive, or co-dominant?
It depends on how you look at it. If we look at the proteins the two alleles code for, the picture becomes a little more clear. The affected protein is hemoglobin, the oxygen-carrying molecule that fills red blood cells. The sickle-cell allele codes for a slightly modified version of the hemoglobin protein.
The modified hemoglobin protein still carries oxygen, but under low-oxygen conditions the proteins stick together. When a person has two sickle cell alleles, all of their hemoglobin is the sticky form, and the proteins form very long, stiff fibers that distort red blood cells. When someone has one sickle-cell allele and one normal allele, only some of the hemoglobin is sticky.
Non-sticky hemoglobin is made from the normal allele, and sticky hemoglobin is made from the sickle-cell allele every cell has a copy of both alleles. The protist that causes malaria grows and reproduces in red blood cells. Just exactly how the sickle-cell allele leads to malaria resistance is complex and not completely understood.
However, it appears that the parasite reproduces more slowly in blood cells that have some modified hemoglobin. And infected cells, because they easily become misshapen, are more quickly removed from circulation and destroyed. To see more examples of how variations in genes influence traits, visit The Outcome of Mutation. Dominant and recessive are important concepts, but they are so often over-emphasized.
For example, the allele for brown eyes is dominant, therefore you only need one copy of the 'brown eye' allele to have brown eyes although, with two copies you will still have brown eyes. If both alleles are dominant, it is called codominance. The resulting characteristic is due to both alleles being expressed equally. An example of this is the blood group AB which is the result of codominance of the A and B dominant alleles.
Recessive alleles only show their effect if the individual has two copies of the allele also known as being homozygous.
For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the 'blue eye' allele. Related Content:. What is a gene? What is inheritance? What is genetic variation? In humans, this is often the observed signs and symptoms of a condition. Individuals usually have two copies of each gene, with the exception of genes on the sex chromosomes in males.
Dominant alleles will produce the phenotype associated with them, even in the presence of another allele. Developing people for health and healthcare. This site uses cookies. By continuing to browse the site, you are agreeing to our use of cookies. We may request cookies to be set on your device.
0コメント