What kind of alleles are there

Below are examples of alleles and how they are expressed. The allelic expression may follow a Mendelian pattern of inheritance — as in the case of complete dominance. As for nonMendelian inheritance, the alleles may be expressed through codominance , incomplete dominance , or polygenic inheritance. One classic example explaining how alleles work is the alleles responsible for the flower color in garden peas, one of the seven traits studied by Gregor Mendel in his experiments that later established the principles of genetics.

The color of garden peas flowers can be either purple or white. The gene coding for this trait, responsible for producing a purple pigment, is found in two forms, allele for purple color and allele for white color.

Alleles are designated by letters to describe genotypes. Hence, the resulting genotype is written as Pp. Other genotypes that can be found for this trait are PP Purple flower and pp white flower. However, one P allele can produce enough pigment to make the flower purple color in this particular example.

Therefore, plants with either PP or Pp genotypes will have purple flowers, and only plants with the pp genotype have white flowers. In this situation, the allele that expresses the trait over the other trait is called the dominant allele the P allele in this case. The other allele p is called recessive and is only expressed when present in two copies. However, this is not always the case. In some genes, two different alleles heterozygous genotype are equally expressed, a phenomenon called codominance.

One of the most common examples of this type of inheritance is ABO blood grouping alleles. The genotype of this gene determines the antigens on the red blood cells, hence the blood group phenotype.

The table below shows the inheritance pattern of the ABO gene. Since the locus has three alternative alleles, six possible genotypes can result from mating. Two inheritance patterns can be seen in blood groups. Both A and B alleles are dominant when they are found with the O allele. However, If a person has an AB genotype, both alleles are equally expressed codominance , resulting in a phenotype where both A and B antigens are found on the surface of red blood cells.

Some genes could have more than two alternative allelic forms. Many know human genes have multiple alleles. Each individual can have only two alleles of each gene. However, some genes could have hundreds of alleles within a population. The alleles that we receive from each parent might be the same homozygous or they might differ heterozygous.

Some individuals are genetically prone to form multiple lipomas. Since there are two copies of each gene, there are two alleles, which may be the same or different. The figure below shows a hypothetical example in which there is an allele for red hair on one chromosome and an allele for brown hair on the other. Note that there may be many alleles for some genes, but normally we each have two alleles for each gene on our autosomes. Note also that in the hypothetical illustration to the right the alleles for the multiple lipoma trait are also different.

The obvious question that arises is, what happens when the two alleles that are present differ? What will the phenotype be? The answer depends on whether one allele is dominant over the other. A dominant allele is one that is expressed to a greater degree than the other allele that is present. Instead, the genes that are expressed result in the phenotype , which is how genes are expressed in observable characteristics. How does the body know which alleles to express?

This comes down to the properties of alleles that are paired. Every individual has two copies, or alleles, or a single gene.

When the alleles are the same, they are known as homozygotes. When they are different, they are called heterozygotes. Homozygotes code for the same trait, for instance, blue eyes. If you have two blue eye alleles, your eyes will be blue. But if you have one allele for blue eyes and another for brown eyes, your eye color will be dictated by whichever allele is dominant.

A dominant allele is one that always determines the phenotype when present. On the other hand, a recessive allele is one that is not expressed when its paired allele is dominant. With eye color, the brown eye allele is dominant to the blue eye allele. If you have any other comments or suggestions, please let us know at comment yourgenome. Can you spare minutes to tell us what you think of this website?

Open survey. In: Facts In the Cell. Since human cells carry two copies of each chromosome they have two versions of each gene. These different versions of a gene are called alleles.

Alleles can be either dominant or recessive. Dominant alleles show their effect even if the individual only has one copy of the allele also known as being heterozygous. 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.