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.
Therefore, multiple alleles of a given gene do not necessarily mean variation in the phenotype. Multiple alleles are also used to describe all mutations found in a gene in a population. For example, multiple loss-of-function mutations can be reported in a population of patients suffering from a genetic disease. Although different mutations lead to the same result, loss of protein function, each mutation constitutes a different mutant allele as long as it is located in a different gene site.
Beta-thalassemia is characterized by the reduction or absence of beta-globin chains of the hemoglobin. They are non-coding DNA sequences but can be linked to specific genetic conditions.
However, the primary and most successful application of STR markers is forensic analysis. The number of repeats in each STR locus is highly variable between individuals. Scientists used these variations within a population and developed a test for forensic identification using multiple STR markers.
Going back to the discussion of the ABO blood groups, genetic variation between alleles could lead to a protein variation. Even a small variation in protein level, four amino acids, in this case, can result in drastic effects on the phenotypes. Therefore, it can be said that one of the significant factors contributing to the genetic variation observed between individuals is the allelic variation between their genes.
This inheritance means that individuals have two gene copies for a given trait, one inherited from their mother and the other from their father.
These are known as maternal alleles and paternal alleles. It is how these alleles interact that is responsible for unique characteristics. But not all gene variations will be expressed. For instance, you might have one allele for brown eyes and another for blue eyes, but you will not therefore have one blue and one brown eye. Individuals do not display the characteristics encoded on each matching pair of genes. 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.
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