How do alleles relate to dna

It contains chemical codes which guide life processes, written by using those four letters. Along the length of DNA molecule, only some stretches are 'meaningful' and are called genes. Hence, several genes are located on same chromosome. Homologous chromosomes carry same genes, generally in same order. This is why one gene may have different alleles. For example, at the genetic locus where gene for our eye colour is located, so much variations are present that you see various different colours of iris in different people.

Related questions Why is the number of chromosomes an organisms has an even number? 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.

An allele is a variant form of a gene. Some genes have a variety of different forms, which are located at the same position, or genetic locus, on a chromosome. Humans are called diploid organisms because they have two alleles at each genetic locus, with one allele inherited from each parent. Each pair of alleles represents the genotype of a specific gene.

An example of this would be seen if, when crossing Antirrhinums — flowers with incompletely dominant "red" and "white" alleles for petal color — the resulting offspring had pink petals.

Another exception is co-dominance, where both alleles are active and both traits are expressed at the same time; for example, both red and white petals in the same bloom or red and white flowers on the same plant. A person with one "A" blood type allele and one "B" blood type allele would have a blood type of "AB". A wild type allele is an allele which is considered to be "normal" for the organism in question, as opposed to a mutant allele which is usually a relatively new modification.

Note that with the advent of neutral genetic markers, the term 'allele' is now often used to refer to DNA sequence variants in non-functional, or junk DNA. For example, allele frequency tables are often presented for genetic markers, such as the DYS markers. Also there are many different types of alleles.

Reference Terms. An individual's genotype for that gene is the set of alleles it happens to possess. One version might result in red petals, while another might result in white petals.