Where is allele found

In heterozygous instances, however, the phenotype of the organism is determined by which one is dominant, meaning that the one overrides the other. In the case of eye color, if someone inherits a blue and a brown allele, his or her eyes will be brown, because brown is a dominant genetic trait, requiring only one allele for expression. However, if that person had a child with someone who also carried a blue allele and both parents passed the blue trait down, the child would have blue eyes.

This explains why blue-eyed children sometimes randomly pop up in a brown-eyed family: because someone in the family's genetic history had blue eyes. Researchers are constantly identifying new alleles, and developing specific tests to look for certain ones, especially those linked with genetic conditions or genetic predispositions to disease. In genetic testing for conditions like Huntington's Disease, a medical lab can search for the specific spot on chromosome four where the Huntington's allele resides.

Unfortunately, Huntington's is a dominant trait , so it only takes one allele to develop the condition. Crime labs, for example, test DNA evidence from crime scenes against known DNA databases and potential suspects, and DNA tests are also used to test the parentage of children. Such testing is often extremely accurate, as long as the samples are handled properly and they are of good quality.

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.

Genotypes are described as homozygous if there are two identical alleles at a particular locus and as heterozygous if the two alleles differ. Alleles contribute to the organism's phenotype, which is the outward appearance of the organism. Some alleles are dominant or recessive. When an organism is heterozygous at a specific locus and carries one dominant and one recessive allele, the organism will express the dominant phenotype. Alleles can also refer to minor DNA sequence variations between alleles that do not necessarily influence the gene's phenotype.

Further Exploration Concept Links for further exploration gene recessive dominant test cross genotype phenotype haplotype DNA chromosome Hardy-Weinberg equilibrium evolution mutation speciation penetrance SNP allele frequency Hardy-Weinberg equation population bottleneck principle of segregation principle of independent assortment dihybrid cross genetic drift lethal allele principle of uniformity Principles of Inheritance.