Determine which alleles are dominant

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.

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.

Therefore, it is impossible to identify the genotype of an organism with a dominant trait by visually examining its phenotype. To identify whether an organism exhibiting a dominant trait is homozygous or heterozygous for a specific allele, a scientist can perform a test cross. The organism in question is crossed with an organism that is homozygous for the recessive trait, and the offspring of the test cross are examined. If the test cross results in any recessive offspring, then the parent organism is heterozygous for the allele in question.

If the test cross results in only phenotypically dominant offspring, then the parent organism is homozygous dominant for the allele in question.

Further Exploration Concept Links for further exploration genetic cross phenotype allele dominant recessive dihybrid cross genotype Mendelian test cross. Related Concepts 8. Individuals receive two versions of each gene, known as alleles, from each parent. If the alleles of a gene are different, one allele will be expressed; it is the dominant gene.

The effect of the other allele, called recessive, is masked. Dominant refers to a relationship between two versions of a gene. 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.