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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.
In addition, different genotypes can lead to the same phenotype. This happens because genes have different alleles. For some genes and traits, certain alleles are dominant while others are recessive. Some aspects of eye color work this way. Brown eyes, for instance, are dominant over blue eyes. This is because a pigment called melanin produces the brown color, while having no pigment leads to blue eyes. Having just one allele for the dark pigment is enough to make your eyes brown.
There actually are several different pigments that affect eye color, each pigment resulting from a particular gene. This is the reason why people can have green eyes, hazel eyes, or any of a range of eye colors apart from blue or brown. When discussing genotype, biologists use uppercase letters to stand for dominant alleles and lowercase letters to stand for recessive alleles.
An organism with two dominant alleles for a trait is said to have a homozygous dominant genotype. Using the eye color example, this genotype is written BB. An organism with one dominant allele and one recessive allele is said to have a heterozygous genotype. In our example, this genotype is written Bb. Finally, the genotype of an organism with two recessive alleles is called homozygous recessive.
Since these providers may collect personal data like your IP address we allow you to block them here. Please be aware that this might heavily reduce the functionality and appearance of our site. Changes will take effect once you reload the page. Pronunciation: [ dom -ih-nuhnt] [al- leel ]. Definition When two copies of a Gene. A segment of DNA that encodes for a particular polypeptide.
Use in clinical context Individuals usually have two copies of each gene, with the exception of genes on the sex chromosomes in males.
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