Alleles are different variants of a gene. Alleles are classified as dominant or recessive based on their related characteristics. For example, if one allele causes brown eyes and another allele causes blue eyes, then those people would be considered to have brown eyes because they have one copy of the brown-eye-causing allele and one copy of the blue-eye-causing allele. If both alleles cause the same trait, then these individuals would be considered to have two copies of that allele (i.e., homozygous for that allele). Alleles can also be classified as good or bad based on whether they help or hurt an individual's ability to survive and reproduce. For example, some alleles may increase an animal's risk of developing diseases while other alleles may enhance its resistance to diseases.
In humans, there are three main types of alleles: wild-type, mutant, and transgenic. The wild-type allele is the normal version of the gene that is present in 50% of the human population. People who carry two wild-type alleles for a particular gene are called heterozygotes. Those people who do not have any copies of this gene are called null mutants.
Alleles are different versions of a gene. A single gene has two alleles, one dominant and one recessive. The phenotype is determined by which allele is present in greater number. For example, if a person carries two copies of the recessive allele, they will appear phenotypically like those with no copies of this allele.
A mutant gene can be defined as any genetic variation that affects the structure or expression of a gene and that does not occur naturally within populations. A mutation can be caused by exposure to radiation, certain chemicals, or when using laboratory techniques to alter genes. Natural mutations arise spontaneously within living organisms. They can also be created artificially by scientists working in the field of genetics. Mutations can be beneficial or detrimental depending on how they affect the function of the gene they are in contact with.
A polymorphism is a variant of a genetic marker.
A gene can have two or more alleles. Every gene in a person has two alleles, or variations. Humans are classified as diploid creatures since each gene has two gene variations. One of these variations is called the dominant allele and the other is called the recessive allele. When one of each pair of chromosomes contains a mutation, the person is said to be heterozygous for that mutation. If both of those chromosomes contain the mutation it will be present in all cells of that individual causing the disease. There are several ways for two individuals to inherit the same mutation in their genes and yet not suffer any effects from it. The first way is if they are homozygous for different mutations in the same gene. The second way is if they are heterozygous for different mutations in different genes.
In humans, most genetic diseases result from only one of the two copies of a chromosome being mutated. A mutation can be anything that changes the DNA code; therefore, mutations can include alterations such as deletions, insertions, repeats, or mismatches between pairs of nucleotides. A deletion occurs when a section of DNA is removed from one region of a chromosome and placed in another region where it does not belong. This can happen during meiosis, when the cell divides without copying its entire genome.
A variant version of a gene is referred to as an allele. Some genes have many versions that are all situated at the same place, or genetic locus, on a chromosome. Alleles contribute to an organism's phenotype, which is its external appearance. Some alleles are dominant while others are recessive. A dominant allele will always produce a phenotype effects even if it is not the one inherited from their parent. A recessive allele will only show up in cases where both copies of the gene are present.
A variant version of a gene can be due to any of several things including mutation (change in DNA sequence), duplication (copying of segment of DNA), transposition (movement of segment of DNA within genome), or recombination (cross-over between strands of DNA). Variants can also arise because some genes are active in embryonic development but are turned off after they are no longer needed. These so-called "junk" DNA sequences may appear to be meaningless strings of chemicals with no effect on health but they can alter protein production and therefore be important for evolutionarily adapting organisms to their environment.
For example, two versions of the COL5A1 gene code for proteins that are very similar but not identical. One version tends to cause disease when it occurs alone but does not affect the ability of cells to divide. The other version causes disease when it occurs alone but helps cells divide more easily than they would if they were only equipped with one copy of this gene.
Each variant of a gene is referred to as an allele (pronounced "AL-eel"). These two copies of the gene in your chromosomes determine how your cells function. A gene pair's two alleles are inherited, one from each parent. If one of the genes is defective, its allele will be passed on to its offspring.
All living organisms inherit their genetic material from their parents. In general, this inheritance follows what is called a "biallelic" model, which means that each gene resides on a pair of homologous chromosomes and is expressed in a bipolar fashion from these pairs. The two copies of each gene in humans are called alleles. Alleles can be identified by their chemical composition or sequence. For example, if you have two copies of the mutant allele GCG instead of the normal allele GCT at position 514 in exon 7 of the beta-globin gene, you would know that you are a carrier of Sickle Cell Disease.
During gamete formation, the genome of each parent contributes a set of chromosomes that together make up the complete set needed for a successful pregnancy. The exact number of chromosomes in human eggs and sperm is the same - 23 - but some chromosomes are present in higher than expected amounts while others are missing. As we will see below, this unequal distribution affects the ability of eggs and sperm to join together during fertilization.