Allele
From CreationWiki, the encyclopedia of creation science
An allele or allelomorph is one of two alternate forms of a gene responsible for heritble traits at the same position or locus on a chromosome or are also described as belonging to the same gene family. Through mutation new alleles arise from existing ones and can equally be considered the cause of heredity variation for organisms within evolution and creationism.
The different set of alleles for a given gene control the degree to which the specific hereditary characteristic is manifested. The allele which causes a characteristic to be expressed in a normal or essentially mutated form is, at times referred to as a wild-type allele. Alleles usually occur in pairs but multiple alleles are also found to affect the expression of a particular trait. If paired alleles are the same and there is no difference in expression of the trait or gene between the two they are said to be homozygous for that trait. If they are different then the organism is heterozygous.
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Dominant vs. Recessive
- Main Article: Genetic dominance
A dominant allele is an allele that is almost always expressed, even if only one copy is present. Dominant alleles express their phenotype even when paired with a different allele, that is, when heterozygous. In this case, the phenotype appears the same in both the heterozygous and homozygous states. Just how the dominant allele overshadows the other allele depends on the gene, but in some cases the dominant gene produces a gene product that the other allele does not. Well-known dominant alleles occur in the human genes for Huntington disease, a form of dwarfism called achondroplasia, and polydactylism (extra fingers and toes).
On the other hand, a recessive allele will be expressed only if there are two identical copies of that allele, or for a male, if one copy is present on the X chromosome. The phenotype of a recessive allele is only seen when both alleles are the same. When an individual has one dominant allele and one recessive allele, the trait is not expressed because it is overshadowed by the dominant allele. The individual is said to be a carrier for that trait. Examples of recessive disorders in humans include sickle cell anemia, Tay-Sachs disease, and phenylketonuria (PKU).[1]
Sex-linkage
Sex chromosomes (X and Y) not only carry the genes that determine male and female traits, but also those for some other characteristics as well. Genes that are carried by either sex chromosome are said to be sex linked. Men normally have an X and a Y combination of sex chromosomes, whereas women have two X's. Because only men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Both men and women can have X-linked traits because both inherit X chromosomes.
X-linked traits not related to feminine body characteristics are primarily expressed in the phenotype of men. This is because men have only one X chromosome. Subsequently, genes on that chromosome that do not code for gender are expressed in the male phenotype, even if they are recessive. In women, a recessive allele on one X chromosome is often masked in their phenotype by a dominant normal allele on the other. This explains why women are frequently carriers of X-linked traits but more rarely have them expressed in their own phenotypes. In humans, at least 320 genes are X-linked. These include the genes for hemophilia, red–green color blindness, and congenital night blindness. There are at least a dozen Y-linked genes, in addition to those that code for masculine physical traits.[2]
References
- Allele by National Human Genome Research Institute
- What is a Genome by the National Center for Biotechnology Information
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