Allele

An allele is one of the variant forms of a gene at a particular locus (or location) on a chromosome. Different alleles produce variation in inherited characteristics such as hair color or blood type. Although alleles occur in pairs, multiple pairs of alleles are found to affect some traits such as eye color. If the alleles are the same, the organism is said to be homozygous for that trait. If they are different, then it is termed heterozygous.

Alleles may dominate others, and in doing so, control the degree to which a particular characteristic is manifested. In an individual, one form of the allele (the dominant one) may be expressed more than another form (the recessive one).

New alleles may arise from existing ones through either mutation or genetic recombination. While evolutionists claim that random errors are ultimately responsible for all new alleles, creationists assert that the accumulation of new alleles occurs by design through naturally directed cellular mechanisms.

Genetic dominance

 * Main Article: Genetic dominance

Gene expression, as reflected in an organism's phenotype, is based on conditions specific for each copy of a gene. For some alleles, their influence on phenotype takes precedence over all other alleles. For others, expression depends on whether the gene appears in the homozygous or heterozygous state. Still other phenotypic traits are a combination of several alleles from several different genes. Determining the allelic condition used to be accomplished solely through the analysis of pedigrees, much the way Gregor Mendel carried out his experiments on peas. However, this method can leave many questions unanswered, particularly for traits that are a result of the interaction between several different genes. Today, molecular genetic techniques exist that can assist researchers in tracking the transmission of traits by pinpointing the location of individual genes, identifying allelic variants, and identifying those traits that are caused by multiple genes.

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's 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).

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.