The dominance relationships between the alleles control which traits are and are not expressed.Īn example of an autosomal dominant human disorder is Huntington's disease, which is a neurological disorder resulting in impaired motor function. Consequently, a child may inherit a blue eye allele from their mother and a brown eye allele from their father. The specific variations possible for a single gene are called alleles: for a single eye-color gene, there may be a blue eye allele, a brown eye allele, a green eye allele, etc. Each chromosomal pair has the same genes, although it is generally unlikely that homologous genes from each parent will be identical in sequence. It is estimated that the human genome contains 20,000-25,000 genes " ". Humans have 23 homologous chromosome pairs (22 pairs of autosomal chromosomes and two distinct sex chromosomes, X and Y).
The alleles present in a locus are usually separated by a slash in the Mc - mc case, the dominant trait is the "mackerel-stripe" pattern, and the recessive one the "classic" or "oyster" tabby pattern, and thus a classical-pattern tabby cat would carry the alleles mc/mc, whereas a mackerel-stripe tabby would be either Mc/mc or Mc/Mc. Alleles producing dominant traits are denoted by initial capital letters those that confer recessive traits are written with lowercase letters. Genes are indicated in shorthand by a combination of one or a few letters - for example, in cat coat genetics the alleles Mc and mc (for "mackerel tabby") play a prominent role. Nomenclature File:KittenAgiosGeorgiosCrete.jpgĪ kitten with an mc/mc genotype: the recessive tabby pattern is expressed 4.1.1 Simple dominance or complete dominance.2 Relationship to other genetics concepts.The relationship described as codominance describes a relationship where the distinct phenotypes caused by each allele are both seen when both alleles are present. Relationships described as incomplete or partial dominance are usually more accurately described as giving an intermediate or blended phenotype. Geneticists often use the term dominance in other contexts, distinguishing between simple or complete dominance as described above, and other relationships. When geneticists loosely refer to a dominant allele or a recessive allele, they mean that the allele is dominant or recessive to the standard allele. Although this usage is convenient it is misleading, because dominance is not a property of an allele considered in isolation but of a relationship between the effects of two alleles.
B b for alleles causing brown and blue eyes). This is the case for the eye color alleles described above, where a single functional copy of the ‘brown’ allele causes enough melanin to be made in the iris that the eyes appear brown even when paired with the non-melanin-producing ‘blue’ allele.ĭominance was discovered by Mendel, who introduced the use of uppercase letters to denote ‘dominant alleles’ and lowercase to denote ‘recessive alleles’, as is still commonly used in introductory genetics courses (e.g. In these cases a single copy of the normal allele produces enough of the gene’s product to give the same effect as two normal copies, and so the normal allele is described as being dominant to the defective allele. We now know that in most cases a dominance relationship is seen when the recessive allele is defective. But having one copy of each allele leads to a brown iris, thus the brown allele is said to be dominant over the blue allele (and the blue allele is said to be recessive to the brown allele). But if the combined effect is the same as the effect of having two copies of one of the alleles, we say that allele’s effect is dominant over the other.įor example, having two copies of one allele of the EYC元 gene causes the eye’s iris to be brown, and having two copies of another allele causes the iris to be blue. If the two copies are not identical (not the same allele), their combined effect may be different than the effect of having two identical copies of one or the other allele. Animals (including humans) and plants are ‘diploid’ (see ploidy), with two copies of each gene, one inherited from each parent. Autosomal dominance in Genetics, describes a specific relationship between the effects of different versions of a gene ( alleles) on a trait or phenotype.