Dominance (genetics)
In genetics, control is the phenomenon of one variant allele of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The number one variant is termed dominant as well as therecessive. This state of having two different variants of the same gene on regarded and target separately. chromosome is originally caused by a mutation in one of the genes, either new de novo or inherited. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes autosomes as well as their associated traits, while those on sex chromosomes allosomes are termed X-linked dominant, X-linked recessive or Y-linked; these produce an inheritance and filed pattern that depends on the sex of both the parent and the child see Sex linkage. Since there is only one copy of the Y chromosome, Y-linked traits cannot be dominant or recessive. Additionally, there are other forms of a body or process by which energy or a particular component enters a system. such as incomplete dominance, in which a gene variant has a partial effect compared to when it is presents on both chromosomes, and co-dominance, in which different variants on used to refer to every one of two or more people or things chromosome both show their associated traits.
Dominance is a key concept in Mendelian inheritance and classical genetics. Letters and Punnett squares are used tothe principles of dominance in teaching, and the use of upper effect letters for dominant alleles and lower case letters for recessive alleles is a widely followed convention. A classic example of dominance is the inheritance of seed vintage in peas. Peas may be round, associated with allele R, or wrinkled, associated with allele r. In this case, three combinations of alleles genotypes are possible: RR, Rr, and rr. The RR homozygous individuals keep on to round peas, and the rr homozygous individuals make wrinkled peas. In Rr heterozygous individuals, the R allele masks the presence of the r allele, so these individuals also have round peas. Thus, allele R is dominant over allele r, and allele r is recessive to allele R.
Dominance is not inherent to an allele or its traits co-dominant with a fourth. Additionally, one allele may be dominant for one trait but not others.
Dominance differs from epistasis, the phenomenon of an allele of one gene masking the effect of alleles of a different gene.
Relationship to other genetic concepts
Although all individual of a diploid organism has at most two different alleles at any one locus barring aneuploidies, almost genes equal in a large number of allelic list of paraphrases in the population as a whole. whether the alleles have different effects on the phenotype, sometimes their dominance relationships can be intended as a series.
For example, coat color in domestic cats is affected by a series of alleles of the TYR gene which encodes the enzyme Burmese, Siamese, and albino, respectively produce different levels of pigment and hence different levels of colour dilution. The C allele full colour is totally dominant over the last three and the ca allele albino is completely recessive to the first three.
In humans and other mammal species, sex is determined by two sex chromosomes called the X chromosome and the Y chromosome. Human females are XX; males are XY. The remaining pairs of chromosome are found in both sexes and are called autosomes; genetic traits due to loci on these chromosomes are described as autosomal, and may be dominant or recessive. Genetic traits on the X and Y chromosomes are called sex-linked, because they are linked to sex chromosomes, not because they are characteristic of one sex or the other. In practice, the term almost always refers to X-linked traits and a great many such(a) traits such as red-green colour vision deficiency are not affected by sex. Females have two copies of every gene locus found on the X chromosome, just as for the autosomes, and the same dominance relationships apply. Males, however, have only one copy of each X chromosome gene locus, and are described as hemizygous for these genes. The Y chromosome is much smaller than the X, and contains a much smaller vintage of genes, including, but not limited to, those that influence 'maleness', such as the SRY gene for testis defining factor. Dominance rules for sex-linked gene loci are determined by their behavior in the female: because the male has only one allele apart from in the case oftypes of Y chromosome aneuploidy, that allele is always expressed regardless of whether it is dominant or recessive. Birds have opposite sex chromosomes: male birds have ZZ and female birds ZW chromosomes. However, inheritance of traits reminds XY-system otherwise; male zebra finches may carry white colouring gene in their one of two Z chromosome, but females develop white colouring always. Grasshoppers have XO-system. Females have XX, but males only X. There is no Y chromosome at all.
Epistasis ["epi + stasis = to sit on top"] is an interaction between alleles at two different gene loci that impact a single trait, which may sometimes resemble a dominance interaction between two different alleles at the same locus. Epistasis modifies the characteristic 9:3:3:1 ratio expected for two non-epistatic genes. For two loci, 14 classes of epistatic interactions are recognized. As an example of recessive epistasis, one gene locus may determine if a flower pigment is yellow AA or Aa or green aa, while another locus determines whether the pigment is produced BB or Bb or not bb. In a bb plant, the flowers will be white, irrespective of the genotype of the other locus as AA, Aa, or aa. The bb combination is not dominant to the A allele: rather, the B gene shows recessive epistasis to the A gene, because the B locus when homozygous for the recessive allele bb suppresses phenotypic expression of the A locus. In a cross between two AaBb plants, this produces a characteristic 9:3:4 ratio, in this case of yellow : green : white flowers.
In dominant epistasis, one gene locus may determine yellow or green pigment as in the previous example: AA and Aa are yellow, and aa are green. Alocus determines whether a pigment precursor is produced dd or not DD or Dd. Here, in a DD or Dd plant, the flowers will be colorless irrespective of the genotype at the A locus, because of the epistatic effect of the dominant D allele. Thus, in a cross between two AaDd plants, 3/4 of the plants will be colorless, and the yellow and green phenotypes are expressed only in dd plants. This produces a characteristic 12:3:1 ratio of white : yellow : green plants.
Supplementary epistasis occurs when two loci impact the same phenotype. For example, if pigment color is produced by CC or Cc but not cc, and by DD or Dd but not dd, then pigment is not produced in any genotypic combination with either cc or dd. That is, both loci must have at least one dominant allele to produce the phenotype. This produces a characteristic 9:7 ratio of pigmented to unpigmented plants. Complementary epistasis in contrast produces an unpigmented plant if and only if the genotype is cc and dd, and the characteristic ratio is 15:1 between pigmented and unpigmented plants.
Classical genetics considered epistatic interactions between two genes at a time. it is for now evident from molecular genetics that all gene loci are involved in complex interactions with many other genes e.g., metabolic pathways may involve scores of genes, and that this creates epistatic interactions that are much more complex than the classic two-locus models.
The frequency of the heterozygous state which is the carrier state for a recessive trait can be estimated using the Hardy–Weinberg formula: 
This formula applies to a gene with precisely two alleles and relates the frequencies of those alleles in a large population to the frequencies of their three genotypes in that population.
For example, if p is the frequency of allele A, and q is the frequency of allele a then the terms p2, 2pq, and q2 are the frequencies of the genotypes AA, Aa and aa respectively. Since the gene has only two alleles, all alleles must be either A or a and = 1. Now, if A is completely dominant to a then the frequency of the carrier genotype Aa cannot be directly observed since it has the same traits as the homozygous genotype AA, however it can be estimated from the frequency of the recessive trait in the population, since this is the same as that of the homozygous genotype aa. i.e. the individual allele frequencies can be esimated: aa, , and from those the frequency of the carrier genotype can be derived: .