A genetic trait is considered dominant if it is expressed in a person who has only one copy of that gene. In genetic terms, a dominant trait is one that is phenotypically expressed in heterozygotes. A dominant trait is opposed to a recessive trait which is expressed only when two copies of the gene are present. In genetic terms, a recessive trait is one that is phenotypically expressed only in homozygotes.
Introduction to genetics The concept of dominance was introduced by Gregor Johann Mendel. Though Mendel, "The Father of Genetics", first used the term in the s, it was not widely known until the early twentieth century.
Mendel observed that, for a variety of traits of garden peas having to do with the appearance of seeds, seed pods, and plants, there were two discrete phenotypes, such as round versus wrinkled seeds, yellow versus green seeds, red versus white flowers or tall versus short plants. When bred separately, the plants always produced the same phenotypes, generation after generation.
However, when lines with different phenotypes were crossed interbredone and only one of the parental phenotypes showed up in the offspring green, or round, or red, or tall.
However, when these hybrid plants were crossed, the offspring plants showed the two original phenotypes, in a characteristic 3: Mendel reasoned that each parent in the first cross was a homozygote for different alleles one parent AA and the other parent aathat each contributed one allele to the offspring, with the result that all of these hybrids were heterozygotes Aaand that one of the two alleles in the hybrid cross dominated expression of the other: Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced later.
He did introduce Definition of dominant and recessive gene notation of capital and lowercase letters for dominant and recessive alleles, respectively, still in use today. Chromosomes, genes, and alleles[ edit ] See also: Ploidy and Zygosity an autosomal dominant pattern.
Most animals and some plants have paired chromosomesand are described as diploid. They have two versions of each chromosome, one contributed by the mother's ovumand the other by the father's spermknown as gametesdescribed as haploid, and created through meiosis.
These gametes then fuse during fertilization during sexual reproductioninto a new single cell zygotewhich divides multiple times, resulting in a new organism with the same number of pairs of chromosomes in each non-gamete cell as its parents.
Each chromosome of a matching homologous pair is structurally similar to the other, and has a very similar DNA sequence locisingular locus. The DNA in each chromosome functions as a series of discrete genes that influence various traits.
Thus, each gene also has a corresponding homologue, which may exist in different versions called alleles.
The alleles at the same locus on the two homologous chromosomes may be identical or different. The blood type of a human is determined by a gene that creates an A, B, AB or O blood type and is located in the long arm of chromosome nine.
There are three different alleles that could be present at this locus, but only two can be present in any individual, one inherited from their mother and one from their father.
The genetic makeup of an organism, either at a single locus or over all its genes collectively, is called its genotype. The genotype of an organism directly and indirectly affects its molecular, physical, and other traits, which individually or collectively are called its phenotype.
At heterozygous gene loci, the two alleles interact to produce the phenotype. Complete dominance[ edit ] In complete dominance, the effect of one allele in a heterozygous genotype completely masks the effect of the other. The allele that masks the other is said to be dominant to the latter, and the allele that is masked is said to be recessive to the former.
A classic example of dominance is the inheritance of seed shape pea shape 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 and rr are homozygous and Rr is heterozygous. The RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals the R allele masks the presence of the r allele, so these individuals also have round peas.
Thus, allele R is completely dominant to allele r, and allele r is recessive to allele R.Dominant alleles mask the effects of recessive alleles, so a recessive trait is only expressed when an organism has two recessive alleles for a gene.
Mendelian Inheritance Gregor Mendel was an Austrian monk who did research on pea plants in the 19 th century. recessive gene n. A gene that is phenotypically expressed in the homozygous state but has its expression masked in the presence of a dominant gene. being or produced by a form of a gene whose effect can be hidden by a dominant gene and which can produce a noticeable effect only when two copies of the gene are present.
Genetics Of, relating to, or designating an allele that does not produce a characteristic effect when present with a dominant allele. b. Of or relating to a trait that is expressed only when the determining allele is present in the homozygous condition. X-linked gene a gene carried on the X chromosome; the corresponding trait, whether dominant or recessive, is always expressed in males, who have only one X chromosome. the term “X-linked” is sometimes used synonymously with “sex-linked,” since no genetic disorders have as yet been associated with genes on the Y chromosome. Dominance in genetics is a relationship between alleles of one gene, in which the effect on phenotype of one allele masks the contribution of a second allele at the same locus. The first allele is dominant and the second allele is ashio-midori.com genes on an autosome (any chromosome other than a sex chromosome), the alleles and their associated traits are autosomal dominant or autosomal recessive.
Recessive alleles only show their effect if the individual has two copies of the allele (also known as being homozygous).
For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the 'blue eye' allele.
pertaining to or describing a gene, the effect of which is masked or hidden if there is a dominant gene at the same locus. If both genes are recessive and produce the same trait, the trait is .
X-linked gene a gene carried on the X chromosome; the corresponding trait, whether dominant or recessive, is always expressed in males, who have only one X chromosome. the term “X-linked” is sometimes used synonymously with “sex-linked,” since no genetic disorders have as yet been associated with genes on the Y chromosome.