Incomplete dominance, codominance & multiple alleles (article) | Khan Academy
Phenotype determines dominance. The "rules" you state are just general possibilities that are most likely; they are not correct definitions. Alleles are described as either dominant or recessive depending on their Dominant alleles show their effect even if the individual only has. The terms dominant and recessive describe the inheritance patterns of certain traits. That is, they describe how likely it is for a certain phenotype to pass from.
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 the 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.
What are Dominant and Recessive?
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. Incomplete dominance[ edit ] This Punnett square illustrates incomplete dominance.
In this example, the red petal trait associated with the R allele recombines with the white petal trait of the r allele. The plant incompletely expresses the dominant trait R causing plants with the Rr genotype to express flowers with less red pigment resulting in pink flowers.
The colors are not blended together, the dominant trait is just expressed less strongly. Incomplete dominance also called partial dominance, semi-dominance or intermediate inheritance occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes.
For example, the snapdragon flower color is homozygous for either red or white. When the red homozygous flower is paired with the white homozygous flower, the result yields a pink snapdragon flower. The pink snapdragon is the result of incomplete dominance. A similar type of incomplete dominance is found in the four o'clock plant wherein pink color is produced when true-bred parents of white and red flowers are crossed. In quantitative geneticswhere phenotypes are measured and treated numerically, if a heterozygote's phenotype is exactly between numerically that of the two homozygotes, the phenotype is said to exhibit no dominance at all, i.
Multiple alleles, incomplete dominance, and codominance
When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1: The long, pointy blood cells get caught in capillaries, where they block blood flow. The disease has a recessive pattern of inheritance: People with just one copy are healthy.
In addition to causing disease, the sickle-cell allele makes people who carry it resistant to malaria, a serious illness carried by mosquitos. Malaria resistance has a dominant inheritance pattern: This is the very same allele that, in a recessive inheritance pattern, causes sickle-cell disease!Wild & Mutant Allele-concept behind dominant recessive relationship.
People with two copies of the sickle-cell allele have many sickled red blood cells. People with one sickle-cell allele and one normal allele have a small number of sickled cells, and their cells sickle more easily under certain conditions. So we could say that red blood cell shape has a co-dominant inheritance pattern. That is, individuals with one copy of each allele have an in-between phenotype. So is the sickle cell allele dominant, recessive, or co-dominant? It depends on how you look at it.
Protein function If we look at the proteins the two alleles code for, the picture becomes a little more clear. The affected protein is hemoglobin, the oxygen-carrying molecule that fills red blood cells. The sickle-cell allele codes for a slightly modified version of the hemoglobin protein.
genetics - Dominance and recessive relationship of alleles - Biology Stack Exchange
The modified hemoglobin protein still carries oxygen, but under low-oxygen conditions the proteins stick together. When a person has two sickle cell alleles, all of their hemoglobin is the sticky form, and the proteins form very long, stiff fibers that distort red blood cells. When someone has one sickle-cell allele and one normal allele, only some of the hemoglobin is sticky. Non-sticky hemoglobin is made from the normal allele, and sticky hemoglobin is made from the sickle-cell allele every cell has a copy of both alleles.
The protist that causes malaria grows and reproduces in red blood cells. The manifestation of each allele can depend on the second allele in the given locus for the particular individual. It very often happens that a particular allele is recessive, i. A dominant allele is the opposite of a recessive allele.
Its presence is manifested in the same way both in a carrier of two copies of the given allele, i. The degree to which semi-dominant alleles, i. In co-dominance, the two alleles present are manifested to the same degree to which they would be manifested in the relevant homozygotes. While, in partial dominance, the degree of manifestation of the two alleles in a heterozygote is less than for one or the other homozygote, in super-dominance, the expression of the given trait is greater in a heterozygote than in either of the two homozygotes.