Recessive
Encyclopedia
In genetics
, the term "recessive gene" refers to an allele
that causes a phenotype
(visible or detectable characteristic) that is only seen in a homozygous genotype
(an organism that has two copies of the same allele) and never in a heterozygous genotype
. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father. If a genetic trait is recessive, a person needs to inherit two copies of the gene for the trait to be expressed. Thus, both parents have to be carriers of a recessive trait in order for a child to express that trait. If both parents are carriers, there is a 25% chance with each child to show the recessive trait. Thus if the parents are closely related (in-breeding) the probability of both having inherited the same gene is increased and as a result the probability of the children showing the recessive trait is increased as well.
The term "recessive gene" is part of the laws of Mendelian inheritance
created by Gregor Mendel
. Examples of recessive genes in Mendel's famous pea plant experiments include those that determine the color and shape of seed pods, and plant height.
of genetic traits located on the autosome
s (the 22 non-sex determining chromosome
s).
In opposition to autosomal dominant trait, a recessive trait only becomes phenotypically apparent when two copies of a gene (two alleles) are present. In other words, the subject is homozygous for the trait. Recessive genes will also show a horizontal inheritance on a pedigree chart.
The frequency of the carrier state can be calculated by the Hardy-Weinberg formula
:
(p is the frequency of one pair of alleles, and q = 1 − p is the frequency of the other pair of alleles.)
Recessive genetic disorder
s occur when both parents are carriers and each contributes an allele
to the embryo
. As both parents are heterozygous for the disorder, the chance of two disease alleles being inherited by one of their offspring is 25% (in autosomal dominant traits this is higher). 50% of the children (or 2/3 of the remaining ones) are carriers. When one of the parents is homozygous, the trait will only show in his/her offspring if the other parent is also a carrier. In that case, the chance of disease in the offspring is 50%.
that is recessive but the phenotype (or trait). It should also be noted that the concepts of recessiveness and dominance were developed before a molecular understanding of DNA
and before molecular biology
, thus mapping many newer concepts to "dominant" or "recessive" phenotypes is problematic. Many traits previously thought to be recessive have mild forms or biochemical abnormalities that arise from the presence of the one copy of the allele. This suggests that the dominant phenotype is dependent upon having two dominant genes and the presence of one dominant and one recessive gene creates some blending of both dominant and recessive traits.
Recessive genes are usually represented by a lowercase letter in a Punnett square
, as opposed to the uppercase letters of dominant genes (see example at right). Using the letter "T" as an example, only in "tt" (the homozygous recessive genotype, indicated by blue) would the recessive physical trait appear. The form "Tt" is called heterozygous (indicated by magenta with a red border), and, even though a recessive allele is present, the dominant gene is the one that appears (becomes the phenotype). The homozygous dominant genotype is "TT" (indicated by red).
performed many experiments on pea plants (Pisum sativum) while researching traits, chosen because of the simple and low variety of characteristics, as well as the short period of germination. He experimented with color (green vs. yellow), size (short vs. tall), pea texture (smooth vs. wrinkled), and many others. By good fortune, the characteristics displayed by these plants clearly exhibited a dominant and recessive form. This is not true for many organisms.
For example, when testing the color of the pea plants, he chose two yellow plants, since yellow was more common than green. He mated them, and examined the offspring. He continued to mate only those that appeared yellow, and eventually, the green ones would stop being produced. He also mated the green ones together and determined that only green ones were produced.
Mendel determined that this was because green was a recessive trait which only appeared when yellow, the dominant trait, was not present. Also, he determined that the dominant trait would be displayed whether or not the recessive trait was there.
), sickle=GTG (val
). There are several phenotypes associated with the sickle genotype: -
This example demonstrates that one can only refer to dominance/recessiveness with respect to individual phenotypes.
Other recessive disorders:
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
, the term "recessive gene" refers to an allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
that causes a phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
(visible or detectable characteristic) that is only seen in a homozygous genotype
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
(an organism that has two copies of the same allele) and never in a heterozygous genotype
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father. If a genetic trait is recessive, a person needs to inherit two copies of the gene for the trait to be expressed. Thus, both parents have to be carriers of a recessive trait in order for a child to express that trait. If both parents are carriers, there is a 25% chance with each child to show the recessive trait. Thus if the parents are closely related (in-breeding) the probability of both having inherited the same gene is increased and as a result the probability of the children showing the recessive trait is increased as well.
The term "recessive gene" is part of the laws of Mendelian inheritance
Mendelian inheritance
Mendelian inheritance is a scientific description of how hereditary characteristics are passed from parent organisms to their offspring; it underlies much of genetics...
created by Gregor Mendel
Gregor Mendel
Gregor Johann Mendel was an Austrian scientist and Augustinian friar who gained posthumous fame as the founder of the new science of genetics. Mendel demonstrated that the inheritance of certain traits in pea plants follows particular patterns, now referred to as the laws of Mendelian inheritance...
. Examples of recessive genes in Mendel's famous pea plant experiments include those that determine the color and shape of seed pods, and plant height.
Autosomal recessive gene
Autosomal recessive or autorecessive is a mode of inheritanceInheritance
Inheritance is the practice of passing on property, titles, debts, rights and obligations upon the death of an individual. It has long played an important role in human societies...
of genetic traits located on the autosome
Autosome
An autosome is a chromosome that is not a sex chromosome, or allosome; that is to say, there is an equal number of copies of the chromosome in males and females. For example, in humans, there are 22 pairs of autosomes. In addition to autosomes, there are sex chromosomes, to be specific: X and Y...
s (the 22 non-sex determining chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s).
In opposition to autosomal dominant trait, a recessive trait only becomes phenotypically apparent when two copies of a gene (two alleles) are present. In other words, the subject is homozygous for the trait. Recessive genes will also show a horizontal inheritance on a pedigree chart.
The frequency of the carrier state can be calculated by the Hardy-Weinberg formula
Hardy-Weinberg principle
The Hardy–Weinberg principle states that both allele and genotype frequencies in a population remain constant—that is, they are in equilibrium—from generation to generation unless specific disturbing influences are introduced...
:
(p is the frequency of one pair of alleles, and q = 1 − p is the frequency of the other pair of alleles.)
Recessive genetic disorder
Genetic disorder
A genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....
s occur when both parents are carriers and each contributes an allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
to the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. As both parents are heterozygous for the disorder, the chance of two disease alleles being inherited by one of their offspring is 25% (in autosomal dominant traits this is higher). 50% of the children (or 2/3 of the remaining ones) are carriers. When one of the parents is homozygous, the trait will only show in his/her offspring if the other parent is also a carrier. In that case, the chance of disease in the offspring is 50%.
Nomenclature
Technically, the term "recessive gene" is imprecise because it is not the geneGene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
that is recessive but the phenotype (or trait). It should also be noted that the concepts of recessiveness and dominance were developed before a molecular understanding of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
and before molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
, thus mapping many newer concepts to "dominant" or "recessive" phenotypes is problematic. Many traits previously thought to be recessive have mild forms or biochemical abnormalities that arise from the presence of the one copy of the allele. This suggests that the dominant phenotype is dependent upon having two dominant genes and the presence of one dominant and one recessive gene creates some blending of both dominant and recessive traits.
| T | t | |
| T | T T | T t |
| t | T t | t t |
Recessive genes are usually represented by a lowercase letter in a Punnett square
Punnett square
The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach, and is used by biologists to determine the probability of an offspring's having a particular genotype...
, as opposed to the uppercase letters of dominant genes (see example at right). Using the letter "T" as an example, only in "tt" (the homozygous recessive genotype, indicated by blue) would the recessive physical trait appear. The form "Tt" is called heterozygous (indicated by magenta with a red border), and, even though a recessive allele is present, the dominant gene is the one that appears (becomes the phenotype). The homozygous dominant genotype is "TT" (indicated by red).
Pea Plant
Gregor MendelGregor Mendel
Gregor Johann Mendel was an Austrian scientist and Augustinian friar who gained posthumous fame as the founder of the new science of genetics. Mendel demonstrated that the inheritance of certain traits in pea plants follows particular patterns, now referred to as the laws of Mendelian inheritance...
performed many experiments on pea plants (Pisum sativum) while researching traits, chosen because of the simple and low variety of characteristics, as well as the short period of germination. He experimented with color (green vs. yellow), size (short vs. tall), pea texture (smooth vs. wrinkled), and many others. By good fortune, the characteristics displayed by these plants clearly exhibited a dominant and recessive form. This is not true for many organisms.
For example, when testing the color of the pea plants, he chose two yellow plants, since yellow was more common than green. He mated them, and examined the offspring. He continued to mate only those that appeared yellow, and eventually, the green ones would stop being produced. He also mated the green ones together and determined that only green ones were produced.
Mendel determined that this was because green was a recessive trait which only appeared when yellow, the dominant trait, was not present. Also, he determined that the dominant trait would be displayed whether or not the recessive trait was there.
Autosomal recessive disease
Dominance/recessiveness refers to phenotype, not genotype. An example to prove the point is sickle cell anemia. The sickle cell genotype is caused by a single base pair change in the beta-globin gene: normal=GAG (gluGlutamic acid
Glutamic acid is one of the 20 proteinogenic amino acids, and its codons are GAA and GAG. It is a non-essential amino acid. The carboxylate anions and salts of glutamic acid are known as glutamates...
), sickle=GTG (val
Valine
Valine is an α-amino acid with the chemical formula HO2CCHCH2. L-Valine is one of 20 proteinogenic amino acids. Its codons are GUU, GUC, GUA, and GUG. This essential amino acid is classified as nonpolar...
). There are several phenotypes associated with the sickle genotype: -
- anemia (a recessive trait)
- blood cell sickling (co-dominant)
- altered beta-globin electrophoretic mobility (co-dominant)
- resistance to malaria (dominant)
This example demonstrates that one can only refer to dominance/recessiveness with respect to individual phenotypes.
Other recessive disorders:
- Cystic fibrosisCystic fibrosisCystic fibrosis is a recessive genetic disease affecting most critically the lungs, and also the pancreas, liver, and intestine...
- Tay-Sachs diseaseTay-Sachs diseaseTay–Sachs disease is an autosomal recessive genetic disorder...
- Chronic granulomatous diseaseChronic granulomatous diseaseChronic granulomatous disease is a diverse group of hereditary diseases in which certain cells of the immune system have difficulty forming the reactive oxygen compounds used to kill certain ingested pathogens...
- ThalassemiaThalassemiaThalassemia is an inherited autosomal recessive blood disease that originated in the Mediterranean region. In thalassemia the genetic defect, which could be either mutation or deletion, results in reduced rate of synthesis or no synthesis of one of the globin chains that make up hemoglobin...
- Bloom's Syndrome
- Alpha 1-antitrypsin deficiencyAlpha 1-antitrypsin deficiencyAlpha 1-antitrypsin deficiency is an autosomal recessive genetic disorder caused by defective production of alpha 1-antitrypsin , leading to decreased A1AT activity in the blood and lungs, and deposition of excessive abnormal A1AT protein in liver cells...
- HaemochromatosisHaemochromatosisHaemochromatosis type 1 is a hereditary disease characterized by excessive intestinal absorption of dietary iron resulting in a pathological increase in total body iron stores. Humans, like most animals, have no means to excrete excess iron...
types 1-3 - Wilson's diseaseWilson's diseaseWilson's disease or hepatolenticular degeneration is an autosomal recessive genetic disorder in which copper accumulates in tissues; this manifests as neurological or psychiatric symptoms and liver disease...
- Most types of MucopolysaccharidosisMucopolysaccharidosisMucopolysaccharidoses are a group of metabolic disorders caused by the absence or malfunctioning of lysosomal enzymes needed to break down molecules called glycosaminoglycans - long chains of sugar carbohydrates in each of our cells that help build bone, cartilage, tendons, corneas, skin and...
- Most Glycogen storage diseaseGlycogen storage diseaseGlycogen storage disease is the result of defects in the processing of glycogen synthesis or breakdown within muscles, liver, and other cell types. GSD has two classes of cause: genetic and acquired. Genetic GSD is caused by any inborn error of metabolism involved in these processes...
s - HomocystinuriaHomocystinuriaHomocystinuria, also known as cystathionine beta synthase deficiency or CBS deficiency, is an inherited disorder of the metabolism of the amino acid methionine, often involving cystathionine beta synthase...
- Congenital adrenal hyperplasiaCongenital adrenal hyperplasiaCongenital adrenal hyperplasia refers to any of several autosomal recessive diseases resulting from mutations of genes for enzymes mediating the biochemical steps of production of cortisol from cholesterol by the adrenal glands ....
- Dubin-Johnson syndromeDubin-Johnson syndromeDubin–Johnson syndrome is an autosomal recessive disorder that causes an increase of conjugated bilirubin in the serum without elevation of liver enzymes . This condition is associated with a defect in the ability of hepatocytes to secrete conjugated bilirubin into the bile, and is similar to...
- Fanconi anemiaFanconi anemiaFanconi anemia is a genetic disease with an incidence of 1 per 350,000 births, with a higher frequency in Ashkenazi Jews and Afrikaners in South Africa.FA is the result of a genetic defect in a cluster of proteins responsible for DNA repair...
- GalactosemiaGalactosemiaGalactosemia is a rare genetic metabolic disorder that affects an individual's ability to metabolize the sugar galactose properly. Although the sugar lactose can metabolize to galactose, galactosemia is not related to and should not be confused with lactose intolerance...
- Glucose-6-phosphate dehydrogenase deficiencyGlucose-6-phosphate dehydrogenase deficiencyGlucose-6-phosphate dehydrogenase deficiency is an X-linked recessive hereditary disease characterised by abnormally low levels of glucose-6-phosphate dehydrogenase , a metabolic enzyme involved in the pentose phosphate pathway, especially important in red blood cell metabolism. G6PD deficiency is...
- PhenylketonuriaPhenylketonuriaPhenylketonuria is an autosomal recessive metabolic genetic disorder characterized by a mutation in the gene for the hepatic enzyme phenylalanine hydroxylase , rendering it nonfunctional. This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine...
- AlbinismAlbinismAlbinism is a congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...
- Rotor syndromeRotor syndromeRotor syndrome, also called Rotor type hyperbilirubinemia, is a rare, relatively benign autosomal recessive bilirubin disorder of unknown origin...
- Familial Mediterranean feverFamilial Mediterranean feverFamilial Mediterranean fever is a hereditary inflammatory disorder. FMF is an autoinflammatory disease caused by mutations in MEFV, a gene which encodes a 781–amino acid protein denoted pyrin....
- Pendred syndromePendred syndromePendred syndrome or Pendred disease is a genetic disorder leading to congenital bilateral sensorineural hearing loss and goitre with occasional hypothyroidism . There is no specific treatment, other than supportive measures for the hearing loss and thyroid hormone supplementation in case of...
- Certain forms of Spinal muscular atrophySpinal muscular atrophySpinal Muscular Atrophy is a neuromuscular disease characterized by degeneration of motor neurons, resulting in progressive muscular atrophy and weakness. The clinical spectrum of SMA ranges from early infant death to normal adult life with only mild weakness...
- Xeroderma pigmentosumXeroderma pigmentosumXeroderma pigmentosum, or XP, is an autosomal recessive genetic disorder of DNA repair in which the ability to repair damage caused by ultraviolet light is deficient. In extreme cases, all exposure to sunlight must be forbidden, no matter how small. Multiple basal cell carcinomas and other skin...
- Friedrich's ataxia
See also
- Dominant gene
- Incomplete dominance
- Dominance relationshipDominance relationshipDominance in genetics is a relationship between two variant forms of a single gene, in which one allele masks the effect of the other in influencing some trait. In the simplest case, if a gene exists in two allelic forms , three combinations of alleles are possible: AA, AB, and BB...
- X-linked
- mitochondrialMitochondrial DNAMitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...