Trait (biology)
Encyclopedia
A trait is a distinct variant of a phenotypic character of an organism that may be inherited, environmentally determined or be a combination of the two. For example, eye color
is a character or abstraction
of an attribute, while blue, brown and hazel are traits.
A trait may be any single feature or quantifiable measurement of an organism. However, the most useful traits for genetic analysis
are present in different forms in different individuals.
A visible trait is the final product of many molecular
and biochemical
processes. In most cases, information starts with DNA
traveling to RNA
and finally to protein
(ultimately affecting organism structure and function). This is the central dogma of molecular biology
as stated by Francis Crick
.
This information flow may also be followed through the cell
as it travels from the DNA
in the nucleus
, to the cytoplasm
, to the ribosomes and the endoplasmic reticulum
, and finally to the Golgi apparatus
, which may package the final products for export outside the cell.
Cell products are released into the tissue
, and organ
s of an organism
, to finally affect the physiology
in a way that produces a trait.
. A gene is a portion of a chromosome
. An important reference point along a chromosome, which is a very long and compacted
string of DNA
, is the centromere
; the distance from a gene to the centromere is referred to as the gene's locus
or map location. A chromosomal region known to control a trait while the responsible gene within not being identified is referred to as a quantitative trait locus
.
The nucleus of a diploid cell contains two of each chromosome, with homologous
(mostly identical) pairs of chromosomes having the same genes at the same loci
.
Different phenotypic traits are caused by different forms of genes, or allele
s, which arise by mutation
in a single individual and are passed on to successive generations.
sequence; the slightly different variations of that sequence are called alleles. Allele
s can be significantly different and produce different product RNA
s.
Combinations of different alleles thus go on to generate different traits through the information flow charted above. For example, if the alleles on homologous chromosome
s exhibit a "simple dominance" relationship, the trait of the "dominant" allele shows in the phenotype.
Gregor Mendel
pioneered modern genetics. His most famous analyses were based on clear-cut traits with simple dominance. He determined that the heritable
units, what he called "gene
s", occurred in pairs and could exhibit linkage
. His tool was statistics
: long before the molecular model of DNA
was introduced by James D. Watson
and Francis Crick
.
He is also known to have 'cheated' on his experiments, removing outliers and generally 'tidying' datasets - statistics, and repeating the experiments, shows that his results were just 'too perfect', a point first noted by the Godfather of statistics, RA Fisher, in 1936: http://www.cs.brown.edu/courses/csci1950-l/presentations/REVISED--AFCB%202007%20Course%20Fisher%20and%20Mendel.ppt
of the intermediate proteins determines how they interact in the cell. Therefore, biochemistry predicts
how different combinations of alleles will produce varying traits.
Extended expression patterns seen in diploid organisms include facets of incomplete dominance, codominance
, and multiple alleles.
Incomplete dominance is the condition in which neither allele dominates the other in a heterozygote.
Instead the phenotype is intermediate in heterozygotes. Thus you can tell that each allele is present in
the heterozygote.
Codominance refers to the allelic relationship that occurs when two alleles are both expressed in the
heterozygote, and both phenotypes are seen simultaneously.
Multiple alleles refers to the situation when there are more than 2 common alleles of a particular gene.
Blood groups in humans is a classic example. The ABO blood group proteins are important in
determining blood type in humans, and this is determined by different alleles of the one locus.
Eye color
Eye color is a polygenic phenotypic character and is determined by two distinct factors: the pigmentation of the eye's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris....
is a character or abstraction
Abstraction
Abstraction is a process by which higher concepts are derived from the usage and classification of literal concepts, first principles, or other methods....
of an attribute, while blue, brown and hazel are traits.
Definition
A phenotypic trait is an obvious and observable trait; it is the expression of genes in an observable way. An example of a phenotypic trait is hair color, there are underlying genes that control the hair color, which make up the genotype, but the actual hair color, the part we see, is the phenotype. The phenotype is the physical characteristics of the organism. The phenotype is controlled by the genetic make-up of the organism and the environmental pressures the organism is subject to.A trait may be any single feature or quantifiable measurement of an organism. However, the most useful traits for genetic analysis
Genetic analysis
Genetic analysis can be used generally to describe methods both used in and resulting from the sciences of genetics and molecular biology, or to applications resulting from this research....
are present in different forms in different individuals.
A visible trait is the final product of many molecular
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...
and biochemical
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
processes. In most cases, information starts with 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...
traveling to RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
and finally to protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
(ultimately affecting organism structure and function). This is the central dogma of 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...
as stated by Francis Crick
Francis Crick
Francis Harry Compton Crick OM FRS was an English molecular biologist, biophysicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson...
.
This information flow may also be followed through the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
as it travels from the 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...
in the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
, to the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
, to the ribosomes and the endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
, and finally to the Golgi apparatus
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
, which may package the final products for export outside the cell.
Cell products are released into the tissue
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
, and organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
s of an organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
, to finally affect the physiology
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
in a way that produces a trait.
Genetic origin of traits in diploid organisms
The heritable unit that may influence a trait is called a 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...
. A gene is a portion of a 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...
. An important reference point along a chromosome, which is a very long and compacted
Histone
In biology, histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. They are the chief protein components of chromatin, acting as spools around which DNA winds, and play a role in gene regulation...
string 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...
, is the centromere
Centromere
A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...
; the distance from a gene to the centromere is referred to as the gene's locus
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
or map location. A chromosomal region known to control a trait while the responsible gene within not being identified is referred to as a quantitative trait locus
Quantitative trait locus
Quantitative traits refer to phenotypes that vary in degree and can be attributed to polygenic effects, i.e., product of two or more genes, and their environment. Quantitative trait loci are stretches of DNA containing or linked to the genes that underlie a quantitative trait...
.
The nucleus of a diploid cell contains two of each chromosome, with homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
(mostly identical) pairs of chromosomes having the same genes at the same loci
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
.
Different phenotypic traits are caused by different forms of genes, or 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...
s, which arise by mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
in a single individual and are passed on to successive generations.
Mendelian expression of genes in diploid organisms
A gene is only a DNA codeGenetic code
The genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells....
sequence; the slightly different variations of that sequence are called alleles. 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...
s can be significantly different and produce different product RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
s.
Combinations of different alleles thus go on to generate different traits through the information flow charted above. For example, if the alleles on homologous 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 exhibit a "simple dominance" relationship, the trait of the "dominant" allele shows in the phenotype.
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...
pioneered modern genetics. His most famous analyses were based on clear-cut traits with simple dominance. He determined that the heritable
Inheritance
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...
units, what he called "gene
Gene
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...
s", occurred in pairs and could exhibit linkage
Genetic linkage
Genetic linkage is the tendency of certain loci or alleles to be inherited together. Genetic loci that are physically close to one another on the same chromosome tend to stay together during meiosis, and are thus genetically linked.-Background:...
. His tool was statistics
Statistics
Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....
: long before the molecular model 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...
was introduced by James D. Watson
James D. Watson
James Dewey Watson is an American molecular biologist, geneticist, and zoologist, best known as one of the co-discoverers of the structure of DNA in 1953 with Francis Crick...
and Francis Crick
Francis Crick
Francis Harry Compton Crick OM FRS was an English molecular biologist, biophysicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson...
.
He is also known to have 'cheated' on his experiments, removing outliers and generally 'tidying' datasets - statistics, and repeating the experiments, shows that his results were just 'too perfect', a point first noted by the Godfather of statistics, RA Fisher, in 1936: http://www.cs.brown.edu/courses/csci1950-l/presentations/REVISED--AFCB%202007%20Course%20Fisher%20and%20Mendel.ppt
Biochemistry of dominance and extensions to expression of traits
The biochemistryBiochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
of the intermediate proteins determines how they interact in the cell. Therefore, biochemistry predicts
Philosophy of science
The philosophy of science is concerned with the assumptions, foundations, methods and implications of science. It is also concerned with the use and merit of science and sometimes overlaps metaphysics and epistemology by exploring whether scientific results are actually a study of truth...
how different combinations of alleles will produce varying traits.
Extended expression patterns seen in diploid organisms include facets of incomplete dominance, codominance
Codominance
Codominance refers to a relationship between two alleles of a gene. It occurs when the contributions of both alleles at a single locus are visible and do not overpower each other in the phenotype....
, and multiple alleles.
Incomplete dominance is the condition in which neither allele dominates the other in a heterozygote.
Instead the phenotype is intermediate in heterozygotes. Thus you can tell that each allele is present in
the heterozygote.
Codominance refers to the allelic relationship that occurs when two alleles are both expressed in the
heterozygote, and both phenotypes are seen simultaneously.
Multiple alleles refers to the situation when there are more than 2 common alleles of a particular gene.
Blood groups in humans is a classic example. The ABO blood group proteins are important in
determining blood type in humans, and this is determined by different alleles of the one locus.