seanyuki
04-27-2009, 11:27 PM
Just sharing.....:)
Genes, The Basis of Inheritance
Genetics is the science that deals with transmission of traits from parents to offsprings. Every individual of all species tend to produce their off-springs as Xerox copies of themselves. The traits or characters are transfered from parents to offsprings through genes. Genes may be considered as a transmission medium through which parents can transfer their characters (referred as traits in genetic terms) to their offsprings. Each gene contributes to a specific trait. For eg., in discus one gene may contribute to the colour of the Iris and another can contribute to body colour and another can contribute to striations. Every trait has a corresponding gene in the chromosome that expresses it. The specific location in which genes are located are called Gene Loci.
Alleles:
Sometimes more than one gene can contribute to a single character. A group of genes that are associated with each other to bring about a single trait. Alternate forms of genes are called Alleles. Genes are located in alleles which are placed in linear fashion in a chromosome. All genes located in a Gene Loci are related to a single trait but bring out different effect. For eg, in a spotted discus, one gene may represent the colour of the spot, one gene may represent the size, one gene may represent the density of the spot and all these genes are related to a single 'Spotted' trait and will be located in the same Gene Locus.
Mendel's Principle:
Law of segregation:
All animal cells have one pair of chromosomes which means they have two copies of each gene. This state where two copies of a single gene are present is called Diploid. But the gamete or sex cells alone have only one copy of each gene and this state is called a Haploid state. When the gametes meet, two Haploids (one from the male & one from the female) segregate when a zygote (in simple terms, a fertilized egg) is formed. So the offspring has the tendency to inherit traits from both the parents. This is Mendel's law of segregation. When both the alleles for a single trait are similar, the zygote formed is called a Homozygote and the offspring is called a Homozygous offspring. For eg, if we breed a pair of wild caught Tefe' Red Spotted Green Discus then all offsprings will be Homozygous ie., all offsprings will be Tefe Red Spotted Greens.
When both the alleles for a single trait are different, the zygote formed is called a Heterozygote and the offspring produced is called a Heterozygous offspring. For eg, if we breed a Pigeon Blood discus with a blue diamond discus the off-springs may have both the PB and the BD genes. Also heterozygosity is not constant ie., all offsprings will not be similar. In a batch of say 100 fry, say around 70-80 fry will be PBs and the remaining will be BDs. So in heterozygosity, the gene that expresses itself more than the other gene is called a dominant gene and that trait is called Dominant Trait. The gene that is suppressed by the dominant gene is called a recessive gene. In the PB-BD example, the PB gene is the dominant gene and the Blue diamond gene is called a Recessive Gene. Not all the offsprings are either PBs or BDs. Some offsprings may be inbetween these two ie., will express both the traits. The combination obeys the laws of probability and is expressed in a random manner.
In the above example, the PB parent have two alleles for 'redness' which we could denote by AA(Dominant) and similarly the BD has two alleles for 'blueness' which can be denoted by aa (Recessive). So when these two are bred the offsprings may be of the following types.
AA x aa -> AA Aa aA aa
In the above expression, when AA (dominant PB Gene) is crossed with aa (recessive BD Gene), the offsprings may be of four types viz.,
AA-Homozygous for dominant trait
Aa-Heterozygous having both traits
aA-Heterozygous having both traits
aa-Homozygous for recessive trait
Law of Independent Assortment:
This law states that when the gamates divide, each allele is segregated independently during gamete formation ie., the presence or absence or the degree in which it is expressed does not affect another gene. For eg, let us consider crossing parents differing in two traits, body base colour and iris colour. A red discus with red iris is denoted by AABB (A being the dominant allele for base colour and B being the dominant allele for iris colour) and a discus with blue body white eye is denoted by aabb. So the dominant allele for body colour is AA(red) and dominant allele for iris is BB(red iris). Similarly the recessive allele for body colour is aa(blue) and recessive allele for iris is white is bb(white). When these are crossed the offsprings will be formed in the following types.
AB x ab -> AB Ab aB ab
AB-Red with red iris
Ab-Red with white iris
aB-blue with red iris
ab-blue with white iris
In the above example it could be understood that the allele for body colour was not affected by the allele for iris colour and vice versa and combine in a random manner.
Genetics Glossary:
Aquired character:
The physical or morphological modification that a species undergoes with respect to ecological factors is called aquired character.
Albinism:
Absence or removal of colour pigments is called albinism. In other words removal of pigments responsible for bringing out colours is called albinism.
Back Crossing:
Crossing of progencies (offsprings) back to their parents to bring about a desired trait is called back crossing. The classical definition of a Back Cross is offspring crossed with its homozygous recessive parent.
Mutation:
Change in the structure of a gene with respect to ecological factors is called a Mutation.
Pure Line:
In breeding of parents of the same population of completely homozygous gene pool capeable of producing 100% identical offsprings is called pure lines.
Co-Dominance:
Sometimes both alleles of a gene in a heterozygote lack either dominance or recession ie., both dominant and recessive genes may contribute in equal amount or degree. As a result the progencies will have the traits of both the parents in equal degree. This phenomenon is called Co-dominance.
Polygenes or Multiple factor:
-Qualitative trait is the presence or absence of a trait and Quantitative trait is the degree to which the trait is brought out in the offsprings.
-In qualitative trait, the presence or absence of an allele brings about a trait
-In some cases, in contrast to qualitative traits, when several genes contribute to a specific trait, each gene contributing in small quantities, then the particular trait is exhibited in a measurable quantity depending upon the genes that are involved. This contribution of several genes in small quantities each to bring about a trait cummulatively is referred to Multiple factor or polygenes.
-Polygenes makes the base of evolutionary and ecological genetics.
Inbreeding:
-The process of mating of individuals which are more closely related than the average of the population to which they belong is called Inbreeding.
-Inbreeding tends to produce homozygous offsprings.
-In other words it could also be called as mating of pure lines.
-A pureline population is one which breeds true when bred within itself produces offsprings without any mutation or genetic variability in any form.
-In the wild Inbreeding occurs by restrictions in population size bringing about close relatives to interbreed and bring about a homozygous population.
-Inbreeding brings about a recessive gene to appear in a homozygous state (aa).
-The measure or degree of the expected proportion of homozygous offsprings at which both the alleles can be tracked down to a single ancestor is called the Inbreeding coefficient.
-When the Inbreeding coefficient is higher, the homozygosity and hence the appearance of the recessive gene goes higher.
-Line breeding is one type of Inbreeding where the offsprings are crossed back with the parents or their grand parents.
-Breeding brothers & sisters is called Full Sibling Mating.
-Breeding brothers & sisters of a consecutiy batch is called Half Sibble Mating.
-Generally each generation of inbreeding brings about roughly 50% of loss of heterozygosity.
-Approx. after 10 generations the offsprings will be pure line ie., 100% of the brood will become homozygous.
Flaws of Extensive Inbreeding:
-Reduced Fertility.
-Increased inherited genital defects.
-Fluctuating asymmetry in shape esp. Facial asymmetry.
-Lower birth rate.
-Higher neo natal mortality.
-Slower growth rates.
-Smaller adult size.
-Loss or reduction of immune system function.
Panmixis or Random Mating:
-If there is no restriction or constraint assigned upon a selective individual and when the gamates meet randomly without any restriction in population, then the offsprings are said to have greater genetic diversity among themselves.
-Panmixis leads to Outbreeding.
-Unline inbreeding it brings about good traits in the offsprings.
-The Inbreeding coefficient goes considerably low when specimen are subjected to random mating.
-Panmaxis gives rise to heterozygosity and in many cases a strong Gene Pool.
Outbreeding:
-When mating involves individuals that are more distantly related than the average of the population, then it is called Outbreeding.
-Outbreeding specimen can be from a different family or race or a different population.
-Outbreeding increases heterozygosity and out bred progenies are more physically fit, show remarkable growth & vigour. The superiority of such outbred hybrids is called Heterosis.
-Outbred specimen can produce more stable gene pools, high fertility rate and diverse offsprings.
-Cross breeding is an extreme level of outbreeding where two different species are made to mate. Cross breeding produces more no.of sterile males and even if fertile produces offsprings with low fertility rate.
-Outbreeding occurs very commonly in nature whereas cross breeding occurs very rarely, ie., twice or thrice in a century.
Evolutionary Significance of Inbreeding & Outbreeding:
-Inbreeding brings about a recessive gene in a homozygous state and natural selection acts on it ie., those specimen or race is selected for extinction.
-Inbreeding also brings about speciation and specialisation to micro-habitats. So it is not all negative.
-Outbreeding brings about a good or an advantageous trait and makes the offsprings physically fit.
-In out bred specimen, the dominant gene masks the recessive genes to a greater extend.
-This may be concluded as Inbreeding & Outbreeding forms the basis of natural selections, providing new allelic combinations or recessive alleles which may be good or bad for Natural Selection.
-Inbred fish often lose vigour over time. An inbred race can often be improved by outbreeding with other, unrelated populations.
unquote
Cheers
Francis:)
Genes, The Basis of Inheritance
Genetics is the science that deals with transmission of traits from parents to offsprings. Every individual of all species tend to produce their off-springs as Xerox copies of themselves. The traits or characters are transfered from parents to offsprings through genes. Genes may be considered as a transmission medium through which parents can transfer their characters (referred as traits in genetic terms) to their offsprings. Each gene contributes to a specific trait. For eg., in discus one gene may contribute to the colour of the Iris and another can contribute to body colour and another can contribute to striations. Every trait has a corresponding gene in the chromosome that expresses it. The specific location in which genes are located are called Gene Loci.
Alleles:
Sometimes more than one gene can contribute to a single character. A group of genes that are associated with each other to bring about a single trait. Alternate forms of genes are called Alleles. Genes are located in alleles which are placed in linear fashion in a chromosome. All genes located in a Gene Loci are related to a single trait but bring out different effect. For eg, in a spotted discus, one gene may represent the colour of the spot, one gene may represent the size, one gene may represent the density of the spot and all these genes are related to a single 'Spotted' trait and will be located in the same Gene Locus.
Mendel's Principle:
Law of segregation:
All animal cells have one pair of chromosomes which means they have two copies of each gene. This state where two copies of a single gene are present is called Diploid. But the gamete or sex cells alone have only one copy of each gene and this state is called a Haploid state. When the gametes meet, two Haploids (one from the male & one from the female) segregate when a zygote (in simple terms, a fertilized egg) is formed. So the offspring has the tendency to inherit traits from both the parents. This is Mendel's law of segregation. When both the alleles for a single trait are similar, the zygote formed is called a Homozygote and the offspring is called a Homozygous offspring. For eg, if we breed a pair of wild caught Tefe' Red Spotted Green Discus then all offsprings will be Homozygous ie., all offsprings will be Tefe Red Spotted Greens.
When both the alleles for a single trait are different, the zygote formed is called a Heterozygote and the offspring produced is called a Heterozygous offspring. For eg, if we breed a Pigeon Blood discus with a blue diamond discus the off-springs may have both the PB and the BD genes. Also heterozygosity is not constant ie., all offsprings will not be similar. In a batch of say 100 fry, say around 70-80 fry will be PBs and the remaining will be BDs. So in heterozygosity, the gene that expresses itself more than the other gene is called a dominant gene and that trait is called Dominant Trait. The gene that is suppressed by the dominant gene is called a recessive gene. In the PB-BD example, the PB gene is the dominant gene and the Blue diamond gene is called a Recessive Gene. Not all the offsprings are either PBs or BDs. Some offsprings may be inbetween these two ie., will express both the traits. The combination obeys the laws of probability and is expressed in a random manner.
In the above example, the PB parent have two alleles for 'redness' which we could denote by AA(Dominant) and similarly the BD has two alleles for 'blueness' which can be denoted by aa (Recessive). So when these two are bred the offsprings may be of the following types.
AA x aa -> AA Aa aA aa
In the above expression, when AA (dominant PB Gene) is crossed with aa (recessive BD Gene), the offsprings may be of four types viz.,
AA-Homozygous for dominant trait
Aa-Heterozygous having both traits
aA-Heterozygous having both traits
aa-Homozygous for recessive trait
Law of Independent Assortment:
This law states that when the gamates divide, each allele is segregated independently during gamete formation ie., the presence or absence or the degree in which it is expressed does not affect another gene. For eg, let us consider crossing parents differing in two traits, body base colour and iris colour. A red discus with red iris is denoted by AABB (A being the dominant allele for base colour and B being the dominant allele for iris colour) and a discus with blue body white eye is denoted by aabb. So the dominant allele for body colour is AA(red) and dominant allele for iris is BB(red iris). Similarly the recessive allele for body colour is aa(blue) and recessive allele for iris is white is bb(white). When these are crossed the offsprings will be formed in the following types.
AB x ab -> AB Ab aB ab
AB-Red with red iris
Ab-Red with white iris
aB-blue with red iris
ab-blue with white iris
In the above example it could be understood that the allele for body colour was not affected by the allele for iris colour and vice versa and combine in a random manner.
Genetics Glossary:
Aquired character:
The physical or morphological modification that a species undergoes with respect to ecological factors is called aquired character.
Albinism:
Absence or removal of colour pigments is called albinism. In other words removal of pigments responsible for bringing out colours is called albinism.
Back Crossing:
Crossing of progencies (offsprings) back to their parents to bring about a desired trait is called back crossing. The classical definition of a Back Cross is offspring crossed with its homozygous recessive parent.
Mutation:
Change in the structure of a gene with respect to ecological factors is called a Mutation.
Pure Line:
In breeding of parents of the same population of completely homozygous gene pool capeable of producing 100% identical offsprings is called pure lines.
Co-Dominance:
Sometimes both alleles of a gene in a heterozygote lack either dominance or recession ie., both dominant and recessive genes may contribute in equal amount or degree. As a result the progencies will have the traits of both the parents in equal degree. This phenomenon is called Co-dominance.
Polygenes or Multiple factor:
-Qualitative trait is the presence or absence of a trait and Quantitative trait is the degree to which the trait is brought out in the offsprings.
-In qualitative trait, the presence or absence of an allele brings about a trait
-In some cases, in contrast to qualitative traits, when several genes contribute to a specific trait, each gene contributing in small quantities, then the particular trait is exhibited in a measurable quantity depending upon the genes that are involved. This contribution of several genes in small quantities each to bring about a trait cummulatively is referred to Multiple factor or polygenes.
-Polygenes makes the base of evolutionary and ecological genetics.
Inbreeding:
-The process of mating of individuals which are more closely related than the average of the population to which they belong is called Inbreeding.
-Inbreeding tends to produce homozygous offsprings.
-In other words it could also be called as mating of pure lines.
-A pureline population is one which breeds true when bred within itself produces offsprings without any mutation or genetic variability in any form.
-In the wild Inbreeding occurs by restrictions in population size bringing about close relatives to interbreed and bring about a homozygous population.
-Inbreeding brings about a recessive gene to appear in a homozygous state (aa).
-The measure or degree of the expected proportion of homozygous offsprings at which both the alleles can be tracked down to a single ancestor is called the Inbreeding coefficient.
-When the Inbreeding coefficient is higher, the homozygosity and hence the appearance of the recessive gene goes higher.
-Line breeding is one type of Inbreeding where the offsprings are crossed back with the parents or their grand parents.
-Breeding brothers & sisters is called Full Sibling Mating.
-Breeding brothers & sisters of a consecutiy batch is called Half Sibble Mating.
-Generally each generation of inbreeding brings about roughly 50% of loss of heterozygosity.
-Approx. after 10 generations the offsprings will be pure line ie., 100% of the brood will become homozygous.
Flaws of Extensive Inbreeding:
-Reduced Fertility.
-Increased inherited genital defects.
-Fluctuating asymmetry in shape esp. Facial asymmetry.
-Lower birth rate.
-Higher neo natal mortality.
-Slower growth rates.
-Smaller adult size.
-Loss or reduction of immune system function.
Panmixis or Random Mating:
-If there is no restriction or constraint assigned upon a selective individual and when the gamates meet randomly without any restriction in population, then the offsprings are said to have greater genetic diversity among themselves.
-Panmixis leads to Outbreeding.
-Unline inbreeding it brings about good traits in the offsprings.
-The Inbreeding coefficient goes considerably low when specimen are subjected to random mating.
-Panmaxis gives rise to heterozygosity and in many cases a strong Gene Pool.
Outbreeding:
-When mating involves individuals that are more distantly related than the average of the population, then it is called Outbreeding.
-Outbreeding specimen can be from a different family or race or a different population.
-Outbreeding increases heterozygosity and out bred progenies are more physically fit, show remarkable growth & vigour. The superiority of such outbred hybrids is called Heterosis.
-Outbred specimen can produce more stable gene pools, high fertility rate and diverse offsprings.
-Cross breeding is an extreme level of outbreeding where two different species are made to mate. Cross breeding produces more no.of sterile males and even if fertile produces offsprings with low fertility rate.
-Outbreeding occurs very commonly in nature whereas cross breeding occurs very rarely, ie., twice or thrice in a century.
Evolutionary Significance of Inbreeding & Outbreeding:
-Inbreeding brings about a recessive gene in a homozygous state and natural selection acts on it ie., those specimen or race is selected for extinction.
-Inbreeding also brings about speciation and specialisation to micro-habitats. So it is not all negative.
-Outbreeding brings about a good or an advantageous trait and makes the offsprings physically fit.
-In out bred specimen, the dominant gene masks the recessive genes to a greater extend.
-This may be concluded as Inbreeding & Outbreeding forms the basis of natural selections, providing new allelic combinations or recessive alleles which may be good or bad for Natural Selection.
-Inbred fish often lose vigour over time. An inbred race can often be improved by outbreeding with other, unrelated populations.
unquote
Cheers
Francis:)