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LINKAGE

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Nethravathi Siri
Nethravathi Siri

GENETICS CYTOGENETICS Definition of Linkage, Coupling and Repulsion hypothesis, Linkage group- Drosophila, maize and man, Types of linkage-complete linkage and incomplete linkage, Factors affecting linkage- distance between genes, age, temperature, radiation, sex, chemicals and nutrition, Significance of linkage. The tendency of two or more genes to stay together (i.e., the co-existence of two or more genes) in the same chromosome during inheritance is known as LINKAGE. The linked genes are present on the same chromosome are said to be SYNTENIC. The linked genes do not show independent assortment. LINKAGE v/s INDEPENDENT ASSORTMENT The frequency of linkage or the strength recombination is influenced by several factors (agents).

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GENETICS Page 1 of 12 CYTOGENETICS LINKAGE LINKAGE Genes (hereditary units) which determines the character of the individual are carried in the chromosomes. The genes for the different character may be either situated in the same chromosomes or different chromosomes. When the genes are situated in different chromosome, the characters they control appear in the next generation together or apart, depending upon the Mendel’s law of Independent assortment. During meiosis, all chromosomes are inherited as one gametic unit and all the genes (which may specify to the same or various phenotypic traits) located on the same chromosomes tend to be inherited together. All genes on same chromosomes are said to be linked to one another and they belong to same LINKAGE GROUP. The tendency of two or more genes to stay together (i.e., the co-existence of two or more genes) in the same chromosome during inheritance is known as LINKAGE. The linked genes are present on the same chromosome are said to be SYNTENIC. The linked genes donot show independent assortment. LINKAGE v/s INDEPENDENT ASSORTMENT LINKAGE INDEPENDENT ASSORTMENT Gene location Same chromosome Different chromosome Dihybrid cross ratio Highly varies 9:3:3:1 Test cross ratio Highly Varies 1:1:1:1 Example Coloured full seeds (CCSS) and Colour-less shrunk seeds of maize Round yellow peas (RRYY) and Wrinkled Green peas (rryy) COUPLING AND REPULSION HYPOTHESIS • In linkage, there are two phases; they are coupling phase & Repulsion Phase. • BATESON & PUNNETT (1905) proposed the concept of Coupling and repulsion phase, but they could not provide exact interpretation of these terms. • THOMAS H MORGAN (1910) conducted series experimental research on Drosophila which enabled him to explain that coupling & repulsion are two aspects of the same phenomenon known as LINKAGE.
GENETICS Page 2 of 12 CYTOGENETICS LINKAGE COUPLING • The Concept of coupling explains about; 1. Two or more dominant genes which are present on one of the homologous chromosomes. 2. Two or more recessive genes which are present on one of the homologous chromosomes. • The linkage between two or more dominant genes on one homologous chromosome and all recessive genes on another homologous chromosome. • It is also known as Cis-arrangement • The linkage between two or more dominant alleles of different genes on one homologous chromosome and all recessive alleles of different genes on another homologous chromosome during the process of inheritance is called as Coupling. The best example for Coupling is been reported by HUTCHINSON. In maize, gene for seed color as dominant allele ‘C’ which produces colored seeds, while its recessive allele ‘c’ produces colorless seeds. Another gene for seed shape as dominant allele ‘S’ which produces full – round seeds, while its recessive allele‘s’ produces shrunken seeds. HUTCHINSON made a experimental cross between plants having Colored-full seeds (CCSS) and colorless- shrunken seeds (ccss). The F1 seeds were Colored-full. When the F1 was test crossed with double recessive parent, the following result were obtained, instead of 1:1:1:1. CROSS: Parental phenotype: Colored-full seeds X colorless- shrunken seeds Parental genotype : CCSS X ccss Gametes : CS X cs F1 generation : CcSs Colored-full seeds Test cross : F1 X double recessive parent Test cross phenotype : Colored-full seeds X colorless- shrunken seeds Test cross genotype : CcSs X ccss
GENETICS Page 3 of 12 CYTOGENETICS LINKAGE Test cross progeny: Phenotype Genotype Progeny count type Colored-full CS 4032 (~4000) Parental type Colored- shrunken Cs 149 (~100) Recombinant type colorless-full cS 152 (~100) Recombinant type colorless- shrunken cs 4035(~4000) Parental type Total = 8368 INTERPRETATION In the experimental results; the phenotypic classes - Colored-full and colorless- shrunken have a much higher frequency than the expected. These two character combinations are referred as Parental combination (parental phenotypes or Parental types). (Since, they are the same character combinations that were present in the two parents of F1 generation). The remaining other phenotypic classes - Colored- shrunken and colorless-full are reported in low frequency than expected. These two character combinations are called Recombinant combination (Recombinant phenotypes or Recombinant types). (Since, they are obtained by reshuffling of the characters present in two parents of F1) Henceforth, the parental combinations are higher than recombinant combinations, indicating the presence of linkage and it is clear that, if the two dominant genes ‘C’ and ‘S’ have a strong affinity for each other. The frequencies of Colored-full and colorless- shrunken phenotypes are greater than expected. This situation is referred as COUPLING PHASE, due to the presence of genes ‘C’ and ‘S’ in the same chromosome.
GENETICS Page 4 of 12 CYTOGENETICS LINKAGE REPULSION • The Concept of Repulsion explains about; Two or more dominant and recessive alleles of different genes which are present on one of the homologous chromosomes. Vice-versa • The linkage between two or more dominant and recessive alleles of different genes on one homologous chromosome and respective counter-alleles on another homologous chromosome. • Trans arrangement • The linkage between two or more dominant and recessive alleles of different genes on a homologous chromosome and vice-versa during the process of inheritance is called as Repulsion. The best example for Repulsion is been reported by HUTCHINSON. In maize, gene for seed color as dominant allele ‘C’ which produces colored seeds, while its recessive allele ‘c’ produces colorless seeds. Another gene for seed shape as dominant allele ‘S’ which produces full – round seeds, while its recessive allele‘s’ produces shrunken seeds. HUTCHINSON made a experimental cross between plants having Colored- shrunken seeds (CCss) and colorless- full seeds (ccSS). The F1 seeds were Colored-full. When the F1 was test crossed with double recessive parent (ccss), the following result were obtained, instead of 1:1:1:1. CROSS: Parent-1 X Parent-2 Parental phenotype: Colored- shrunken seeds X colorless- full seeds Parental genotype : CCss X ccSS Gametes : Cs X cS F1 generation : CcSs Colored-full seeds Test cross : F1 X double recessive parent Test cross phenotype : Colored-full seeds X colorless- shrunken seeds Test cross genotype : CcSs X ccss
GENETICS Page 5 of 12 CYTOGENETICS LINKAGE Test cross progeny: Phenotype Genotype Progeny count type Colored-full CS 639 (~650) Recombinant type Colored- shrunken Cs 21,379 (~21,300) Parental type colorless-full cS 21,906 (~21,500) Parental type colorless- shrunken cs 672 (~650) Recombinant type Total = 44,596 INTERPRETATION In the experimental results; the phenotypic classes - Colored-shrunken and colorless- full have a much higher frequency than the expected. These two character combinations are referred as Parental combination (parental phenotypes or Parental types). (Since, they are the same character combinations that were present in the two parents of F1 generation). The remaining other phenotypic classes - Colored- full and colorless- shrunken are reported in low frequency than expected. These two character combinations are called Recombinant combination (Recombinant phenotypes or Recombinant types). (Since, they are obtained by reshuffling of the characters present in two parents of F1) Henceforth, the parental combinations are higher than recombinant combinations, indicating the presence of linkage and it is clear that, if the dominant allele for color genes ‘C’ and and recessive allele for seed shape gene ‘s’ have a strong affinity for each other. The frequencies of Colored- shrunken and colorless- full phenotypes are greater than expected. This situation is referred as Repulsion PHASE, due to the presence of genes ‘C’ and ‘s’ in the same chromosome.
GENETICS Page 6 of 12 CYTOGENETICS LINKAGE LINKAGE GROUP A linkage group is a linearly arranged group of linked genes which are normally inherited together except for crossing over. It corresponds to a chromosome which bears a linear sequence of genes linked and inherited together. Because the two homologous chromosomes which possess either similar or different allelic genes on the same loci, they constitute to the same linkage group. Therefore, the number of linkage groups present in an individual corresponds to number of chromosomes in its one genome (all the chromosomes if haploid or homologous pairs if diploid). However, the organisms in which the female or the male have heterogametic chromosomes, the linkage group is haploid plus one. It is known as principle of limitation of linkage groups. The size of the linkage group depends upon the size of chromosome: The smaller chromosome will naturally have smaller linkage group while a longer one has longer linkage group. This is subject to the amount of heterochromatin present in the chromosome. Thus Y-chromosome of man possesses 231 genes while human chromosome 1 has 2969 genes. Example: 1. LINKAGE GROUPS IN HUMANS Humans can have 2 linkage groups based on gender. Male have 22 pairs of autosomes and 1X and 1Y. So, there are in all 24 linkage groups. While females have 22 pairs autosomes and 1X pair. So, total 23 linkage groups 1a) Human Females = 22 pairs of autosomes + 1 pair of homogametic X-chromosome = 22 + 1 = 23 linkage groups 1b) Human Males = 22 pairs of autosomes + 1 pair of heterogametic allosome = 22 + 1 x-chromosome + 1 Y-chromosome = 24 linkage groups
GENETICS Page 7 of 12 CYTOGENETICS LINKAGE 2. LINKAGE GROUPS IN Drosophila melanogaster In fruit fly, there are four pair of chromosomes: three are autosomal pairs and one pair of sex chromosomes. Genes present on one chromosome form a linkage group. Homologous chromosomes carry same set of genes (the alleles may differ), hence same linkage group. 2a) Female Drosophila melanogaster (the fruit fly) consists of FOUR pairs of chromosomes and hence they have FOUR linkage groups, among which three are large linkage groups and one is small linkage group. 2b) The male Drosophila’s have one extra linkage group (FIVE linkage groups) because they are heterogametic. This is because other than the three autosomes there are a pair of sex chromosomes which are not homologous, i.e. genes on X chromosome are not same as the genes on Y chromosome. 3. LINKAGE GROUPS IN MAIZE (Zea mays) Corn (Zea mays or maize) has 10 pairs of chromosomes and 10 linkage groups. Note; • Pea seven linkage groups (7 pairs of chromosomes) • Neurospora 7 linkage groups (7 chromosomes) • Escherichia coli one linkage group (one pro-chromosome or nucleoid)
GENETICS Page 8 of 12 CYTOGENETICS LINKAGE TYPES OF LINKAGE COMPLETE LINKAGE AND INCOMPLETE LINKAGE COMPLETE LINKAGE • The linked genes on a homologous chromosome which fail to undergo crossing over and exhibit absolute linkage is called complete linkage. • In this case, the parental combinations of characters appear together for two or more generations in a continuous or regular fashion. • In this type of linkage, genes are strongly associated and tend to transmit together. X EXAMPLE: COMPLETE LINKAGE in Male Drosophila In most of the organisms crossing over takes place between both males and females. But, in case of male Drosophila and female Silk moth (Bombyx mori), the crossing over takes place very rarely or not at all. This becomes clear from T H morgan’s experimental results from Drosophila. T H morgan’s experiment for complete linkage in male Drosophila In 1919, T.H.Morgan crossed Gray-bodied and Vestigial-winged fly (b+vg/b+vg) with Black-body and normal wing (bvg+/bvg+) fly. F1 progeny had gray-body and normal wings (b+vg/bvg+), thereby indicating these characters are dominant in nature. For the test cross, The F1 male flies were crossed to double recessive females (bvg/bvg) black-body and vestigial-wing. Gray-body Black-body Normal-wing Vestigial-wing b+ b vg+ vg
GENETICS Page 9 of 12 CYTOGENETICS LINKAGE CROSS: Parental phenotype : Gray-bodied and Vestigial-winged X Black-body and normal wing Parental genotype : b+ vg/b+vg X bvg+/bvg+ Gametes : b+ vg X bvg+ F1 generation : b+ vg/bvg+ gray-body and normal wings TEST CROSS : F1 male X double recessive female parent Test cross phenotype : gray-body and normal wings X black-body and vestigial-wing Test cross genotype : b+vg/bvg+ X bvg/bvg Test cross Gametes: b+vg bvg+ X bvg Test cross progeny: bvg b+vg b+vg/ bvg Gray-bodied and Vestigial-winged 50% or ½ bvg+ bvg+ / bvg Black-body and normal wing 50% or ½ Test cross ratio= ½ Gray-bodied and Vestigial-winged : ½ Black-body and normal wing There are only two types of Test cross progenies in 1:1 ratio (rather than four types of test cross progenies in 1:1:1:1 ratio). This is due to the complete linkage (absolute linkage) and absence of crossing over in male Drosophila.
GENETICS Page 10 of 12 CYTOGENETICS LINKAGE INCOMPLETE LINKAGE • If some frequency of crossing over takes place among the linked genes, it is known as Incomplete linkage. In other words, when recombinants are produced in the test cross progeny, besides parental combinations, it refers to Incomplete linkage. • The linked genes which are widely located in a chromosome and have chances of separation by crossing over are called the incompletely linked genes and the phenomenon of their inheritance is called Incomplete linkage. EXAMPLE : INCOMPLETE LINKAGE IN FEMALE Drosophila T H Morgan’s experiment for Incomplete linkage in female Drosophila T.H.Morgan crossed Gray-bodied and Vestigial-winged fly ( b+ vg/b+vg) with Black- body and normal wing fly (bvg+/bvg+). F1 progeny had gray-body and normal wings (b+vg/bvg+), thereby indicating these characters are dominant in nature. For the test cross, The F1 female flies were crossed to double recessive males (bvg/bvg) black-body and vestigial-wing. CROSS: Parental phenotype : Gray-bodied and Vestigial-winged X Black-body and normal wing Parental genotype : b+ vg/b+vg X bvg+/bvg+ Gametes : b+ vg X bvg+ F1 generation : b+ vg/bvg+ gray-body and normal wings Test cross : F1 female X double recessive male parent Test cross phenotype : gray-body and normal wings X black-body and vestigial-wing Test cross genotype : b+vg/bvg+ X bvg/bvg Test cross Gametes : b+vg bvg+ b+vg+ bvg X bvg Non crossovers recombinants Test cross progeny: bvg Experimental values b+vg b+vg /bvg Gray-body and Vestigial- winged 41.5% 83% parental combination showing linkage bvg+ bvg+/bvg Black-body and normal wing 41.5% b+vg+ b+vg+/bvg gray-body and normal wings 8.5% 17% recombinants due to crossover bvg bvg /bvg black-body and vestigial-wing 8.5% There was both parental combination and recombinant ones in the test cross progenies. Henceforth, female Drosophila flies exhibit incomplete linkage. Different test cross ratios showing complete linkage in male Drosophila and incomplete linkage in female Drosophila were obtained. Thus, proves the association of linked genes in different patterns of both genders in Drosophila flies.
GENETICS Page 11 of 12 CYTOGENETICS LINKAGE FACTORS AFFECTING LINKAGE • The frequency of linkage or the strength recombination is influenced by several factors (agents). • Most of the influential agents or factors data documented based on experiments conducted on Drosophila 1. DISTANCE BETWEEN GENES • The strength of linkage between two genes would increase with the decrease in distance between them. • Crossing over will be relatively more frequent, if distance between two genes is more. • The less distance between the genes the more is the linkage strength, greater the distance between genes lower is a chance of linkage. • 𝟏/∝ • CO ∝ Distance between genes • Linkage 𝟏/∝ C.O • Linkage 𝟏/∝ Distance between genes 2. AGE • Generally, the linkage increases with advancement in the age in the female Drosophila. • Crossing over shows a progressive decline with advancing age of female Drosophila this was experimentally proved Drosophila crosses. • Age ∝ Linkage in female Drosophila 3. TEMPERATURE • H H Plough (a student of T H Morgan) identified that the rate of linkage remains constant in 22 degree Celsius. • In Drosophila, the lowest frequency of recombination (strongest linkage) is observed when female drosophila flies are cultured at 22 degrees Celsius. • Above and below 22 degree Celsius there is fluctuation in linkage, showing increased Recombinant progenies. 4. RADIATION • An increase in crossing over is observed when Drosophila females and many other organisms are irradiated with X-rays and gamma-rays. • The irradiation reduces the frequency of linkage. • Drosophila male provides evidences for destruction of linkage after irradiation with X-rays. • Radiation 𝟏/∝ linkage strength
GENETICS Page 12 of 12 CYTOGENETICS LINKAGE 5. SEX / GENDER • Linkage is markedly influenced by gender or sex of an organism and varies in heterozygotes. • Heterochromatic genders show relatively stronger linkage when compared to homogametic genders of the same species. • There is lack of crossing over in Drosophila male and female Silk moth, which means rate of linkage also varies among different genders for a given species. Female Drosophila exhibit incomplete linkage. 6. CHEMICALS • Treatment with mutagenic chemicals like alkylating agents would decrease the frequency of linkage in female Drosophila • For example; When female Drosophila’s are injected with certain antibiotics like actinomycin D have reported to reduce linkage, while promoting the crossing over. 7. NUTRITION • Presence of metallic ions like calcium (Ca2+) and magnesium (Mg2+) in the food& nutrition will result in reduction of recombination in Drosophila. • For example; When female Drosophila’s are fed with high calcium ions diet, the subsequent generation showed reduced recombinants. • However, removal of such ions from the diet increased the rate of crossing over. Note: 1. Centromeric effect: genes located adjacent to the centromere show enhanced frequency of linkage 2. Cytoplasmic genes/plasmogenes: in some species presence of cytoplasmic genes would reduce the linkage strength. 3. Chromosomal aberrations / structural changes: they enhance linkage SIGNIFICANCE OF LINKAGE. • The phenomenon of linkage has one of the great significance for the living organisms in that it reduces the possibility of variability in gametes unless crossing over occurs. (Linkage prevents the incidence of recombination) • Linkage plays an important role in determining the nature of scope of hybridization and selection programme. • Linkage reduces the chance of recombination of genes and thus helps to hold parental characteristics together. It thus helps organism to maintain its parental, racial and other characters. For this reason plant and animal breeders find it difficult to combine various characters.

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LINKAGE

  • 1. GENETICS Page 1 of 12 CYTOGENETICS LINKAGE LINKAGE Genes (hereditary units) which determines the character of the individual are carried in the chromosomes. The genes for the different character may be either situated in the same chromosomes or different chromosomes. When the genes are situated in different chromosome, the characters they control appear in the next generation together or apart, depending upon the Mendel’s law of Independent assortment. During meiosis, all chromosomes are inherited as one gametic unit and all the genes (which may specify to the same or various phenotypic traits) located on the same chromosomes tend to be inherited together. All genes on same chromosomes are said to be linked to one another and they belong to same LINKAGE GROUP. The tendency of two or more genes to stay together (i.e., the co-existence of two or more genes) in the same chromosome during inheritance is known as LINKAGE. The linked genes are present on the same chromosome are said to be SYNTENIC. The linked genes donot show independent assortment. LINKAGE v/s INDEPENDENT ASSORTMENT LINKAGE INDEPENDENT ASSORTMENT Gene location Same chromosome Different chromosome Dihybrid cross ratio Highly varies 9:3:3:1 Test cross ratio Highly Varies 1:1:1:1 Example Coloured full seeds (CCSS) and Colour-less shrunk seeds of maize Round yellow peas (RRYY) and Wrinkled Green peas (rryy) COUPLING AND REPULSION HYPOTHESIS • In linkage, there are two phases; they are coupling phase & Repulsion Phase. • BATESON & PUNNETT (1905) proposed the concept of Coupling and repulsion phase, but they could not provide exact interpretation of these terms. • THOMAS H MORGAN (1910) conducted series experimental research on Drosophila which enabled him to explain that coupling & repulsion are two aspects of the same phenomenon known as LINKAGE.
  • 2. GENETICS Page 2 of 12 CYTOGENETICS LINKAGE COUPLING • The Concept of coupling explains about; 1. Two or more dominant genes which are present on one of the homologous chromosomes. 2. Two or more recessive genes which are present on one of the homologous chromosomes. • The linkage between two or more dominant genes on one homologous chromosome and all recessive genes on another homologous chromosome. • It is also known as Cis-arrangement • The linkage between two or more dominant alleles of different genes on one homologous chromosome and all recessive alleles of different genes on another homologous chromosome during the process of inheritance is called as Coupling. The best example for Coupling is been reported by HUTCHINSON. In maize, gene for seed color as dominant allele ‘C’ which produces colored seeds, while its recessive allele ‘c’ produces colorless seeds. Another gene for seed shape as dominant allele ‘S’ which produces full – round seeds, while its recessive allele‘s’ produces shrunken seeds. HUTCHINSON made a experimental cross between plants having Colored-full seeds (CCSS) and colorless- shrunken seeds (ccss). The F1 seeds were Colored-full. When the F1 was test crossed with double recessive parent, the following result were obtained, instead of 1:1:1:1. CROSS: Parental phenotype: Colored-full seeds X colorless- shrunken seeds Parental genotype : CCSS X ccss Gametes : CS X cs F1 generation : CcSs Colored-full seeds Test cross : F1 X double recessive parent Test cross phenotype : Colored-full seeds X colorless- shrunken seeds Test cross genotype : CcSs X ccss
  • 3. GENETICS Page 3 of 12 CYTOGENETICS LINKAGE Test cross progeny: Phenotype Genotype Progeny count type Colored-full CS 4032 (~4000) Parental type Colored- shrunken Cs 149 (~100) Recombinant type colorless-full cS 152 (~100) Recombinant type colorless- shrunken cs 4035(~4000) Parental type Total = 8368 INTERPRETATION In the experimental results; the phenotypic classes - Colored-full and colorless- shrunken have a much higher frequency than the expected. These two character combinations are referred as Parental combination (parental phenotypes or Parental types). (Since, they are the same character combinations that were present in the two parents of F1 generation). The remaining other phenotypic classes - Colored- shrunken and colorless-full are reported in low frequency than expected. These two character combinations are called Recombinant combination (Recombinant phenotypes or Recombinant types). (Since, they are obtained by reshuffling of the characters present in two parents of F1) Henceforth, the parental combinations are higher than recombinant combinations, indicating the presence of linkage and it is clear that, if the two dominant genes ‘C’ and ‘S’ have a strong affinity for each other. The frequencies of Colored-full and colorless- shrunken phenotypes are greater than expected. This situation is referred as COUPLING PHASE, due to the presence of genes ‘C’ and ‘S’ in the same chromosome.
  • 4. GENETICS Page 4 of 12 CYTOGENETICS LINKAGE REPULSION • The Concept of Repulsion explains about; Two or more dominant and recessive alleles of different genes which are present on one of the homologous chromosomes. Vice-versa • The linkage between two or more dominant and recessive alleles of different genes on one homologous chromosome and respective counter-alleles on another homologous chromosome. • Trans arrangement • The linkage between two or more dominant and recessive alleles of different genes on a homologous chromosome and vice-versa during the process of inheritance is called as Repulsion. The best example for Repulsion is been reported by HUTCHINSON. In maize, gene for seed color as dominant allele ‘C’ which produces colored seeds, while its recessive allele ‘c’ produces colorless seeds. Another gene for seed shape as dominant allele ‘S’ which produces full – round seeds, while its recessive allele‘s’ produces shrunken seeds. HUTCHINSON made a experimental cross between plants having Colored- shrunken seeds (CCss) and colorless- full seeds (ccSS). The F1 seeds were Colored-full. When the F1 was test crossed with double recessive parent (ccss), the following result were obtained, instead of 1:1:1:1. CROSS: Parent-1 X Parent-2 Parental phenotype: Colored- shrunken seeds X colorless- full seeds Parental genotype : CCss X ccSS Gametes : Cs X cS F1 generation : CcSs Colored-full seeds Test cross : F1 X double recessive parent Test cross phenotype : Colored-full seeds X colorless- shrunken seeds Test cross genotype : CcSs X ccss
  • 5. GENETICS Page 5 of 12 CYTOGENETICS LINKAGE Test cross progeny: Phenotype Genotype Progeny count type Colored-full CS 639 (~650) Recombinant type Colored- shrunken Cs 21,379 (~21,300) Parental type colorless-full cS 21,906 (~21,500) Parental type colorless- shrunken cs 672 (~650) Recombinant type Total = 44,596 INTERPRETATION In the experimental results; the phenotypic classes - Colored-shrunken and colorless- full have a much higher frequency than the expected. These two character combinations are referred as Parental combination (parental phenotypes or Parental types). (Since, they are the same character combinations that were present in the two parents of F1 generation). The remaining other phenotypic classes - Colored- full and colorless- shrunken are reported in low frequency than expected. These two character combinations are called Recombinant combination (Recombinant phenotypes or Recombinant types). (Since, they are obtained by reshuffling of the characters present in two parents of F1) Henceforth, the parental combinations are higher than recombinant combinations, indicating the presence of linkage and it is clear that, if the dominant allele for color genes ‘C’ and and recessive allele for seed shape gene ‘s’ have a strong affinity for each other. The frequencies of Colored- shrunken and colorless- full phenotypes are greater than expected. This situation is referred as Repulsion PHASE, due to the presence of genes ‘C’ and ‘s’ in the same chromosome.
  • 6. GENETICS Page 6 of 12 CYTOGENETICS LINKAGE LINKAGE GROUP A linkage group is a linearly arranged group of linked genes which are normally inherited together except for crossing over. It corresponds to a chromosome which bears a linear sequence of genes linked and inherited together. Because the two homologous chromosomes which possess either similar or different allelic genes on the same loci, they constitute to the same linkage group. Therefore, the number of linkage groups present in an individual corresponds to number of chromosomes in its one genome (all the chromosomes if haploid or homologous pairs if diploid). However, the organisms in which the female or the male have heterogametic chromosomes, the linkage group is haploid plus one. It is known as principle of limitation of linkage groups. The size of the linkage group depends upon the size of chromosome: The smaller chromosome will naturally have smaller linkage group while a longer one has longer linkage group. This is subject to the amount of heterochromatin present in the chromosome. Thus Y-chromosome of man possesses 231 genes while human chromosome 1 has 2969 genes. Example: 1. LINKAGE GROUPS IN HUMANS Humans can have 2 linkage groups based on gender. Male have 22 pairs of autosomes and 1X and 1Y. So, there are in all 24 linkage groups. While females have 22 pairs autosomes and 1X pair. So, total 23 linkage groups 1a) Human Females = 22 pairs of autosomes + 1 pair of homogametic X-chromosome = 22 + 1 = 23 linkage groups 1b) Human Males = 22 pairs of autosomes + 1 pair of heterogametic allosome = 22 + 1 x-chromosome + 1 Y-chromosome = 24 linkage groups
  • 7. GENETICS Page 7 of 12 CYTOGENETICS LINKAGE 2. LINKAGE GROUPS IN Drosophila melanogaster In fruit fly, there are four pair of chromosomes: three are autosomal pairs and one pair of sex chromosomes. Genes present on one chromosome form a linkage group. Homologous chromosomes carry same set of genes (the alleles may differ), hence same linkage group. 2a) Female Drosophila melanogaster (the fruit fly) consists of FOUR pairs of chromosomes and hence they have FOUR linkage groups, among which three are large linkage groups and one is small linkage group. 2b) The male Drosophila’s have one extra linkage group (FIVE linkage groups) because they are heterogametic. This is because other than the three autosomes there are a pair of sex chromosomes which are not homologous, i.e. genes on X chromosome are not same as the genes on Y chromosome. 3. LINKAGE GROUPS IN MAIZE (Zea mays) Corn (Zea mays or maize) has 10 pairs of chromosomes and 10 linkage groups. Note; • Pea seven linkage groups (7 pairs of chromosomes) • Neurospora 7 linkage groups (7 chromosomes) • Escherichia coli one linkage group (one pro-chromosome or nucleoid)
  • 8. GENETICS Page 8 of 12 CYTOGENETICS LINKAGE TYPES OF LINKAGE COMPLETE LINKAGE AND INCOMPLETE LINKAGE COMPLETE LINKAGE • The linked genes on a homologous chromosome which fail to undergo crossing over and exhibit absolute linkage is called complete linkage. • In this case, the parental combinations of characters appear together for two or more generations in a continuous or regular fashion. • In this type of linkage, genes are strongly associated and tend to transmit together. X EXAMPLE: COMPLETE LINKAGE in Male Drosophila In most of the organisms crossing over takes place between both males and females. But, in case of male Drosophila and female Silk moth (Bombyx mori), the crossing over takes place very rarely or not at all. This becomes clear from T H morgan’s experimental results from Drosophila. T H morgan’s experiment for complete linkage in male Drosophila In 1919, T.H.Morgan crossed Gray-bodied and Vestigial-winged fly (b+vg/b+vg) with Black-body and normal wing (bvg+/bvg+) fly. F1 progeny had gray-body and normal wings (b+vg/bvg+), thereby indicating these characters are dominant in nature. For the test cross, The F1 male flies were crossed to double recessive females (bvg/bvg) black-body and vestigial-wing. Gray-body Black-body Normal-wing Vestigial-wing b+ b vg+ vg
  • 9. GENETICS Page 9 of 12 CYTOGENETICS LINKAGE CROSS: Parental phenotype : Gray-bodied and Vestigial-winged X Black-body and normal wing Parental genotype : b+ vg/b+vg X bvg+/bvg+ Gametes : b+ vg X bvg+ F1 generation : b+ vg/bvg+ gray-body and normal wings TEST CROSS : F1 male X double recessive female parent Test cross phenotype : gray-body and normal wings X black-body and vestigial-wing Test cross genotype : b+vg/bvg+ X bvg/bvg Test cross Gametes: b+vg bvg+ X bvg Test cross progeny: bvg b+vg b+vg/ bvg Gray-bodied and Vestigial-winged 50% or ½ bvg+ bvg+ / bvg Black-body and normal wing 50% or ½ Test cross ratio= ½ Gray-bodied and Vestigial-winged : ½ Black-body and normal wing There are only two types of Test cross progenies in 1:1 ratio (rather than four types of test cross progenies in 1:1:1:1 ratio). This is due to the complete linkage (absolute linkage) and absence of crossing over in male Drosophila.
  • 10. GENETICS Page 10 of 12 CYTOGENETICS LINKAGE INCOMPLETE LINKAGE • If some frequency of crossing over takes place among the linked genes, it is known as Incomplete linkage. In other words, when recombinants are produced in the test cross progeny, besides parental combinations, it refers to Incomplete linkage. • The linked genes which are widely located in a chromosome and have chances of separation by crossing over are called the incompletely linked genes and the phenomenon of their inheritance is called Incomplete linkage. EXAMPLE : INCOMPLETE LINKAGE IN FEMALE Drosophila T H Morgan’s experiment for Incomplete linkage in female Drosophila T.H.Morgan crossed Gray-bodied and Vestigial-winged fly ( b+ vg/b+vg) with Black- body and normal wing fly (bvg+/bvg+). F1 progeny had gray-body and normal wings (b+vg/bvg+), thereby indicating these characters are dominant in nature. For the test cross, The F1 female flies were crossed to double recessive males (bvg/bvg) black-body and vestigial-wing. CROSS: Parental phenotype : Gray-bodied and Vestigial-winged X Black-body and normal wing Parental genotype : b+ vg/b+vg X bvg+/bvg+ Gametes : b+ vg X bvg+ F1 generation : b+ vg/bvg+ gray-body and normal wings Test cross : F1 female X double recessive male parent Test cross phenotype : gray-body and normal wings X black-body and vestigial-wing Test cross genotype : b+vg/bvg+ X bvg/bvg Test cross Gametes : b+vg bvg+ b+vg+ bvg X bvg Non crossovers recombinants Test cross progeny: bvg Experimental values b+vg b+vg /bvg Gray-body and Vestigial- winged 41.5% 83% parental combination showing linkage bvg+ bvg+/bvg Black-body and normal wing 41.5% b+vg+ b+vg+/bvg gray-body and normal wings 8.5% 17% recombinants due to crossover bvg bvg /bvg black-body and vestigial-wing 8.5% There was both parental combination and recombinant ones in the test cross progenies. Henceforth, female Drosophila flies exhibit incomplete linkage. Different test cross ratios showing complete linkage in male Drosophila and incomplete linkage in female Drosophila were obtained. Thus, proves the association of linked genes in different patterns of both genders in Drosophila flies.
  • 11. GENETICS Page 11 of 12 CYTOGENETICS LINKAGE FACTORS AFFECTING LINKAGE • The frequency of linkage or the strength recombination is influenced by several factors (agents). • Most of the influential agents or factors data documented based on experiments conducted on Drosophila 1. DISTANCE BETWEEN GENES • The strength of linkage between two genes would increase with the decrease in distance between them. • Crossing over will be relatively more frequent, if distance between two genes is more. • The less distance between the genes the more is the linkage strength, greater the distance between genes lower is a chance of linkage. • 𝟏/∝ • CO ∝ Distance between genes • Linkage 𝟏/∝ C.O • Linkage 𝟏/∝ Distance between genes 2. AGE • Generally, the linkage increases with advancement in the age in the female Drosophila. • Crossing over shows a progressive decline with advancing age of female Drosophila this was experimentally proved Drosophila crosses. • Age ∝ Linkage in female Drosophila 3. TEMPERATURE • H H Plough (a student of T H Morgan) identified that the rate of linkage remains constant in 22 degree Celsius. • In Drosophila, the lowest frequency of recombination (strongest linkage) is observed when female drosophila flies are cultured at 22 degrees Celsius. • Above and below 22 degree Celsius there is fluctuation in linkage, showing increased Recombinant progenies. 4. RADIATION • An increase in crossing over is observed when Drosophila females and many other organisms are irradiated with X-rays and gamma-rays. • The irradiation reduces the frequency of linkage. • Drosophila male provides evidences for destruction of linkage after irradiation with X-rays. • Radiation 𝟏/∝ linkage strength
  • 12. GENETICS Page 12 of 12 CYTOGENETICS LINKAGE 5. SEX / GENDER • Linkage is markedly influenced by gender or sex of an organism and varies in heterozygotes. • Heterochromatic genders show relatively stronger linkage when compared to homogametic genders of the same species. • There is lack of crossing over in Drosophila male and female Silk moth, which means rate of linkage also varies among different genders for a given species. Female Drosophila exhibit incomplete linkage. 6. CHEMICALS • Treatment with mutagenic chemicals like alkylating agents would decrease the frequency of linkage in female Drosophila • For example; When female Drosophila’s are injected with certain antibiotics like actinomycin D have reported to reduce linkage, while promoting the crossing over. 7. NUTRITION • Presence of metallic ions like calcium (Ca2+) and magnesium (Mg2+) in the food& nutrition will result in reduction of recombination in Drosophila. • For example; When female Drosophila’s are fed with high calcium ions diet, the subsequent generation showed reduced recombinants. • However, removal of such ions from the diet increased the rate of crossing over. Note: 1. Centromeric effect: genes located adjacent to the centromere show enhanced frequency of linkage 2. Cytoplasmic genes/plasmogenes: in some species presence of cytoplasmic genes would reduce the linkage strength. 3. Chromosomal aberrations / structural changes: they enhance linkage SIGNIFICANCE OF LINKAGE. • The phenomenon of linkage has one of the great significance for the living organisms in that it reduces the possibility of variability in gametes unless crossing over occurs. (Linkage prevents the incidence of recombination) • Linkage plays an important role in determining the nature of scope of hybridization and selection programme. • Linkage reduces the chance of recombination of genes and thus helps to hold parental characteristics together. It thus helps organism to maintain its parental, racial and other characters. For this reason plant and animal breeders find it difficult to combine various characters.