LINKAGE

Thomas Hunt Morgan

Distance between the two linked gene = Centimorgan

• Chromosomal theory of inheritance states that chromosomes are inherited instead of genes which was explained in Mendel's law of independent assortment.
• Genes of a chromosome are attached to each other and these genes are called linked genes. The group of linked genes of a chromosome is called linkage group. Human has 23 linkage groups.
• The linked genes have tendency to stay together during inheritance. This phenomenon was named as linkage by T.H. Morgan according to him linkage has following characteristics.
• Mendel allowed F1 for selfing and obtain F2 as tall and dwarf pea plant in the ratio of 3:1. It shows that the trait which was not appeared in F1 is reappeared in F2 generation. It can be represented as:

Monohybrid Cross

Tall height Pea (TT) x Dwarf height Pea (tt) -> Parent

T (gamete) x t (gamete)

on crossing

Tt -> F1 (Hybrid tall)

on selfing

-> F2

Genotype: TT : Tt : tt : 1 : 2 : 1

Phenotype: Tall : Dwarf : 3 : 1

DIHYBRID CROSS

• When a cross is made between two homozygous parents two pairs of contrasting traits of two characters is called Dihybrid cross.
• Mendel crossed the Pea having yellow coloured & round shaped seed with the Pea plant having green coloured and wrinkled shaped seed.

and obtained all F1 as yellow coloured and round shaped seeds it shows that yellow colour is dominant over green colour and round shaped is dominant over wrinkled.

• He allowed F1 for selfing and obtain F2 as yellow colour round shaped seed, yellow colour wrinkled seed, Green colour round shaped seed, & Green colour wrinkled shaped seed in the ratio of 9:3:3:1 which is called dihybrid ratio.
• Out of four phenotype two phenotypes Yellow round Green wrinkled are parental traits and others are known parental traits which are also called recombinant. The ratio of parental and non-parental trait is 10:6.

Above explanation can be represented as:

(Note: Student's handwritten table contains transcription errors regarding phenotypes in the bottom right quadrant compared to standard Mendelian genetics.)

Phenotype = Yellow round : Yellow wrinkled : Green round : Green wrinkled

Genetics

Linkage

1. Genes are linearly arranged within chromosome.
2. Each chromosome has fixed number of gene.
3. Each species has fixed number of chromosome.
4. The strength of linked genes depend on the distance between them. The distance is measured in Centimorgan.

Types of linkage

It is of 2 types: Complete & Incomplete based on position of linked gene.

a) Complete Linkage:

When the linked gene of a chromosome are always inherited together over the generation the linkage is called Complete Linkage. Due to this only parental traits are appeared in offspring.

• T.H Morgan performed an experiment on Drosophila melanogaster which have two varieties: one with grey body colour and long wing and another with black body colour and vestigial wing. He made cross betn them and obtain all F1 Drosophila grey body colour and long wing. It shows that grey colour and long wing size are dominant over black colour and vestigial wing.
• He test crossed F1 and obtain all Drosophila as grey colour long wing and black colour vestigial wing. It shows that the gene for body colour is Completely linked with gene for wing size. These genes are closely linked in a chromosome and are never separated therefore this gene only show parental traits.

Incomplete Linkage:

The genes of a chromosome have tendency to stay together but they might be separated due to crossing over this phenomenon is called incomplete linkage.

In this case both parental and non-parental traits are formed but the frequency of parental traits is always higher.

• The genes which shows incomplete linkage are distantly located and have higher Centimorgan.
• Hutchinson had formed coloured and colourless seeds and also found full and shrunken seeds.

it means he considered two genes one for colour and another for shape.

• He crossed a maize having coloured and full seeds with the maize having Colourless and shrunken seeds be obtain all F1 as coloured and full seeds. It shows that coloured and full traits are dominant over colourless and shrunken seeds.
• He test crossed F1 and obtain offspring as Coloured full seeds, coloured shrunken seeds, Colourless full seed and Colourless shrunken seeds. Out of above four Coloured full seeds and Colourless shrunken seeds are parental traits and remaining others are non-parental traits after calculation he found the ratio of parental and non-parental.

CROSSING OVER:

• It occurs in germ cells of gonads which are testis in male and ovaries in female.
• By the process of meiosis which develops gametes sperm and ova.
• Gametes have half numbers of chromosome then of mother cell.
• Meiosis has two sequential divisions, meiosis I and meiosis II.
• Crossing over take place in prophase I of meiosis I.
• Crossing over is characterized with exchange of segment of non-sister chromatids of homologous chromosome.

-> Crossing Over is a genetic phenomenon in which segment of non-sister chromatids are exchange between homologous chromosomes. The homologous chromosome is a pair of chromosomes, one from father & another from mother and has genes for same characters.

• Crossing over takes place at prophase I of meiosis I and each a significant process that change chromosome over the generation therefore crossing over is a source of evolution. It follows following events.

1. Synapsis:

   [Diagram of non-sister chromatids]

   During leptotene the chromosomes are condense and become short and distinct structure. Each chromosome has made its copy and has 4 arms this arms are called sister chromatids.

   • The nucleus of each cell has 2 sets of chromosomes, one set for paternal and another set for maternal. In humans 23 chromosome from father form a set and 23 chromo some from mother form another set.
   • The Homologous chromosomes are paired and this pairing is called Synapsis. The pairing is assisted by a protein and forms a complex called Synaptonemal Complex.
2. Overlapping of Non-sister chromatide

   [Diagram of overlapping chromosomes]

• Synapsis is followed by Overlapping of non-sister chromatids of homologous chromosome this process is called crossing over the overlapping may occur at one or more site based on it crossing over is called single crossing over, double crossing over and multiple crossing over.

3. Exchange of segments of Non-sister chromatids

   [Diagram showing Chiasmata and Endonuclease action]

   Endonuclease enzyme cuts the segments of non-sister chromatid at the point of crossing over this segments are exchange a membrane is formed and the genes of a chromosome at few places are exchange.

4. Terminalization:

   The centromere of homologous chromosome repels each other. It makes chiasmata move towards end of chromosome. This phenomenon is called terminalization.

Sex Linked Inheritance

• Each organism has two kinds of chromosome they are autosome and sex chromosome. The chromosome which contain gene's for body traits like height, skin colour, hair colour etc is called autosomes. In human 22 pairs of chromosomes are autosomes.
• The chromosomes which contains genes for sex and sex related character is called sex chromosome. In human sex chromosome is one pair, XX in female and XY in male therefore female is Homomorphic for sex chromosome and male is Heteromorphic for the same.
• Imp: The genes for body characters might be located in Sex-chromosome instead of autosomes this genes are called sex linked gene. The inheritance pattern of sex linked gene is called sex linked inheritance.
• Sex-linked gene are of two types x-linked gene and y-linked gene when sex-linked genes are located in x-chromosome they are called x-linked gene. Most of sex-linked gene are located in y-chromosome. These genes are called y-linked gene.
• T.H Morgan studies sex-linked inheritance in Drosophila with reference to eye colour the reasons behind selection of drosophila are given below.

• They are easily cultured even in bottle having sugar solution. The life cycle is short about 15 days.
• Male and female drosophila are easily distinguished male is small and has pointed abdomen. Female is broad and has blunt abdomen.
• They have only four pairs of chromosome. Three pairs autosome and one pair of sex-chromosome.

Cross I:

Mating betn Red eyed female Drosophila and white eyed male Drosophila.

• Morgan was culturing red eye drosophila in his lab for many generation. In one batch he obtain white eye Drosophila white were male he named red eye as wild type and white eyes as mutant type. The wild type is dominant and Synthesizes protein whereas mutant type is recessive and doesn't synthesizes protein.
• He made a cross betn red eyed Drosophila with white eyed male drosophila and obtain all F1 as red eye colour is dominant trait.
• He allow F1 for selfing and obtain ratio 3:1 but all white eyed drosophila where male. It shows that the gene for eye colour is not located in autosomes but is located in x-chromosome.

they received from red eyed mother. It can be represented as: male drosophila with white eye.

From above process it is show that sex linked character of a sex are transmitted to same sex through opposite sex it is called Criss-cross. Therefore, sex linked inheritance is also called Criss-Cross inheritance.

Cross 2:

Mating betn white eyed female Drosophila and red eyed male Drosophila.

• To verify gene for eye colour in Drosophila is located in x-chromosome morgan crossed white eyed female Drosophila with red eyed male Drosophila. He obtain F1 as red eyed as well as white eyed all the male Drosophila where white eyed because they have Single x-chromosome which was received from mother who was white eyed female. Drosophila were red eyed but carrier because they have x-chromosomes, one from father and another from mother.
• He allows F1 for selfing and obtain F2 as red eyed and white eyed in both sexes in the ratio of 1:1. It can be represented as:

Sex-linked inheritance in human

In human two cases of sex-linked inheritance are well-studied. They are colour blindness and haemophilia.

Colour blindness:

• The inability of a person to differenciate red and green colours from other is called colour blindness. Human eye has retina which has rod and cone. The cones are responsible for colour vision for this purpose cones are equipped with pigment which are chemically protein. When gene for the protein is recessive it cannot be expressed whereas dominant gene for the protein is expressed.

• When normal colour-vision women marriage with colour blind man the daughter and son are normal to their colour vision. The daughter in this case act as carriers.
• When carriers women marriage to normal man all the daughter are normal but 50% of son are Colourblindness.
• When colourblind women marriage normal man All the daughter are normal but carrier for colour blindness where as all the sons are colour blind because they have only one x-chromosome which is received from mother who is colour blindness.

Haemophilia:

• The inability of a person to clot blood at the mouth of wound is called haemophilia. The blood of a person has a protein called fibrinogen which becomes fibrin in presence of atmospheric air the fibrins are attached to each other and form a membrance that lies over the mouth of wound and prevents bleeding.
• When the gene for fibrinogen is recessive then fibrin is not form and profuse bleeding occurs. Experiments have proved that gene for fibrinogen is located in x-chromosome and shows sex-linked inheritance.

Characteristics of sex-linked Inheritance

• It is Criss Cross type which means sex linked characters of a sex are transmitted to same sex through Opposite sex.
• Sex-link disease are generally transfer to son because Son has only one x-chromosome when x-chromosome is recessive it shows the disease.
• Female is not generally suffered because she has two x-chromosomes one from father and another from mother.
• Female are suffered only when father is suffered and mother either suffered or Carrier.
• Mostly recessive Sex-linked genes cause sex-linked diseases.