SEX LINKED INHERITANCE

·         The chromosomes present in the diploid cells of the majority of the sexually reproducing animals are of two types: autosomes bearing genes for somatic characters and sex chromosomes bearing genes for sex.

·         Sex chromosomes also carry some genes for non-sexual characters such as colour blindness and haemophilia.

·         Such genes which are always associated with sex chromosomes are called sex-linked genes.

·         In man and Drosophila the sex chromosomes (X and Y) are unequal in size and shape, X being larger and rod shaped whereas Y is small and slightly curved.

·         In birds and butterflies the sex chromosomes (Z and W) are also unequal in shape and size, Z being larger than W.

·         In Mendelian pattern of inheritance, the genes for contrasting characters were located on autosomes but not on the sex chromosomes.

·         Secondly, the result of reciprocal cross is same as normal cross which is not the case with sex linked inheritance.

·          There are three types of sex-linked genes depending upon their association with particular chromosome.

·         They are as follows:

·         (i) The genes which are located on X-chromosomes are called X-linked genes or sex linked genes.

·         (ii) The genes which are located on Y chromosomes are called Y-linked genes or holandric genes.

·         (iii) Certain genes are found to occur in both X and Y chromosomes. Such genes are called incomplete sex-linked genes.

·         Characteristics of Sex Linked Inheritance:

·         (a) It is a criss-cross inheritance as the father passes its sex-linked character to his daughter who in turn passes it to the grandson.

·         (b) Daughter does not express the recessive trait but act as carrier in the heterozygous condition.

·         (c) Female homozygous for recessive trait expresses the trait.

·         (d) Any recessive gene borne by the X chromosome of male is immediately expressed as Y chromosome has no allele to counteract.

Sex Linked inheritance in Man:

In man about fifty six sex-linked genes have been reported, the most common examples are:

1. Red green colour blindness.

2. Haemophilia.

1. Red Green Colour Blindness:

·         Colour blindness is an example of sex linked character.

·         Those who suffer from red green colour blindness cannot distinguish between red and green colour.

·         The gene for this defect is located on X chromosome. It was first studied by Horner (1876).

·         Colour blindness is recessive to normal vision.

(i) Normal Woman and Colour Blind Man:

·         When a normal woman is married to a colour blind man, their children (daughters and sons) have normal colour vision.

·         But when their daughters were married to normal man, 50% of their sons are colour blind and the remaining 50% are normal, while the daughters were all normal.

(ii) Colour Blind Woman and Normal Man:

·         If a colour blind woman marries a normal man, their daughters are normal but all their sons are colour-blind.

·         When these F1 daughters are married to colour blind men, colour blind sons and daughters are born in equal number.

2. Haemophilia (Bleeder’s Disease):

·         Haemophila is another popular example of sex linked inheritance in human beings.

·         It is caused by a mutant gene (h) present in X chromosome and recessive to normal gene and is, therefore, suppressed in heterozygous condition.

·         Individuals suffering from this disease lack a factor responsible for clotting of blood.

·          So in the absence of blood clotting substance, a minor cut or injury may cause prolonged bleeding leading to death.

·          This disease in man is generally restricted to male members.

·         If a haemophilic man marries a normal woman, the daughter are all carriers (phenotypically normal but carries haemophilic gene in one on her X chromosome) but sons are normal.

·         Such a carrier daughter, when marries a normal man transmits the haemophilic gene to half of her son.

·         A haemophilic woman is produced only if a carrier woman is married to a haemophilic man.

SEX LINKED INHERITANCE OF HAEMOPHILIA

·         Haemophilia is also called ‘Royal disease’ as it is found in certain royal families of Europe.

·         Apparently the gene for haemophilia (h) arose as a mutation in a reproductive cell which produced Queen Victoria of England.

DIFFUSION, OSMOSIS, PLASMOLYSIS, IMBIBITION

1. Diffusion

If a small bottle filled with some gas or vapours is opened at a certain place in the room, very soon its molecules become evenly distributed throughout the available space in that room.

 Similarly, if a solute is placed in its solvent, it is dissolved and its particles move so that they are evenly distributed throughout the container.

 This movement of particles or molecules from a region of higher concentration to a region of lower concentration is called as diffusion.

 The rate of diffusion of gases is faster than liquids or solutes.

The diffusing particles have a certain pressure called as the diffusion pressure which is directly proportional to the number or concentration of the diffusing particles.

Therefore, the diffusion takes place always from a region of higher diffusion pressure to a region of lower diffusion pressure i.e., along a diffusion pressure gradient.

The rate of diffusion increases if:

(i) The diffusion pressure gradient is steeper.

(ii) The temperature is increased.

(iii) The density of the diffusing particles is lesser.

(iv) The medium through which diffusion occurs is less concentrated.

Diffusion of more than one substance at the same time and place may be at different rates and in different directions but is independent of each other. A very common example of this is the gaseous exchange in plants.

Role of Diffusion in the life of the plants:

i. It is an essential step in the exchange of gases during respiration and photosynthesis.

ii. During passive salt uptake, the ions are absorbed by simple process of diffusion.

iii. Last step in stomatal transpiration is the diffusion of water vapours from the intercel­lular spaces into the outer atmosphere through open stomata.

2. Osmosis

If a solution and its pure solvent are separated by a semi permeable membrane (which allows only solvent and not the solute to pass through it) the solvent molecules diffuse into the solution. This diffusion of solvent molecules into the solution through a semi permeable membrane (SPM) is called as osmosis.

In case, there are two solutions of different concentrations separated by the semi-permeable membrane, the diffusion of solvent will take place from the less concentrated solution into the more concentrated solution till both the solutions attain equal concentration.

The phenomenon of osmosis can be demonstrated by the following simple experiment:

Mouth of a thistle funnel is tied with goat bladder (it acts as semi-permeable membrane) and concentrated sugar solution is filled in it whose level is marked on its narrow neck. It is now placed in a beaker of water. After sometime the level of the sugar solution in the thistle funnel rises. 

Osmotic Pressure:

As a result of the separation of solution from its solvent or the two solutions by the semi-permeable membrane, a pressure is developed in solution due to the presence of dissolved solutes in it. This is called as osmotic pressure (O.P.).

Osmotic pressure is measured in terms of atmospheres.

Osmotic pressure is directly proportional to the concentration of dissolved solutes in the solution. More conc. solution has higher osmotic pressure.

Osmotic pressure of solution is always higher than its pure solvent.

Osmotic pressure does not increase by the addition of insoluble solute in the solution.

Thus, during osmosis the movement of solvent molecules takes place from the solution whose osmotic pressure is lower (i.e., less concentrated or hypotonic) into the solution whose osmotic pressure is higher (i.e., more concentrated or hypertonic).

Osmotic diffusion of solvent molecules will not take place if the two solutions separated by the semi-permeable membrane are of equal concentrations having equal osmotic pressures (i.e., they are isotonic).

Plant Cells as Osmotic Systems:

Living cells in plants form osmotic systems due to the presence of semi-permeable plasma membrane and the cell sap having a certain osmotic pres­sure. Plasma-membrane actually is not truly semi-permeable as it allows certain solutes to pass through it and hence, it is known as selectively permeable or differentially permeable mem­brane. The tonoplast or the vacuolar membrane also possesses the same nature. The solvent in case of plants is always water. The cell wall is permeable.

If a living plant cell or tissue is placed in water or hypotonic solution (whose O.P. is lower than that of cell sap) water enters into the cell sap by osmosis. This process is called as end- osmosis. As a result of entry of the water into the cell sap, a pressure is developed which presses the protoplasm against the cell wall and the cell becomes turgid. This pressure is called as turgor pressure. Consequence of the turgor pressure is the wall pressure which is exerted by the elastic cell wall against the expanding protoplasm. At a given time turgor pressure (T.P.) equals the wall pressure (W.P.).

T.P. = W.P.

If on the other hand, the plant cell or the tissue is placed in hypertonic solution (whose O.P. is higher than that of cell sap) the water comes out of the cell sap into the outer solution and the cell becomes flaccid. This process is known as ex-osmosis. Cell or tissue will remain as such in isotonic solution.

Significance of Osmosis in Plants:

(1) Large quantities of water are absorbed by roots from the soil by osmosis.

(2) Cell to cell movement of water and other substances dissolved in it involves this pro­cess.

(3) Opening and closing of stomata depend upon the turgor pressure of the guard cells.

(4) Due to osmosis the turgidity of the cells and hence the shape or form of their organs is maintained.

(5) The resistance of plants to drought and frost increases with increase in osmotic pres­sure of their cells.

(6) Turgidity of the cells of the young seedlings allows them to come out of the soil.

3. Plasmolysis

In normal condition the protoplasm is tightly pressed against the cell wall. If this plant cell or tissue is placed in a hypertonic solution, water comes out from the cell sap into the outer solution due to ex-osmosis and the protoplasm begins to contract from the cell wall. This is called as incipient plasmolysis.

If the outer hypertonic solution is very much concentrated in comparison to the cell sap, the process of ex-osmosis and contraction or shrinkage of protoplasm continues and ultimately the protoplasm separates from the cell wall and assumes a spherical form. This phenomenon is called as plasmolysis and the cell or the tissue is said to be plasmolysed. Because of the per­meable cell wall the space in between the cell wall and plasma-membrane in plasmolysed cells is filled with outer hypertonic solution.

If a plasmolysed cell or tissue is placed in water, process of end-osmosis takes place. Water enters into the cell sap, the cell becomes turgid, and the protoplasm again assumes its normal shape and position. This phenomenon is called deplasmolysis.

Advantages of Plasmolysis:

1. It indicates the semi-permeable nature of the plasma-membrane.

2. This phenomenon is utilized in salting of meat and fishes and addition of concentrated sugar solution to jams and jellies to check the growth of fungi and bacteria which become plasmolysed in conc. solution.

3. It is also used in determining the O.P. of the cell sap.

4. Imbibition

The term imbibition means the absorption of water by the solid particles of an adsorbent without forming a solution.

The absorption of water by the solid particles of an adsorbent without forming a solution is called Imbibition.

In other words the adsorption of water by hydrophilic colloids is known as Imbibition.

 Solid substance or adsorbent which take part in imbibition are called Imbibants.

The liquid which is imbibed is known as Imbibate.

Imbibition of water of increases the volume of the imbibant due to which pressure is created known as Imbibitional Pressure (IP).

Role of Imbibition In Plants:

(i) Imbibition is the first step in the absorption of water by the roots and cells,

(ii) Imbibition of water by cell walls helps to keep the cells moist, and

(iii) Imbibition pressure is helpful in seed germination, growth of seedling through the soil, ascent of sap in plants, etc.

Significance of Imbibition:

1. It is the dominant and first step of water absorption.

2. Imbibition is the first step of seed germination.

3. Seedling is able to come out of soil due to development of imbibitional pressure.