What is sex linkage? Describe the chromosomal theory of sex linkage.

     Ans. : A) The genes linked with the sex chromosomes are called sex linked and their linkage is called as sex linkage.

      B) The different postulates of chromosomal theory of sex determination can be given as follows -

C)  Morgan and castle proposed theory of sex linkage in 1911

     1) Linked genes occur in the same chromosome.

     2) Gene are present in linear fashion on the chromosome.

     3) There is a tendency to maintain the parental combinations of genes except for occasional cross overs.

    4)  Strength of the linkage between two genes is inversely proportional to the distance between the two genes.

     It means that higher the distance between the two linkage genes frequency of crossing over be more and if the small distance is present the tendency to cross over lowers.

Linked genes & Type of linkage:-

1) Linked genes are those genes which occur over the same chromosome.

2) The genes which do not show linkage are present over different chromosomes.

3) The phenomenon of independent assortment is shown by unlinked genes.

4) Independent assortment is not observed in case of linked genes, rather they remain together at the time of inheritance and are inherited en block resulting into development of parental type of progeny.

5) Types of Linkage:-

    There are two main types of linkage.

a) Complete linkage (Morgan, 1919) :-

1) The genes located in the same chromosome do not seggregate and are inherited together over the generations due to abscence of crossing over.

2) Complete linkage allows the combination of parental traits to be inherited as such.

3) Such a case is rare but can be explained in male Drosophila and some other heterogametic organisms.

b) Incomplete linkage :-

1) Genes present in the same chromosome have a tendency to separate due to crossing over and hence produce recombinant progeny besides the parental types.

2) The number of recombinant individuals is usually less than the expected number in independent assortment.

3) In case of independent assortment out of four phenotypes two are parental and two are recombinant i.e. each is 25%.

4) In case of linkage parentall types are more than 25% and recombinant types are always less than 25%

Linkage Groups:-

1) A linearly arranged group of linked genes which are normally inherited together except for crossing over is known as linkage group.

2) It corresponds to a chromosome which bears a linear sequence of genes linked and inherited together.

3) Because the two homologous chromosomes possess either similar or allelic genes on the same loci, they constitute the same linkage group.

4) The number of linkage groups present in an individual corresponds to number of chromosomes in its one genome (all the chromosomes if haploid or hologous pairs if diploid).

5) It is known as principle of limitation of linkage groups.

6) Importance of linkage groups:-

a) Number of linkage groups is equivalent to number of chromosomes present in a genome which proves that genes are present over the chromosomes.

b) Linkage prevents or reduces the chances of recombination which helps to retain the varietal as racial characters over generations.

c) Can be used to maintain the good characters of newly developed variety.

d) Linkage is the main hurdle for breeders because it never helps to combine all the desirable characters in single variety.

e) It dilutes the use of desirable traits if the undesirable traits are also present on the same linkage group.

f) Marker genes or the genes which express their effect in early growth can indicate the effect of linked gene which is to express late.

Q.: Explain the types of sex linkage

    Ans.: 1) The transmission of body characters from parents to offsprings along with    sex is called as sex linked inheritance or sex linkage.

     2) It shows two types as -

     a) Incomplete sex linkage

     b) Complete sex linkage

     A) Incomplete sex linkage:

     1) Incomplete sex linked genes are located on the homologous region of X and    Y chromosome.

2)  These are loosely linked together.

3)  Due to this reason they do not inherit together because crossing over may take place in these regions.

4)  Such gene are incompletely sex linked genes and mode of inheritance is incomplete sex linked inheritance or incomplete sex linkage.

5)  The different diseases are total colourblindness, Retinitis pigmentosa, Nephritis etc.

B) Complete sex linkage:-

1) It is shown by genes present over non homologous parts of sex chromosomes.

2) As crossing over does not take place they are inherited together.

3) hemophilia, Red green colourblindness ,myopia are examples of X-linked traits.

4) Y-linked characters are hypertrichoisis and ichthyosis. 

Q. : Write a note on crossing over.

Ans.: 1) Crossing over is the process of exchange of genetic material or segments  between non sister chromatids of two homologous chromosomes.

      2) The process of crossing over shows different steps which can be explained as follows:

A) Synapsis:

1)   Replicated but apparently single homologous chromosomes come to lie side    by side with same gene locus of two opposite chromosomes.

2)  The phenomenon takes place during zygotene stage of prophase-I and known as synapsis.

     3) The synapsed pairs of bivalent chromosomes are called as bivalents.

     4) The two homologous chromosomes are held together by synaptinemal complex.

     5) If any amount of unreplicated material is present undergoes replication.

    B) Crossing Over:

     1) It takes place during pachytene stage of meiosis - I

     2) The process is carried out at four strand stage using various enzymes like endonucleases, exonuclease, R-protein and recombinase.

3)  The step is characterised by breaking of chromatid segments, exchange of non-sister chromatids and later their fusion in new places.

C)  Tetrad Stages:

1)   There is dissolution of synaptinemal complex except the region of crossing over.

2)  Due to this there is separation of chromosomes and chromatid become distinct at most of the places.

     3) If this stage, bivalent appears to have four chromatids producing tetrad stage.

4)  The synaptinemal point of attachments between the homologous       chromosomes are called as chiasmata.

D) Terminalization:

1)   The tendency of chiasmata to shift to the sides is known as Terminalization.

2)  Many of such chiasmata disappear before metaphase I.

 

      Q.:  Explain Morgan’s experiment showing linkage and Recombination.

      Ans.:  1)  The different experiments were carried out by Morgan on Drosophila. These were basically dihybrid crossers.

2)  The crosses were conducted to find out whether genes were sex linked.

3)  The experiments were very similar to those dihybrid crosses in pea plants conducted by Mendel.

4)  Morgan hybridized yellow bodied white eyed female with brown bodied red eyed male and they inter crossed F1 progeny.

     5) The result deviated from Mendelian Ratio 9:3:3:1.

     6) Morgan stated that when the two genes in a dihybrid cross were situated on the same chromosomal the proportion of parental gene combination were     much higher than non-parental type.

7)  The reason is the linkage between the two genes and non-parental gene combination is described as recombination.

8)  He also concluded that, when grouping of the genes is on the same chromosomes some genes which were tightly linked (low recombination) while others were loosely linked (High recombination).

     9) It can be explained with following chart.