Linkage:

The phenomenon of tendency of linked genes to inherit together in the same combination for more than two generation is called linkage.

Morgan’s View:

The degree of linkage between two genes depends on the distance between location of genes and they vary and form crossing over, if they are located at the distance. This phenomenon is explained by T.H. Morgan in 1911 in Drosophila melanogaster with grey body long wing and black body, vestigial wing. He stated that the pairs of genes of homozygous parents tried to enter the same gametes and to remain together, whereas same genes from heterozygous parents tend to enter different gametes and remain apart from each other.

Chromosomes Theory of linkage:

According to Morgan and Castle,

1.They concluded that chromosomes bear many genes.

2.The genes which show linkage are situated in the same chromosomes are bounded by the chromosomal material.

3.Gene are arranged in a linear fashion.

4.The strength of linkage depends upon the distance between the linked genes in the chromosomes.

5.Linked gene remained in their original combination during the course of inheritance.

Types of Linkages:

1.Complete linkage:

If the parental combination of characters appear together for two or more generation in a continuous manner and regular manner. Such linked is called complete linkage.

Example: Drosophila melanogaster

Here, gene are closely associated and tend to transmit together.

Parents:                      Grey, vestigial        x                   Black long

                                         (BBvv)                                        (bbVV)

Gametes:                        (BV)                                            (bV)

F1 generation :         All grey, long

                                       (BbVv)

Test cross:      F1 male Grey, long              x             Female Black, vestigial

                                        (BbVv)                                          (bbvv)

Gametes:                      (BV) (bv)                                        (bv)

(Due to complete linkage only two types of gametes are formed).

Test cross ratio: Grey, vestigial:   Black long (1:1)

                                         (Bvbv)          (bVbv)

The results shows complete linkage.

Incomplete linkage :

Incomplete linkage produces new combinations of the genes in the progeny due to the formation of chiasma or crossing over in between the linked genes present on homologous chromosomes. When in sweet peas a cross is made between blue flower and long pollen (BBLL) with red flower and round pollen (bbll) in F1 expected blue flower and long pollen (BbLl) heterozygous condition is got.

                             Fig. A case of incomplete linkage.

However, test cross between blue and long (BbLl) and double recessive (bbll) gave blue long (43.7%), red round (43.7%), blue round (6.3%) and red long (6.3%). The parent combinations are 87.4% are due to linkage in genes on two homologous chromosomes, while in case of new combinations (12.6%) the genes get separated due to breaking of chromosomes at the time of crossing over in prophase-I of meiosis. New combinations in the progeny appeared due to incomplete linkage.

Significance of Linkage:

1. Linkage does not permit the breeders to bring the desirable characters in one variety.

2. Linked characters are maintained for generations because linkage prevents the incidence of recombination.

linkage group,  in genetics, all of the genes on a single chromosome. They are inherited as a group; that is, during cell division they act and move as a unit rather than independently. The existence of linkage groups is the reason some traits do not comply with Mendel’s law of independent assortment (recombination of genes and the traits they control); i.e., the principle applies only if genes are located on different chromosomes. Variation in the gene composition of a chromosome can occur when a chromosome breaks, and the sections join with the partner chromosome if it has broken in the same places. This exchange of genes between chromosomes, called crossing over.

Crossing over:

Crossing over may be defined as an exchange of genetic material between non- sister chromatids of homologous chromosomes resulting in a new combination of genes.

The crossing over takes place during the early stage of prophase first of meiosis ce;; division.

Mechanism of crossing over:

The process of crossing over involves the following stages:

1.Synapsis

2.Duplication of chromosomes

3.Crossing over

4.chiasmata formation

5.Terminalisation

1.Synapsis:

During zygotene substage of prophase I, the maternal and paternal homologous chromosomes come close to each other and start pairing along their length. The pairing of homologous chromosomes is called synapsis. They paired homologous chromosomes are called bivalents. It is mechanical basis of crossing over.

2.Duplication of chromosomes:

The synapsis is followed by duplication of chromosomes. During pachytene substage of prophase I , the chromotids of each homologous chromosome splits lengthwise and forms two identical sister chromatids.Thus each bivalent contains four chromatids so it is known as tetrad.

3.Crossing over:

The non-sister chromatids of homologous pair twist over each other at one or more points. The chromatid segments break at the corresponding points and the segment of one side fuses with the segment of the opposite side due to the action of enzyme. Thus the crossing over includes breaking of chromatid segments, their transposition and fusion.

4.Chiasmata Formation:

Chiasmata are the points of attachment between two homologous chromosomes, where the crossing over occurs. The number of chiasmata depends on the length of the chromosomes;  greater the length greater is the number.The crossing over may take place at one or several points in one tetrad and may result in the formation of one or more chiasma.

5.Terminalisation:

After the process of crossing over, the non- sister chromatids start to repel each other due to lack of attraction force between them. The repulsion of chromatids starts from the centromere towards the chiasma and the chiasma itself moves in a zipper fashion towards the end of the tetrad. The movement of chiasma is known as terminalization. Due to terminalisation the hlomologous are separated completely.

Types of crossing over:

Significance of Crossing over:

1.It affords a proof for the linear arrangement of genes in the chromosomes.

2.Result of crossing over, new gene combination are produced play an important role in evolution.

3.It is necessary for natural selection due to changes of variation increase.

4.Useful combination can be formed , which may be used in breeding.

Sex- linked inheritance:

Sex linked inheritance is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance. Since humans have many more genes on the X than the Y, there are many more X-linked traits than Y-linked traits.

In mammals, the female is the homozygous sex, with two X chromosomes (XX), while the male is heterozygous, with one X and one Y chromosome (XY). Genes on the X or Y chromosome are called sex linked genes. In birds, the opposite is true: the male is the homozygous sex, having two Z chromosomes (ZZ), and the female (hen) is heterozygous, having one Z and one W chromosome (ZW).


Sex – linked inheritance in Dorsophila:

A. A cross between Red eyed female and white eyed male:

Morgan crossed a normal red eyed female with a white eyed male. He found only red eyed males and female in F1 generation. When the red eyed male and red eyed female individuals of F1 generations are intercrossed, he got :

a.All F2 females were red eyed.

2.Half of F2 males were red eyed.

c.Half of F2 males were white eyed.

 Thus, F2 generation shows red eyed and white eyed individuals in 3:1 ratio. The ratio of male and female flies was 2:2 or 50% each. Thus, appearance of white eyed character is linked with the male sex, but is inherited through the female.

B.Cross between white eyed female and red eyed male:

The white eyed females were crossed with red eyed males. The result that all males were white eyed, all females were re eyed in F1 generation. He got;

a.Half of F2 population were red eyed.

b.Halsf of F2 population were white eyed.

c.Half of F2 males were red eyed.

d.Half of F2 males were white eyed.

Morgan concluded that the eye colour in Dorsophila is a sex- linked character and follows a criss-cross inheritance. The male transmits sex-linked characters through his daughters to grandsons and never through his sons.

Sex-linked inheritance in man:

The inheritance patterns shown below for red-green colour blindness can equally well apply to other sex-linked genes such as haemophilia. Since most sex-linked genes are usually only carried on the large X chromosome, a dash ( - ) signifies the presence of the relatively inert Y chromosome. The colour blindness gene is recessive to the gene for normal sight. Because the males have only one X chromosome and, therefore, only a single sex-linked gene at each locus, they are said to be hemizygous. The females, however, with two sex-linked genes, can be either homozygous or heterozygous

1.Colour blindness:

a.Marriage between colour blind man and normal visioned woman:

When a normal visioned woman marries a colour blind man, all their male and female children will have normal vision, but the female children are heterozygous, who, if married with a normal man, will produce two normal visioned female, one normal visioned male and one colour blind male.

b.Marriage between normal visioned man and colour blind woman:

when a colour blind woman is married with a normal visioned man, in F1 generation they are produce all colour blind sons and normal visioned daughters because the son receives one X- linked recessive gene for colour blindness from their mother and Y chromosome from father; while the daughter receives one X- linked recessive gene for colour blindness from mother and another X- linked dominant gene for normal vision from father.

2.Haemophilia:

It is a disease in which blood doesn’t clot properly. It is very rare found in only few humans also known as bleeder’s disease. A person who contain the recessive gene for hemophilia lacks in normal clotting substances  antihemophilic globutian in blood. It is well known in the royal families of Europe. The gene of haemophilia is recessive and present in the X-chromosome. Thus, a single gene produces the disease in males while two genes will produce the disease in females.