Lecture notes on linkage

An individual  has many genes for the determination of various different characters on the chromosomes.If the genes are situated in the same chromosome and are fairly close to each other, they tend to be inherited together. This type of coexistence of two or more genes in the same chromosome is known as linkage.

Chromosome Theory of Linkage

Morgan along with Castle formulated the chromosome theorynof linkage which is as follows:

1. The genes which show the phenomenon of linkage are situated in the same chromosomes and these linked genes usually remain bounded by the chromosomal material so that they cannot be

separated during the process of inheritance.

2. The distance between the linked genes determines the strength of linkage. The closely located genes show strong linkage than the widely located genes which show the weak linkage.

3. The genes are arranged in linear fashion in the chromosomes.

T.H. Morgan and his co-workers by their investigation on the Drosophila and other organisms have found two types of linkage, viz., complete linkage and incomplete linkage.

1. Complete Linkage

The complete linkage is the phenomenon in which parental combinations of characters appear

together for two or more generations in a continuous and regular fashion. In this type of linkage genes are closely associated and tend to transmit together.

Example. The genes for bent wings (bt) and shaven bristles (svn) of the fourth chromosome

mutant of Drosophila melanogaster exhibit complete linkage.

Complete linkage in male Drosophila.But in male Drosophila and female silkworm, Bombyx mori crossing-over takes place either very rarely or not at all. This becomes clear from Morgan’s experimental results from Drosophila. In 1919, T.H. Morgan mated gray bodied and vestigial winged (b+vg/b+vg) fruit flies with flies having black bodies and normal wings (bvg+/bvg+). F1 progeny had gray bodies and normal long wings (b+vg/bvg+), indicating thereby that these characters are dominant.When F1 males (b+vg/bvg+), were backcrossed (i.e., test crossed) to double recessive females (bvg/bvg or black vestigial), only two types of progeny (one with gray bodies and vestigial wings, b+vg/bvg and the other with black bodies and normal wings, to bvg+/bvg instead of four types of phenotypes were obtained :

Parents : Gray, Vestigial X Black, Long

b+vg/b+vgX bvg+/bνg+

Gametes :       (b+vg)                        (bvg+)

F1 : All Gray, Long(b+vg/bvg+)

Test cross : F1 male Gray, Long × Female Black, Vestigial

                                    b+vg/bvg+ x  bvg/bvg

Gametes :           (b+vg) (bvg+)                (bvg)

(only two types of gametes due to complete linkage and lack of crossing over in male Drosophila)

Test cross ratio : ½ Gray, Vestigial : ½ Black, Long or 1 : 1.(b+vg/bvg , bvg+/bvg)

2. Incomplete Linkage

The linked genes do not always stay together because homologous non-sister chromatids may

exchange segments of varying length with one another during meiotic prophase. This sort of exchange of chromosomal segments in between homologous chromosomes is known as crossing over .The linked genes which are widely located in chromosomes and have chances of separation by crossingover are called incompletely linked genes and the phenomenon of their inheritance is calledincomplete linkage.

Example. The incomplete linkage has been reported in female Drosophila and various other

organisms such as tomato, maize, pea, mice, poultry and man, etc. Here, the examples of incomplete linkage have been considered only for Drosophila and maize.

Incomplete linkage in female Drosophila. When F1 females of the Morgan’s classical cross in

Drosophila between gray, vestigial (b+vg/b+vg) and black, normal or long (bvg+/bvg+) were testcrossed to double-recessive (bvg/bvg) males, all four types of progeny were obtained in following ratio, showing occurrence of crossing-over :

Parents : Gray,            Vestigial × Black, Long

b+vg/b+vg X  bvg+/bvg+

Gametes :                               (b+vg) (bvg+)

F1 :                 Gray, Long

b+vg/bvg+

Test cross : F1 Female Gray, Long × Male Black, Vestigial

b+vg/bvg+                               bvg/bvg

↓                                  ↓

Gametes :       (b+vg) (bvg+) = Non-crossovers (bvg)

(b+vg+) = Recombinants

Test cross ratio :

1. Gray, Vestigial; b+vg/bvg = 41.5%

2. Black, Long; bvg+/bvg = 41.5%

3. Gray, Long ; b+vg+/bvg = 8.5%

4. Black, Vestigial; bvg/bvg = 8.5%

Similar different test cross ratios (showing complete linkage in males and incomplete linkage in

females) were obtained for F1 males and females of Drosophila by Bridges, one of the student of Morgan. He made a cross between fruit flies having wild dominant alleles for red eye colour and normal wings (pr+vg+/pr+vg+) and having mutant recessive alleles for purple eye colour and vestigial wings (pr vg/pr vg) .

LINKAGE GROUPS

All the linked genes of a chromosome form a linkage group. Because, all the genes of a chromosome have their identical genes (alleomorphs) on the homologous chromosome, therefore linkage groups of a homologous pair of chromosome is considered as one. The number of linkage group of a species, thus, corresponds with haploid chromosome number of that species.

Example.

 1. Drosophila has 4 pairs of chromosomes and 4 linkage groups.

2. Man has 23 pairs of chromosomes and 23 linkage groups.

3. Corn (Zea mays) has 10 pairs of chromosomes and 10 linkage groups.

However, in organisms the female or male sex having dissimilar sex chromosomes (e.g., human beings, Drosophila, fowl, etc.), one more linkage group occur than the haploid number .

Example.

1. Female human beings = 22 pairs of autosomes or non-sex chromosomes + 1 pair of

: homomorphic X chromosomes= 22 autosomal linkage groups + 1 X chromosomal linkage group = 23 linkage groups.

2. Male human beings : = 22 pairs of autosomes + 2 heteromorphic sex chromosomes, i.e., 1 X chromosome + 1 Y chromosome= 22 autosomal linkage group + 1 X chromosomal linkage group + 1 Y chromosomal linkage group= 24 linkage groups.