Gene which are located on the same chromosome and are transmitted together in the offsprings are called linked genes. This phenomenon of inheritance of linked genes together so as to retain the parental combinations in the offsprings is known as linkage. The characters controlled by linked genes are called linked characters. All those which located in a single chromosome constitute a linked group.

1. LINKAGE
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2. WHAT IS LINKAGE?
• Gene which are located on the same chromosome and are transmitted together
in the offsprings are called linked genes. This phenomenon of inheritance of
linked genes together so as to retain the parental combinations in the offsprings
is known as linkage.
• The characters controlled by linked genes are called linked characters.
• All those which located in a single chromosome constitute a linked group.

3. HISTORY
• Walter Sutton and theodor boveri (1902), Sutton (1903), and Bateson and
Punnett (1906)pointed out the likelhood that many genes are located in each
chromosome.
• In certain crosses they observed that the characters did not show independent
assortment and f2 ratio was much different from dihybrid ratio (9:3:3:1).
• However, the theory of linkage and advanced by T.H.Morgan (1911).

4. ARRANGEMENT OF LINKED GENES
• In an individual, which is heterozygous for two pairs of linked genes, the linkage
can be either of the two types:
• The dominant allele of both the gene pairs are located in one member of the
chromosome of the pair. This arrangement is known as cis-arrangement and genes
are said to be in coupling state. The heterozygous with such arrangement (AB/ab) are
known as cis-heterozygotes.
• the dominant alleles of one genes and the recessive allele of other gene are located on
one chromosome of the pair and the recessive allele of first gene and the dominant
allele of the second gene are located on the other homologous chromosome of the
pair.this arrangement of a dominant and a recessive gene in the same chromosome
(Ab/aB) of homologous pair of chromosome is known as trans arrangement.the
heterozygotes with such arrangements are called trans heterozygotes.in this
combination genes are said to be in repulsive state.

6. TYPES OF LINKAGE
•Complete linkage
•Incomplete linkage

7. COMPLETE LINKAGE
• A linkage is said to be complete when two or multiple characteristics are
inherited and normally surface in two or further generations in their parental or
original combinations, they are known as complete linkage.
• These particular genes do not generate combinations that are non-parental.
• The genes that exhibit these linkages are located nearby in the same
chromosome. Examples – genes for long wings and grey body in male Drosophila

8. • Example.
• The genes for bent wings (bt) and shaven bristles (svn) of the
fourth chromosome mutant of Drosophila melanogaster exhibit
complete linkage. 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.

9. INCOMPLETE LINKAGE
• It is displayed by genes that generate some portion of non-parental
combinations.
• These genes are situated at a distance on the chromosomes which can be
attributed to the occasional or accidental deconstruction of chromosomal
segments while crossing over.

10. • Example
• The incomplete linkage has been reported in female Drosophila and
various other organisms such as tomato, maize, pea, mice, poultry and
man, etc. Incomplete Linkage in maize. In Zea mays (maize) a case of
incomplete linkage between the alleles for colour and shape of the
seed has been observed by Hutchison. When a maize plant with seeds
having coloured and full endosperm (CS/CS) is crossed with another
plant having recessive alleles for colourless, shrunken seeds (cs/cs), the
F1 heterozygotes are found with the phenotype of coloured full and
genotype of CS/cs. When F1 hybrid is test crossed with double
recessive parent (cs/cs) four classes of descendants are obtained
instead of two. The test cross results are clearly showing that parental
combination of alleles (e.g., CS/cs and cs/ cs) are those expected from
complete linkage and appear in 96% cases, the other two are new
combinations (e.g., Cs/cs and cS/cs) and appear in 4% cases. Thus, in
4% cases crossing over has occurred between linked genes.

12. SIGNIFICANCE
• The phenomenon of linkage has one of the great
significance for the living organisms in that it reduces the
possibility of variability in gametes unless crossing over
occurs.

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