Chromosomal-Basis-Inheritance-Linkage-Crossing-over | uksir-notes | Genetics-5
Chromosomal Basis of Inheritance:
 |
| Chromosome |
Want to know about Deviation from Mendel's work , Click the link below:
https://uksirnotes.blogspot.com/2021/07/Genetics-Deviation-Mendelian-Inheritance-uksir.htmlParallelism between Chromosome and Mendelian Factors:
 |
| Chromosome and gene |
Theory:
-
Every diploid organism performs Meiosis and
fertilization during reproduction.
-
The chromosomal basis of inheritance proves
that, Chromosomes are bearer of hereditary units or factors or genes.
-
Also each pair of mendelian factors is carried
by a pair of chromosome separately.
-
Thus the mendelian factors or genes have
specific loci on chromosomes (site where they exist)
-
 |
| Chromosome |
 |
| Chromosomal transfer |
Linkage :
·
There must be independent assortment during the gamete
formation, according to Mendel.
·
But sometime pairs of traits /characters are
coupled and do not show independent assortment. (Do not separate generation
after generation)
·
Thus the genes responsible for such characters
are present in some chromosome and said to be linked.
·
This phenomenon of remaining together without
separation of linked genes is called as linkage.
·
The linked genes of a chromosome form the
linkage group.
·
Linkage was demonstrated by T.H. Morgan in 1910.
·
He performed a test cross in Drosophila (fruit
fly) and got the result, described below.
Test
Cross
Heterozygous x Homozygous recessive
(Gg
Ww) (ggww)
4
Type of phenotype found:
1. Gray
body long winged (GW) – 41.5%
2. Gray body vestigial winged (Gw) - 8.5%
3. Black
body long winged (gW) - 8.5%
4. Black
body vestigial winged (gw) - 41.5%
·
But, The result of Test cross must be 1 : 1 : 1
: 1 (i.e. 25% of every phenotype) according to Mendel.
·
It clearly indicates that, there are some traits
(gene) do not want to separate from each other, generation after generation.
·
They have strong affinities and show linkage.
·
Here gray body trait is linked to long winged,
likewise black body linked to vestigial wing.
Here
two type of linkage can be found
(A) Complete linkage :
·
If both the gene for the traits present on the
same pair of chromosome (Homologous chromosome), then there will be formation
of only two type of phenotype :
Gray
long & black vestigial (1 :1)
Cross:
 |
| Complete Linkage |
·
Here the change is 50% to both the phenotype.
·
These will not be any recombination.
·
So theoretically the result will be 1:1
(B)
Incomplete linkage :
·
If both the dominant gene for the traits are on
different homologous chromosome. Then there is more chances of separation.
·
In this case the theoretical ratio of phenotype
will be 1 : 1 : 1 : 1.
·
As the gene can separate, there will be
formation of 4 type of phenotype.
We
can illustrate using alphabets :
(Example
incomplete linkage)
Gary
body long wing Black
body vestigial wing
 |
| Incomplete Linkage |
Crossing Over:
·
The deviation from linkage result in crossing
over.
·
During meiosis homologous chromosome undergo a
process called synapsis & after that exchange of chromosome fragments
occurs. (Chiasma formation)
·
Here the alleles of the traits (genes) are
exchanged, which is called as crossing over.
·
Due to this- two type of gametes are produced,
one is parental type and other is recombinant type.
·
The recombinant type of gametes fuse to produce
new variety of offspring (new trait)
Mechanism :
 |
| Crossing-over |
 |
| Crossing over |
·
The two parental combination – have
approximately equal proportion, likewise the two recombinant also have the
same.
·
This result can be seen in through Morgan’s
experiment.
Morgan Exp :
 |
| Morgan Experiment |
Generally
recombinant types are produced in different proportion, depending on the
linkage distance between the genes.
Recombination Frequency:
·
If complete linkage do not occur, then there
will be formation of recombinants.
·
The frequency for occurrence of recombinant, is
called as Recombination frequency or Crossover Frequency or Cross over Value.
·
The rate of crossing over is directly
proportional to the distance between two gene.
·
That means if the Genes are more apart, hen more
will be recombination frequency.
·
If genes are less apart, recombination frequency
will be less.
 |
| Linkage-Map |
Here
the genes are arranged in linearly.
-
A and B genes are linked together, so there is
less recombination frequency between them.
-
Whereas Between A and E there is maximum chance
of recombination.
Formula for calculation of recombination frequency :
· Recombination frequency =
 |
| Formula |
·
Calculation of recombination frequency for
linked genes help, to know the genetic linkage map in a chromosome.
·
Genetic linkage map is nothing but a map showing
relative position of genes on a chromosome.
·
The distance between genes in gene map/linkage
map is expressed as map unit/morgan unit (cM)
Example:
·
If recombination frequency between A & B is
6 cm, B & C is 7 cm x A& C is 13 cm, Then the gene loci/Locus will be :
 |
| Gene Loci |
Want to know about Deviation from Mendel's work , Click the link below:
https://uksirnotes.blogspot.com/2021/07/Genetics-Deviation-Mendelian-Inheritance-uksir.html
No comments