Chromosomal Theory of
Inheritance:
1)Gregor
Johann Mendel published
his work on inheritance of traits in 1866 but for some reasons, it remained
unnoticed till 1900, as communication was not easy in those days.
2)His
work was not widely recognized.
3)
His approach of using mathematics and statistics to explain biological
phenomenon was totally new and unacceptable to the biologists. .
4)As
continuous variations were observed in nature, Mendel's concept of factors
(genes) as stable .
5)Discrete
unit which controlled the expression of characters, and that a pair of alleles
did not "blend" with each other, was not accepted by another
biologist.
6)He
also did not know the physical location of the 'factors' (genes) in the gametic
cell.
7)
In 1900, .three scientists Hugo de Vries, Correns and von Tschermak,
independently rediscovered Mendel's work on the inheritance of traits.
8)Due
to advancements in microcopy, scientists were able to observe cell division and
the structure of chromosomes under microscope.
9)Walter
Sutton along with Theodor Boveri (1903) studied the parallel behavior of
Mendel's factors (genes) and behavior of chromosomes, at the time of meiosis.
10)
Based on these observations, chromosomal theory of inheritance was put
forth by Sutton and Boveri.
11)This
theory identifies chromosomes as the carriers of genetic material.
12)This
theory states that the chromosomes are present in pairs in somatic cells.
13)
During gamete formation homologous chromosomes pair, segregate and assort
independently during meiosis.
14)Thus,
each gamete contains only one chromosome from a pair.
15)
Nucleus of gametes contains chromosomes, which carry all hereditary traits.
16)Male
and female gametes (sperms and eggs) carry all the hereditary traits.
17)They
are the link between parents and offsprings.
18)The
fusion of haploid male gamete and haploid female gamete, restores the diploid
number of chromosomes of the species.
1)
The chroosomes, which have same genes but they contain different alleles.
2)
In Anaphase -I .
3)Genes
are located on the chromosomes inside the nucleus.
4)DNA
is present on chromosomes, and genes are segment of DNA. so, they are
indirectly related and both contain genetic material of the parent and passed
on to the progeny. thus, they both have
similar behavior.
Chromosomes:
1)Chromosomes
are filamentous bodies present in the eukaryotic nucleus. The term
chromosomes (Gr. Chromo = colour, soma = body) was coined by W. Waldeyer (1888).
2)
The size of chromosome varies from species to species.
3)
Each metaphase chromosome varies from 0.1 to 33 mm in length and 0.2 to 2 mm in
thickness.
4)Chromosomes
are visible during cell division.
5)They
are capable of self-replication and play vital role in heredity, mutation,
variation, and evolutionary development of eukaryotic species.
6) Chemically eukaryotic chromosomes are made of DNA, histone and non-histone proteins. Function:Chromosomes are carriers of heredity.
Number of chromosomes:
1)The
number of chromosomes is specific and constant for a particular species,
therefore it is of great importance in the study of phylogeny and taxonomy
of the species.
2)The
term Ploidy speak for the degree
of repitation of the primary basic number of chromosomes (x) in a cell.
3)When
the chromosome number in a cell is the exact multiple of the primary basic
number, then it is called euploidy.
4)Euploids
include monoploid/ haploid (with one set of chromosomes where x=n), diploids
(2n-two sets of chromosomes), triploids (3n-three sets of chromosomes),
tetraploid (4n-four sets of chromosomes) and so on.
5)When
the chromosome number is not the exact multiple of the haploid set, it is
described as Aneuploidy.
6)
Aneuploidy is either addition or deletion of one or more chromosome (s) to the
total number of chromosomes in a cell.
Structure of chromosome:
1)
Chromosomes are best visible under microscope,When the cell is at metaphase
stage.
2)It
is because at this stage chromosomes are highly condensed.
3)Typical
chromosome consists of two chromatids joined together at centromere or primary
constriction.
4)Primary
constriction consists of a disk shape plate called kinetochore.
5)It
is at the kinetochore, spindle fibres get attached during cell division.
6)Besides
primary constriction, some few chromosomes possess additional one or two
constrictions called secondary constriction.
7)At
secondary constriction I, nucleolus becomes organized during interphase.
8)In
very few chromosomes, satellite body (SAT body) is attached at secondary
constriction II.
9)Each
chromatid in turn contains a long, unbranched, slender, highly coiled DNA
thread, called Chromonema, extending through the length of chromatid.
101)Chromatid
consists a double stranded DNA molecule which extends from one end of
chromosomes to other.
Depending
upon the position of centromere there are four types (shapes) of chromosomes -.
1)Metacentric
( V shaped). Centromere is middle
2)Submetacentric
( L shaped) Centromere is Away from middle
3)Acrocentric
( J shaped) Centromere is Near the
end
4)Telocentric
( i shaped) Centromere is at the end
The
ends of chromosome (i.e. chromatids) are
known as telomeres.
Sex Chromosomes:
1)The
chromosomes which are responsible for the determination of sex are known as sex
chromosomes (Allosomes) .
2)Human
being and other mammals have X and Y Chromosomes as sex chromosomes.
3)X
chromosome is straight, rod like and longer than Y chromosome.
4)
X chromosome is metacentric, Y chromosome is acrocentric.
5)X
chromosome has large amount of euchromatin (extended region) and small amount
of heterochromatin (highly condensed region).
6)In
X chromosome Euchromatin has large amount of DNA material, hence genetically
active.
7)Y
chromosome has small amount of euchromatin and large amount of heterochromatin,
hence it is genetically less active or inert.
8)Both
X and Y chromosome show homologous and non-homologous regions.
9)Homologous
regions show similar genes while non-homologous regions show dissimilar genes
10)Crossing
over occurs only between homologous regions of X and Y chromosomes.
11)Non-homologous
regicm of X chromosome is longer and contains more genes than that of nonhomologous
region ot Y chromosome.
12)X-linked
genes are present on non-homologous region of X-chromosome while Y linked genes
are present on non-homologous region of Y-chromosome.
Linkage and Crossing Over:
Linkage -
1)It is known fact that Several genes are present on
the chromosome.
2)As chromosomes are carriers of heredity, these
genes have tendency to be inherited together.
3)Such genes are called linked genes.
4) The tendency of two or more genes present on the
same chromosomes that are inherited together is known as linkage.
5)Linkage was discovered in plants by Bateson and
Punnett and in animals by T. H. Morgan.
Linkage is of two kinds - complete and incomplete
linkage:
I)Complete linkage:
1)The linked genes which are closely located on the
chromosome do not separate (no crossing over) and inherit together.
2)They are called completely linked (strongly
linked) genes and the phenomenon of their inheritance is called complete
linkage.
3)Thus, the parental traits are inherited in
offsprings.
4)e.g. X chromosome of Drosophifa males- show
complete linkage.
II) Incomplete linkage:
1)The linked genes which are distantly located on the
same chromosome and have chances of separation by crossing over, are known as
incompletely linked (weakly linked) genes.
2) The phenomenon of their inheritance, is called
incomplete linkage
3)Thus, new traits occur in offsprings.
4)e.g. In Zea maize - colour and shape of
grain show incomplete linkage
Linkage Groups:
1)All the linked genes in a particular chromosome,
constitute a linkage group.
2)The number of linkage groups of a particular
species corresponds to its haploid number of chromosomes.
3)e.g. Drosophila melanogaster has 4 linkage
groups that correspo d to the 4 pairs of chromosomes.
4)Garden pea has 7 linkage groups and 7 pairs of
chromosomes.
Sex-linkage:
The
transmission (inheritance) of X -linked and Y-linked genes from parents to
offspring, is called sex-linked inheritance.
Sex-linked inheritance is of-three types viz.
X-linked, Y-Iinked and XYlinked
Sex
linkage is of two kinds-
a) Complete sex linkage:
It is exhibited by g chromosomes.
They inherit together because crossing 0 er does not
occur in this region.
Examples of X-linked traits are hemopbilia,
red-green colour blindness, myopia (near sighted ness) and for Y -linked are hypertrichosis, Ichthyosis, etc.
b)Incomplete sex linkage:
. It is exhibited by genes located on homologous
regions of X and Y chromosomes.
They d no, inherit together because crossing over
occurs in this region.
Exam les of X-Y linked tr is are total colour
blindness, ne hritis, retinitis i mentosa, etc.
Crossing Over:
1)Crossing over is a process that produces new
combinations (recombination's) of genes by interchanging and exchanging of
corresponding segments between non-sister chromatids of homologous chromosomes.
2)It occurs during pachytene of prophase-I of
meiosis. The term crossing over was coined by Morgan.
3)The mechanism of crossing over consists four
sequential steps such as synapsis, tetrad formation, crossing over and
terminalization.
4)The phenomenon of crossing over is universal and
it is necessary for the natural selection, because it increases the chances of
variation.
Morgan's Experiments showing linkage and crossing
over:
Morgan
used Drosophila melanogaster (fruit
fly) for his experiments because-
1)Drosophila
can easily be cultured in laboratory.
2)Its
life span is short, about two weeks. More over, it has high rate of
reproduction.
3)Morgan
carried out several dihybrid cross experiments in fruit fly to study genes that
are sex-linked.
4)The
crosses were similar to dihybrid crosses, as carried out by Mendel in Pea.
5)For
example, Morgan and his group crossed yellow-bodied, white eyed female to
the wild type with brownbodied, red eyed males and
intercrossed their F1
progeny.
6)He
observed that the two genes did not segregate independently of each other and
F2 ratio is not same as to dihybrid cross 9:3:3:1 ratio.
7)Morgan
and his group knew that the genes were located on X chromosome and stated that
when two genes in a dihybrid cross are situated on the same chromosome, then
the proportion of parental combination is much higher than non-parental type.
8)This
occurs due to physical association or linkage of the two genes
9)He
also found that, when genes are grouped on the same chromosome, some genes are
strongly linked. . .
10)They
show very few recombination's (1.3 %).
11)When
genes are loosely linked i.e. present far away from each other on chromosome,
they show more (higher) recombination's (37,2 %).
12)For
example, the genes for yellow body and white eye were strongly linked and
showed only 1.3 percent recombination (in cross-I).
13)White
bodied and miniature wings showed 37.2 percent recombination (in cross-II).
14)Cross
I show crossing over between genes y and w, Cross II shows crossing over
between genes white (w) and miniature wing (rn), Here dominant wild type
alleles are represented with (+) sign.
