The document discusses heredity and genetics, explaining how traits are passed from parents to offspring through genes located on chromosomes. It details historical perspectives on the chromosomal theory of inheritance established by Mendel, Sutton, and Morgan, reinforcing that genetic material is carried by chromosomes and elucidating mechanisms such as sex-linked traits and genetic variation. Key experiments and discoveries, including nondisjunction and gene mapping, are highlighted as evidence supporting the association of genes with specific chromosomes.

1. SANDESH,G.M

2.  Heredity or Hereditary is the process of passing the
traits and characteristics from parents to offsprings.
 The offspring cells get their features and
characteristics aka genetic information from their
mother and father.
 Heredity and genetics are the reason you look so
much like your parents

3.  When Gregor Mendal formulated his laws of
heredity, he postulated a particulate nature for the
units of inheritance. What exactly these particles
were he did not know.
 The unraveling of the physical basis of heredity led
to understand location of genes and their transfer.
 Today scientists understand not only the physical
location of hereditary units (i.e., the genes) but their
molecular composition as well.

4.  In the year 1900, Mendel’s paper was rediscovered by Correns, de Vries and
Tshermack. This paper of Mendel outlined the essiential features of the
behavior of genes during inheritance.
 In the same year, the second edition of “The Cell in Development and
Inheritance” by Wilson was published.
At that time it was well established that.
 Each species has a constant chromosome number
 Each chromosome divides longitudinally during mitosis.

5.  It was generally supposed that genes
were located on chromosomes, but a
critical assessment of the supposition
was lacking.
 It was not recognized that chromosome
occurred in pairs and that different
chromosomes of the haploid set were
distinct from each other.

6. 1902-Correns proposed that genes are located on chromosomes.
 The idea of chromosomal basis of segregation and independent
assortment was put forth by Sutton.
1903- The hypothesis that genes are located in chromosomes was
designated as Sutton-Boveri hypothesis.
Chromosomal theory of Inheritance
1865- Mendels work was published.
Hypothetical units of inheritance - Factors
1900- Reconstitution of mendels work was done.
W. Sutton T.Boveri

7. Mendel
 Factors are in pairs
 Factors separate at the time of
gamete formation.
 Alleles or factors assort
independently
Sutton and Boveri
 Chromosomes which have these
genes (factors) are in pair
(Homologous)
 Homologous chromosomes
separate at the time of gamete
formation.
 Chromosome assort independently

8. 1. Both chromosomes as well as genes occur in pairs in the somatic
or diploid cells.

9. 2. A gamete contains only one chromosome of a type and only one of
the two alleles of a character.

10. 3. The paired condition of both chromosomes as well as Mendelian
factors is restored during fertilization.

11. Chromosomes are also transferred from one generation to the next
as in the case of genes (Mendelian factors).
The number of chromosomes is fixed in each living species. These
are found as homologous pairs in diploid cells.
One chromosome from father and the other contributed by the
mother constitute a homologous pair.

12.  Before cell division, each chromosome as a whole and the alleles of
genes get replicated and are separated during mitotic division.
 Meiosis takes place during gamete formation.
 Homologous chromosomes form synapses during prophase-I stage which
in later course get separated and transferred to daughter cells.
 Each gamete or a haploid cell has only one allele of each gene present in
the chromosome.
 A characteristic diploid number is again established by the union of the
two haploid gametes.
 Both chromosomes and the alleles (Mendelian factors) behave in
accordance to Mendel's law of segregation.

13.  Sex chromosomes.
 White eye gene of drosophila (sex linkage).
 Non-disjunction of X chromosomes (sex linkage).
 Attached X chromosomes.
 Bar eye gene (A duplication)
 Y-linked inheritance
 Structural and numerical chromosome changes
 Linkage and chromosome maps.
 Genetic material
 In situ hybridization
 Transgenic organisms

14. Sex Determination by chromosomes :
Those chromosomes which are involved in the determination of sex of an
individual are called sex chromosomes while the other chromosomes are called
autosomes.
22 +
X
22 +
XX
(b) The X-0 system
(c) The Z-W system
76 +
ZW
76 +
ZZ
32
(Diploid)
16
(Haploid)
(d) The haplo-diploid system

15.  In 1902, Mc Clung discovered the accessory or X
chromosome of grasshopper and postulated that
chromosome was involved in sex determination.
 This was the first demonstration that a character was
associated with a specific chromosome.
 Sex is also governed by genes.
 Since a specific chromosome is involved in sex
development, the genes governing this trait is located
in the X chromosome.

16.  In 1910, Morgan presented a more direct evidence
supporting chromosome theory of heredity.
 Morgan first observed and noted
 Wild type, or normal, phenotypes that were
common in the fly populations
 Traits alternative to the wild type are called
mutant phenotypes.
 In males, only one copy of a sex-linked gene is
required in order for them to express a trait,
such as white eyes.
 Two copies are required in females.

17.  Morgan mated a wild-type (red-eyed) female with a
mutant white-eyed male. The F1 offspring all had red eyes
• The F2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However,
no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes,
and half had red eyes.
• Morgan then bred an F1 red-eyed female to an
F1 red-eyed male to produce the F2 generation.

18.  Since all F1 offspring had red eyes, the mutant
white-eye trait (w) must be recessive to the
wild-type red-eye trait (w+).
 Since the recessive trait—white eyes—was
expressed only in males in the F2 generation,
Morgan hypothesized that the eye-color gene
(w) is located on the X chromosome and that
there is no corresponding locus on the Y
chromosome.
 This explains chromosome theory of
inheritance.
P
Generation
F1
Generation
F2
Generation
Ova
(eggs)
Ova
(eggs)
Sperm
Sperm
X
X
X
X
Y
WW+
W+
W
W+
W+ W+
W+
W+
W+
W+
W+
W
W+
W W
W

19.  From 1913 to 1916, Calvin Bridges performed experiments that indicated
genes are found on chromosomes.
 His experiments like unusual transmission of a sex-linked gene in drosophila
were a part of his doctoral thesis advised by Thomas Hunt Morgan.
 Nondisjunction, as described by Bridges, caused sperm or egg cells to contain
abnormal amounts of chromosomes. In some cases, that caused the offspring
produced by the sperm or eggs to display traits that they would typically not
have. Failure of homologues to separate in Anaphase I
 His research on nondisjunction provided evidence that chromosomes carry
genetic traits, including those that determine the sex of an organism

20.  Non disjunction gives rise
to some cytological
expectations.
 Those having vermilion
eye should have XXY
chromosome constitution.
 This was later confirmed
by cytological studies.
 This was a concrete proof in favor of chromosomal theory of inheritance.

21.  Attached X chromosome ( X^X) consists of two
full lenth X chromosome sharing common
centromere so that thay are always inherited
together.
 In 1922, L.V Morghan discovered a female fly,
which regularly produced unusual transmission of
a sex linked gene (Yellow body colour).
 She concluded two X chromosomes of the fly
attached together.
 Cytological studies confirmed presence of the sex
linked genes are located in the X chromosome.
 Hence its also a basis for chromosomal theory of
inheritance.

22.  Bar eyes is a trait in which flies have a reduced number of facets.
 Ultra-bar (double bar) is a trait in which flies have even fewer facets than the
normal bar homozygote.
 Both traits are X-linked and show complete dominance.

23.  Sturtevant demonstrated that phenotype of flies with two
bar genes on one chromosome and none on the other
(double bar heterozygous), was different from those with
one bar gene on each of the two chromosomes.
 This indicates that position of gene with respect to
adjacent regions also influences its expression. This was
called position effect.
 Later C.B. Bridges and his group, through a study of
salivary gland chromosomes demonstrated cytologically
that Bar character was associated with two such repeats of
16A region in X-chromosome and that ultrabar (double
bar) had three such repeats in the same region.
 Thus confirmed that genetic effect was localized to a
distinctly recognizable small segment of a chromosome.

24.  Some genes like H-Y genes in humans are located in
the short arm of Y chromosome (Ys).
 Such genes are transmitted from father to son and
absent in females.
 Such a characters are known as holandric characters
since they are confined to males only.
 This clearly indicates H-Y genes are located on Y
chromosome , thus heredity to such genes through Y
chromosome proves the physical basis of heredity.

25. Structural changes:
 If genes are located in chromosomes, structural changes should
lead to a corresponding change in linkage relationship of
concerned genes.
 Ex: when a segment is inverted, the inversion heterozygote
shows a inverted gene sequence to corresponding inverted
segment.
 Similarly in reciprocal translocation homozygotes, a
corresponding change in the two linkage groups is obtained.
 Since structural changes in chromosomes are always
accompanied by corresponding changes in the concerned
linkage groups genes must be located on the chromosomes.

26. Numerical changes:
 It also furnish evidence in favour of chromosome theory of heredity.
 In Monosomic plants for a specific chromosomes, genes located in the chromosome
should be of single copy. So they show a deviation from normal inheritance pattern.
 The same is the case of trisomics where they carry three copies of the concerned
genes than two.
 This property of monosomics and trisomics is exploited in assigning the different
linkage groups to individual chromosomes of the haploid set.
 Thus genes on present on the chromosomes that favours heredity can be understood.

27.  In 1910, Morgan described the phenomenon f linkage and recombination
between two sex linked genes in drosophila.
 Proposed that strength of linkage between any genes depends on the distance
between them in the chromosome.
 Based on these Sturtevant proposed preparation of chromosome map based on
frequencies of recombination between linked genes.
 The number of different linkage groups is the same as the number of
chromosomes in the haploid set.
 There are no single case is known where it differs.
 This is exactly what would be expected if the genes are located in
chromosomes.

28.  In 1931, Stern demonstrated that
recombination between the linked genes
indeed involved the exchange of chromatin
material between homologous
chromosomes.
 This was rather direct demonstration that
genes are located in chromosomes.
 Linkage maps of bacteria and viruses were
demonstrated to be circular. This led to
expectation of chromosomes of both
bacterial and viral would also be circular.
The isolation of same proved it.
Mutant phenotypes
Short
aristae
Black
body
Cinnabar
eyes
Vestigial
wings
Brown
eyes
Long aristae
(appendages
on head)
Gray
body
Red
eyes
Normal
wings
Red
eyes
Wild-type phenotypes
II
Y
I
X IV
III
0 48.557.5 67.0 104.5

29.  It is accepted beyond any
reasonable doubt that DNA is the
genetic material in eukaryotes and
most prokaryotes.
 By far the vast majority of DNA
occurs in chromosomes of
eukaryotes and prokaryotes.
 Therefore it is self evident that the
genetic information or genes must
be an integral part of chromosomes

30.  Gene expression occurs as follows:
1. RNA polymerase produces RNA copies (mRNA)
of one strand of the gene.
2. The mRNA direct synthesis of specific protein.
3. The protein participates in cellular activities to produce the phenotype specified by
the gene.
 Radioactive labelled DNA copies of different mRNA molecules have been shown
to base-pair with DNA in different but specific location of chromosomes in case of
IN SITU hybridization.
 This demonstrates beyond any doubt that genes are present in chromosomes and
they are the physical basis of heredity.

31.  The mRNA produced by gene can be used to obtain copies of the concerned
genes as follows.
1. The enzyme reverse transcriptase can be used t produce a DNA copy (cDNA)
of the mRNA using latter as a template.
2. Alternatively, the genomic DNA of the organism may be isolated and the
genomic DNA segment complementary to the mRNA may be obtained .
 Genes obtained by either technique have been transferred on to a different
organisms, where they are expressed.
 Copies of gene obtained by both the approaches produce identical proteins.
 This clearly shows that the genes are located in chromosomes.

32. The foregoing considerations clearly demonstrate that the genes are
located on chromosomes and the chromosomes that carry genes to
off springs are the physical basis of heredity.