in this presentation how chromosomes are basis of inheritance is explained....i have combined 3 or 4 presentations..i hope so it will be helpfull

2. CHROMOSOMAL
BASIS OF
INHERITANCE

4. Genes and Chromosomes
Genes
 Are located on chromosomes
 Can be visualized using certain
techniques

5. INTRODUCTION
 In 1875 process of Mitosis
and in 1890 Meiosis was
worked out
 Independently, Karl Correns,
Erich von Tschermak, and
Hugo de Vries all found that
Mendel had explained the
same results 35 years before
 But there was still resistance

6.  Around 1900, cytologists and
geneticists began to see parallels
between the behavior of
chromosomes and the behavior
of Mendel’s factors
 Chromosomes and genes are
both present in pairs in
diploid cells
 Homologous chromosomes
separate and alleles
segregate during meiosis
 Fertilization restores the
paired condition for both
chromosomes and genes.

7. Chromosomal Theory of
Inheritance
In 1902, Walter Sutton, Theodor
Boveri, and others noted these
parallels and a chromosome theory
of inheritance began to take form
The chromosome theory of
inheritance states that
Mendelian genes have specific
loci on chromosomes
Chromosomes undergo
segregation and independent
assortment

8. Evidence that Genes Associated to
Chromosomes
 Thomas Hunt Morgan was the first to associate a
specific gene with a specific chromosome
 He provided convincing evidence that
chromosomes are the location of Mendel’s
heritable factors
 Experimental animal, Drosophila melanogaster, a fruit
fly species that eats fungi on fruit.
• Fruit flies  prolific breeders  generation time of
two weeks.
• Fruit flies  three pairs of autosomes and a pair of
sex chromosomes (XX in females, XY in males).

9.  Thomas Hunt Morgan was the first to
associate a specific gene with a specific
chromosome
 He provided convincing evidence that
chromosomes are the location of
Mendel’s heritable factors
Evidence that Genes Associated to
Chromosomes

10.  In one experiment Morgan mated male flies with
white eyes (mutant) with female flies with red
eyes (wild type)
• The F1 generation all had red eyes
• The F2 generation showed the 3:1 red:white
eye ratio
 The white-eyed trait appeared only in males.
 All the females and half the males had red eyes
Correlating Behavior of a Gene’s Alleles with
Behavior of a Chromosome Pair
 Morgan concluded that a fly’s eye color was
linked to its sex

11. Morgan concluded that a fly’s eye color was
linked to its sex
 Females (XX) may have two
red-eyed alleles and have
red eyes or may be
heterozygous and have red
eyes.
 Males (XY) have only a
single allele and will be red
eyed if they have a red-eyed
allele or white-eyed if they
have a white-eyed allele.

12. Morgan’s Discovery
 That transmission of the X chromosome in fruit
flies correlates with inheritance of the eye-color
trait
 Was the first solid evidence indicating that a
specific gene is associated with a specific
chromosome

13. Linkage of Genes Affects
Inheritance
 Each chromosome
 Has hundreds or thousands of genes
 Genes
 located on the same chromosome, linked genes,
tend to be inherited together because the
chromosome is passed along as a unit
 Morgan did other experiments with fruit flies
 To see how linkage affects the inheritance of
two different characters

14. Linked genes
Located on the same chromosome often do NOT
assort independently:
Parental type
B
N
B b
N n
B
n
Parental type
b
N
b
n
Recombinant types
47% 5% 5% 43%

15. Unlinked genes
B b
Y y
Located on different chromosomes assort
independently:
B
Y
B
y
b
Y
b
y
25% 25% 25% 25%

16. Genetic Recombination and Linkage
 Recombinant offspring are those that show new
combinations of the parental traits
 Morgan discovered that genes can be linked but due
to the appearance of recombinant phenotypes, the
linkage appeared incomplete
 Morgan proposed that some process must
occasionally break the physical connection between
genes on the same chromosome
 Crossing over of homologous chromosomes was
the mechanism

17. Recombinant Types
Produced when a crossover occurs between the 2
genes being studied
AA
B
a
b
B
A a
b
b
Recombinant types Non recombinant types
B a
b
aA
B

18. The Chromosomal Basis
of Sex
 The Chromosomal basis for determining sex is
rather simple
 The X-Y System
Female homogameticXX
Male heterogametic XY
Sex of offspring depends on whether the sperm has an X
e.g human

19. The Z-W System
The X-O System
In insects Only one type of
chromosome
Females  XX, Males XO
Sex of off spring sperm has X or O
e.g
In birds, some fishes and some insects
Females ZW, Males ZZ
Sex chromosome is in the ovum
e.g birds , fishes ,insect etc

20. Human Sex linked
Disorders
 Some recessive alleles found on the X chromosome in
humans cause certain types of disorders
 Color blindness

21.  Hemophilia
 Sex linked recessive trait
 Absence of one or more clotting factors in the blood
 Gene that controls formation of clotting factors are
recessive and present on the X chromosome
 Individuals bleed excessively when injured
 Approx. 1/10,000 males are affected
 Female Hemophiliacmust have this trait on both the XX
 Male only one X chromosome from mother, will be a
hemophiliac if mother has this trait

22. Alteration of Chromosome Number
and Genetic Disorders
 Sex-linked traits are not the only notable deviation
from the inheritance patterns observed by Mendel
 Also, gene mutations are not the only kind of
changes to the genome that can affect phenotype
 Physical and chemical disturbances, errors in
meiosis damage chromosomes and alter #s
 Large-scale chromosomal alterations lead to
spontaneous abortions or cause a variety of
developmental disorders

23. Trisomy 21
 Results from an error during meiosis
 Affects 1/700 children born in the US
Downs Syndrome
 Extra chromosome on # 21
 This affects phenotype
 Facial features
Broad round face
Flattened nose
Small irregular teeth
 Stature usually short
 Heart defects
 Susceptible to infection, leukemia, Alzheimer’s
 Life span shorter
 Varying degrees of mental retardation

24. Aneuploidy
Klinefelter’s syndrome, an XXY male, occurs once in every 2000 live
births.
 These individuals have male sex organs, but are sterile.
 There may be feminine characteristics, but their intelligence is
normal
 Males with an extra Y chromosome (XYY) tend to somewhat taller
than average
Monosomy X or Turner’s syndrome (X0), occurs once in
every 5000 births, produces phenotypic, but immature
females.

25. Alterations of Chromosome
Structure
Breakage of a chromosome can lead to four types of changes in
chromosome structure
Deletion
A deletion occurs when a chromosome fragment lacking a
centromere is lost during cell division.
This chromosome will be missing certain genes.

26. Duplication
A duplication occurs
when a fragment
becomes attached as an
extra segment to a sister
chromatid
Inversion
Occurs when a
chromosomal fragment
reattaches to the original
chromosome but in the
reverse orientation.

27. Translocation
A chromosomal fragment joins a non homologous
chromosome.
Some translocations are reciprocal, others are not.