1. - Dr. Gourav Thakre
Jr-3
Dept. Of Anatomy
Dr.S.C.G.M.C. Nanded

2. Mode of inheritance
“ It Describes the different ways in which
genes handed down from parents to their
offspring through several generations may
express themselves.”

3. Genetic disorders classified into:
 Chromosomal disorders: abnormality in the number or
structure of chromosomes.
 Single gene disorders: called as Mendelian disorders,
due to a single mutant gene. Four basic pattern of
inheritance.
 Multifactorial inheritance: disorder is a result of
interaction of gene and environmental factors.
 Mitochondrial disorders: some neuropathies

4. Analysis of Genetic disorder
While dealing with genetic case:
1) Recording family history of the index case.
2) Index case/ proband: is an affected person who has
bought the attention of the family to the clinician.
3) Also called as propositus or proposita if female.
4) All the information is taken abt. Proband.
5) In family history
– does any of relative has similar complaints.
- does any relative show any other disease which
is not present in proband.
- any condition with which any of the relative has
suffered or is suffering which might have been
unnoticed.

5. 6) Is proband an outcome of consanguineous
marriage?
7) What is the ethnic group of the family?
8) Record addresses of the relevant family members
as this is very important in order to contact them.

6. Terminology
 Allele or Allelomorph :
One or more alternative forms of a gene found
at the same (corresponding) locus on homologous
chromosomes in an individual &/or a population.
 Homologous genes:
Identical alleles occupying the same locus on
homologous chromosomes.

7.  Homozygous :
An individual (homozygote) who possesses two
identical alleles at one particular locus on
homologous chromosomes.
 Heterozygous:
An individual (heterozygote) who possesses
two different alleles at one particular locus on
homologous chromosomes.

8.  Dominant:
An allele that is always expressed, both in
homozygous and heterozygous conditions
 Recessive:
An allele that is expressed only when it is
homozygous.

9.  Genotype:
The total genetic constitution (genome) of an
individual, or more specifically ,the alleles present at
one locus for a particular trait.
 Phenotype:
The appearance (physical, biochemical &
physiological) of an individual produced by
expression of the genotype under the influence of
the environment.

10. Pedigree
 Depicts the family data.
 Shorthand method of giving relevant information.
 Also the mode of transmission of the disorder in the
family.

12. Punnett square
 The Punnett square is a diagram that is used to
predict an outcome of a particular cross or breeding
experiment.
 Reginald C. Punnett
 To determine the probability of an offspring's having
a particular genotype.

13. Mendelian Inheritance
 Caused by single mutanat gene.
 Follow one of the following four pattern of
inheritance.
- Autosomal Dominant.
- Autosomal Recessive
- Sex/ X- linked Dominant
- X- linked Recessive inheritance.

14. Sex - linked inheritance
 Sex linked inheritance means X – linked or Y- linked
inheritannce.
 Genes on Y chromosome shows holandric
inheritance.
 Y chromosome has H – Y gene which shows single
gene mutation and trait is hairy pinna.
 So sex – linked inheritance means X – linked .
 It can be either Recessive or Dominant.

15. X – linked recessive inheritance.
 Males are said to be hemizygous in respect to X –
linked gene.
 The trait is determined by the gene carried on X –
chromosome.
 It manifests in females only when it is in double
dose therefore females are rarely affected.
 A heterozygous female forms a carrier.
 In males even single mutant gene on their X-
chromosome (heterozygous state) are affected.

17. Examples of X – linked recessive traits
 Haemophilia.
 Duchenne muscular dystrophy.
 Colour blindness.
 G6PD deficiency.

18. Duchenne muscular dystrophy
X – linked recessive trait, characterized by
progressive muscular weakness.
 Incidence : 1 in 3500 males.
 Clinical features :
- present between 3 to 5 years.
-muscular weakness characterized by awkward
gait, difficulty in climbing and inability to run.
-while standing from sitting position the child
climbs on itself, called as Grower’s sign.

19. -Lumbar lordosis in advanced cases.
-Inability to walk by the age of 11 years.
-Death, age ~20 from respiratory infection or cardiac
failure
 DMD gene on the end of X chromosome
 Encodes protein dystrophin that supports plasma
membrane during contraction
 If dystrophin absent or defective, cells are torn apart
 Diagnosis : elevated creatine kinase in serum.
muscle biopsy.

22. Pedigree analysis:

23. Haemophilia - A
Deficiency of factor VIII in blood
Incidence – 1 in 5000- 10,000
Clinical features:
 Increased prolonged bleeding
 Hemorrhages in joints and muscles
 Intracranial bleeding
Cytogenetics:
Distal region on long arm of chromosome X.
186kb DNA & 26 exons.

25. Punnett square

26. Pedigree

27. ex. Hemophilia in European royalty

28. Colour blindness
 Inability to perceive the colour Green (deuteranopia) &
Red (protanopia)
 Incidence: 1-8 %
Cytogenetics :
 Distal portion of long arm of chromosome X
 Genes for red & green opsin proteins are located
adjacent to each other
 Deletion of genes
 Unequal crossing over in meiosis.

29. Pedigree

32. X – linked dominant inheritance
 X – linked dominant trait occurs more frequently in
females.
 Twice common in females than males.
 Affected male transmits the trait to all his daughters
and none to his sons.
 Examples :
-vitamin D resistant rickets.
-Hypophosphataemia.
-Alports syndrome.
-charcot marie tooth disease.

33. Pedigree analysis